Patent Publication Number: US-9895135-B2

Title: Freehand ultrasound imaging systems and methods providing position quality feedback

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of Application No. 61/180,050 filed on 20 May 2009 and entitled ULTRASOUND SYSTEMS INCORPORATING SPATIAL POSITION SENSORS AND ASSOCIATED METHODS and Application No. 61/252,377 filed on 16 Oct. 2009 and entitled ULTRASOUND SYSTEMS INCORPORATING SPATIAL POSITION SENSORS AND ASSOCIATED METHODS, which are both hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     This invention relates to ultrasound imaging. The invention has particular application in the field of medical ultrasonography and oncology. 
     BACKGROUND 
     Ultrasound imaging is widely used in a range of medical applications. One area in which ultrasound imaging is used is to guide biopsy procedures. A biopsy typically involves identifying an abnormality of interest, such as suspicious solid mass, a distortion in the structure of a body tissue, or an area of abnormal tissue change. A needle or other fine member may be inserted into the abnormality and used to withdraw a small tissue sample for investigation. 
     Various types of needles may be used for biopsies. In fine needle aspiration, small hollow needles are used to extract cells from an abnormality. A core needle is a larger diameter needle which may be used to withdraw larger samples of tissue. Vacuum assisted devices may be used to collect multiple tissue samples during one needle insertion. In some cases ultrasound is used to assist in placing a guide wire into an abnormality to assist a surgeon in locating the abnormality for a surgical biopsy. 
     A problem with the use of ultrasound to guide a needle or wire in any of these procedures, or like procedures, is that the thin needles are often very difficult to see in an ultrasound image. This makes it difficult for a person taking the biopsy to ensure that the needle has reached its target. Also, guiding the needle to place the tip of the needle at an area of abnormality shown in an ultrasound image takes a significant amount of skill because the image does not always provide good feedback to the practitioner regarding exactly where the needle is placed and how the needle should be manipulated to cleanly enter the abnormality. Also, the needle may not be visible in the ultrasound image because the needle is out of the plane of the ultrasound image. 
     The following US patents and publications disclose technology in the general field of this invention:
         U.S. Pat. No. 7,221,972 to Jackson et al.;   U.S. Pat. No. 5,161,536 to Vilkomerson et al.;   U.S. Pat. No. 6,216,029 to Palteili;   U.S. Pat. No. 6,246,898 to Vesely et al.;   U.S. Pat. No. 6,733,458 to Stein et al.;   U.S. Pat. No. 6,764,449 to Lee et al.;   U.S. Pat. No. 6,920,347 to Simon et al.;   2004/0267121 to Sarvazyan et al.;   WO 94/24933 to Bucholz;   WO 97/03609 to Paltieli;   WO 99/27837 to Paltieli et al.;   WO 99/33406 to Hunter et al.;     Freehand  3 D Ultrasound Calibration: A Review , P-W. Hsu, R. W. Prager A. H. Gee and G. M. Treece CUED/F-INFENG/TR 584, University of Cambridge Department of Engineering, December 2007       

     SUMMARY 
     The following aspects and embodiments thereof are described and illustrated in conjunction with systems, apparatus and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described problems have been reduced or eliminated, while other embodiments are directed to other improvements. 
     An aspect of the invention provides an ultrasound system for use in directing a fine elongate instrument, the instrument having a longitudinal axis and a tip, towards a reference position located in a body, the ultrasound system comprising an ultrasound transducer operable to receive ultrasound echo signals returning from a portion of the body, a position sensing system operable to monitor a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, a controller communicatively coupled to the ultrasound transducer and the position sensing system, and a display communicatively coupled to the controller, wherein the controller is configured to generate a two dimensional ultrasound image based on the ultrasound echo signals display the ultrasound image on the display, determine a location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, and generate on the display a marker corresponding to a projection of the reference position onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the projection of the reference position onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the projection of the reference position onto the ultrasound image comprises a projection parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the controller is configured to compute a distance between the reference position and the plane of the ultrasound image, and to indicate on the display the distance between the reference position and the plane of the ultrasound image by applying a coded color and/or a coded luminosity to the marker corresponding to the projection of the reference position onto the plane of the ultrasound image. In some embodiments according to this aspect, the controller is configured to compute a distance between the reference position and the plane of the ultrasound image, and to indicate on the display the distance between the reference position and the plane of the ultrasound image using a coded marker forming part of the marker of the projection of the reference position onto the plane of the ultrasound image. 
     In some such embodiments, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is orthogonal to the plane of the ultrasound image. In other such embodiments, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the controller is configured to determine a location of a reference plane that contains the reference position and is parallel to the plane of the ultrasound image, determine a location of an axis-reference position plane intersection of the longitudinal axis of the instrument with reference plane, and to generate on the display a marker corresponding to a projection of the axis-reference position plane intersection onto the plane of the ultrasound image. In some such embodiments, the projection of the axis-reference position plane intersection onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the controller is configured to determine a location of an axis-image plane intersection of the longitudinal axis of the instrument with the plane of the ultrasound image, and to generate on the display a marker indicating the location of the axis-image plane intersection. In some such embodiments, the controller is configured to determine an angle between the longitudinal axis of the instrument and the plane of the ultrasound image, and to indicate on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image by applying a coded color and/or a coded luminosity to the marker indicating the location of the axis-image plane intersection. 
     In some embodiments according to this aspect, the controller is configured to determine an angle between the longitudinal axis of the instrument and the plane of the ultrasound image, and to indicate on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image using a coded marker comprised in the marker indicating of the location of the axis-image plane intersection. In some such embodiments, the coded marker comprises two lines meeting at a vertex and forming an angle corresponding to the angle between the longitudinal axis of the instrument and the plane of the ultrasound image, the angle of the marker bisected by an orthogonal projection of the longitudinal axis of the instrument onto the plane of the ultrasound image, the vertex located at the axis-image plane intersection. 
     In some embodiments according to this aspect, the controller is configured to determine a location of the tip of the instrument based on the spatial location and orientation of the instrument, compute a distance between the location of the tip of the instrument and the plane of the ultrasound image, and to indicate on the display the distance between the tip of the instrument and the plane of the ultrasound image by applying a coded color and/or a coded luminosity to the marker indicating indication of the location of the axis-image plane intersection. 
     In some embodiments according to this aspect, the controller is configured to determine a location of the tip of the instrument based on the spatial location and orientation of the instrument, compute a distance between the location of the tip of the instrument and the plane of the ultrasound image, and to indicate on the display the distance between the tip of the instrument and the plane of the ultrasound image by a coded size of the marker indicating the location of the axis-image plane intersection. 
     In some such embodiments, the distance between the tip of the instrument and the ultrasound image comprises the distance along a line from the tip of the instrument to the plane of the ultrasound image that is orthogonal to the plane of the ultrasound image. In other such embodiments, the distance between the tip of the instrument and the plane of the ultrasound image comprises the distance along a line from the tip of the instrument to the plane of the ultrasound image that is parallel to the longitudinal axis of the instrument. 
     Another aspect of the invention provides a method for generating a display useful in directing a fine elongate instrument, the instrument having a longitudinal axis and a tip, towards a reference position located in a body, the method comprising receiving ultrasound echo signals returning from a portion of the body, monitoring a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, generating a two dimensional ultrasound image based on the ultrasound echo signals, displaying the ultrasound image on the display, determining a location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, and generating on the display a marker corresponding to a projection of the reference position onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the projection of the reference position onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. In other embodiments according to this aspect, the projection of the reference position onto the ultrasound image comprises a projection parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the method comprises computing a distance between the reference position and the plane of the ultrasound image, and indicating on the display the distance between the reference position and the plane of the ultrasound image by applying a coded color and/or a coded luminosity to the indication of the projection of the reference position onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the method comprises computing a distance between the reference position and the plane of the ultrasound image, and indicating on the display the distance between the reference position and the plane of the ultrasound image using a coded marker forming part of the indication of the projection of the reference position onto the plane of the ultrasound image. 
     In some such embodiments, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is orthogonal to the plane of the ultrasound image. In other such embodiments, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the method comprises determining a location of an axis-reference position plane intersection of the longitudinal axis of the instrument with a plane that contains the reference position and is parallel to the plane of the ultrasound image, and generating on the display a marker corresponding to a projection of the axis-reference position plane intersection onto the plane of the ultrasound image. In some such embodiments, the projection of the axis-reference position plane intersection onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the method comprises determining a location of an axis-image plane intersection of the longitudinal axis of the instrument with the plane of the ultrasound image, and generating on the ultrasound image a marker indicating the location of the axis-image plane intersection. In some such embodiments, the method comprises determining an angle between the longitudinal axis of the instrument and the plane of the ultrasound image, and indicating on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image by applying a coded color and/or coded luminosity to the indication of the location of the axis-image plane intersection. In some embodiments, the method comprises indicating on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image using a coded marker forming part of the indication of the location of the axis-image plane intersection. In some such embodiments, the coded marker comprises two lines meeting at a vertex and forming an angle corresponding to the angle between the longitudinal axis of the instrument and the plane of the ultrasound image, the angle of the marker bisected by an orthogonal projection of the longitudinal axis of the instrument onto the plane of the ultrasound image, the vertex located at the axis-image plane intersection. 
     In some embodiments according to this aspect, the method comprises determining a location of the tip of the instrument based on the spatial location and orientation of the instrument, computing a distance between the location of the tip of the instrument and the plane of the ultrasound image, and indicating on the display the distance between the tip of the instrument and the plane of the ultrasound image using a coded color and/or coded luminosity applied to the indication of the location of the axis-image plane intersection. In some embodiments, the method comprises indicating on the display the distance between the tip of the instrument and the plane of the ultrasound image using a coded size of the marker indicating the location of the axis-image plane intersection. 
     In some embodiments, the distance between the tip of the instrument and the ultrasound image comprises a distance along a line from the tip of the instrument to the plane of the ultrasound image that is orthogonal to the plane of the ultrasound image. In other embodiments, the distance between the tip of the instrument and the plane of the ultrasound image comprises a distance along a line from the tip of the instrument to the plane of the ultrasound image that is parallel to the longitudinal axis of the instrument. 
     Yet another aspect of the invention provides an ultrasound system for use in directing a fine elongate instrument in a body, the instrument having a longitudinal axis and a tip, the ultrasound system comprising an ultrasound transducer operable to receive ultrasound echo signals returning from a portion of the body, a position sensing system operable to monitor a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, a controller communicatively coupled to the ultrasound transducer and the position sensing system, and a display communicatively coupled to the controller, wherein the controller is configured to generate a two dimensional ultrasound image based on the ultrasound echo signals, display the ultrasound image on the display, determine a location of the ultrasound image based on the spatial location and orientation of the ultrasound transducer, determine an angle between the longitudinal axis of the instrument and the plane of the ultrasound image, generate on the display a marker corresponding to a projection of at least a portion of the longitudinal axis of the instrument onto the plane of the ultrasound image based on the spatial location and orientation of the instrument, and indicate on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image using a coded appearance characteristic of the marker corresponding to projection of the longitudinal axis of the instrument onto the ultrasound image. 
     In some embodiments according to this aspect, the controller is configured to determine a location of an axis-image plane intersection of the longitudinal axis of the instrument and the plane of the ultrasound image, and the portion of the longitudinal axis of the instrument whose projection onto the ultrasound image is indicated by the marker comprises the axis-image plane intersection. In some embodiments, the coded appearance characteristic of the marker comprises two lines meeting at a vertex and forming an angle corresponding to the angle between the longitudinal axis of the instrument and the plane of the ultrasound image, the angle of the marker bisected by an orthogonal projection of the longitudinal axis of the instrument onto the plane of the ultrasound image, the vertex located at the axis-image plane intersection. 
     In some embodiments according to this aspect, the coded appearance characteristic of the marker comprises a coded luminosity indicative of the angle between the longitudinal axis of the instrument and the plane of the ultrasound image. In some embodiments according to this aspect, the coded appearance characteristic of the marker comprises a coded color indicative of the angle between the longitudinal axis of the instrument and the plane of the ultrasound image. 
     In some embodiments according to this aspect, the controller is configured to determine a location of the tip of the instrument based on the spatial location and orientation of the instrument, and to generate on the display a marker indicating the location of the tip of the instrument. 
     In some embodiments according to this aspect, the controller is configured to register a reference position and indicate on the display a marker corresponding to a projection of the reference position onto the plane of the ultrasound image. In some such embodiments, the ultrasound system comprises a user interface operable to register a user-indicated reference position, and the reference position comprises the user-indicated reference position registered by the user interface. 
     In some embodiments according to this aspect, the projection of the reference position onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. In other embodiments according to this aspect, the projection of the reference position onto the ultrasound image comprises a projection parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the controller is configured to compute a distance between the reference position and the plane of the ultrasound image, and to indicate on the display the distance between the reference position and the plane of the ultrasound image by applying a coded color and/or coded luminosity to the marker corresponding to the projection of the reference position onto the plane of the ultrasound image. In some embodiments according to this aspect, the controller is configured to indicate on the display the distance between the reference position and the plane of the ultrasound image using a coded marker forming part of the marker of the projection of the reference position onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is orthogonal to the plane of the ultrasound image. In other embodiments according to this aspect, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the controller is configured to determine a location of an axis-reference position plane intersection of the longitudinal axis of the instrument with a plane that contains the reference position and is parallel to the plane of the ultrasound image, and to generate on the display a marker corresponding to a projection of the axis-reference position plane intersection onto the plane of the ultrasound image. In some such embodiments, the projection of the axis-reference position plane intersection onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. 
     A further aspect of the invention provides a method for use in generating a display useful for directing a fine elongate instrument in a body, the instrument having a longitudinal axis and a tip, the method comprising receiving ultrasound echo signals returning from a portion of the body, determining a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, generating a two dimensional ultrasound image based on the ultrasound echo signals, displaying the ultrasound image on the display, determining a location of the ultrasound image based on the spatial location and orientation of the ultrasound transducer, determining an angle between the longitudinal axis of the instrument and the plane of the ultrasound image, generating on the display a marker corresponding to a projection of at least a portion of the longitudinal axis of the instrument onto the plane of the ultrasound image based on the spatial location and orientation of the instrument, and indicating on the display the angle between the longitudinal axis of the instrument and the plane of the ultrasound image using a coded appearance characteristic of the marker corresponding to projection of the longitudinal axis of the instrument onto the ultrasound image. 
     In some embodiments according to this aspect, the method comprises determining a location of an axis-image plane intersection of the longitudinal axis of the instrument and the plane of the ultrasound image, and the portion of the longitudinal axis of the instrument whose projection onto the ultrasound image is indicated by the marker comprises the axis-image plane intersection. 
     In some embodiments according to this aspect, the coded appearance characteristic of the marker comprises two lines meeting at a vertex and forming an angle corresponding to the angle between the longitudinal axis of the instrument and the plane of the ultrasound image, the angle of the marker bisected by an orthogonal projection of the longitudinal axis of the instrument onto the plane of the ultrasound image, the vertex located at the axis-image plane intersection. 
     In some embodiments according to this aspect, the coded appearance characteristic of the marker comprises a coded color and/or a coded luminosity indicative of the angle between the longitudinal axis of the instrument and the plane of the ultrasound image. 
     In some embodiments according to this aspect, the method comprises determining a location of the tip of the instrument based on the spatial location and orientation of the instrument, and generating on the display a marker indicating the location of the tip of the instrument. 
     In some embodiments according to this aspect, the method comprises registering a reference position, and indicating on the display a marker corresponding to a projection of the reference position onto the plane of the ultrasound image. In some such embodiments, the method comprises obtaining a user-indicated reference position via a user interface, and the reference position comprises the user-indicated reference position obtained via the user interface. 
     In some embodiments according to this aspect, the projection of the reference position onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. In other embodiments according to this aspect, the projection of the reference position onto the ultrasound image comprises a projection parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the method comprises computing a distance between the reference position and the plane of the ultrasound image, and indicating on the display the distance between the reference position and the plane of the ultrasound image by applying a coded color and/or coded luminosity to the marker corresponding to the projection of the reference position onto the plane of the ultrasound image. In some embodiments according to this aspect, the method comprises computing a distance between the reference position and the plane of the ultrasound image, and indicating on the display the distance between the reference position and the plane of the ultrasound image using a coded marker forming part of the marker of the projection of the reference position onto the plane of the ultrasound image. 
     In some embodiments according to this aspect, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is orthogonal to the plane of the ultrasound image. In other embodiments according to this aspect, the distance between the reference position and the plane of the ultrasound image comprises a distance along a line that is parallel to the longitudinal axis of the instrument. 
     In some embodiments according to this aspect, the method comprises determining a location of an axis-reference position plane intersection of the longitudinal axis of the instrument with a plane that contains the reference position and is parallel to the plane of the ultrasound image, and generating on the display a marker corresponding to a projection of the axis-reference position plane intersection onto the plane of the ultrasound image. In some such embodiments, the projection of the axis-reference position plane intersection onto the plane of the ultrasound image comprises an orthogonal projection onto the plane of the ultrasound image. 
     Yet another aspect of the invention provide an ultrasound system for use in locating a reference position located in a body, the ultrasound system comprising a memory operable to contain a spatial description of the reference position, an ultrasound probe operable to receive ultrasound echo signals returning from a portion of the body, an ultrasound image processor communicatively coupled to the ultrasound probe, the ultrasound image processor operable to generate an ultrasound image based on the ultrasound echo signals, a position sensing system operable to determine a spatial location and orientation of the ultrasound probe, an image plane locator communicatively coupled to the position sensing system, the image plane locator operable to determine a spatial description of a plane of the ultrasound image based on the spatial location and orientation of the ultrasound probe, a geometry computer communicatively coupled to the image plane locator and the memory, the geometry computer operable to determine a spatial relationship between the reference position and the plane of the ultrasound image based on the spatial description of the reference position and the spatial description of the plane of the ultrasound image, a graphics processor communicatively coupled to the geometry computer, the marker generator operable to generate a marker indicative of the spatial relationship between the reference position and the plane of the ultrasound image, and a display communicatively coupled to the ultrasound image processor and the marker generator, the display operable to display the ultrasound image and the marker. 
     Yet a further aspect of the invention provides an ultrasound system for use in guiding medical interventions in a body, the ultrasound system comprising an ultrasound transducer operable to receive ultrasound echo signals returning from a portion of the body, a fine elongate instrument insertable in the body, the instrument defining a longitudinal axis, a position sensing system operable to monitor a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, a controller communicatively coupled to the ultrasound transducer and the position sensing system; and a display communicatively coupled to the controller, wherein the controller is configured to generate a two dimensional ultrasound image based on the ultrasound echo signals, display the ultrasound image on the display, determine a location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, determine a first distance between a first line in the plane of the portion of the body depicted in the ultrasound image and a first point along the longitudinal axis at which the longitudinal axis of the instrument traverses the first line, determine a second distance between a second line in the plane of the portion of the body depicted in the ultrasound image and a second point along the longitudinal axis at which the longitudinal axis of the instrument traverses the second line, generate on the display a first needle-image alignment indicator having a first coded appearance characteristic indicative of the first distance, and generate on the display a second needle-image alignment indicator having a second coded appearance characteristic indicative of the second distance. 
     In some embodiments according to this aspect, the first line comprises a first edge of the portion of the body depicted in the ultrasound image and the second line comprises a second edge of the portion of the body depicted in the ultrasound image. 
     In some embodiments according to this aspect, the controller is configured to generate the first needle-image alignment indicator at a first location on the display adjacent to a first line of the ultrasound image corresponding to the first line in the plane of the portion of the body depicted in the ultrasound image, and to generate the second needle-image alignment indicator at a second location on the display adjacent to a second line of the ultrasound image corresponding to the second line in the plane of the portion of the body depicted in the ultrasound image. In embodiments according to this aspect, the first and second coded appearance characteristics may each comprise a color, a fill pattern and/or a feature of shape. In some embodiments according to this aspect, the first and second coded appearance characteristics are selected from a discrete set of different coded appearance characteristics. In some such embodiments, the controller is configured to determine the first coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based at least in part on whether the first distance is greater than a first threshold distance, and to determine the second coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based at least in part on whether the second distance is greater than a second threshold distance. 
     In some embodiments according to this aspect, the controller is configured to determine the first and second threshold distances based at least in part on a maximum angular separation between the longitudinal axis and the plane of the ultrasound image. 
     In some embodiments according to this aspect, the controller is configured to determine a first side of a plane of the portion of the body depicted in the ultrasound image which the first point along the longitudinal axis is on, determine a second side of the plane of the portion of the body depicted in the ultrasound image the second point along the longitudinal axis is on, determine the first coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based on the first side, and to determine the second coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based on the second side. 
     In some embodiments according to this aspect, the first and second coded appearance characteristics of the respective first and second needle-image alignment indicators each comprise a combination of a color and a feature of shape. In some such embodiments, the controller is configured to determine the color of the first needle-image alignment indicator based at least in part on the first distance, determine the feature of shape of the first needle-image alignment indicator based at least in part on the first side, determine the color of the second needle-image alignment indicator based at least in part on the second distance, and determine the feature of shape of the second needle-image alignment indicator based at least in part on the second side. 
     Still another aspect of the invention provides a method for providing a display for use in guiding a fine elongate instrument, the instrument defining a longitudinal axis, the method comprising receiving ultrasound echo signals returning from a portion of the body, determining a spatial location and orientation of the instrument and a spatial location and orientation of the ultrasound transducer, generating a two dimensional ultrasound image based on the ultrasound echo signals, displaying the ultrasound image on the display, determining a location of the ultrasound image based on the spatial location and orientation of the ultrasound transducer, determining a location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, determining a first distance between a first line in the plane of the portion of the body depicted in the ultrasound image and a first point along the longitudinal axis at which the longitudinal axis traverses the first line, determining a second distance between a second line in the plane of the portion of the body depicted in the ultrasound image and a second point along the longitudinal axis at which the longitudinal axis traverses the second line, generating on the display a first needle-image alignment indicator having a first coded appearance characteristic indicative of the first distance, and generating on the display a second needle-image alignment indicator having a second coded appearance characteristic indicative of the second distance. In some embodiments according to this aspect, the first line comprises a first edge of the portion of the body depicted in the ultrasound image and the second line comprises a second edge of the portion of the body depicted in the ultrasound image. 
     In some embodiments according to this aspect, the method comprises generating the first needle-image alignment indicator at a first location on the display adjacent to a first line of the ultrasound image corresponding to the first line in the plane of the portion of the body depicted in the ultrasound image, and generating the second needle-image alignment indicator at a second location on the display adjacent to a second line of the ultrasound image corresponding to the second line in the plane of the portion of the body depicted in the ultrasound image. 
     In embodiments according to this aspect, the first and second coded appearance characteristics may each comprise a color, a fill pattern, and/or a feature of shape. In some embodiments according to this aspect, the first and second coded appearance characteristics are selected from a discrete set of different coded appearance characteristics. In some such embodiments, the method comprises determining the first coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based at least in part on whether the first distance is greater than a first threshold distance, and determining the second coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based at least in part on whether the second distance is greater than a second threshold distance. 
     In some embodiments according to this aspect, the method comprises determining the first and second threshold distances based at least in part on a maximum angular separation between the longitudinal axis and the plane of the ultrasound image. 
     In some embodiments according to this aspect, the method comprises determining a first side of a plane of the portion of the body depicted in the ultrasound image which the first point along the longitudinal axis of the instrument is on, determining a second side of the plane of the portion of the body depicted in the ultrasound image which the second point along the longitudinal axis of the instrument is on, determining the first coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based on the first side, and determining the second coded appearance characteristic by selecting it from the discrete set of different coded appearance characteristics based on the second side. 
     In some embodiments according to this aspect, determining each of the first and second coded appearance characteristics of the respective first and second needle-image alignment indicators comprises determining a combination of a color and a feature of shape. Some such embodiments comprise determining the color of the first needle-image alignment indicator based at least in part on the first distance, determining the feature of shape of the first needle-image alignment indicator based at least in part on the first side, determining the color of the second needle-image alignment indicator based at least in part on the second distance, and determining the feature of shape of the second needle-image alignment indicator based at least in part on the second side. 
     Still a further aspect of the invention provides an ultrasound system comprising an ultrasound transducer operable to receive ultrasound echo signals returning from a portion of the body, at least one first position marker connectable to the ultrasound transducer, at least one second position marker connectable to the instrument, a controller communicatively coupled to the ultrasound transducer and the first and second position markers, and a display communicatively coupled to the controller, wherein, when the at least one first position marker is connected to the ultrasound transducer and the at least one second position marker is connected to the instrument, the controller is configured to monitor a spatial location and orientation of the ultrasound transducer based on a first position signal from the at least one first position marker, monitor a spatial location and orientation of the instrument based on a second position signal from the at least one second position marker, monitor a quality of the first position signal, monitor a quality of the second position signal, generate a two dimensional ultrasound image based on the ultrasound echo signals, determine a spatial location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, and to display on the display a plurality of display elements including the ultrasound image, and a marker corresponding to a projection of at least a portion of the longitudinal axis of the instrument onto the plane of the ultrasound image based on the spatial location and orientation of the instrument and the spatial location of the portion of the body depicted in the ultrasound image; and, when at least one of the quality of the first position signal is below a first quality threshold and the quality of the second position signal is below a second quality threshold, to generate an alert on the display by doing one or more of inhibiting display of at least one of the plurality of display elements and changing an appearance characteristic of at least one of the plurality of display elements. 
     In some embodiments according to this aspect, the controller is configured to inhibit display of the ultrasound image when the quality of the first position signal is below the first quality threshold. In some embodiments according to this aspect, the controller is configured to change an appearance characteristic of the ultrasound image when the quality of the first position signal is below the first quality threshold. 
     In some embodiments according to this aspect, the controller is configured to inhibit display of the marker when the quality of the second position signal is below the second quality threshold. In some embodiments according to this aspect, the controller is configured to change an appearance characteristic of the marker when the quality of the second position signal is below the second quality threshold. 
     An aspect of the invention provides a method in an ultrasound system comprising receiving ultrasound echo signals returning from a portion of the body, monitoring a spatial location and orientation of an ultrasound transducer at which the ultrasound echo signals are received based on a first position signal from at least one first position marker connected to the ultrasound transducer, monitoring a spatial location and orientation of the instrument based on a second position signal from at least one second position marker connected to the instrument, monitoring a quality of the first position signal, monitoring a quality of the second position signal, generating a two dimensional ultrasound image based on the ultrasound echo signals, determining a spatial location of the portion of the body depicted in the ultrasound image based on the spatial location and orientation of the ultrasound transducer, displaying a plurality of display elements including the ultrasound image and a marker corresponding to a projection of at least a portion of the longitudinal axis of the instrument onto the plane of the ultrasound image based on the spatial location and orientation of the instrument and the spatial location of the portion of the body depicted in the ultrasound image, and, when at least one of the quality of the first position signal is below a first quality threshold and the quality of the second position signal is below a second quality threshold, generating an alert on the display by doing one or more of inhibiting display of at least one of the plurality of display elements and changing an appearance characteristic of at least one of the plurality of display elements. 
     In some embodiments according to this aspect, the method comprises inhibiting display of the ultrasound image when the quality of the first position signal is below the first quality threshold. In some embodiments according to this aspect, the method comprises changing an appearance characteristic of the ultrasound image when the quality of the first position signal is below the first quality threshold. 
     In some embodiments according to this aspect, the method comprises inhibiting display of the marker when the quality of the second position signal is below the second quality threshold. In some embodiments according to this aspect, the method comprises changing an appearance characteristic of the marker when the quality of the second position signal is below the second quality threshold. 
     In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following detailed descriptions. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings illustrate non-limiting embodiments. 
         FIG. 1  shows an example ultrasound probe and biopsy assembly as may be used with the invention. 
         FIG. 2  is a perspective view of a probe and a corresponding image plane. 
         FIG. 3  shows example ultrasound images. 
         FIG. 4A  shows a display according to an example embodiment. 
         FIG. 4B  is a perspective schematic illustration of an ultrasound environment. 
         FIG. 5A  is a side elevation schematic illustration of an ultrasound environment. 
         FIG. 5B  shows a display according to an example embodiment. 
         FIG. 6A  is a side elevation schematic illustration of an ultrasound environment. 
         FIG. 6B  shows a display according to an example embodiment. 
         FIG. 7A  is a top plan schematic illustration of an image. 
         FIG. 7B  shows graphical illustrations of computations of in-plane threshold distances according to example embodiments. 
         FIG. 8A  is a perspective schematic illustration of an ultrasound environment. 
         FIG. 8B  shows a graphical illustration of a computation of an in-plane threshold distance according to an example embodiment. 
         FIG. 9  shows a display according to an example embodiment. 
         FIG. 10  show a schematic diagram of an ultrasound operating environment. 
         FIG. 11  shows a schematic diagram of the ultrasound operating environment depicted in  FIG. 10 . 
         FIG. 12  shows an ultrasound image display according to an example embodiment. 
         FIG. 13  shows an ultrasound image display according to an example embodiment. 
         FIG. 13A  is a perspective view of a vacuum biopsy needle. 
         FIG. 13B  shows an ultrasound image display according to an example embodiment. 
         FIG. 13C  shows an ultrasound image display according to an example embodiment. 
         FIG. 14  shows an example shadow representation which could be displayed on a 2D monitor to indicate the 3D relationship of a needle to an anatomical structure. 
         FIG. 15  shows ultrasound equipment according to an embodiment that provides automatic steering of a plane in which ultrasound images are acquired. 
         FIG. 16  is a flow diagram of a method for generating a 3D model. 
         FIG. 17  shows an example system which can generate a 3D model from ultrasound frames. 
         FIG. 18  is a block diagram of apparatus according to an example embodiment that includes a biopsy assembly, an ultrasound probe, and a 3D position sensing system. 
         FIG. 19  shows an example image that may be displayed on a monitor. 
         FIG. 20  shows a method according to one example embodiment. 
         FIG. 21  shows an example system in which a position sensing system determines positions of an ultrasound transducer and a needle and an instruction generator generates instructions to assist a user. 
         FIG. 21A  shows an angle between a needle and an image plane of a transducer. 
         FIG. 22  shows an ultrasound imaging probe having a built-in position sensor. 
         FIG. 23  is a side elevation cross-sectional view of a marker positioning apparatus according to an example embodiment. 
         FIG. 24  is a top plan view of a marker positioning apparatus according to an example embodiment. 
         FIG. 25  shows a real-time display of marker location and orientation according to an example embodiment. 
         FIG. 26  is a perspective view of an example marker. 
         FIG. 27  shows schematically the use of two probe positions to define a volume for a 3D model. 
         FIG. 28  illustrates one way to convert between coordinates of a pixel in a single ultrasound frame and the coordinate system for a 3D model. 
         FIG. 29  illustrates an embodiment involving placement of a needle in a woman&#39;s breast. 
         FIG. 30  illustrates an application involving placing an epidural catheter. 
         FIG. 31  shows an ultrasound system according to an example embodiment. 
         FIG. 32  shows an ultrasound system according to an example embodiment. 
         FIG. 33  shows an ultrasound system according to an example embodiment. 
     
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
       FIG. 1  shows an ultrasound probe  12 . Probe  12  comprises a transducer array  14  that can generate high frequency vibrations and transmit those high frequency vibrations into the body of a patient P. The vibrations are reflected from various structures and interfaces within patient P. Reflected signals are detected at transducer array  14  where they are converted to electronic form and delivered to an ultrasound system (not shown in  FIG. 1 ) for further analysis. Transducer array  14  may comprise a one or two-dimensional array of transducer elements, for example. The particular arrangement of transducer elements in array  14  may be selected based upon the medical application for which the probe  12  will be used. 
     To create a diagnostic image, an ultrasound controller causes electrical excitation signals to be delivered to elements of transducer array  14 . The transducer elements convert the excitation signals into ultrasonic vibrations. The ultrasonic vibrations typically have frequencies in the range of about 2 megahertz to about 15 megahertz. This is not mandatory. Embodiments may employ frequencies outside of this range. 
     The ultrasonic vibrations are scattered and/or reflected by various structures in the patient&#39;s body. Some of the reflected and/or scattered ultrasonic vibrations, which may be called echos, are received at transducer array  14 . The distance from the transducer array  14  to a particular location at which echos are generated may be determined by the time between the transmission of an ultrasonic vibration and the receipt of an echo of that ultrasonic vibration at transducer array  14 . The direction relative to probe  12  of a location at which an echo is generated may be determined by processing the echo signals. Various beam forming techniques may be used to determine the directions from which echos arrive at transducer array  14 . 
     For example, in so-called B-mode imaging, a 2D image of a selected cross-section of the patient&#39;s body is generated. Because the position and orientation of transducer array  14  is fixed in probe  12 , the particular cross section represented by an ultrasound image depends upon the current position and orientation of probe  12  relative to the patient&#39;s body. Moving probe  12  relative to the patient&#39;s body will result in a different cross section being imaged. 
       FIG. 1  shows two scattering locations, P 1  and P 2 . P 1  is located at position R 1 , θ 1 . P 2  is at location R 2 , θ 2 . These locations are both determined with reference to a coordinate system that can be considered to be attached to probe  12 . 
     The position and orientation of probe  12  are monitored by a 3D position sensing system  16 . The 3D position sensing system  16  may include one or more base units and one or more markers carried on probe  12 . In the illustrated embodiment, probe  12  includes a plurality of position markers  15 . In the illustrated embodiment, there are three position markers,  15 A,  15 B, and  15 C. Position markers  15 A,  15 B, and  15 C are not located along a common line. Therefore, if the locations of position markers  15 A,  15 B, and  15 C are known, the position and orientation in space of probe  12  is uniquely determined. Since the particular cross section represented by an ultrasound image depends upon the current position and orientation of probe  12 , the position and orientation of ultrasound images can be determined from the position and orientation in space of probe  12 . 
     The positions of location markers  15  relative to a global coordinate system are measured by 3D position sensing system  16 . In the illustrated embodiment the sensor system includes a position base unit  17 . 3D position base unit  17  and position markers  15  may comprise any suitable technology. For example, 3D position base unit  17  may detect electromagnetic or other fields emitted by position markers  15  or vice versa. In some embodiments the position base unit  17  generates a magnetic field that is sensed by position markers  15 . A 3D position sensing system may, for example, comprise a medSAFE™ or drive BAY™ position sensor available from Ascension Technology corporation of Burlington, Vt., USA. 
     Some 3D position sensing technologies permit both the location and orientation of a single position marker to be determined. Where such 3D position sensing technologies are used, fewer position markers  15  are required to determine the location and orientation of probe  12  than would be the case for position markers for which only position is determined. For example a single 6 degree of freedom position marker may be used in a compatible position sensor to obtain both position and orientation information for a probe  12 . Even in embodiments which detect the orientations of position markers, some redundant position markers  15  may be provided. In embodiments which provide more position markers than are required to identify position and orientation of probe  12 , positions of the additional position markers may be monitored by 3D position base unit  17  and used to provide information regarding the position and orientation of probe  12  of enhanced accuracy. 
       FIG. 1  also shows a biopsy apparatus  19  which includes a handle  20  and a needle  21 . Biopsy apparatus  19  includes one or more position markers  15 . In the illustrated embodiment, there are two position markers  15 , individually identified as  15 D and  15 E. In the illustrated embodiment, position markers  15 D and  15 E are located so that they correspond to reference positions on an extension of a longitudinal axis of needle  21 . Neglecting rotations about the axis of needle  21 , the position and orientation of needle  21  can be uniquely determined if the positions of position markers  15 D and  15 E are known. In the illustrated embodiment, the reference positions of location markers  15 D and  15 E are monitored by 3D position sensing system  16 . 
     In an alternative embodiment, biopsy apparatus  19  has one position marker of a type such that position base unit  17  can determine both a position and orientation of the position marker. The one position marker may, for example, comprise a six degrees of freedom marker. Additional position markers may optionally be provided on biopsy apparatus  19 . 
     In the illustrated embodiment, position markers  15 D and  15 E are built into a handle of biopsy apparatus  19 . Needle  21  is detachably affixable to the handle. 
     In some embodiments, position markers are built into probe  12 , biopsy apparatus  19  and/or needle  21 , such as in the manner described in application Ser. No. 12/703,706 filed on 10 Feb. 2010 and entitled ULTRASOUND SYSTEMS INCORPORATING SPATIAL POSITION SENSORS AND ASSOCIATED METHODS, which is hereby incorporated herein by reference. 
     It can be appreciated that the apparatus illustrated in  FIG. 1  may facilitate the placing of needle  21  into the body of patient P such that needle  21  may be used to acquire a tissue sample or place something at a desired location within patient P. Specifically, when an ultrasound image  23  is generated from ultrasound data acquired by probe  12 , the precise location and orientation of needle  21  relative to that ultrasound image can be determined from the known locations of position markers  15  on probe  12  and biopsy assembly  19 . Having this information allows the location of needle  21  to be illustrated clearly on image  23  (even if the ultrasound echos do not provide a clear image of needle  21 ). In the illustrated embodiment, needle  21  is represented by a computer-generated line  24  that shows the position of needle  21  in image  23 , as calculated based on the relative positions of position markers  15 . 
     In some embodiments, needle  21  is detachable from handle  19 . In such embodiments, needle  21  may be connected to handle  19  using a coupling which fixes the orientation of needle  21  relative to handle  19  such that the axis of needle  21  will have a predetermined alignment with the position markers  15 D and  15 E. Where a replaceable needle  21  is provided then there is a possibility that needles  21  of different lengths may be used. Advantageously, a procedure is provided for calibrating the apparatus to indicate the location of the tip of needle  21  relative to position marker(s)  15 D and  15 E. 
     In one embodiment, a target  25  that has a location known to 3D position sensing system  16  is provided. For example, target  25  may be provided on probe  12  or on 3D position base unit  17 . In some embodiments target  25  is provided on another apparatus having a known position. A user can touch the tip of needle  21  to target  25  (or  25 A). The user can indicate when this has been done by using a control (for example by pressing a button on biopsy assembly  19  or probe  12  or on a connected ultrasound system) or by automatically detecting that needle  21  has contacted target  25  or  25 A. For example, the contact of needle  21  with a target  25  may be detected by providing a pressure sensor in conjunction with target  25  or  25 A that detects the pressure exerted by needle  21  on the target, an electrical sensor that senses an electrical signal delivered to the target by way of needle  21 , a capacitive sensor, or the like. 
     Since needle  21  is used in a surgical procedure it is generally important that needle  21  be sterile and kept sterile. Target  25  should be also sterile. In some embodiments, target  25  is provided on a sterile cover that locks in place over probe  12 . In some embodiments, target  25  includes a marking on probe  12  that can be seen through a transparent portion of the cover. Other embodiments may also be contemplated. 
     In another embodiment, calibrating the apparatus to indicate the location of the tip of needle  21  relative to one or more position markers  15 D and  15 E may be achieved without the use of a target. To calibrate the needle, a user moves the body of the needle while keeping the position of the needle tip unchanged. This can be done, for example, by placing the tip of the needle on any surface and moving the handle around. The positions of position markers  15 D and  15 E are monitored by 3D position sensing system  16  and recorded as the needle is rotated. Since the tip of the needle remains stationary throughout, its location in the global coordinate system (i.e., its position relative to position base unit  17 ) is fixed. The distance between each of position markers  15 D and  15 E and the needle tip is also fixed. Consequently, position markers  15 D and  15 E each move on the surface of a spherical shell. 
     The position of the needle tip relative to position markers  15 D and  15 E may be determined using an algorithm that applies the knowledge that the measured positions of the position markers are on spherical shells (which in this example are centered on the stationary tip of the needle). The algorithm may use the fact that many position measurements can be obtained to reduce the effect of uncertainties in the positions measured for position markers  15 D and  15 E. For example, a recursive least squares position determination algorithm may be employed to estimate the offset between position markers  15 D and  15 E and the needle tip. An example of such an algorithm is explained in P. R. Detmer, G. Basheim, T. Hodges, K. W. Beach, E. P. Filer, D. H. Burns and D. E. Strandness Jr, 3D ultrasonic image feature localization based on magnetic scanhead tracking: in vitro calibration and validation,  Ultrasound Med. Biol.  20, 923-936 (1994) which is hereby incorporated by reference for all purposes. After determining offset, the needle tip position in the global coordinate system can be estimated in real-time through the position and orientation information provided from the position markers  15 D and  15 E. 
     Because position and orientation information from position markers on probe  12  indicates the position of the markers in 3D space, locating the ultrasound image (scan plane) in 3D space requires determining the relationship between the position and orientation of the ultrasound image and the markers on probe  12 . 
     Various methods for calibrating freehand ultrasound probes that may be applied to establish a correspondence between points in an ultrasound image obtained using probe  12  and corresponding points in a global coordinate system are described in  Freehand  3 D Ultrasound Calibration: A Review , P-W. Hsu, R. W. Prager A. H. Gee and G. M. Treece CUED/F-INFENG/TR 584, University of Cambridge Department of Engineering, December 2007 which is hereby incorporated herein by reference. 
     One example method for determining a transform that is characteristic of this relationship involves moving and/or rotating probe  12  to acquire images of a point phantom (e.g. the center of a ball phantom, a string phantom, or the like) from a variety different positions and angles. The position of the fixed feature in each ultrasound image, as measured relative to the ultrasound image coordinate system, is recorded along with the position and orientation of probe  12  in the global coordinate system, which is determined from position markers  15 A,  15 B and  15 C. If the location of the point phantom in the global coordinate system is assumed to correspond to the origin of a phantom coordinate system F, then the relationship between the location (u,v) of the point phantom in the ultrasound image and that its location at the origin of the phantom coordinate system can be expressed as 
                       T     F   ←   W       ⁢     T     W   ←   M       ⁢     T     M   ←   I       ⁢       T   s     ⁡     (         u           v           0         )         =     (         0           0           0         )             (   1   )               
where T F←W  is the transform from the global coordinate space to the phantom coordinate space, T W←M  is the transform from the marker coordinate space to the global coordinate space and T M←I  is the transform from the image coordinate space to the marker coordinate space and T s  is a scaling matrix that accounts for the differences in units of the ultrasound image and the real world (e.g., pixels versus millimeters). Since the position and orientation of the position markers is read from position sensing system  16 , T W←M  is known for each image acquired. Because the orientation of an arbitrary coordinate system relative to the global coordinate system is not relevant, the rotations of T F←W  can be set to arbitrary values (for example zero). This leaves an equation with 11 unknowns: the two unit scaling factors (one for each dimension of the ultrasound image), six calibration parameters (3 translations and 3 rotations from the ultrasound image orientation and position to the marker orientation and position) and three translations for the shift of the point phantom to the origin of the phantom coordinate system. If N images of the point phantom are obtained from a diversity of positions and orientations, the 11 unknowns can be found, for example, by minimizing
 
     
       
         
           
             
               
                 
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     The function of Equation (2) can be minimized using a recursive least squares algorithm, such as the Levenberg-Marquardt algorithm. After finding the transform matrix T M←I , any point on the ultrasound image can be first transformed into the marker coordinate system then into the global coordinate system using the position and orientation information provided from the position sensor system  16 . 
     Finding the optimized transform matrix for the ultrasound probe (the probe calibration process) can be simplified if the position and orientation of the sensor (marker) inside the probe are known to a reasonable degree of accuracy from the start. The following is an example of a simplified probe calibration method which applies a previously calibrated needle. Ideally, if the coordinate axes of the probe marker(s) are aligned with coordinate axes of the probe a rotation matrix is not needed. In this case calibration would only involve determining offsets between the ultrasound image orientation and position to the marker orientation and position. However, in the real-case it is difficult to perfectly align the axes of a marker with those of a probe. In a particular case, each axis might be off by a few degrees. 
     To correct for misalignment of the axes of the marker and probe (transducer array) one can perform a calibration process to arrive at a transformation that will rotate and translate the coordinates in the ultrasound image to corresponding coordinates in 3D space. One example way to achieve this is to separate the rotation matrix into three matrixes targeting each axis and calibrate the axes one-by-one. Such a process can be done automatically or manually. In an example automatic method, one can begin by imaging a needle with different probe angles and directions. Because the needle has been previously calibrated its position and orientation in space are known. Because any misalignments are reasonably small an approximate transformation is also known. For each angle, one can determine the distance between the actual intersection of the needle and the ultrasound imaging plane as identified by locating the needle in the ultrasound data with the predicted intersection of the needle and the ultrasound imaging plane. The predicted intersection may be determined from the known position of the needle, the known position of the probe marker(s) and the approximate transformation. A best-fit algorithm such as a recursive least squares position determination algorithm can then be used to find the best offset and rotation to minimize the total square error. A brute force search minimizing the error can also be used. 
     In an example manual optimization method, the rotation and translation between each axis is calibrated one-by-one.  FIG. 2  is a perspective view of a probe  210  scanning an image plane  211 . The axis definitions in the following explanation correspond to labels shown for axis lines  218 . The X axis extends in a direction away from the face of probe  210  (i.e., out of the plane of the transducer array and into the depth of the image plane). The Y axis extends along the long edge of the face of probe  210  (i.e., along the long dimension of the transducer array; laterally across the image plane). The Z axis extends along the short edge of the face of probe  210  (i.e., along the short dimension of the transducer array; out of the image plane). The ultrasound image corresponds to a planar slice defined between two planes parallel to the plane defined by the X and Y axes. The transducer array lies in a plane defined by the Y and Z axes. 
     To calibrate the X axis, the needle may be placed in a position parallel to the long edge  210 A of probe  210 . The X axis rotation angle is adjusted until the surface angle  212  is 0 or 180 degrees. To calibrate the Y axis, the needle may be put into a position parallel to the short edge  210 B of the probe head  210  and the Y axis rotation angle adjusted until the out-plane angle  214  and surface angle  212  are 90 degrees. To calibrate the Z axis, the needle may be put into a position parallel to the long edge  210 A of probe  210  and the Z axis rotation angle adjusted until the in-plane angle  216  is 0 or 180 degrees. The positioning of the needle may be assisted by locating the needle in the ultrasound image. In some embodiments, the location of the needle in the ultrasound image is determined automatically, such as, for example, by pattern recognition algorithms. The manual calibration process may use a suitable apparatus for positioning the probe and/or needle. 
     Because the axes of the marker(s) may not be aligned with the ultrasound image axes, rotating a marker plane about one marker axis may correspond to rotation about more than one ultrasound image axis. To overcome this difficulty, the marker planes can be rotated through ultrasound image axes instead. This can be achieved by right multiplying an axis adjustment matrix before and after the axis rotation. The right axis adjustment matrix can be estimated by putting the needle parallel to the probe surface and adjusting rotation angles. For example, if rotating the needle about only the Z axis changes the in-plane angle but not the out-plane angle and surface angle, then the marker Z axis is aligned well with the probe Z axis. 
     A simple way to verify the accuracy of the needle calibration is to rotate the needle about its axis 360 degrees (with, for example, 30 degree as the step size) while keeping the position of the probe fixed. The intersection of the needle line with the B-mode image (indicated by the cross mark) should not change by more than a small amount e.g. 1 mm. 
     The offsets and rotations required to calibrate a probe or needle may be stored in an electronic data file. In some embodiments, electronic data files are structured according to the XML format. In some embodiments, a user changing probes or needles can load calibration information for the particular newly attached probe or needle from an electronic data file. This saves the user having to calibrate the newly attached probe or needle. In some embodiments the data file is stored in a memory on the probe or a connector for the probe such that the calibration information is always available to an ultrasound machine to which the probe is connected. 
     Having knowledge of the location of needle  21  relative to the plane at which an ultrasound image  23  is obtained can permit the calculation and display of images and other feedback that helps a user to visualize the relative locations of needle  21  and a targeted abnormality or other location within a patient P.  FIG. 3  shows some examples. In  FIG. 3 , a main ultrasound image  23  shows an image  29  of some anatomical structure into which it is desired to place needle  21 . A plurality of graphical elements, namely line  24 , line  30  and marker  32  are superposed on image  23 . Line  24  represents a projection of needle  21  into the plane of the body represented in image  23 . Line  30  superposed on image  23  indicates the path that will be followed by the tip of needle  21  as it continues to be inserted. Marker  32  is superposed on image  23  at the location corresponding to the intersection of needle  21  with the plane and the body represented by image  23 . 
     In some embodiments, graphical elements, such as, for example, markers, lines and the like, may comprise translucent and/or stippled elements to allow the features of an ultrasound image to be made out by a user when the graphical elements are superposed on the image. In some embodiments, the display of a graphical element may be inhibited and/or the appearance of the graphical element modified to allow the features of an ultrasound image beneath the element to be made out, upon the detection of a condition of the ultrasound environment. For example, in some embodiments, when the tip of needle  21  is in or almost in the plane of the ultrasound image, marker  32  is modified or removed so that the user can see in image  23  the tip of needle  21  (or a line representative of the tip of needle  21 ) approaching the target location. 
     In some embodiments, marker  32 , line  24  and/or line  30  may comprise be coded by luminosity (brightness), color, size, shape, linestyle or other appearance characteristic to indicate a feature or aspect of an ultrasound environment, such as, for example, the distance between marker  32  and the tip of needle  21 . For example, marker  32  may be displayed in a color that transitions along a spectrum from green to red when the tip of the needle  21  nears the plane of image  23 . In some embodiments, marker  32  comprises a circle, the diameter of the circle varying according to the distance between the tip of the needle and the plane of the image). Some embodiments comprise a scale that relates a coded appearance characteristic of marker  32 , line  24  and/or line  30  to distance between the tip of needle  21  and marker  32 . 
     In some embodiments, an appearance characteristic of marker  32 , line  24  and/or line  30  is coded according to which side of the image plane the tip of needle  21  is on. For example, if needle  21  has not passed through the plane of an image marker  32  may be shown in blue, but if needle  21  has passed through the plane of the image, then marker  32  may be shown in yellow. In some embodiments, as the distance between the tip of needle  21  and a first side of the plane of image  23  closes, the color in which marker  32  is shown changes along a first spectrum (e.g., a spectrum running from green to blue), and as the distance between the tip of needle  21  and a second side of the plane of image  23  closes, the color in which marker  32  is shown changes along a second spectrum (e.g., a spectrum running from red to yellow). Some embodiments comprise a scale that relates a coded appearance characteristic of marker  32 , line  24  and/or line  30  to distance between the tip of needle  21  and a side of the plane of image  32 . 
     It may be possible for needle  21  to be oriented such that its trajectory does not intersect the plane of image  23  in the field of view of image  23 . In some embodiments, a coded appearance characteristic of line  30  and/or line  24  indicates the fact that the intersection of the trajectory of needle  21  with the plane of image  23  does not lie in the field of view of image  23 . For example, when the intersection of the axis of needle  21  with the plane of image  23  does not lie in the field of view of image  23 , line  30  may be shown in a different color (e.g., gray) as compared with its color when the intersection of the trajectory of needle  21  with the plane of image  23  does lie in the field of view of image  23 . Other aspects of the display may indicate the fact that the intersection of the trajectory of needle  21  with the plane of image  23  does not lie in the field of view of image  23 . For example, image  23  could be shown with a different range of colors or luminances (e.g., image  23  could be made gray or darker) if the intersection does not lie in the field of view of image  23 . 
     In some embodiments, a coded appearance characteristic of marker  32 , line  24  and/or line  30  indicates the angle that the trajectory of needle  21  makes with the plane of image  23 . For example, the color used to display line  30  may traverse a spectrum of colors as needle  21  moves from parallel to the plane of image  23  to perpendicular to the plane of image  23 . In such embodiments, the projection of needle  21  and its trajectory onto image  23  as lines  24  and  30  indicates the orientation of needle  21  in a first plane, and the brightness of line  30  indicates the orientation of needle  21  in a second plane orthogonal to the first. Accordingly, in such embodiments, a user may be able to ascertain, at least approximately, the orientation of needle  21  relative to features in the plane of image  23  in three dimensions from only line  30 . 
       FIG. 4A  shows a display  330  according to an example embodiment.  FIG. 4B  is a perspective schematic illustration of an ultrasound environment  350 . In environment  350 , trajectories  354  and  364  intersect plane  352 , respectively, at intersections  356  and  366 . Trajectories  354  and  364  intersect plane  352 , respectively, at oblique angles  358  and  368 . Angle  368  is larger than angle  358 . In display  330 , image area  332  corresponds to plane  352 . Lines  334  and  344  represent, respectively, the orthogonal projections of trajectories  354  and  364  onto plane  352 . Coded markers  336  and  346  represent, respectively, intersections  356  and  366 . Coded marker  336  comprises sector  336 A. The acute angle between the radii that define sector  336 A corresponds to angle  358 . Sector  336 A is bisected by the projection of trajectory  344  onto plane  352 . Coded marker  346  comprises sector  346 A. The acute angle between the radii that define sector  346 A corresponds to angle  368 . Sector  346 A is bisected by the projection of trajectory  364  onto plane  352 . 
     In an ultrasound systems comprising display  330 , lines  334  and  344  indicate the orientations of trajectories  354  and  364 , respectively, in plane  352 , and sectors  336 A and  346 A indicate the orientations, respectively, trajectories in planes orthogonal to plane  352 . Advantageously, a user may be able to ascertain, at least approximately, the orientation of trajectory  334  relative to features in the plane of image area  332  in three dimensions from line  334  and coded marker  336 . It will be appreciated that there exist many variations on the configurations of coded markers  334  and  346  that would convey similar information regarding the orientation of trajectories  354  and  364  in planes orthogonal to plane  352 . For example, coded markers  354  and  364  could comprise only the radii defining the sectors (thus appearing as rays of the angles whose vertexes lie, respectively, at the image locations corresponding to the intersection of trajectories  354  and  364  with plane  352  and which are bisected, respectively, the orthogonal projections of trajectories  354  and  364  onto plane  352 ). In other embodiments, coded appearance characteristics of coded markers, such as size, color, intensity, shape, linestyle, of the like, may be used to indicate the orientations of trajectories in planes orthogonal to the image plane. 
     In some embodiments, the angle that an instrument makes with the plane of an ultrasound image is indicated by coded markers whose appearances are related, respectively, to distances between the axis defined by the instrument and two or more lines in the plane of the image that are intersected by the projection of the axis onto the plane of the image. For example, the angle that an instrument makes with the plane of an ultrasound image may be indicated by coded markers whose appearances are related, respectively, to the distances between the axis defined by the instrument and lines at, along or near two or more edges of the image. 
       FIGS. 5A and 6A  show schematic side elevation views of ultrasound environments  380  and  390 , respectively.  FIGS. 5B and 6B  show example displays  386  and  396 , respectively. Displays  386  and  396  depict aspects of environments  380  and  390 , respectively. In environment  380  a needle  382  is at an angle to a middle plane  383  of an ultrasound image slice  384 . Needle  382  crosses middle plane  383  and tip  382 A lies in slice  384 . A trajectory  382 B of needle  382  traverses a first edge  384 A of slice  384 . Where trajectory  382 B traverses edge  384 A of slice  384 , trajectory  382 B is in slice  384  and at a distance  385 A from middle plane  383 . Needle  382  traverses a second edge  384 B of slice  384 . Where needle  382  traverses edge  384 B of slice  384 , needle  382  is in slice  384  and at a distance  385 B from middle plane  383 . 
     In environment  390  a needle  392  is at an angle to a middle plane  393  of an ultrasound image slice  394 . Needle  392  crosses middle plane  393  and tip  392 A lies in slice  394 . A trajectory  392 B of needle  392  traverses a first edge  394 A of slice  394 . Where trajectory  392 B traverses edge  394 A of slice  394 , trajectory  392 B is in slice  394  and at a distance  395 A from middle plane  393 . Needle  392  is out of slice  394  where needle  392  traverses edge  394 B of slice  394 . Where needle  392  traverses edge  394 B of slice  394 , needle  392  is at a distance  395 B from middle plane  393 . 
     In  FIG. 5B , display  386  comprises image area  387 . Image area  387  is bounded by image edges, including opposite image edges  387 A and  387 B, which correspond, respectively, to edges  384 A and  384 B of slice  384 . Image area  387  comprises an image depicting the region in slice  384 . Image area  387  comprises a computer-generated line  388  indicative of the projection of needle  382  onto the plane of the image depicted in image area  387 , and a stippled line  388 A indicative of the projection of trajectory  382 B onto the plane of the image depicted in image area  387 . Display  386  comprises needle-image alignment indicators  389 A and  389 B. 
     In the illustrated embodiment, needle-image alignment indicators  389 A and  389 B are in the shape of elongated bars whose long edges are parallel to the lateral edges of image area  387 . Indicators  389 A and  389 B have a coded fill appearance indicative of proximity between needle  382  and/or its trajectory  382 B and middle plane  383  of image slice  384 . A coded fill appearance may comprise, for example, a color, a pattern, a combination thereof, or the like. In some embodiments, needle-image alignment indicators indicate proximity between a needle axis and an image slice using coded appearance characteristics of color, pattern, shape, size, linestyle or the like. 
     The coded fill appearance of indicators  389 A and  389 B indicate that needle  382  and its trajectory  382 B are close to middle plane  383  where they traverse edges  384 A and  384 B of slice  384 . Since both indicators  389 A and  389 B indicate that needle  382  and its trajectory  382 B are close to middle plane  383  where they traverse edges  384 A and  384 B of slice  384 , a user may infer that the axis defined by needle  382  (i.e., needle  382  and its trajectory  382 A) is substantially parallel to and near middle plane  383 . 
     In  FIG. 6B , display  396  comprises image area  397 . Image area  397  comprises an image depicting the region in slice  394 . Image area  397  is bounded by image edges, including opposite image edges  397 A and  397 B, which correspond respectively, to edges  394 A and  394 B of slice  394 . Image area  397  comprises a computer-generated line  398  indicative of the projection of needle  392  onto the plane of the image depicted in image area  397 , and a stippled line  398 A indicative of the projection of trajectory  392 B onto the plane of the image depicted in image area  397 . Display  396  comprises needle-image alignment indicators  399 A and  399 B. 
     In the illustrated embodiment, needle-image alignment indicators  399 A and  399 B are in the shape of elongated bars whose long edges are parallel to the lateral edges of image area  397 . Indicators  399 A and  399 B have a coded fill appearance indicative of proximity between needle  392  and/or its trajectory  392 B and middle plane  393  of image slice  394 . Indicators  399 A and  399 B have different coded fill appearances. The coded fill appearance of indicator  399 A indicates that trajectory  392 B is close to middle plane  393  where it traverses edge  394 A of slice  394 . The coded fill appearance of indicator  399 B indicates that needle  392  is far from middle plane  393  where needle  392  traverses edge  394 B of slice  394 . Since indicator  399 A indicates trajectory  392 B is close to middle plane  393  where it traverses edge  394 A of slice  394  and indicator  399 B indicates that needle  392  is far from middle plane  393  where it traverses edge  394 B of slice  394 , a user may infer that the axis defined by needle  382  is not substantially parallel to slice  384 . 
     It will be appreciated that the angle that an instrument makes with the plane of an ultrasound image may be indicated by coded markers whose appearance characteristics are related, respectively, to the distances between the axis defined by the instrument and lines in the plane of the image that are intersected by the projection of the axis onto the plane of the image and that themselves intersect. For example, the appearance characteristics of needle-image alignment indicators may be related, respectively, to the distances between the axis defined by the instrument and two edges of an image slice that share a vertex. In some embodiment where the projection of the axis defined by a needle onto an image “cuts the corner” of the image, needle-image alignment indicators may be provided along the edges that form the corner that is cut. 
     Those skilled in the art will understand that the methods and apparatus disclosed herein for generating needle-image alignment indicators for circumstances where a needle axis traverses opposite image edges may be generalized for application to circumstances where a needle axis traverses lines that are not parallel (e.g., lines that intersect each other), such as, for example, lines at, along or adjacent to opposite edges of an image that are not parallel (e.g., as in generally trapezoidal ultrasound images acquired by curved transducer arrays). It will further be appreciated that methods and apparatus disclosed herein for generating needle-image alignment indicators may be applied using lines intersected by the projection of an axis defined by a needle, which lines are not straight, such as, for example, lines comprising image edges that comprise curves, arcs and the like. 
     The coded appearance characteristics of needle-image alignment indicators, such as indicators  389 A,  389 B,  399 A and  399 B may indicate proximity over a continuous range, or may indicate discrete ranges of proximity. In some embodiments, a needle-image alignment indicator comprises an appearance characteristic from a continuous range of appearance characteristics which corresponds to a range of distances between a middle plane of an image slice and the point at which the needle axis traverses an edge of the image slice. For example, a needle-image alignment indicator may comprise a color from a continuous spectrum of color which corresponds to a range of distances between a middle plane of an image slice and the point at which a needle trajectory traverses an edge of the image slice. 
     In some embodiments, a needle-image alignment indicator comprises an appearance characteristic from a discrete set of appearance characteristics, each of which corresponds to a distinct range of distances between a middle plane of an image slice and the point at which a needle axis traverses an edge of the image slice. For example, a coded appearance characteristic may comprise one of a set of two colors (e.g., green and red), with green corresponding to distances between a middle plane of an image slice and the point at a needle axis traverses an edge of the image slice of less than or equal to a pre-determined threshold distance and with red corresponding to distances greater than the predetermined threshold distance. In embodiments where a coded appearance characteristic of a needle-image alignment indicator is drawn from a set of different appearance characteristics (e.g., colors, patterns, symbols, combinations thereof, or the like), one of which is indicative of close proximity to and/or presence within an ultrasound image slice, a user may use the indicators to quickly determine whether the needle axis is in-plane, partially in-plane or out-of-plane. Such a means for quickly determining whether the needle axis is in-plane, partially in-plane, or out-of-plane may be useful where a user is attempting to advance a needle in the plane of an ultrasound image. 
     In some embodiments, a coded appearance characteristic of a needle-image alignment indicator comprises an appearance characteristic that makes the indicator indistinguishable from the background on which it is displayed when the distance between a needle and/or its trajectory is greater than a threshold distance from a point in an image plane. In other words, a needle-image alignment indicator may only appear to a user when a needle and/or its trajectory is sufficiently proximate to an image plane. 
     In some embodiments, a needle-image alignment indicator may indicate a side of the middle plane of the image slice which the needle axis is where it traverses an edge of the image slice. For example, a needle-image alignment indicator may comprise a coded appearance characteristic of a feature of shape selected from a set of shapes. Such a coded appearance characteristic may be selected, for example, from a set of symbols [+,−,=], in which the ‘+’ symbol indicates that the needle/trajectory-image edge crossing is more than a pre-determined distance away from the middle plane of the image on a first side of the image, ‘−’ symbol indicates that the needle/trajectory-image edge crossing is more than the pre-determined distance away from the middle plane of the image on a second side of the image, and the ‘=’ symbol indicates that the needle/trajectory-image edge crossing is within the pre-determined distance from the middle plane of the image. In some embodiments, a needle-image alignment indicator comprises the combination of a feature of shape (e.g., a symbol) indicative of which side of the middle image plane the needle and/or its trajectory is on where it traverses the edge of the image area, and a color indicative of proximity between a middle plane of an image slice and the point at which a needle or its trajectory traverses an edge of the image slice. 
     A pre-determined threshold distance upon which a needle is judged to by in-plane may be, for example, in the range of 0.2 mm to 1 mm. In some embodiments, a pre-determined in-plane threshold distance is 0.5 mm. In some embodiments, an in-plane threshold distance may be specified by a user. Apparatus may provide a user control by which a user can specify an in-plane threshold distance. 
     In some embodiments, an in-plane threshold distance is computed based on a maximum angular separation between an axis of a needle and the plane of an image. In some embodiments, an in-plane threshold distance is computed as a function of the distance between opposite edges of the image and a maximum angular separation between the needle axis and the image plane. For example, given a distance D between the opposite edges of the image and a maximum angular separation between the needle and the image plane of θ, the in-plane threshold distance may be computed as
 
 D  cos θ  (3)
 
       FIG. 7A  shows image  400  bounded by edges  402 N,  402 E,  402 S and  402 W. Opposite edges  402 E and  402 W are spaced apart by a distance D 1  in the plane of image  400 .
 
 f ( D 1,θ1)= D 1 cos θ1  (4)
 
       FIG. 7B  shows a right triangle  410  that illustrates the computation of in-plane threshold distance TD 1  as a function of D 1  and a maximum angular separation θ 1 , namely the function 
     It will be appreciated that an in-plane threshold distance may be calculated by functions having similar form, such as, for example,
 
 f ( D 1,θ1)= aD 1 cos θ1  (5)
 
where a is a pre-determined constant value.
 
     In some embodiments, an in-plane threshold distance is computed in real-time as a function of the distance in an image plane between a first point on a first edge of the image where the needle axis traverses the first edge and a second point on a second edge of the image area where the needle axis traverses the second edge, and a maximum angular separation between the needle axis and the image plane. For example, given a distance D between a first point on a first edge of the image where the needle axis traverses the first edge and a second point on a second edge of the image area where the needle axis traverses the second edge, and a maximum angular separation between the needle axis and the image plane of θ, the in-plane threshold distance may be computed as
 
 D  cos θ  (6)
 
     In  FIG. 7A , point  406  on edge  402 N of image  400  is the point where a needle axis (not shown) traverses edge  402 N, and point  408  on edge  402 W of image  400  is the point where a needle axis (not shown) traverses edge  402 W. Points  406  and  408  are spaced apart by a distance D 2  in the plane of image  400 .  FIG. 7B  shows a right triangle  420  that illustrates the computation of in-plane threshold distance TD 2  as a function of D 2  and a maximum angular separation θ 1 , namely the function
 
 f ( D 2,θ2)= D 2 cos θ2  (7)
 
     It will be appreciated that an in-plane threshold distance may be calculated by functions having similar form, such as, for example,
 
 f ( D 2,θ2)= aD 2 cos θ2  (8)
 
where a is a pre-determined constant value.
 
     In some embodiments, an in-plane threshold distance is computed as a function of the distance along the needle axis between the edges of the image and a maximum angular separation between the needle/trajectory and the image plane. For example, given a maximum angular separation between the needle axis and the image plane of θ and a distance H between the points on the needle axis where the axis traverses the edges of the image, the in-plane threshold distance may be computed as 
     
       
         
           
             
               
                 
                   
                     H 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     sin 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     θ 
                   
                   2 
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
           
         
       
     
     In some embodiments, in-plane threshold distances are computed for edges separately. For example, given a maximum angular separation between the needle and the image plane of θ and a distance D between the intersection of the needle axis and a point on an edge of the image where the needle axis traverses the edge, an in-plane threshold distance for the edge may be computed as
 
 B  cos θ  (10)
 
       FIG. 8A  shows an ultrasound operating environment  430 . An image plane  432  is intersected by an axis  434  of a needle at point  437 . Axis  434  traverses an edge  433  of image plane  432  at point  436  on edge  433 . Point  436  is spaced apart from point  437  by a distance D in the plane of image  432 .  FIG. 8B  shows a right triangle  440  that illustrates the computation of in-plane threshold distance TD as a function of D and a maximum angular separation θ, namely the function.
 
 f ( D 2,θ2)= D 2 cos θ2  (11)
 
     It will be appreciated that an in-plane threshold distance may be calculated by functions having similar form, such as, for example
 
 f ( D ,θ)= aD  cos θ  (12)
 
where a is a pre-determined constant value.
 
     Another example of computing in-plane threshold distances for edges separately comprises computing an in-plane threshold distance based on a maximum angular separation between the needle axis and the image plane of θ and a distance H between a point on the needle axis where the needle axis traverses an edges of the image using the function
 
 f ( H ,θ)= H  sin θ  (13)
 
     In  FIG. 8A , point  438  is the point on needle axis  434  where axis  434  traverses edge  433 . Point  438  is separated from point  437  by a distance H along axis  434 .  FIG. 8B  shows a right triangle  440  that illustrates the computation of in-plane threshold distance TD as a function of H and a maximum angular separation θ, namely the function.
 
 f ( H ,θ)= H  sin θ  (14)
 
     It will be appreciated that an in-plane threshold distance may be calculated by functions having similar form, such as, for example
 
 f ( H ,θ)= aH  sin θ  (15)
 
where a is a pre-determined constant value.
 
     Similar trigonometric relationships for determining an in-plane threshold distance based on the orientation and location of the needle and a maximum angular separation may be used. Maximum angular separation used in determining an in-plane threshold distance may be in the range of 0.1 to 1.5 degrees, for example. In some embodiments, the maximum angular separation is 0.5 degrees. In some embodiments, a maximum angular separation may be specified by a user. Apparatus may provide a user control by which a user can specify a maximum angular separation. 
     Those skilled in the art will appreciate that the dimensions of ultrasound images may depend on the configuration of the ultrasound transducer used to acquire the image and the gating of samples (i.e., the distance of the slice imaged from the transducer). Where this is the case, methods and apparatus may provide for determination of image dimensions in real-time. 
     In some embodiments, a user can mark one or more reference positions on image  23 . A reference position may correspond to a target location at which it is desired to place the tip of needle  21 , a structure that cannot be penetrated by needle  21 , or the like. Reference positions may be defined in terms of coordinates of a global coordinate system, such as, for example, the global coordinate system used by 3D position sensing system  16  to track position markers  15 . Reference positions may comprise a plurality of points in space. 
     A reference position marker corresponding to the marked reference position may be displayed on image  23 . In some embodiments the display indicates a deviation between the reference position and the location  32  at which the further advance of needle  21  would intersect the plane represented by image  23 . An audible signal generated by a suitable transducer, such as a speaker  35  may also be provided to give the user an indication of the proximity of the tip of needle  21  to the reference position at which it is desired to place the tip of the needle and/or an indication of the deviation between the reference position and the location  32  at which the further advance of needle  21  would intersect the plane represented by image  23 . 
     Some embodiments provide a touch screen display and reference positions can be identified by touching an area of an image displayed on the screen. In some embodiments, reference positions can be specified by tracing areas on an image using a finger, a stylus or the like. In some embodiments, users may specify an appearance characteristic for a marker that represents a reference position to differentiate the reference positions from other reference positions. For example, a user may specify that a marker representing a target location at which it is desired to place the tip of needle  21  be displayed in a first color, and that a marker representing a structure that cannot be penetrated by needle  21  be displayed in a second color. 
       FIG. 9  shows a display  220  according to an example embodiment. Display  220  comprises B-mode image  222 . Image  222  includes a line  224  representing a projection of a needle onto the plane of the body represented in image  222 . Another computer generated line  226  indicates the trajectory of the needle. Trajectory-image plane intersection marker  228  indicates the location at which the trajectory of the needle intersects the plane of image  222 . Reference position marker  230  indicates the location of a user-marked reference position. Deviation marker  232  indicates a deviation between the location of reference position marker  230  and trajectory-image plane intersection marker  228 . 
     A reference position marked on an image corresponds to one or more points in three dimensional space (e.g., one or more points lying in the plane of the image on which the reference position is marked). Where free hand probes, such as probe  12 , are used, it is possible that the field of view and/or image plane corresponding to a current position of the probe does not contain a out-of-image plane reference position. In these circumstances, users may find it difficult to re-position the probe so that the reference position is in the plane of the current image. 
     In some embodiments, when the plane of the current image does not contain a out-of-image plane reference position, a reference position projection marker corresponding to a projection onto the current image plane of the out-of-image plane reference position is displayed. The projection of the reference position onto the current image plane may comprise a projection parallel to the trajectory of a needle. A reference position trajectory-projection marker, which comprises a projection parallel to the trajectory of a needle, may be used in conjunction with the orientation of the needle as references for re-positioning the probe and/or needle relative to the out-of-image plane reference position. In some embodiments, a projection of a reference position onto a current image plane comprises an orthogonal projection onto the plane of the current image. A reference position orthogonal-projection marker, which comprises an orthogonal projection onto the current image plane, may be used in conjunction with the orientation of the probe as references for re-positioning the probe and/or needle relative to the out-of-image plane reference position. 
     In some embodiments, when the plane of the current image does not contain an out-of-image plane reference position, a marker is provided to indicate the proximity of the trajectory of a needle to the out-of-image plane reference position. Such a marker may correspond to the projection onto the current image plane of the location where the needle trajectory intersects the plane that contains the reference position and is parallel to the current image plane. The projection of the trajectory-reference position plane intersection onto the current image plane may comprise an orthogonal projection onto the current image plane (i.e., a projection along projector that is orthogonal to the plane of the current image). A user may position a needle so that its trajectory intersects an out-of-image plane target by aligning a trajectory-reference position plane orthogonal-projection marker with a reference position orthogonal-projection marker. 
       FIGS. 10 and 11  show, respectively, schematic diagrams  240  and  250  of an ultrasound operating environment.  FIG. 12  shows an ultrasound image display  260  corresponding to the ultrasound operating environment depicted in diagrams  240  and  250 . Display  260  comprises image  262 . In diagrams  240  and  250 , a reference position  242 , and needles  244 ,  254  and their trajectories  246 ,  256  are depicted relative to lines  248 ,  258 . Line  248  corresponds to edge  262 A of image  262 . Line  258  corresponds to edge  262 B of image  262 . In image  262 , lines  264  and  266  represent the orthogonal projections, respectively, of needle  244  and its trajectory  246  onto the plane of image  262 . Lines  274  and  276  represent the orthogonal projections, respectively, of needle  254  and its trajectory  256  onto the plane of image  262 . 
     In diagrams  240  and  250 , trajectories  246 ,  256  intersect lines  248  and  258  at trajectory-image plane intersections  246 A,  256 A. In image  262 , trajectory-image plane intersection markers  266 A,  276 A indicate where trajectories  246 ,  256  intersect the plane of image  262 , and correspond, respectively, to trajectory-image plane intersections  246 A,  256 A. Trajectory-image plane intersection markers  266 A,  276 A may assist a user in guiding the tips of needles  244  and  254  toward structures depicted in image  262 . 
     In diagrams  240  and  250 , projector  243  extends from reference position  242  to lines  248  and  258  to define reference position projection  243 A. Projector  243  is perpendicular to lines  248  and  258 , and orthogonal to the plane of image  262 . In image  262 , reference position orthogonal-projection marker  263 A represents the projection of reference position  242  onto image plane  262  according to projector  243 , and corresponds to projection  243 A. Reference position orthogonal-projection marker  263 A may assist a user in positioning a probe to acquire an image that contains reference position  242 . 
     In diagrams  240  and  250 , lines  249  and  259  are the orthogonal projections of lines  248  and  258 , respectively, onto a plane that contains reference position  242  and that is parallel to the plane of image  262 . Trajectories  246 ,  256  intersect lines  249  and  259  at trajectory-reference position plane intersections  246 B,  256 B. Projector  247  extends from trajectory-reference position plane intersection  246 B to lines  248  and  258  to define projection  247 A. Projector  247  is perpendicular to lines  248  and  258  and orthogonal to the plane of image  262 . Projector  257  extends from trajectory-reference position plane intersection  256 B to lines  248 , 258  to define projection  257 A. Projector  257  is perpendicular to lines  248  and  258 , and orthogonal to the plane of image  262 . 
     In image  262 , trajectory-reference position plane intersection orthogonal-projection marker  267 A represents the orthogonal projection of trajectory-reference position plane intersection  246 B onto image plane  262  according to projector  247 , and corresponds to projection  247 A. Trajectory-reference position plane intersection orthogonal-projection marker  277 A represents the orthogonal projection of trajectory-reference position plane intersection  256 B onto image plane  262  according to projector  257 , and corresponds to projection  257 A. Trajectory-reference position plane intersection orthogonal-projection markers  267 A and  277 A may assist a user in positioning a needle so that its trajectory is aligned with reference position  242 . For example, a user may re-orient a needle so that its corresponding trajectory-reference position plane intersection orthogonal-projection marker overlaps the reference position orthogonal-projection marker in order to align the needle with an out-of-image plane reference position. 
     In diagrams  240  and  250 , projectors  245 ,  255  extend from reference position  242  to lines  248  and  258  to define reference position projections  245 A,  255 A. Projectors  245 ,  255  are parallel to trajectories  246 ,  256 . In image  262 , reference position trajectory-projection marker  265 A represents the projection of reference position  242  onto image plane  262  according to projector  245 , and corresponds to projection  265 A. Projector marker  265  represents the projection of projector  245  onto the plane of image  262 . Reference position trajectory-projection marker  275 A represents the projection of reference position  242  onto image plane  262  according to projector  255 , and corresponds to projection  255 A. Projector marker  275  represents the projection of projector  255  onto the plane of image  262 . Reference position trajectory-projection markers  265 A,  275 A and/or projector markers  265 ,  275  may assist a user in positioning a needle so that its trajectory is aligned with reference position  242 . For example, a user may interpret the displacement of a reference position trajectory-projection marker (e.g., marker  265 A) relative to a trajectory-image plane intersection marker (e.g., marker  266 A) as indicative of the displacement of needle  244  in a plane parallel to the plane of image  262  that is required to align the trajectory of the needle with the reference position. The spacing between projector markers (e.g., markers  265  and  266 ) may assist users in identifying the required displacement. 
     Advantageously, embodiments in which projections of out-of-image-plane reference positions are displayed on images may assist users in positioning an ultrasound probe to acquire an image that contains the reference position and/or positioning a needle for alignment with a reference position. 
     In some embodiments, coded appearance characteristics are applied to markers to indicate the distance between an out-of-image plane feature (e.g., reference position, trajectory intersection or the like) and the plane of an image. For example, reference position projection marker  263 A may be displayed in different colors along a spectrum to indicate the distance between reference position  242  and its projection  243 A onto the plane of image  262  (e.g., the length of projector  243 ). In some embodiments, when reference position  242  is on a first side of the plane of image  262 , marker  263 A is displayed in different colors of a first spectrum, and when reference position  242  is on a second side of the plane of image  262 , marker  263 A is displayed in different colors of a second spectrum, different from the first. For example, as the distance between reference position  242  and its projection  243 A onto the plane of image  262  closes when reference position  242  is on a first side of the plane of image  262 , the color in which marker  263 A may change along a spectrum running from green to blue, and as the distance between reference position  242  and its projection  243 A onto the plane of image  262  closes when reference position  242  is on a second side of the plane of image  262 , the color in which marker  263 A may change along a spectrum running from red to yellow. 
     In some embodiments, reference position projection marker  263 A may be displayed as a marker whose size varies according to the distance between reference position  242  and its projection  243 A onto the plane of image  262  (e.g., the length of projector  243 ). For example, reference position marker  263 A may comprise a circle centered at the projection  234 A of reference position  242  on the plane of image  262  whose diameter varies inversely with the distance between reference position  242  and its projection  243 A onto the plane of image  262 . Some embodiments comprise a scale that relates coded appearance characteristic(s) of one or more markers to the distance between the corresponding out-of-image plane features and the plane of image  262 . 
     In some embodiments, mutual alignment of out-of-image plane features, in-plane features and/or image planes are signaled by markers having particular coded appearance characteristics. Such use of coded appearance characteristics may assist users in obtaining mutual alignment of out-of-image plane features, in-image plane features and/or image planes. For example, when a reference position lies in an image plane, a marker indicating the location of the reference position in the image may appear differently than a marker used to indicate the projection of the reference position onto the image plane when the reference position does not lie in the image plane. Embodiments according to this example may provide the effect of “lighting-up” the reference position as the image plane is swept through the reference position. 
     In some embodiments, one or more coded appearance characteristics of a trajectory-reference position plane intersection orthogonal-projection marker (e.g., marker  267 A) and/or a reference position orthogonal-projection marker (e.g.,  263 A) is changed to indicate the alignment of the two markers. This feature may serve to alert users to the fact that the current trajectory of the needle intersects an out-of-image plane reference position. 
     Other indicators of the distance between the tip of needle  21  and a user-marked target may be provided. For example,  FIG. 3  shows a bar chart  33  and a read out  34  which both indicate a distance between the tip of needle  21  and the target location in the patient&#39;s body. The distance may be calculated from the known positions of the tip of the needle  21  and the target location. 
     Knowledge of the relative orientations of probe  12  and needle  21  also permits the generation of other views which help the user to visualize the location of needle  21  relative to its intended target. For example, in some embodiments, a display displays both an image taken in the current image plane of transducer  12  and one or more virtual depictions of a needle intersecting a 3D volume as the needle is tracked. The illustrated embodiment shows a top view  37 A and a side view  37 B. In each case, the plane of the ultrasound image is represented by a line  38 A, the position of needle  21  is indicated by a line  38 B, the trajectory of needle  21 , if advanced in a straight line along its axis, is indicated by a line  38 C. The position at which the trajectory of needle  21  will intersect the plane of the ultrasound images is indicated by a marker  38 D and the position of the target is indicated by a marker  38 E. 
     In some cases it is desirable to have the needle  21  inserted into the patient in the same plane of the ultrasound image being taken by probe  12 . In some such cases, a depiction of the projection of needle  21  on the surface of transducer array  14  (i.e., the edge of the current image plane) may be provided to show the relationship of needle  21  to the current image plane.  FIGS. 13 and 13A  show example projections  167  and  167 A of this sort. Projections can also show the needle out of the current image plane, as in projection  167 B of  FIG. 13B . 
     In some cases, an ultrasound apparatus may be configured to determine how needle  21  and/or probe  12  ought to be moved in order to bring needle  21  into the plane of the ultrasound image. In some embodiments, visible or audible feedback is provided to indicate when needle  21  is being moved farther away from the plane of the ultrasound image  23  or closer to the plane of ultrasound image  23 . In some embodiments, the angle of needle  21  relative to the plane of ultrasound image  23  is displayed in a representation in which shadow is used to indicate the 3D relationship between needle  21  and the plane of image  23  on a 2D image. This is illustrated in  FIG. 14 , for example. In some embodiments the plane of the ultrasound image is controlled automatically or semi-automatically to have a desired relationship to the orientation of needle  21 . An example of such an embodiment is described below with reference to  FIG. 15 . 
     In some embodiments, biopsy apparatus  19  comprises vacuum assisted biopsy apparatus, a fire biopsy apparatus or another biopsy apparatus that can be operated to remove selected tissue from a subject. A vacuum assisted biopsy apparatus may comprise a hollow needle with a lateral opening having a sharp edge. An applied vacuum (suction) draws tissue through the opening into the needle bore. When the needle is rotated, the tissue inside the needle bore is cut away by the sharp edge of the opening. The applied vacuum then removes the tissue sample from the needle. A fire biopsy apparatus operates on a similar principal, except that a cutting member is moved along the needle (fired) to cut away the tissue inside the needle bore. Biopsies taken using such apparatus can obtain several tissue samples from different regions during a single needle insertion. In embodiments comprising such biopsy apparatus, an indication of the region that would be sampled (i.e., the region that would be cut away) may be provided. This is illustrated in  FIGS. 13 and 13B , where boxes  160  and  160 A indicate on ultrasound images  162  and  162 A regions of tissue that would be cut into aperture  164  of biopsy needle  166 .  FIG. 13A  shows a region  165  about aperture  164  that corresponds to box  160 . Determination of the region that would be sampled may comprise determining the position and orientation of the lateral opening of the biopsy needle, determining the region of tissue adjacent to the opening that would be drawn into the needle by the applied vacuum, determining the path that the lateral opening of the biopsy needle would trace if it were rotated and/or determining the path that the cutting member would travel if it were fired. 
     In order to indicate the region that would be sampled by a vacuum assisted biopsy, it is desirable to have a description of the needle shaft and features thereon in 3D space. For example, it may be desirable to know the location and orientation of the lateral opening on a vacuum biopsy needle. In embodiments where needle  21  is connected to handle  19  using a coupling which fixes the orientation of needle  21  relative to handle  19  such that the axis of needle  21  has a predetermined alignment with the position markers  15 D and  15 E, the description of the needle shaft in 3D space can be determined analytically from the location of the needle tip, the location of any needle feature relative to the tip and the needle axis, and the predetermined offset of the needle tip relative to position markers  15 D and  15 E. 
     A description of the needle shaft can alternatively be obtained by determining the offset of one or more other points along the needle shaft (i.e., points other than the needle tip, such as the ends of a lateral opening of a vacuum biopsy needle) relative to position marker(s) on the biopsy apparatus. To determine the offset of such a position, the needle may be placed in a slide track or any other suitable device for constraining the movement of the needle to its axis, and the transmit coordinate of the needle tip is determined from its pre-determined offset from position markers  15 D and  15 E. Then the needle is then moved a distance along the direction of its axis. In some embodiments, this distance is greater than 5 mm. 
     Where the needle is advanced in the direction pointed out by the needle tip, the point in space where the needle tip was before the movement will be occupied by a point on the needle shaft that is the distance of the movement from the needle tip. Where the needle has been retracted in the direction opposite the direction pointed out by the needle tip, the needle tip will occupy a point in space where a point on the needle shaft that is the distance of the movement from the needle tip was before the movement. In either movement case, the position of the point along the needle shaft at the movement distance from the needle relative to position markers  15 D and  15 E can be determined by calculating the position of the point along the needle shaft relative to the needle tip from the difference in the needle tip position before and after the movement, and then relating the position of the point along the needle shaft relative to the needle tip to the global coordinate system using the location of the needle-tip in the global coordinate system. 
     It has been found that when positions are determined using some magnetic position sensing systems, excessive separation between position markers  15  and position base unit  17  and/or the presence of metals near position markers  15  and/or position base unit  17  can reduce the quality of the tracking signals that pass between markers  15  and position base unit  17 . As a result, the accuracy of position and orientation information obtained by position sensing system  16  may be diminished. Often accuracy can be restored by adjusting the position markers (i.e. moving and/or rotating the needle and/or probe) or the position base unit and/or by moving the metal away from the markers and/or position base unit. To alert users to the condition of possibly diminished accuracy of position and orientation information, the quality of the tracking signal may be monitored and an indicator thereof provided. Graphical indicator  168  in  FIG. 13  and graphical indicator  168 A in  FIG. 13A  are examples of such indicators. It will be appreciated that tracking signal quality may be represented in many different graphical forms or represented numerically. The quality of the tracking signals may be inferred or estimated from, for example, the mean received power of the tracking signals, the error rate in a known training sequence, or the like. 
     In some embodiments, some or all of the display (e.g., line  24  representing a projection of the needle  21  and line  30  representing the path that will be followed by the tip of needle  21 ) may be hidden or displayed in a different manner (e.g. dimmed, flashed or the like) when the quality of the tracking signal falls below a threshold so that the user is not provided without warning with possibly invalid position information. In some embodiments, an audible and/or visual alarm is triggered when the tracking signal quality falls below a threshold. 
     In some embodiments, the fact that the position and orientation of ultrasound probe  12  may be measured in real time is used to build a 3D model of a patient&#39;s tissues. This is illustrated in  FIG. 16  which shows a flow diagram of a method  40  for generating a 3D model. In method  40 , an ultrasound frame is received at block  41  and positions of the transducer position markers  15  are detected at block  42 . Blocks  41  and  42  are performed essentially simultaneously. The position and orientation of the transducer is determined in block  43  and a transformation between the coordinates of pixels in ultrasound frame  41  and the 3D coordinates of voxels in a 3D model being built is determined at block  44 . In block  45 , the transformation determined in block  44  is used to transform the pixels of the 2D ultrasound frame received at block  41  into corresponding voxels. In block  46 , the transformed data is added to a 3D model  47 . It can be appreciated that if multiple frames of an ultrasound image are acquired over time then, as long as the position of the patient P does not change significantly relative to 3D position base unit  16 , a 3D model of the patient can be readily obtained. 
     The 3D model may comprise a 3D array of voxels. Each voxel may contain a value indicating the degree to which ultrasound signals echo from a corresponding location within the patient. 
     Note that 3D model  47  may continue to be added to while ultrasound frames are received for other purposes. For example, a user may be moving ultrasound transducer  12  over a patient P in order to find a location of interest. While the user is doing so, a system may be building or adding to a 3D model  47  of the portions of the patient that are visualized by transducer  12 . 
     Once a 3D model has been obtained, then the 3D model may be exploited in a wide variety of ways, some of which are described below. For example, the 3D model may be used to generate 2D images on any plane crossing through the 3D model. A representation of needle  21  may also be shown in such images. In some embodiments user controls allow users to select the points of view of the images and the types of images being displayed. Once a 3D model has been acquired, an image showing the approach of needle  21  to a structure of interest may be displayed regardless of the current orientation of transducer  12 . Indeed, as long as the patient P remains in a stable position relative to 3D position base unit  17 , transducer  12  is not even required to guide the introduction of needle  21  to a desired location inside the patient P. 
       FIG. 17  shows an example system  50  which has this functionality. In system  50 , position markers  15 F and  15 G are attached to patient P. For example, position markers  15 F and  15 G may be stuck to a patient&#39;s skin using adhesive. The position markers may be stuck at locations that tend to move with the patient, for example, to the skin overlying a patient&#39;s vertebrae, hips or collar bone. 
     The positions of position markers  15 F and  15 G are monitored by position sensing system  16 . Biopsy assembly  19  is also monitored by position sensing system  16 . Position data from position sensing system  16  is provided to the ultrasound system which displays an image taken from the 3D model. The image may be a 2D image, such as a cross-section derived from the 3D model or a 2D rendering derived from the 3D model. In some embodiments, the display  52  is a 3D display such that a user can see a 3D image. 
     A line  53  representing needle  21  may be shown in the image. If patient P moves slightly then the motion is detected by position sensing system  16  which continually monitors the positions of position markers  15 F and  15 G (there may be additional position markers on the patient P). In general, one or more position markers may be on patient P in an embodiment like embodiment  50 . This information is used to update the location of needle  21  relative to anatomical structures displayed in an image  54  on display  52 . 
       FIG. 18  is a block diagram of apparatus  60  according to an embodiment. Apparatus  60  includes a biopsy assembly  19 , an ultrasound probe  12 , and 3D position sensing system  16 , as discussed above. Ultrasound probe  12  is connected to an ultrasound unit  62  which controls ultrasound probe  12  to generate appropriate ultrasound signals and which processes received ultrasound echos to yield ultrasound scan data. 
     In the illustrated embodiment, the ultrasound scan data comprises a series of 2D ultrasound scans. For example, 2D ultrasound scans may be obtained at a rate of several scans per second. 3D position sensing system  16  generates spatial orientation data for the ultrasound probe  12  that corresponds to each of the ultrasound scans. This spatial orientation data can be used to transform a local coordinate system of each one of the scans to a coordinate system used in a 3D model of a patient. 
     A scan combiner combines scans  63  using spatial orientation data  64  into a 3D model  66 . A 3D image rendering system  67  displays a rendering of 3D model  66  on a display  68 . System  60  also obtains spatial orientation data  69  for biopsy needle  21 . A needle tip position calculation  70  provides a position for the tip of needle  21  in the coordinate space of 3D model  66 . The needle tip position is provided to 3D image rendering system  67 . A needle alignment calculation  72  establishes a line extending along the axis of needle  72 . This line intersects anatomical structures that needle  21  will encounter if it is advanced longitudinally into the patient. The line generated by needle alignment system  72  is also provided to 3D image rendering system  67 . 
     System  60  also includes a 2D image rendering system which can display ultrasound scans  63  and which can super-pose on those ultrasound scans images representing the position of the tip of needle  21  as well as the line which provides information about alignment of needle  21 . 2D images rendered by 2D image rendering system  74  may be displayed on display  68 . Display  68  may comprise a computer monitor, for example. 
       FIG. 19  shows an example image that may be displayed on a display  68 . The image shows anatomical structures, including an anatomical structure of interest. Also shown are a current position of needle  21 , indicated by line  77 , an extension  77 A of line  77  which indicates the trajectory that would be followed by needle  21  and a point  77 B indicating the end of needle  21 . As described in relation to  FIGS. 1 and 3 , various graphical and audible indications may be given to alert the user to the current relationship between the tip of needle  21  and the target position within the patient. 
     In the illustrated embodiment, a zoom control  79 , a multi-axis rotation control  80  and pan-control  82  are provided to allow the user to zoom, rotate and pan the image  76  generated from the 3D model. These controls may be activated by way of a graphical user interface or by way of other user interface elements. In some embodiments, a user can manipulate image  76  using a pointing device such as a mouse, track ball, track pad, touch screen or the like to pan, rotate and zoom image  76  in a desired manner. In some embodiments, zoom and rotation of image  76  are automatically adjusted so that, as the user manages to get the tip of needle  21 , as indicated by line  77  and point  77 B, close to the target the image automatically zooms in and rotates to an angle that best shows the approach of needle  21  to the target. 
     In some locations, a vacuum assisted biopsy needle is provided. In such embodiments, the biopsy apparatus  19  may comprise a control, such as a push button, foot pedal, or the like, which enables a user to trigger the biopsy needle to acquire a biopsy sample. In some such embodiments, the control is interfaced to the ultrasound system in such a manner that the current position of needle  21  is preserved upon actuation of the control. A real-time ultrasound image, a 3D model or an image extracted from a 3D model may also be preserved, if available. This permits later examination of the exact position of the needle at the time each sample was obtained. These images and/or position information may be archived for future reference. 
     In some embodiments, the ultrasound apparatus is configured to automatically determine whether a 3D model of the area of the patient is complete.  FIG. 20  shows a method  90  according to one example embodiment. In block  91 , scan data is acquired in block  91 A and the corresponding probe position and orientation is acquired in block  91 B. In block  92  the data is mapped to voxels in a 3D model. In block  93 , it is determined whether the 3D model is complete. For example, block  93  may comprise considering whether a threshold number or density of voxels have not yet been supplied with image data. If the 3D image is not yet complete then method  90  loops to block  91  on path  94  to acquire more data. Otherwise, method  90  continues to block  95 . In block  95  needle  21  is calibrated, for example as described above. Note that block  95  may be performed before or after acquiring the 3D model. In block  96  the position and orientation of needle  95  are determined. Method  90  may then continue to allow a user to visualize the position of needle  21  while taking a biopsy or doing other medical procedures, as described herein for example. 
     In some embodiments, an ultrasound system is configured to provide instructions that will be useful in helping a user to obtain suitable alignment between a needle  21  and a target location.  FIG. 21  shows an example system  100  in which a position base unit  102  determines positions of a ultrasound transducer and a needle. A block  104  computes an image plane of the transducer from the transducer position data. A block  106  computes a line along which needle  21  is aligned (e.g., a trajectory of needle  21 ) from the needle position data. Block  108  computes an angle θ between the needle and the image plane of the transducer.  FIG. 21A  shows an example of the angle θ. 
     In some embodiments, block  108  determines one or more of the following:
         coordinates of a point at which the line determined in block  106  intersects the image plane of the transducer;   whether the point at which the line determined in block  106  intersects the image plane of the transducer lies in the field-of-view of the image;   a distance along the line determined in block  106  between the image plane of the transducer and the tip of needle  21 ;   a distance along a line normal to the image plane of the transducer between the image plane and the tip of needle  21 ;   which side of the image plane of the transducer the tip of needle  21  is on; and   the like.       

     An instruction generator  110  generates audio and/or visual instructions that assist the user to obtain a desired alignment between the needle and the image plane of the transducer. Instructions may be provided by an audio output system  112  and/or a display  114 . In some embodiments, block  110  determines one or more locations and/or coded appearance characteristics for a marker, line or other display feature shown on display  114  that corresponds to information determined in block  108 . Block  110  may determine such locations and/or coded appearance characteristics by way of a function, a look-up table, or the like. In some embodiments, instruction generator  100  generates a scale for display on display  114  that relates coded appearance characteristics to information determined in block  108 . 
       FIG. 14  shows an example display  11  in which a representation of the needle and the image plane of a transducer are both displayed with an angle indicated. Audio instructions and/or text instructions and/or visual instructions may be provided to assist the user in manipulating the needle to provide the proper needle orientation. 
     Application of the invention is not limited to taking biopsy samples. For example, the systems described herein may be applied to find a correct location within the body for the introduction of a drug, such as a anesthetic, or a radioactive seed for cancer treatment or the like. For example, the system may be used to position a catheter to introduce an epidermal anesthetic. 
     In some embodiments, one or more position markers  15  are built into ultrasound probe  12  and/or needle assembly  19 . In other embodiments, position markers may be mounted to probe  12  and/or needle assembly  19  using a clip or other removable fastener. In some embodiments, needle  21  is large enough in diameter that a position marker  15  can be provided in needle  21  or even at or near the tip of needle  21 . Such embodiments can provide better information regarding the location of the tip of needle  21  in cases where it is possible that needle  21  may bend slightly in use. 
       FIG. 22  shows an example ultrasound probe  130  having built-in position markers  15 . A cable assembly  132  servicing probe  130  includes signal conductors  21 A,  21 B and  21 C that carry signals from position markers  15  to position base unit  17 . 
     In some applications, it is desirable that the probes can be interchanged without having to perform calibration or load new calibration parameter information. This can be facilitated during probe sensor assembly by ensuring that the position/orientation markers that provide location and orientation information for the probes are identically situated with respect to the transducer array of each of the probes. In particular, the position/orientation markers may be placed at the same location in different probes and at the same rotation angle. In some embodiments, one or more axes of the coordinate system according to which the position and orientation of the marker is determined may be aligned with corresponding axes of the ultrasound transducer (e.g, the axes defining the plane of the ultrasound image). 
       FIG. 23  shows a side elevation view of example embodiment of a marker positioning apparatus  170  that may be used for precisely mounting a position marker  182  into a bore  180  of a probe  171 .  FIG. 24  shows a top plan view of positioning apparatus  170 . Probe  171  and a position base unit  172  are mounted on a jig  174 . In some embodiments, the distance between the probe  171  and position base unit  172  is in the range of 8 and 40 cm. In order that position markers can be positioned uniformly with respect to the transducer arrays of different probes, mounting a probe in jig  174  should result in substantially the same spatial relationship between the transducer array of the probe and position base unit  172 . In the illustrated embodiment, jig  174  comprises a seat  175  that conforms to a portion of the exterior of probe  171 . Seat  175  ensures that all probes of the same model as probe  171  will be identically mounted in jig  174 . As a result, the position and orientation of the transducer arrays of such probes with respect to position base unit  172  will be the same as the position and orientation of transducer array  184 . 
     In some embodiments, jig  174  comprises a plurality of seats, each configured to conform to a portion of a different probe model and positioned on jig  174  such that the transducer arrays inside the different model probes are substantially identically situated with respect to position base unit  172 . In some such embodiments, the seats may be removable and interchangeable. It will be appreciated that other means could be employed to facilitate uniform mounting of probes in jig  174 . For example, conductive plates could be provided on jig  174  and probe  171  in locations such that when probe  171  is properly mounted on jig  174  an electric current flows across the plates to trigger a signal indicative of proper mounting. 
     With probe  171  and position base unit  172  mounted on jig  174 , a position marker  182  is inserted into bore  180 . A real-time display of the position and orientation of marker  182  is provided to a user.  FIG. 25  is a screen-capture of an example real-time display  200  of the position  202  and orientation  204  of a marker according to an example embodiment. The user can use this information to guide position marker  182  into a desired relationship with position base unit  172 . Typically, a user will want to guide marker  182  into a pre-determined position (e.g., X, Y, Z coordinate) and orientation (e.g., azimuth, elevation and rotation angles) with respect to position base unit  172 , so that markers for a number of different probes can be guided into the same pre-determined position and orientation. 
     Bore  180  and jig  174  may be configured to facilitate guiding position marker  182  into a desired position and orientation. In such embodiments the body of position marker  182  may be shaped to have at least one feature that corresponds to a coordinate axis by which its position is measured. For example,  FIG. 26  shows a cylindrically shaped position marker  190 . The axis  191  of marker  190  corresponds to one axis  191 X of the three axes ( 191 X,  191 Y and  191 Z) by which the position of marker  190  is measured. Bore  180  can be defined in probe  171  so that a feature of marker  182  that corresponds to a coordinate axis by which its position is measured is aligned (or at least substantially aligned) with an axis of position base unit  172  when marker  182  is inserted in bore  180 . In the embodiment illustrated in  FIG. 23 , the axis of bore  180  is aligned with an axis of position base unit  172  and position marker  182  is generally cylindrical with the axis of its body corresponding to an axis by which position base unit  172  defines the position of marker  182 . Inserting marker  182  along bore  180  causes the axis of the body of marker  182  to be aligned with an axis by which position base unit  172  defines the position of marker  182 . Provided that marker  182  fits snugly inside bore  180 , the axial alignment will substantially determine the azimuth and elevation angles of marker  182  and the position of marker  182  along the other axes by which position base unit  172  defines the position of marker  182 . Thus a user aligning marker  182  to a pre-determined orientation and position need only adjust marker  182  translationally along bore  180  and rotationally about its axis. 
     In the illustrated embodiment, jig  174  and bore  180  are also configured so that end  180 A of bore  180  is a pre-determined distance from transducer array  184 . This allows the position of position marker  182  relative to the position base unit  172  to be fixed precisely at a known distance from position base unit  172  by abutting marker  182  against end  180 A. In some embodiments, a spacer may be deposited at end  180 A of bore  180  in order to increase the minimum separation of position marker  182  from position base unit  172 . In some such embodiments, the depth of bore  180  is ascertained and a spacer of a pre-determined dimension is inserted to make bore  180  have a known effective depth. 
     In some embodiments, bore  180  is slightly larger than the cross-section of marker  182 . This permits marker  182  to be adjusted within bore  180  to more precisely locate marker  182  with respect to position base unit  172 . The ability to adjust marker  182  in bore  180  is desirable where the spatial relationship between bore  180  and transducer array  184  may differ between probes, such as where the bore is formed in an off-the-shelf probe. Where bore  180  is slightly larger than the cross-section of marker  182 , a liquid or gel adhesive can be applied to either or both of bore  180  and marker  182 , and marker  182  held in bore  180  at the desired position and orientation with respect to position base unit  172  until the adhesive has set. 
     Because the quality of the tracking signal is related to the accuracy of the position and orientation information obtained therefrom, it is preferable that the tracking signal be of at least a threshold quality when positioning marker  182  in bore  180 . In some embodiments a very high quality value may indicate either high magnetic field distortion or malfunctioning of position marker  182 . In some such embodiments, it is preferable that the quality value be less than a threshold value during orientation of position marker  182 . Some embodiments provide users with an indication of tracking signal quality. Display  200  in  FIG. 25  includes a numerical indication of tracking signal quality  206 . Other embodiments may provide graphical or audible indications of tracking signal quality. 
     In some embodiments, the plane of an ultrasound image obtained by probe  12  can be steered electronically. In some such embodiments, an operating mode is provided in which the plane of the ultrasound image is automatically steered so that the needle lies perpendicular to the plane of the ultrasound image. This can enhance visibility of the needle. In other embodiments the plane of the 2D ultrasound image is steered automatically to be in the same plane as the needle. Steering may be accomplished, for example, by selecting a group of transducers from a transducer array in probe  12  and/or performing beamforming on transmitted and received ultrasound signals. 
       FIG. 15  shows schematically ultrasound apparatus  140  which includes a beam-steering capability. In the illustrated embodiment, an angle θ between the plane of the ultrasound image and a reference plane  141  of transducer  142  can be electronically varied. In some embodiments, θ is controlled to make the plane of the ultrasound image perpendicular to the axis of needle  21 . 
     In some cases, 3D data may also be available from another source. For example, there may be magnetic resonance imaging (MRI) or computed tomography (CT) results for the patient that provide 3D volume data. In some embodiments, the 3D model derived from the ultrasound data is co-registered to the other 3D image data. This permits various alternatives. For example, the position of the needle may be shown relative to structures shown in an MRI or CT image. In some embodiments, a composite image is created from one or more of the ultrasound images, the magnetic resonance image and the CT image. The composite image may comprise high contrast boundaries shown in any one or more of these imaging modalities. The position of needle  21  may then be shown in comparison to the composite image. 
       FIG. 14  shows an example shadow representation which could be displayed on a 2D monitor to indicate the 3D relationship of a needle to an anatomical structure. 
     In some embodiments, the origin of the coordinate system for the 3D model is set automatically. There are various ways to achieve this. For example, a user may move probe  12  over a patient P to acquire ultrasound data. In the course of doing so, a number of ultrasound frames are acquired. The ultrasound frames may provide reasonably complete coverage of a particular region in space. The ultrasound unit may be configured to identify a region of space that is reasonably completely covered by the acquired ultrasound data (for example, a region in which a density of voxels for which there are one or more corresponding pixels in the ultrasound data) exceeds some threshold density. The system may then establish an origin for the 3D model with reference to the region identified. The origin may be determined for example with reference to a centroid of the identified volume. 
     In another embodiment, a coordinate system for the 3D model is established with reference to specific positions for probe  12 . In an example embodiment illustrated in  FIG. 27 , a user places probe  12  at a first location  150 A on a first side of a region of patient P that is of interest and indicates by triggering a control when the transducer is in the desired position. The user may then move probe  12  to a second location  150 B on another side of the region of interest and trigger the same or different control to indicate that the transducer is now located at the other side of the region of interest. The origin of the coordinate system for the 3D model may then be set with reference to the two positions. For example, the origin may be set at one of the positions. The orientation of an axis, such as the X-axis in  FIG. 27  may be set with reference to the difference vector between the two positions. The orientation of another coordinate, such as the Z-axis shown in  FIG. 27  may be determined with reference to the orientation of probe  12  in one or both of the positions. Once the X and Z axises are determined, the Y-axis is determined automatically. 
     The 3D model is not limited to having a Cartesian coordinate system. The 3D model may also be defined using some other coordinate system such as a spherical coordinate system or a cylindrical coordinate system. 
     It is not mandatory that the 3D model be built in real time. In some embodiments, the 3D model is built in real time as new ultrasound frames are received. In other embodiments, a plurality of ultrasound frames are received and saved and the 3D model is then built from the received and saved ultrasound frames. Position/orientation information corresponding to each ultrasound frame may be saved. In either case, where a particular voxel of the 3D model corresponds to pixels in multiple different ultrasound frames, then the value for the voxel may be set in a way that selects one of the ultrasound frames or, in the alternative, combines the values for the corresponding pixels in the different ultrasound frames in some manner (such as taking an average or weighted average or median of values corresponding to the voxel from two or more ultrasound frames). 
     Some example ways in which a single one of the ultrasound frames may be selected to provide the value for a voxel are:
         the voxel value may be set to a value corresponding to a most-recent ultrasound frame having a pixel corresponding to the voxel;   the voxel may be set to a value determined from a best-quality one of a plurality of ultrasound frames according to a suitable image quality measure;   etc.       

     Some example ways in which pixel values from multiple ultrasound frames may be combined to yield a voxel value include:
         averaging the pixel values;   performing a weighted average of the pixel values;   where there are three or more corresponding pixel values, rejecting any outlying pixel values and combining the rest in a suitable manner (such as by averaging or weighted averaging);   etc.       

       FIG. 28  illustrates a possible relationship between the coordinates of a pixel in a single ultrasound frame and the coordinate system for a 3D model. In this example, the coordinate system for each ultrasound frame is a polar coordinate system having an origin located at a point  15 A at coordinates X 0 , Y 0 , Z 0  in the coordinate space of the 3D model. The location of point  15 A is determined by the position of probe  12  when the ultrasound frame is acquired. This position is indicated by vector K 4 . The plane of the ultrasound image is oriented such that the normal to the plane is defined in this example by the vector K 7 . The direction of the ultrasound image is defined by the vector O 1 . Given this relationship then a pixel identified by the coordinates R, θ in the coordinate system of the ultrasound frame corresponds to the voxel at a location X′, Y′, Z′ in the coordinate space of the 3D model. Conversion between coordinates of the ultrasonic image and voxels of the 3D model may be implemented as matrix multiplications, for example. 
     Some anatomical structures, such as breasts, may move around during image acquisition. In such cases, a 3D image may not be true to the current position of the anatomical structure. In such cases, the probe  12  itself may be used as reference location. The user may be instructed to keep the probe in one place on the breast or other moveable tissue. 
     In an example embodiment illustrated in  FIG. 29 , imaging of movable tissue, such as a breast, is performed while the breast or other tissue is held in a rigid or semi-rigid form. For breast imaging the form may be conical, for example. In the embodiment illustrated in  FIG. 29 , form  120  is a conical form. One or more position markers  15  is provided on form  120 . Form  120  is made of a material, such as a suitable plastic, that is essentially acoustically transparent at ultrasound frequencies so that ultrasound images may be acquired through form  120 . Apertures  122  may be provided in form  120  to permit introduction of a needle such as a needle  21  of biopsy apparatus  19 . In some embodiments form  120  is mesh-like or penetrated by a regular array of apertures. In the alternative, a needle  21  may simply pierce form  120  at a desired location. Form  120  may be a disposable item. 
     In an example application, a woman&#39;s breast is inserted into form  120  such that the breast tissue fills and conforms substantially to the shape of form  120 . A 3D model of the breast tissue is acquired by running an ultrasound probe  12  over the outer surface of form  120 . The voxels of the 3D model are referenced to the locations of the position markers  15  on form  120 . After the 3D model has been acquired, form  120  prevents the breast tissue from shifting relative to the position marker(s). 
     A practitioner can then introduce needle  21  of biopsy apparatus  19  into a desired location in the woman&#39;s breast while following progress of the needle  21  on a display. Optionally, ultrasound images may be taken as needle  21  is introduced to provide real-time ultrasound images of the tissues through which needle  21  is being introduced. 
     In another example application, an ultrasound image of a portion of a patient&#39;s spinal column is obtained and the techniques described herein are applied to follow a catheter as its tip is inserted into the patient&#39;s epidural space.  FIG. 30  illustrates such an application. One or more position markers  15  may be adhered to the patient during this procedure. For example, the position markers may be adhesively but removably affixed to the patient&#39;s skin. Positions of the one or more position markers on the patient may be monitored and used to maintain correspondence between sensed positions of a needle and a 3D model. 
       FIG. 31  shows an ultrasound system  310  according to an example embodiment. System  310  comprises a controller  311  connected to an ultrasound probe  312 , a display  313 , a user input device  314 , a 3D position sensing system  316 , a needle  317 , and a position base unit  305 . 
     Ultrasound probe  312  emits ultrasound pulses into the body of patient P. Ultrasound pulses emitted by probe  312  are reflected off of structures in the body of patient P. Probe  312  receives reflected ultrasound pulses that return in its direction. Controller  311  may be configured to control aspects of the operation of ultrasound probe  312 . For example, controller  311  may control the transmission of pulses from ultrasound probe  312  and/or the gating of samples of reflected pulses received at ultrasound probe  312 . 
     Controller  311  may comprise one or more central processing units (CPUs), one or more microprocessors, one or more field programmable gate arrays (FPGAs), application specific integrated circuits, logic circuits, or any combination thereof, or any other suitable processing unit(s) comprising hardware and/or software capable of functioning as described herein. 
     Controller  311  comprises a memory  315 . In some embodiments, memory  315  is external to controller  311 . Controller  311  may be configured to store data representative of signals acquired by probe  312  in memory  315 . Controller  311  processes ultrasound data acquired from ultrasound transducer  312 . In some embodiments, controller  311  is configured to generate ultrasound image data from ultrasound data acquired by probe  312 . For example, memory  315  may comprise instructions that when executed by controller  311  or when used to configure controller  311  cause controller  311  to generate a B-mode image from ultrasound data acquired by probe  312 . Controller  311  may comprise an analog or digital beamformer for use in processing echo signals to yield image data. Controller  311  may be configured to store image data that it generates in memory  315 . 
     Either or both of controller  311  and probe  312  may optionally be part of an ultrasound machine that is commercially available. Controller  311  and probe  312  may be of any known or future developed type. 
     3D position sensing system  316  includes one or more position markers (not shown) on each of probe  312  and needle  317 . The position markers on probe  312  and needle  317  communicate with position base unit  305 . 3D position base unit  305  measures the locations of the position markers relative to a global coordinate system. Where three position markers are located on a rigid body and not located along a common line, the orientation of the rigid body is uniquely determined by the positions of the three position markers. In some embodiments, probe  312  and needle  317  comprise rigid bodies having at least three position markers that are not located along a common line. 
     In some embodiments, 3D position sensing system  316  comprises 3D position sensing technology that permits both the location and orientation of a single position marker to be determined by 3D position base unit  305 . In some such embodiments, the location and orientation of probe  312  and/or needle  317  may be determined from information provided by as few as one position marker. 
     3D position base unit  305  is connected to controller  311 . In some embodiments, 3D position base unit provides location and/or orientation information for markers on probe  312  and/or needle  317  to controller  311 . In some embodiments, position base unit  305  determines a spatial description of probe  312 , needle  317  and/or features thereof (e.g., tip of needle  317 , the plane of an ultrasound image derived from ultrasound data acquired by probe  312 , etc.) based on information provided by the position markers, and provides such description(s) to controller  311 . A spatial description may comprise information specifying location and/or orientation in space. The spatial description of probe  312 , needle  317  and/or features thereof may be specified in terms of any suitable global coordinate system (e.g., Cartesian, spherical, cylindrical, conical, etc.). 
     Controller  311  is configured to generate images from ultrasound data and display such images on display  313 . In some embodiments, controller  311  is be configured to generate and display on display  313  images comprising graphical overlay elements that represent features of an ultrasound operating environment relative to the plane of an ultrasound image. Such graphical overlay elements may comprise, for example, lines, markers and the like of the type shown herein in images/views/displays  23 ,  37 A,  37 B,  68 ,  162 ,  162 A,  220 ,  260 . 
     Input device  314  provides user input to controller  311 . In the illustrated embodiment, input device  314  comprises keyboard  314 A and computer mouse  314 B. Input device  314  may comprise other user interfaces. In some embodiments, display  313  comprises a touch screen, which may form part of input device  314 . 
     A user may use input device  314  to control aspects of the operation of controller  311 . Input device  314  may provide controls for manipulating images generated by controller  311 . For example, a user may interact with input device  314  to control the gating of samples received at ultrasound probe  312  and thereby change the image displayed by controller  311  on display  313 . Control may be provided for other aspects of controller  311 , such the selection, determination and/or appearance of graphical elements overlaid on ultrasound images (e.g., lines and markers of images/views/displays  23 ,  37 A,  37 B,  68 ,  162 ,  162 A,  220 ,  260 ). 
     In some embodiments, input device  314  is operable to indicate a reference position on an image displayed on display  313 . In some such embodiments, controller  314  is configured to register a location of a reference position indicated by a user on an image displayed on display  313 . The location of the reference position may comprise locations of one or more pixel in an image display on display  313  and/or one or more coordinates in a global coordinate system. 
     In some embodiments, controller  311  may be configured to determine spatial descriptions of features of an ultrasound operating environment based on information provided by position base unit  305  (e.g., location and/or orientation information for position markers on probe  312  and/or needle  317 , spatial descriptions of probe  312  and/or needle  317 , or the like). For example, controller  311  may be configured to determine one or more of:
         a location of probe  312 ;   a location of needle  317 ;   a location of an image acquired by probe  312 ;   a plane of a location of an image acquired by probe  312 ;   a trajectory of needle  317 ;   a location of the longitudinal axis of needle  317 ;   a location of an intersection of needle  317  with the plane of an image acquired by probe  312 ;   a location of an intersection of the longitudinal axis of needle  317  with the plane of an image acquired by probe  312 ;   an angle between the longitudinal axis of needle  317  and the plane of an image acquired by probe  312 ;   a location of a reference position indicated on an image;   a plane containing a reference position and parallel to a plane of an image;   a location of an intersection of the longitudinal axis of needle  317  with an arbitrary plane, such as, for example, a plane containing a reference position indicated on an image and parallel to a plane of another image;   a projector that is orthogonal to a plane of an image acquired by probe  312  and that extends from a feature of an ultrasound environment to the plane of the image;   a projector that is parallel to a trajectory of needle  317  and that extends from a feature of an ultrasound environment to the plane of an image acquired by probe  312 ;   a distance between a reference position and an image acquired by probe  312  along a projector from the reference position to the image that is orthogonal to the plane of the image;   a distance between a reference position and an image acquired by probe  312  along a projector from the reference position to the image that is parallel to the longitudinal axis of needle  317 ;   a distance between the location of an intersection of the longitudinal axis of needle  317  with an arbitrary plane and an image acquired by probe  312  along a projector from the intersection to the image that is orthogonal to the plane of the image;   a distance between the location of an intersection of the longitudinal axis of needle  317  with an arbitrary plane and an image acquired by probe  312  along a projector from the intersection to the image that is parallel to the longitudinal axis of the instrument.
 
Controller  311  may determine such spatial descriptions using any suitable combination of mathematical operations and/or techniques, such as mapping (e.g., between image pixels and points in a global coordinate system), interpolation, extrapolation, projection or the like. Spatial descriptions of features of an ultrasound operating environment may comprise locations of pixels in an image, and/or coordinates in a global coordinate system.
       

     In some embodiments, controller  311  is configured to determine projections of features of an ultrasound operating environment onto planes of ultrasound images. Controller  311  may be configured to determine projections along various projectors, such as, for example, projectors orthogonal to a plane of an ultrasound image acquired by probe  312 , projectors parallel to a trajectory of needle  317 , or the like. For example, controller  311  may be configured to determine one or more of:
         an orthogonal projection of at least a portion of the longitudinal axis of needle  317  onto a plane of an ultrasound image;   an orthogonal projection of the tip of needle  317  onto a plane of an ultrasound image;   an orthogonal projection of a reference position onto a plane of an ultrasound image;   an orthogonal projection of an intersection between the longitudinal axis of needle  317  and an arbitrary plane, such as, for example, a plane containing a reference position indicated on an image and parallel to a plane of another image, onto a plane of an ultrasound image;   a projection of a reference position onto a plane of an ultrasound image according to a projector that is parallel to the longitudinal axis of needle  317 ; and   an orthogonal projection of a projector onto a plane of an ultrasound image, such as, for example, an orthogonal projection of a projector that is parallel to the longitudinal axis of needle  317  and that extends from a feature of an ultrasound environment to a plane of an ultrasound image.       

     In some embodiments, controller  311  is configured to indicate on display  313  features of an ultrasound operating environment and/or projections of such features onto a plane of an ultrasound image. For example, controller  311  may be configured to indicate such features and/or projections on display  313  by causing graphical elements to be displayed on display  313 . Such graphical elements may be displayed as overlays on ultrasound images displayed on display  313 . In some embodiments, controller  311  is configured to determine an image location for display of a graphical overlay element corresponding to such a feature or projection by mapping a spatial description of the feature or projection to pixel(s) of an ultrasound image. For example, controller  311  may be configured to graphically overlay a marker representing an orthogonal projection of a reference position onto a plane of an ultrasound image at an image location corresponding to the spatial description of the projection. 
     In some embodiments, controller  311  is configured to determine if all or part of a feature of an ultrasound operating environment, or a projection thereof onto a plane of an ultrasound image, lies in the field-of-view of an ultrasound image. For example, controller  311  may be configured to determine if an intersection of a trajectory of needle  317  with a plane of an ultrasound image shown on display  313  lies in the field-of-view of the ultrasound image. 
     In some embodiments, controller  311  is configured to determine displacements between features of an ultrasound operating environment. For example, controller  311  may be configured to determine the displacement between two planes, between a plane and a point in space, between two points in space, and the like. Controller  311  may be configured to display graphical and/or numerical indications of such displacements on display  313 . 
     In some embodiments, controller  311  is configured to determine one or more coded appearance characteristics of markers or lines to convey spatial relationship information pertaining to features of an ultrasound operating environment. Coded appearance characteristics may comprise, for example, size, size, color, intensity, shape, linestyle, or the like that vary according to the information the characteristics are meant to convey. Coded appearance characteristics may vary continuously (e.g., color, brightness or the like along a spectrum; e.g., size, thickness, length, etc.) to convey continuous information (e.g., distance, angle, etc.) or may vary discretely (e.g., color from among a selection of primary and secondary colors, marker shape, etc.) to convey discrete information (e.g., alignment of an instrument trajectory and a reference position, intersection of an instrument and an image plane, etc.). Controller  311  may be configured to determine coded appearance characteristics of an indication, such as a graphical element, based on any combination of:
         a location of the corresponding ultrasound operating environment feature, or projection thereof, relative to the field-of-view of the ultrasound image;   a location of the corresponding ultrasound operating environment feature, or projection thereof, relative to the plane of the ultrasound image;   a displacement between the corresponding ultrasound operating environment feature, or projection thereof, and another feature or projection;   alignment of the corresponding ultrasound operating environment feature, or projection thereof, and another feature or projection;   alignment of the corresponding ultrasound operating environment feature and the plane of an ultrasound image (e.g., the plane of an ultrasound image currently displayed on display  313 );   an angle at which a corresponding ultrasound operating environment feature intersects the plane of an ultrasound image (e.g., an angle between the longitudinal axis of needle  317  and the plane of an image acquired by probe  312 );   a distance between a reference position and an image acquired by probe  312  along a projector from the reference position to the image that is orthogonal to the plane of the image;   a distance between a reference position and an image acquired by probe  312  along a projector from the reference position to the image that is parallel to a longitudinal axis of needle  317 ;   a distance between the location of an intersection of needle  317  with an arbitrary plane and an image acquired by probe  312  along a projector from the intersection to the image that is orthogonal to the plane of the image;   a distance between the location of an intersection of needle  317  with an arbitrary plane and an image acquired by probe  312  along a projector from the intersection to the image that is parallel to the longitudinal axis of the instrument; and   the like.
 
For example, in some embodiments, controller  311  is configured to display a line representing the projection of the trajectory of needle  317  onto the plane of the ultrasound image with a first intensity when the intersection of the trajectory and the plane of the ultrasound image lies outside the field-of-view of the ultrasound image and with a second intensity when the intersection lies within the field-of-view of the ultrasound image.
       

     In some embodiments, position base unit  305  is configured to monitor the quality of the tracking signal(s) from marker(s) on probe  312  and/or needle  317 . Position base unit  305  may provide an indication of the quality of such tracking signal(s) to controller  311 . Position base unit  305  may infer or estimate the quality of the tracking signal(s) from, for example, the mean received power of the tracking signal(s), the error rate in a known training sequence, or the like. 
     In some embodiments, controller  311  is configured to display a graphical or numerical indicator of the quality of tracking signals on display  313 . Graphical indicators of tracking signal quality may comprise, for example, bar graphs, pivoting needles, or the like. 
     In some embodiments, controller  311  is configured to generate an alert when tracking signal quality falls below a threshold. A tracking signal quality threshold may comprise, for example, a mean received power level, an error rate, or the like. An alert generated by controller  311  may comprise, for example,
         a textual warning message displayed on display  313 ;   an audible tone;   a verbal warning message;   a change in the appearance (e.g., attributes of color, intensity, shape, linestyle, or the like) of some or all of the display elements shown on display  313 ;   causing some or all of the display elements shown on display  313  to be hidden; or   the like.
 
In some embodiments, controller  311  is configured to cause lines representing projections of needle  317  and its trajectory onto the plane of an ultrasound image to disappear from display  313  when tracking signal quality is below a threshold.
       

       FIG. 32  shows a portion  320  of an ultrasound system according to an example embodiment. System  320  comprises a controller  321  connected to a 3D position sensing system  325 . Controller  321  comprises a position sensing system interface  322 , a display controller  323  and a position information quality monitor  324 . Position sensing system  325  comprises a position signal receiver  326  configured to receive position signals from markers (not shown) rigidly connected to a needle  328  and an ultrasound probe  329 . The position markers provide position signal receiver  326  with tracking signals indicative of their location and/or orientation. Position sensing system  325  also comprises position base unit  327 , which is communicatively coupled to position signal receiver  326 . 
     Position sensing system interface  322  is communicatively coupled to position sensing system  325 . Position sensing system interface  322  provides information to controller  321  that is indicative of the position and orientation of needle  327  and ultrasound probe  328 , and indicative of the quality of the position and orientation information. 
     Display controller  323  is operable to generate data for driving a display (not shown). Display controller  323  is operable to generate data for driving a display to show an image generated from ultrasound data (e.g., from ultrasound data acquired by probe  329 ) and to generate data for driving a display to show graphics indicative of features of the environment. In  FIG. 32 , the details of how ultrasound data is provided to controller  321  and/or display controller  323  and the details of how controller  321  and/or display controller  323  interface with a display (not shown) are omitted for clarity. Display controller  323  comprises an image display inhibitor  323 A, an image display appearance control  323 B, a graphics display inhibitor  323 C, a graphics display appearance control unit  323 D and a signal quality graphics generator  323 E. 
     Position signal receiver  326  provides location and/or orientation information concerning the position markers to position base unit  327 . Position signal receiver  326  may relay the tracking signals provided by the position markers to position base unit  327 . In some embodiments, position signal receiver  326  processes tracking signals from the position markers to generate location and/or orientation information concerning the position markers, and provides this information to position base unit  327 . 
     In some embodiments, position signal receiver  326  is configured to monitor the quality of the tracking signals from markers on probe  329  and/or needle  328 . Position signal receiver  326  may be configured to provide an indication of the quality of the tracking signals to position base unit  327 . Position signal receiver  326  may be configured to infer or estimate the quality of the tracking signals from, for example, the mean received power of the tracking signals, an error rate in a known training sequence, or the like. Position signal receiver  326  may be configured to communicate a tracking signal quality indicator, such as, for example, mean received power, error rate, a computed quality measure or the like, to position base unit  327 . Position signal receiver  326  may be configured to communicate tracking signal quality indicators that are specific to particular markers or particular instruments (e.g., probe  329  or needle  328 ). 
     In some embodiments, position signal receiver  326  is configured to determine whether the quality of one or more of the tracking signals from markers on probe  329  and/or needle  328  is below a threshold. Such a threshold may comprise, for example, a received power value, an error rate, or the like. Position signal receiver  326  may be configured to provide position base unit  327  with a low signal quality indication when it determines that the quality of one or more of the tracking signals from markers on probe  329  and/or needle  328  is below a threshold. A low signal quality indication may be specific to a particular marker whose tracking signal quality is below a threshold or be specific to a particular instrument (e.g., probe  329  or needle  328 ) for which a marker mounted thereon has a tracking signal quality below a threshold. A low signal quality indication may comprise position signal receiver  326  not providing location and/or orientation information for one or more position markers to position base unit  327 . 
     In some embodiments, position base unit  327  is configured to monitor the quality of the tracking signals from markers on probe  329  and/or needle  328 . Position base unit  327  may be configured to provide an indication of the quality of the tracking signals to position sensing system interface  322 . Position base unit  327  may be configured to infer or estimate the quality of the tracking signals from, for example, the mean received power of the tracking signals, the error rate in a known training sequence, a tracking signal quality indicator provided by position signal receiver  326 , or the like. Position base unit  327  may be configured to communicate a tracking signal quality indicator, such as, for example, mean received power, error rate, a computed quality measure, a tracking signal quality indicator generated by position signal receiver  326 , or the like, to position sensing system interface  322 . Position base unit  327  may be configured to communicate tracking signal quality indicators that are specific to particular markers or particular instruments (e.g., probe  329  or needle  328 ). 
     In some embodiments, position base unit  327  is configured to determine whether the quality of one or more of the tracking signals from markers on probe  329  and/or needle  328  is below a threshold. Such a threshold may comprise, for example, a received power value, an error rate, or the like. Position base unit  327  may be configured to provide position sensing system interface  322  with a low signal quality indication when it determines that the quality of one or more of the tracking signals from markers on probe  329  and/or needle  328  is below a threshold. A low signal quality indication may be specific to a particular marker whose tracking signal quality is below a threshold or be specific to a particular instrument (e.g., probe  329  or needle  328 ) for which a marker mounted thereon has a tracking signal quality below a threshold. A low signal quality indication may comprise position base unit  327  not providing location and/or orientation information for one or more position markers to position sensing system interface  322 . 
     Position sensing system interface  322  is configured to receive signals from position base unit  327 . Position sensing system interface  322  may be configured to receive a tracking signal quality indicator and/or a low signal quality indication from position base unit  327 . In some embodiments, position information quality monitor  324  is configured to determine whether the quality of one or more of the tracking signals from markers on probe  329  and/or needle  328  is below a threshold. Such a threshold may comprise, for example, a received power value, an error rate, a computed quality measure value, or the like. For example, position information quality monitor  324  may be configured to determine whether a tracking signal quality indicator provided by position based unit  327  is below a threshold. 
     Position information quality monitor  324  of controller  321  is configured to cause controller  321  to generate an alert when tracking signal quality is below a threshold, such as when a tracking signal quality indication is below a threshold, when a low quality indication is received from position base unit  327 , or in like circumstances. Controller  321  may be configured to generate visual and/or audible alerts. 
     In some embodiments, controller  321  is configured to generate a visual alert by causing display controller  323  to change an aspect of the display driving data it generates. Display controller  323  may be configured to change an aspect of the display driving data it generates by doing one or more of the following:
         causing image display inhibitor  323 A to inhibit display of the ultrasound image;   causing image display appearance controller  323 B to change an appearance of the ultrasound image;   causing graphics display inhibitor  323 C to inhibit display of one or more previously displayed graphics elements;   causing graphics display appearance controller  323 D to change an appearance (e.g., attributes of color, intensity, shape, linestyle, or the like) of one or more previously displayed graphics elements;   causing signal quality graphics generator  323 E to generate a graphic element indicative of the low tracking signal quality condition, such as, for example, a textual message, or the like; or   the like.       

       FIG. 33  shows an ultrasound system  370  according to an example embodiment. Ultrasound system  370  is useful for locating a reference position located in the body of patient P. An ultrasound probe  372  is operable to receive ultrasound echo signals returning from a portion of the body of a patient P. An ultrasound image processor  374  is communicatively coupled to ultrasound probe  372 . Ultrasound image processor  374  is operable to generate an ultrasound image based on the ultrasound echo signals received by ultrasound probe  372 . A position sensing system  376  is operable to determine a spatial location and orientation of ultrasound probe  372 . An image plane locator  378  is communicatively coupled to position sensing system  376 . Image plane locator  378  is operable to determine a spatial description of a plane of the ultrasound image generated by ultrasound image processor  374  from the ultrasound echo signals received by ultrasound probe  372  based on the spatial location and orientation of the ultrasound probe  372  determined by ultrasound position sensing system  376 . A geometry computer  380  is communicatively coupled to image plane locator  378  and a memory  382 . Memory  382  is operable to contain a spatial description of the reference position. Geometry computer  380  is operable to determine a spatial relationship between the reference position and the plane of the ultrasound image based on the spatial description of the reference position in memory  382  and the spatial description of the plane of the ultrasound image determined by image plane locator  378 . A graphics processor  384  is communicatively coupled to geometry computer  380 . Graphics processor  384  is operable to generate a marker indicative of the spatial relationship, determined by geometry computer  380 , between the reference position and the plane of the ultrasound image. A display  386  is communicatively coupled to ultrasound image processor  374  and graphics processor  384 . Display  386  is operable to display the ultrasound image generated by ultrasound image processor  374  and the marker generated by graphics processor  384 . 
     It is not mandatory that the ultrasound scans result in two-dimensional images (as are commonly obtained, for example, in B-mode imaging). In some embodiments, the ultrasound scans obtain three-dimensional (volumetric) images. 
     The techniques described herein are not limited to B-mode ultrasound scans. 
     Ultrasound imaging may be performed in other modes. For example, the ultrasound imaging may be performed in modes such as: 
     Doppler modes; 
     color Doppler modes; 
     elastography; 
     etc. 
     In some embodiments, the 3D model is made up of ultrasound images acquired in a plurality of different modes. Or, equivalently, multiple 3D models sharing the same coordinate system or having coordinate systems that can be related to one another by some known transformation are provided for different ultrasound imaging modes. In such embodiments, the apparatus may be configured to provide a 3D image that combines information from two or more different modes. A user control may be provided, for example, to control the blending of the information in the displayed image. This may permit a user to identify specific anatomical features that are more readily visible to some modes of ultrasound imaging than they are to others. 
     It can be appreciated that the apparatus and methods described herein have application in a wide range of ultrasound imaging applications. For example, the methods and apparatus may be applied to: 
     obtaining biopsy samples; 
     placing radioactive seeds for cancer treatment or the like; 
     placing electrodes; 
     injecting drugs at specific locations; 
     inserting an epidural catheter, for example for the introduction of an anaesthetic; 
     injecting epidural anaesthetic; 
     positioning surgical tools for minimally-invasive surgery; 
     etc. 
     Certain implementations of the invention comprise computer processors which execute software instructions which cause the processors to perform a method of the invention. For example, one or more processors in an ultrasound system may implement methods as described above by executing software instructions in a program memory accessible to the processor(s). The invention may also be provided in the form of a program product. The program product may comprise any medium which carries a set of computer-readable signals comprising instructions which, when executed by a data processor, cause the data processor to execute a method of the invention. Program products according to the invention may be in any of a wide variety of forms. The program product may comprise, for example, physical media such as magnetic data storage media including floppy diskettes, hard disk drives, optical data storage media including CD ROMs, DVDs, electronic data storage media including ROMs, flash RAM, or the like. The computer-readable signals on the program product may optionally be compressed or encrypted. 
     In addition to or as an alternative to performing methods by way of software executed in a programmable processor, such methods may be implemented in whole or in part by suitable logic circuits. The logic circuits may be provided in hard-wired form such as by hard-wired logic circuits or one or more application specific integrated circuits. The logic circuits may in whole or part be provided by configurable logic such as suitably-configured field-programmable gate arrays. 
     Where a component (e.g. a software module, processor, assembly, device, circuit, etc.) is referred to above, unless otherwise indicated, reference to that component (including a reference to a “means”) should be interpreted as including as equivalents of that component any component which performs the function of the described component (i.e., that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure which performs the function in the illustrated exemplary embodiments of the invention. 
     Those skilled in the art will appreciate that certain features of embodiments described herein may be used in combination with features of other embodiments described herein, and that embodiments described herein may be practised or implemented without all of the features ascribed to them herein. Such variations on described embodiments that would be apparent to the skilled addressee, including variations comprising mixing and matching of features from different embodiments, are within the scope of this invention. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations, modifications, additions and permutations are possible in the practice of this invention without departing from the spirit or scope thereof. The embodiments described herein are only examples. Other example embodiments may be obtained, without limitation, by combining features of the disclosed embodiments. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such alterations, modifications, permutations, additions, combinations and sub-combinations as are within their true spirit and scope.