Patent Publication Number: US-9895133-B2

Title: System and methods for enhanced imaging of objects within an image

Description:
REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 12/790,109, entitled “Systems and Methods for Enhanced Imaging of Objects Within An Image”, filed May 28, 2010, now U.S. Pat. No. 8,861,822, which claims priority to U.S. Provisional Patent Application Ser. No. 61/321,666, entitled “Systems and Methods for Enhanced Imaging of Objects Within An Image”, filed Apr. 7, 2010, and is related to commonly assigned U.S. patent application Ser. No. 11/749,319 entitled “System and Method for Optimized Spatio-Temporal Sampling,” filed May 16, 2007, and Ser. No. 11/854,371 entitled “System and Method for Spatial Compounding Using Phased Arrays,” filed Sep. 12, 2007, the disclosures of which are hereby incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The invention relates generally to ultrasound imaging and, more particularly, to enhancing the visualization of objects within an image. 
     BACKGROUND OF THE INVENTION 
     Ultrasound imaging systems have gained wide acceptance for use in providing images of objects and areas which are not otherwise visible to an observer. Such ultrasound imaging systems are typically configured with various imaging parameters selected to produce the best overall ultrasound image quality and not the best visualization of individual objects that may be present in a volume being imaged. As a result, the visualization of individual objects is typically compromised to achieve an overall satisfactory ultrasound image quality. 
     Objects visualized and represented in ultrasound images may comprise biological structures, such as human tissue and organs, and man-made structures such as implantable devices, instruments, etc. The various biological and man-made structures may require specific imaging parameters to achieve high quality ultrasound visualization of the structures that are different from parameters selected to achieve overall image quality. In addition, the imaging parameters chosen to achieve high quality visualization of one type structure may be significantly different than the parameters chosen to achieve high quality visualization of a different type of structure. Thus, it is not a simple task to provide high quality visualization of one or more individual objects within an overall high quality ultrasound image. 
     It is now common practice to use ultrasound imaging systems to aid in the guidance and placement of man-made instruments and other objects. For example, interventional instruments, such as needles, catheters, etc., may be used to deliver medication or other fluids directly into a nerve, an artery, or a vein deep within or internal to a patient&#39;s body. Such procedures may require precise positioning of an instrument internal to a patient thus requiring high quality ultrasound visualization of both biological structures and man-made instruments. 
     Using ultrasound imaging systems configured with imaging parameters selected to optimize overall image quality, it is often difficult, and sometimes impossible, to provide adequate visualization of instruments inserted at a steep angle with respect to an ultrasound transducer used to generate an ultrasound image. The problem of poor visualization of instruments inserted at steep angles results, at least in part, from the fact that representations of such instruments in ultrasound images are based on ultrasound echoes that are reflected from the instruments in a specular fashion. The principles of specular reflection indicate that for steep insertion angles the ultrasound echoes reflected from the instruments do not sufficiently intersect the ultrasound transducer elements to produce a clear representation of the instrument in the resulting ultrasound image. 
     Due to the generally poor representation in ultrasound images of instruments inserted at steep angles, a clinician must often rely on secondary artifacts to visualize or “guess” where the interventional instrument is within a volume (e.g., within a patient&#39;s anatomy). For example, a clinician may rely upon movement of tissue, or other structures visible within the resulting image, caused by pressure from a needle as the needle is inserted or otherwise moved, to visualize where the needle is within the patient&#39;s anatomy. Visualization of the location of an interventional instrument based upon the movement of nearby structures generally does not provide for precise location determinations. 
     Another technique used for visualizing the location of an interventional instrument requires injecting fluid through the interventional instrument and observing the resulting image as the fluid moves through the media of the volume being imaged (e.g., as the fluid moves into and through tissue). This technique thus also relies on secondary artifacts and has not been found to be particularly satisfactory. 
     Several specially-designed echogenic needles have been introduced to address the problem of poor visualization of instruments inserted at steep angles. These specialty needles are typically designed and constructed in a way that the ultrasound waves reflected from the needle reach the ultrasound transducer elements even when the needle is inserted at steep angles. However, there are a number of factors that reduce the effectiveness and desirability of such needles. For example, the increased cost associated with such special needles reduces their clinical acceptance and widespread use. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is directed to systems and methods providing ultrasound imaging signatures for generating ultrasound images. For example, a plurality of ultrasound imaging signatures, wherein each imaging signature is related to an object in an imaged volume, may be utilized according to embodiments of the invention. Using such ultrasound imaging signatures, an ultrasound imaging system may provide improved or optimized imaging of various objects in the imaged volume. 
     According to embodiments of the present invention, an ultrasound imaging signature comprises one or more ultrasound imaging parameter values, where each imaging parameter is associated with either the acquisition or processing of ultrasound data. Each imaging signature of a plurality of imaging signatures may be associated with the same ultrasound modality, e.g., B-mode, color-flow, power-doppler, elastography and others. Moreover, each ultrasound imaging signature of embodiments has one or more imaging parameters set at values tailored to the high quality ultrasound visualization of a particular object of interest. 
     The ultrasound imaging signatures of embodiments of the invention are optimized or otherwise configured for imaging particular objects, structures, aspects, etc. of a volume being imaged. For example, a first imaging signature may be configured for providing relatively high quality images with respect to subsurface regions of living tissue (e.g., general patient anatomy), whereas a second imaging signature may be configured for providing relatively high quality images with respect to interventional instruments (e.g., a needle) inserted into living tissue at a steep angle. 
     Sub-frames generated using different ones of such ultrasound imaging signatures are preferably combined to form a frame providing a relatively high quality image of the various structures, attributes, aspects, etc., collectively referred to as objects (e.g., general patient anatomy and interventional instrument) within the volume being imaged. Frames formed according to embodiments are preferably combined or blended to form a final image. 
     According to a preferred embodiment, two ultrasound imaging signatures are used wherein one ultrasound imaging signature is tailored to provide high image quality for human tissues and another is tailored to provide high image quality for interventional instruments such as needles inserted at steep insertion angles. According to aspects of the embodiments, an ultrasound imaging signature of the foregoing two ultrasound imaging signatures comprises a predetermined set of steering angles specifically targeted for high quality interventional instrument visualization at steep insertion angles. According to other aspects of the embodiments, one or more ultrasound imaging parameter values are changed between the two ultrasound imaging signatures, where the one or more ultrasound parameters include transmit waveform, ratio of transmit to receive beamformed lines, steering angle, receive line density, number of focal zones, location of focal zones, quadrature bandpass filter type and coefficients, compression curve, speckle reduction parameters, etc. 
     Other embodiments of the invention operate to identify an area or block of interest within a frame for combining or blending with one or more other frames to form a final image. For example, a block in which an interventional instrument is to be disposed may be known or determined. Thus, embodiments of the invention may crop, or otherwise render insignificant, portions outside of the block prior to blending an interventional instrument frame with an anatomical structure frame when forming a final image. Such embodiments may be utilized in mitigating or avoiding image clutter, artifacts, etc. associated with the use of an interventional instrument imaging signature. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       For a more complete understanding of the present invention, reference is now made to the following descriptions taken in conjunction with the accompanying drawing, in which: 
         FIGS. 1A and 1B  show an embodiment of an ultrasound imaging system adapted according to an embodiment of the invention; 
         FIGS. 2A-2C  show different sub-frames utilized according to an embodiment of the invention; 
         FIG. 2D  shows a frame generated from sub-frames of  FIGS. 2A-2C  according to an embodiment of the invention; 
         FIGS. 3A and 3B  show different sub-frames according to an embodiment of the invention; 
         FIG. 3C  shows a frame generated using the sub-frames of the ultrasound imaging signature of  FIGS. 3A and 3B  according to an embodiment of the invention; 
         FIG. 4  shows a final ultrasound image generated using the frames of  FIGS. 2D and 3C  according to an embodiment of the invention; 
         FIG. 5  shows a schematic diagram of operation of the ultrasound imaging system of  FIGS. 1A and 1B  operating to generate a final ultrasound image using a multiple ultrasound imaging signature technique of an embodiment of the invention; 
         FIG. 6  shows a high level flow diagram of operation of the ultrasound imaging system of  FIGS. 1A and 1B  operating to generate a final ultrasound image using a multiple ultrasound imaging signature technique of an embodiment of the invention; 
         FIG. 7  shows a coordinate system for detecting straight lines in sub-frames according to embodiments of the invention; and 
         FIG. 8  shows graphics in a final image to indicate the coverage area defined by the highly steered sub-frame of embodiments of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1A  shows an ultrasound imaging system adapted according to an embodiment of the invention. Specifically, ultrasound imaging system  100  is shown comprising system unit  110  coupled to transducer  120 . System unit  110  of embodiments comprises a processor-based system operable to control transducer  120  to transmit and receive ultrasound signals, to process the received ultrasound signals, to generate an image using the processed received ultrasound signals, and to display the generated image (e.g., on display  111 ). Transducer  120  comprises an array of ultrasound elements operable to controllably transmit and receive ultrasound signals. Detail with respect to imaging systems which may be adapted according to the concepts of the present invention is provided in co-pending and commonly assigned U.S. patent application Ser. No. 12/467,899 entitled “Modular Apparatus for Diagnostic Ultrasound,” the disclosure of which is hereby incorporated herein by reference. 
     In operation, ultrasound imaging system  100  implements an imaging technique known as “spatial compounding” wherein a number of different signal steering angles are used to illuminate a volume being imaged. Additional detail regarding spatial compounding techniques is found in the above referenced patent applications entitled “System and Method for Optimized Spatio-Temporal Sampling” and “System and Method for Spatial Compounding Using Phased Arrays.” Using typical spatial compounding techniques, the data collected with respect to an imaged volume using a single steering angle are processed to form a sub-frame and all the sub-frames of the imaged volume are then compounded to produce a frame. A frame may be formed, for example, using two sub-frames, three sub-frames, four sub-frames or more, corresponding to the use of two, three, four or more steering angles respectively. 
       FIGS. 2A-2D  illustrate the use of three sub-frames in generating a frame. Frame  200  of  FIG. 2D , for example, illustrates a top-down view into volume being imaged  101 , such as may comprise living tissue of a patient&#39;s anatomy. In the illustrated example, frame  200  of  FIG. 2D  may be generated by ultrasound imaging system  100  using sub-frames  201 - 203  of  FIGS. 2A-2C . Sub-frame  201  of the illustrated embodiment implements a straight down view (e.g., a steering angle of 0°) with respect to transducer  120  and provides sub-frame data for the un-steered sub-frame. Sub-frames  202  and  203  of the illustrated embodiment implement offset steering angles. For example, the steering angle for sub-frame  202  may be directed to the left at approximately −14° and the steering angle for sub-frame  203  may be directed to the right at approximately +14°. The data provided by these sub-frames is compounded to generate frame  200  having improved image characteristics over a frame generated using a single steering angle (e.g., a frame based solely on the data available for sub-frame  201 ). The resulting frame  200  produces a desired higher image quality by, for example, mitigating effects of shadowing, reducing speckle noise, and improving boundary delineation. 
     Ultrasound imaging systems employing spatial compounding are typically configured with various imaging parameters selected to produce the best overall ultrasound image quality and not the best visualization of individual objects within a volume being imaged. In general, the term “imaging parameters” refers to parameters associated with either the acquisition or processing, or both, of ultrasound data. Examples of such imaging parameters comprise transmit waveform, ratio of transmit to receive beamformed lines, imaging steering angle, receive line density, number of focal zones, location of focal zones, quadrature bandpass filter type and coefficients, compression curve, and speckle reduction parameters, among others. Because imaging parameters are typically selected to produce best overall image quality, the visualization quality of individual objects within the volume being imaged may be compromised. However, from the clinical standpoint it is beneficial that a single ultrasound image is presented to a clinician where there is high quality visualization of all objects of interest, including different biological structures and man-made objects. 
     An exemplary case illustrating this problem is the visualization of instruments, such as interventional instrument  130  comprising needles, catheters, etc. when inserted at relatively steep angles relative to transducer  120  (as depicted in  FIG. 1B ). For example, a portion of interventional instrument  130 , such as may comprise a needle, a catheter, a stent, a percutaneous tool, etc., is inserted into volume being imaged  101 , to perform an interventional procedure. Exemplary procedures might require injecting pharmaceuticals into target  102 , performing a biopsy with respect to target  102 , etc. Typically, when the insertion angle (represented by angle α in  FIG. 1B ) is approximately 20° or greater, it is considered to be a steep angle of insertion. 
     When interventional instruments are inserted at steep angles, the imaging steering angles of the sub-frames configured for imaging target  102  may not provide satisfactory imaging of interventional instrument  130 . For example, the effects of specular reflection may cause the signal reflected from the interventional instrument to not be readily visible in the resultant image. The failure to provide satisfactory imaging with respect to interventional instrument  130  is particularly problematic where ultrasound system  100  is being used to generate images to facilitate a clinician&#39;s performance of an interventional procedure using interventional instrument  130 . 
     Although it might appear that the foregoing spatial compounding technique may be easily modified to include one or more sub-frames configured for imaging interventional instrument  130 , the various different imaging signal steering angles of the sub-frames which may be compounded using spatial compounding techniques is not without restriction if image quality is to be maintained or improved. For example, it has been discovered that the use of sub-frames implementing more acute steering angles (e.g., in excess of ±15° as may be desirable for use in imaging an interventional instrument inserted at a steep angle) in combination with the sub-frames of the foregoing imaging signature implementing less acute steering angles results in image degradation. That is, particular sub-frames are not compatible for generation of a frame using spatial compounding techniques because their compounding results in undesired or unacceptable image clutter, artifacts, etc. By “compatible,” as used with reference to imaging parameters, it is meant that various imaging parameters (e.g., optimized or otherwise configured for use with respect to a particular object) may be used together (e.g., in a spatial compounding technique) without resulting in undesired or unacceptable image clutter, artifacts, etc. Conversely, by “not compatible” it is meant that imaging parameters (e.g., optimized or otherwise configured for use with respect to a particular object) may result in undesired or unacceptable image clutter, artifacts, etc. when used with respect to other imaging parameters (e.g., optimized or otherwise configured for use with other objects) and thus may be considered not to be compatible with such other imaging parameters. 
     In order to achieve high quality ultrasound visualization of particular objects within an imaged volume, embodiments of the present invention employ ultrasound imaging signatures which comprise one or more ultrasound imaging parameter values tailored to produce high quality ultrasound visualization of particular objects of interest. Such ultrasound imaging signatures comprise imaging parameters associated with either the acquisition and/or the processing of ultrasound data such that the parameter values are tailored to the high quality ultrasound visualization of a particular object of interest. In a preferred embodiment, a particular ultrasound imaging signature is defined for an object of interest without compromise in favor of the visualization of other objects being scanned. 
     According to embodiments of the present invention, the particular sub-frames utilized in generating a frame are selected so as to be optimized or otherwise configured for imaging particular objects according to the ultrasound imaging signature defined for a particular object. For example, such a collection of sub-frames may be configured for providing relatively high quality images with respect to subsurface regions of living tissue (e.g., general patient anatomy), particular tissue (e.g., heart, liver, uterus, stomach, etc.), particular structure (e.g., bone joints, artery bifurcation, nerve cluster, etc.), or man-made objects such as implants and instruments, and/or the like. According to embodiments of the present invention, the collection and processing of sub-frames, e.g., sub-frames  201 - 203 , and their combination to produce a frame, e.g., frame  200 , is associated with an ultrasound imaging signature associated with target  102 . 
     Therefore, embodiments of the ultrasound imaging system of the present invention are operable to implement a plurality of different ultrasound imaging signatures wherein each ultrasound imaging signature of the plurality of imaging signatures implemented provides high quality visualization of a corresponding object of interest within a scanned volume. According to an embodiment of the invention, a first ultrasound imaging signature (e.g., comprising sub-frames  201 - 203  of  FIGS. 2A-2C ) is configured for providing relatively high quality images with respect to subsurface regions of volume being imaged  101 . Correspondingly, a second ultrasound imaging signature (e.g., comprising sub-frames  301 - 302  of  FIGS. 3A-3B ) is configured for providing relatively high quality images with respect to a portion of interventional instrument  130  inserted at a steep angle into volume being imaged  101 . 
     Sub-frames  301  and  302  of  FIGS. 3A and 3B  are configured for providing relatively high quality images with respect to interventional instrument  130 , inserted at a steep angle, by implementing more acute steering angles to provide imaging data which reduces specular reflection disadvantages associated with interventional instrument  130 . As has been discussed, interventional instrument  130  of the illustrated embodiment has a relatively steep angle with respect to the face of transducer  120  and is visible in frame  300  of  FIG. 3C  because the steering angles of at least one of sub-frames  301  and  302  is highly steered in one or more appropriate directions. It should be appreciated that, for clarity, artifacts from item to be imaged  101  (e.g., tissue and other structure) are not shown in sub-frame  300 . Features of item to be imaged  101  would be highly degraded due to clutter and other factors associated with the use of sub-frames  301  and  302 . 
     In configuring sub-frames  301  and  302  according to embodiments of the invention, the steering angles may be selected to substantially correspond to the insertion angle of interventional instrument  130  so as to provide transmission of signals more normal to the surface of interventional instrument  130 . By way of example, interventional instrument  130  may be inserted at an angle of approximately 60° and correspondingly the steering angles implemented with respect to sub-frames  301  and  302  may be ±30° so as to provide transmission of acoustic waves which result in approximately a 90° angle of incidence with a face of interventional instrument  130 . It should be appreciated that other angles of incidence, other than 90°, may be utilized according to embodiments of the invention. Preferred embodiments operate to facilitate angles of incidence in the range of 75° degrees to 105° degrees with respect to a surface of an object for providing high quality images of the object. 
     It should be appreciated that the insertion angle of an interventional instrument being imaged may be known or assumed for configuring and/or selecting the parameter values for an appropriate imaging signature for use therewith by a number of means. For example, an interventional instrument guide may be utilized which provides for a particular angle of insertion. The angle of insertion provided by a selected interventional instrument guide may be provided automatically to a processor of system unit  110  using various sensor or other feedback techniques, such as those shown in U.S. patent application Ser. No. 11/216,735 entitled “Medical Device Guide Locator,” filed Aug. 31, 2005, assigned to Sonosite, Inc. the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference. Where an interventional instrument is inserted free-hand (e.g., without the aid of a guide), an insertion angle used for configuration and/or selection of an appropriate imaging signature may be estimated as the intended insertion angle, a typical insertion angle, a calculated insertion angle, etc. Additionally or alternatively, an insertion angle may be determined through analysis of collected data (e.g., imaging data), such as through identifying attributes of an interventional instrument within a generated image. 
     The ultrasound imaging signatures utilized in generating the final image may have differing numbers and types of imaging parameters. For example, the first of the foregoing exemplary imaging signatures comprises 3 sub-frames while the second of the foregoing exemplary imaging signatures comprises 2 sub-frames. There is no limitation regarding the illustrative number of sub-frames or other imaging parameters and thus imaging signatures of embodiments may implement fewer or more sub-frames than that illustrated. For example, an imaging signature configured for use with respect to a particular object or feature, such as an ultrasound imaging signature configured for use with respect to interventional instrument  130 , may implement a single sub-frame according to embodiments of the invention. 
     Although embodiments of ultrasound imaging signatures comprising sub-frames implementing various steering angles, embodiments of the invention may utilize ultrasound imaging signatures comprising various additional or alternative imaging parameters. For example, ultrasound imaging signatures of embodiments may provide specification configurations of imaging parameters which include transmit waveforms, ratio of transmit to receive beamformed lines, steering angle, receive line density, number of focal zones, location of focal zones, quadrature bandpass filter type and coefficients, compression curve, speckle reduction parameters, etc. to enhance the particular objects of interest. 
     Irrespective of the particular ultrasound imaging signatures used, system unit  110  of embodiments operates to generate image frames (e.g., frames  200  of  FIG. 2D and 300  of  FIG. 3C ) using signals received and processed with respect to each of the ultrasound imaging signatures invoked. Such image frames formed according to embodiments are preferably combined or blended to form a final image.  FIGS. 5 and 6  illustrate operation of ultrasound system  100  in generating final image  400  using a plurality of imaging signatures, and image frames generated there from, as described herein. 
     As shown in block diagram  600  of the embodiment illustrated in  FIG. 6 , a first ultrasound imaging signature for imaging a first object (e.g., object  102 ) is established at block  601 . For example, ultrasound imaging signature  502  of  FIG. 5  (comprising sub-frames  201 - 203 ) may be established for use in imaging a first object. Additionally, a second ultrasound imaging signature for imaging a second object (e.g., interventional instrument  130 ) is established at block  602  of the illustrated embodiment. For example, ultrasound imaging signature  503  of  FIG. 5  (comprising sub-frames  301 - 302 ) may be established for use in imaging a second object. 
     At block  603  of the illustrated embodiment imaging data is collected and processed using the first ultrasound imaging signature. For example, system unit  110  and transducer  120  may cooperate to implement the first ultrasound imaging signature and collect and process imaging data. Additionally, at block  604  of the illustrated embodiment imaging data is collected and processed using the second ultrasound imaging signature. For example, system unit  110  and transducer  120  may cooperate to implement the second ultrasound imaging signature and collect and process imaging data. 
     As shown in the embodiment of  FIG. 5 , data collected by transducer  120  operating in accordance with ultrasound imaging signature  502  is provided to system unit  110 . The sub-frames collected and processed using the first imaging signature are further processed by system unit  110  to provide a frame including the first object at block  603  of the illustrated embodiment. For example, spatial compounding processing  511  of embodiments, as may comprise a processor operating under control of an instruction set defining operation as described herein, is operable with respect to the data collected using ultrasound imaging signature  502  to generate frame  200  comprising a high quality image of target  102 . 
     Correspondingly, data collected by transducer  120  operating in accordance with ultrasound imaging signature  503  is provided to system unit  110 . The sub-frames collected and processed using the second ultrasound imaging signature are further processed by system unit  110  to provide a frame including the second object at block  604  of the illustrated embodiment. For example, interventional instrument detection  512 , as may be provided by algorithms operable upon a processor of system unit  110 , is operable with respect to the data collected using ultrasound imaging signature  503  to generate frame  300  comprising a high quality image of a portion of interventional instrument  130 . 
     Interventional instrument detection  512  of embodiments, as may comprise a processor operating under control of an instruction set defining operation as described herein, provides operation to detect interventional instrument  130  or other objects of interest. For example, algorithms of interventional instrument detection  512  may analyze data collected by transducer  120  to identify attributes of interventional instrument  130  therein for configuration and/or selection of a particular ultrasound imaging signature for use in providing a high quality image thereof. Additionally or alternatively, embodiments of interventional instrument detection  512  operate to identify interventional instrument  130  within one or more sub-frames, e.g., sub-frames  301  and  302 , to provide isolation of the interventional instrument for generation of frame  300  where the interventional instrument and its surrounding areas are shown. For example, the shape or other characteristics of interventional instrument  130  may be known (e.g., an interventional instrument in the form of a needle has a known shape, e.g., a linear segment) and may be readily identified by using an appropriate algorithm. In one embodiment, the mathematical framework of the Hough Transform is used to detect the interventional instrument. This is a well known framework for detecting lines and any curve that can be expressed in a parametric form. Using this approach the object of interest can be modeled as a straight line or a parametric curve and the algorithm would determine where the object is within a sub-frame, e.g., sub-frames  301  and  302 . 
     Irrespective of the particular technique implemented, operation of interventional instrument detection  512  of embodiments provides a segmentation of the interventional instrument  130  and its immediate surrounding area, the result of which is mask  530 . The segmentation result, e.g., mask  530 , enables the isolated use of interventional instrument  530  and its surrounding area in generating the final image  400  without degrading the tissue image (e.g., as may be provided by frame  102 ). 
     As previously mentioned, the Hough Transform for detecting straight lines is used with sub-frames  301  and  302  according to embodiments. According to one implementation of the Hough Transform and directing attention to  FIG. 7 , line  701  in coordinate system  700  {x,y}, can be expressed by the equation:
 
ρ= x ·cos θ+ y ·sin θ  (1)
 
where ρ is the distance of the line from the origin of coordinate system  700  {x,y} and θ is the angle between a line that is perpendicular to the line of interest  701  and the axis x of the coordinate system {x,y}. During an initialization process of the Hough Transform according to embodiments a 2D array that contains the sub-frame pixels in the coordinate system {x,y}, and a second 2D array called the accumulator array are defined. Each cell in the accumulator array corresponds to a particular set of parameters (ρ 0 , θ 0 ) which represent a single line in the processed sub-frame, as shown by equation (1). The size of the 2D accumulator array depends on the range of shortest distances ρ and angles θ that are of interest and the resolution by which they need to be defined. Once initialization is complete, the main mechanism of the Hough Transform is to sequence through all the pixels of a sub-frame and for each pixel (x 0 ,y 0 ) that satisfies a set of criteria, such as intensity threshold, gradient strength etc, a counter is increased in the accumulator array for all the cells (ρ, θ) that satisfy equation (1) for the pixel (x 0 ,y 0 ). In operation according to embodiments, after the entire sub-frame is sequenced the cell in the accumulator array with the highest counter value corresponds to the 2D line in the image sub-frames  301  or  302  representing the interventional instrument.
 
     To further identify the particular segment along the 2D line where the interventional instrument is located, the intensity and gradient magnitude, along the perpendicular direction to the detected line, are examined for each pixel along the 2D line identified in the sub-frame according to embodiments. The pixels along the detected 2D line for which both the aforementioned intensity and gradient magnitude exceed a certain threshold define the particular location of the interventional instrument along the 2D line detected in the sub-frame and can be used to define mask  530  in frame  300  shown in  FIG. 5  (e.g., a margin, such as a margin of a predetermined number of pixels, a predetermined distance, a percentage of the object&#39;s size, etc., may be used around the object to define the mask). According to one embodiment interventional instrument detection  512  can include a preprocessing step for removing sub-frame artifacts and noise, before the Hough Transform or other object identification technique is applied. 
     It should be appreciated that, although shown as being separate in the illustrated embodiment, spatial compounding processing  511  and interventional instrument detection  512  may be provided in combined circuitry according to embodiments. For example, a same processor may operate under control of an instruction set defining operation of spatial compounding processing  511  and an instruction set defining operation of interventional instrument detection  512 , if desired. 
     Having generated frames  200  and  300 , each providing a high quality image with respect to different objects of area to be imaged  101 , system unit  110  of the illustrated embodiment utilizes frame blending  513  to blend frames  200  and  300  to form final image  400  providing a high quality image of various objects within volume being imaged  101  at block  605 . For example, mask  530  of frame  300  defining the appropriate location for interventional instrument  130  may be blended into frame  200  to form final image  400 . In the illustrated embodiment frames  200  and  300  are aligned based on the acquisition and processing used to generate them, whereas mask  530  serves as a means to identify the portion of frame  300  that will be blended with frame  200 . For pixels outside of the region identified by mask  530  the value of the pixels in final image  400  are identical to those of frame  200  according to embodiments. For each pixel location within mask  530  the resulting pixel value for final image  400  may be a combination of the corresponding pixel values found in frames  200  and  300 . 
     A blending function may be implemented by frame blending  513  to provide the foregoing blending of frames to generate a final image. Assuming that a pixel (x 0 , y 0 ) is within the region identified by mask  530  and that f 1 (x 0 , y 0 ), f 2 (x 0 , y 0 ), and f(x 0 , y 0 ), are the pixel values of frames  200 ,  300  and final image  400  respectively, we can write the following blending function equation:
 
 f ( x   0   ,y   0 )=(1− b )* f   1 ( x   0   ,y   0 )* f   2 ( x   0   ,y   0 )  (2)
 
where b is a coefficient between 0 and 1 that can be constant, e.g., 0.5 to produce an average of the two pixel values. In another embodiment coefficient b can be a function of the location within the mask where coefficient b shall have higher values for locations in mask  530  closer to interventional instrument  130 , and lower values for locations in mask  530  farther from the interventional instrument  530 . Alternatively, instead of applying a linear operation shown by equation (2), a non-linear operation can be used in a blending function of embodiments. In one example of such a case, f(x 0 , y 0 ) can be the maximum of values f 1 (x 0 , y 0 ) and f 2 (x 0 , y 0 ).
 
     While only two ultrasound imaging signatures are shown with respect to the illustrated embodiment, it should be appreciated that any number of ultrasound imaging signatures can be used as appropriate to particular embodiments. For example, where 3 objects of interest (e.g., a nerve, an interventional instrument, and a main or underlying volume) are to be imaged, embodiments may implement 3 ultrasound imaging signatures (e.g., configured for a corresponding one of the nerve, interventional instrument, and main volume) may be used to generate sub-frames blended to form a frame. 
     Through frame generation and blending according to embodiments of the invention, a final image is generated which provides a high quality image of the objects, substantially free of undesired or unacceptable image clutter, artifacts, etc., when using imaging parameters which are otherwise not compatible. For example, using an ultrasound imaging signature frame technique of embodiments of the invention, imaging is not degraded although image processing is performed to render an interventional instrument which is inserted at a steep angle clearly visible in the resulting image. 
     Operation of system unit  110  according to embodiments may provide image processing in addition to the foregoing spatial compounding, interventional instrument detection, and frame blending. For example, embodiments of the invention may provide speckle reduction and/or other types of image processing algorithms with respect to frames and/or final image. 
     Additionally or alternatively, directing attention to  FIG. 8 , operation of system  110  of embodiments includes graphics  800  (shown here as a trapezoid indicated by dashed lines) in final image  400 , such as to indicate the coverage area  801 . Graphics  800  may be controlled, for example, by parameters used to generate frame  300 , to indicate coverage area  801  defined by the highly steered sub-frame  301  in  FIG. 5 . Coverage area  801 , as indicated by graphics  800 , comprises an area where high quality visualization of the interventional instrument is enabled. Such graphics allow the clinician or other operator to know that items of interest will only appear with high quality visualization in certain portions, e.g., coverage area  801 , of the final image and if they do not, e.g. if they appear in region  802  of the final image  400 , then care should be used because the object (such as the interventional instrument) is out of the high quality visualization field of view (although the interventional instrument may still be within the volume being imaged). 
     If desired, certain attributes (such as the end point, the mid point, etc.) of the interventional instrument or other target could be coded so that different portions show up differently in the final image (e.g., different intensities, color, etc.), such as to alert the clinician or other operator as to which portion of an object (e.g., the interventional instrument) is being overlaid. Such image processing may be provided by frame blending  513  of embodiments, for example. 
     It should be appreciated that application of an ultrasound multi-imaging signature technique of embodiments of the invention is not limited to use with interventional instruments. For example, an ultrasound imaging signature may be configured to image a nerve, while another ultrasound imaging signature is configured to image another object, such as a tumor, whereby these ultrasound imaging signatures are used in cooperation to form frames which are blended to provide a final image showing a high quality image of each such object. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.