Abstract:
The invention relates to a method and an ultrasonic imaging apparatus ( 20 ) for imaging a specular object (such as a biopsy needle) and a target anatomy in a tissue, whereby the specular object remains visible even when its location deviates from a target plane ( 21 ) including the target anatomy.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to a method of imaging a specular object, such as for example a needle-like instrument, and a target anatomy in a tissue using ultrasound, the method comprising a tissue imaging step of transmitting first sound waves into a volumetric region of the tissue, the volumetric region including the target anatomy located in a target plane, of receiving echoes of said first sound waves from the target plane, and of processing said received echoes to produce a tissue image, said tissue imaging step using a tissue mode specific set of parameters, and a specular object imaging step of transmitting second sound waves into said volumetric region of the tissue, the volumetric region including the specular object, of receiving echoes of said second sound waves from the target plane, and of processing said received echoes to produce a specular object image, said specular object imaging step using a specular object mode specific set of parameters, and a display step of displaying a combined tissue image and specular object image. 
         [0002]    The invention further relates to an apparatus implementing the method according to the invention. 
         [0003]    More specifically the invention relates to a method of imaging a biopsy needle using ultrasound and to an ultrasonic imaging apparatus for imaging a biopsy needle. 
       BACKGROUND OF THE INVENTION 
       [0004]    Ultrasound is commonly used for imaging needle insertion during interventional surgical procedures. Such needles may, for example, be paracentetic needles for withdrawal of fluid from a target anatomy in a body or biopsy needles for the removal of samples from a target anatomy in a body. Needles may also be used to administer drugs or other substances into a specific location relative to a target anatomy in the body. 
         [0005]    During such procedures the precision of needle trajectory is highly important. The needle should not disturb or damage anatomy not connected with the procedure being undertaken. Some procedures may be directed towards small regions in the body e.g. artery or an area close to a major anatomy, such as the heart. Accuracy of needle placement is therefore vital. 
         [0006]    Ultrasound is often used as an imaging method for imaging a needle and a target anatomy in support of the interventional procedure. Other imaging alternatives, such as Xray or EM (electromagnetic) guidance, suffer from drawbacks such as risk of radiation exposure, 2D view only, additional set-up time, tracking errors or lack of visibility of the specular object, such as a biopsy needle. 
         [0007]    A method according to the preamble for imaging a specular needle together with a target anatomy in a tissue is described in “Enhancement of needle visibility in ultrasound-guided percutaneous procedures”; Cheung and Rohling&#39;s; Ultr. Med. Biol. (2004) 30: 617-624. Here the visualization of a needle in the ultrasound image is enhanced, without compromising the quality of the visualization of the target anatomy in the tissue, by displaying the combination of a specular object image (also referred to as a needle-specific image) and a tissue image. The tissue image, obtained using a tissue mode specific set of parameters, depicts the target anatomy in the tissue but may depict the needle in a poor quality, or even do not depict the needle at all, whereas the specular object image, obtained using a specular object mode specific set of parameters, depicts the needle with great contrast, but may depict almost no background tissue. Combining both images, by taking their weighted average, yield an image (often referred to as a needle-enhanced image) showing both the needle and the target anatomy. 
         [0008]    The characteristics of the ultrasound imaging in the tissue imaging step and in the specular object imaging step are optimized to allow the best possible responses. These characteristics are set by using respectively the tissue mode specific set of parameters or the specular object mode specific set of parameters. These sets of parameters may comprise, for example, the frequency of the ultrasound waves, beam steering and beam density related parameters, the focal depth, the rate at which the echoes are received (the frame rate), and parameters controlling the type of processing of the received echoes, such as for example aperture processing and temporal filtering. 
         [0009]    In the above described method the favored geometry for the needle to be displayed is the so-called “in-plane” geometry wherein the long axis of the needle is contained in the target plane including the target anatomy. However, in a significant number of cases the needle location deviates from the target plane due to, for example, tissue heterogeneities and bevel asymmetry. 
         [0010]    It is now a problem of the known method that such an out-of-plane needle is not shown (or not sufficiently shown) in the combined (needle-enhanced) images. The clinician has to move the imaging transducer, transmitting and receiving the sound waves, to locate the needle, but then loses the target anatomy located in the original target plane. Furthermore, since the clinician does not know where the needle is located in relation to the original target plane, he has no indication how to move the transducer to find the needle. 
       SUMMARY OF THE INVENTION 
       [0011]    It is an object of the present invention to overcome the above problems of the prior art. According to the present invention this object is achieved by providing a method according to the preamble in which the specular object imaging step is adapted i) to receive echoes of said second sound waves from a plurality of image planes, said plurality of image planes separated in an elevation direction at selected elevations around the target plane, and ii) to process the echoes received from the target plane and from the plurality of image planes to produce the specular object image. 
         [0012]    The inventors have realized that it is important to keep imaging the target plane including the target anatomy in the tissue imaging step, and at the same time image a volumetric region of the tissue surrounding that target anatomy without a need to move the transducer in the specular object imaging step. 
         [0013]    According to the invention a plurality of image planes separated in an elevation direction is acquired in the specular object imaging step using the specular object mode specific set of parameters. This gives a 3-dimensional set of (needle-specific) images. From this 3-dimensional set of (needle-specific) images the specular object image is obtained providing a detailed image of the object (such as a needle) which can be pinpointed in three dimensions and quantified in length and direction. 
         [0014]    The plurality of image planes may comprise the target plane including the target anatomy. This target plane may fall in the centre of the plurality of image planes, but this is not essential. Nor it is essential to centre an apex of the elevation planes at the centre of the array transducer producing the ultrasound waves, or for the elevation planes to be parallel. 
         [0015]    In the tissue imaging step a 2-dimensional tissue image of the target anatomy located in a target plane is obtained, while in the specular object imaging step the specular object image (such a a needle) is obtained from a 3-dimensional set of (needle-specific) images. Thus the relation between them allows combining of these images to display the combined (needle-enhanced) image in which the object (such as a needle) is shown in relation to the target anatomy. Such a combined image allows the operator to see the object (such as a needle) even when it does not directly coincide with the target plane. It should be noted that it is advantageous that these images have been obtained without moving the transducer. 
         [0016]    In the display step the planar tissue image (showing the target anatomy) is combined with a volumetric rendering of the specular object. In this way a simultaneous display of the specular object together with the target anatomy is maintained, even when the specular object moves away from the target plane. 
         [0017]    The data from the individual planes in the plurality of image planes may be combined by averaging to form the specular object image. However, in an embodiment a 3-dimensional segmentation algorithm is used to segment the specular object (such as a needle) out of the plurality of image planes to create a specular object image only showing that object. In a further embodiment each of the planes in the plurality of image planes are used as individual snapshots to produce separate images showing (part of) the specular object which are then fitted together in jigsaw-like fashion. 
         [0018]    According to the invention a plurality of image planes separated in an elevation direction at selected elevations around the target plane are acquired in the specular object imaging step. It is noted that the target plane may fall in the centre of the plurality of image planes (resulting in image planes symmetrically arranged about the target plane), but that this is not essential to the present invention. The angular range of elevation of many commercially available array transducers may be as much wide as +45 to −45 degrees. If the elevation planes are parallel to each other, the lateral range spanned by all the planes may be as wide as the elevational probe footprint. According to an embodiment of the present invention a range between −10 degrees and +10 degrees in relation to the target plane is preferred. Such a range has been found to be most efficient in terms of imaging quality and imaging rate, while at the same time being large enough to capture the specular object. For the same reasons an imaging range between −15 mm to +15 mm is preferred when a plurality of parallel image planes is acquired (using, for example, a linear array transducer) in the specular object imaging step. 
         [0019]    In a further embodiment of the present invention the image planes in the plurality of image planes are being imaged in the specular object imaging step at a lower imaging rate than the imaging rate at which the target plane is being imaged in the tissue imaging step. This allows for maintaining an optimized frame rate for displaying the combined tissue end specular object image without undue burden on the image processor hardware. The tissue image is preferably imaged at a high imaging rate to give a real-time and most detailed image of the tissue in the target plane. The specular object image needs less detail and thus a lower imaging rate of the plurality of images from which this specular object image is derived is possible. 
         [0020]    A further embodiment of the present invention involves multiple tissue imaging steps and multiple specular object imaging steps which are performed in an interleaved manner. This embodiment has the advantage that it reduces any temporal lag between the tissue image and the specular object image. 
         [0021]    The object of the present invention is further achieved by providing an ultrasonic imaging apparatus for of imaging a specular object and a target anatomy in a tissue, the apparatus comprising 
         [0000]    an array transducer for transmitting sound waves into a volumetric region of the tissue and for receiving echoes of said sound waves, said array transducer adapted to have elevation steering capabilities;
 
a beamformer for controlling the transmission of sound waves by the array transducer such that first sound waves are transmitted into the volumetric region including the target anatomy located in a target plane and echoes are received of said first sound waves from the target plane, and for controlling the transmission of sound waves by the array transducer such that second sound waves are transmitted into the volumetric region including the specular object and echoes are received of said second sound waves from the target plane, said beamformer adapted to control the reception of said second sound waves from a plurality of image planes, said plurality of image planes separated in an elevation direction at selected elevations around the target plane;
 
a data processor for processing said received echoes of said first sound waves to produce a tissue image and for processing said received echoes of said second sound waves to produce a specular object image, said data processor adapted to process the echoes received from the target plane and from the plurality of image planes to produce the specular object image; and
 
an image processor for displaying a combined tissue image and specular object image.
 
         [0022]    It is noted that the data processor and the image processor may be formed by dedicated hardware as well as by a generic processor implementing the required functions through appropriate firmware. 
         [0023]    It is noted that international patent application WO 2008/126015 describes an ultrasonic diagnostic imaging system that scans a plurality of planar slices in a volumetric region which are parallel to each other. The image data of the individual slices is combined by projecting the data in the elevation dimension to produce a so-called “thick slice” image. The thick slice image is then displayed at a high frame rate by combining a single newly acquired slice with slices previously acquired. The described method thus aims at providing, in the tissue imaging step, spatially compounded images which can be generated at a high frame rate. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0024]    The principle of the invention and preferred embodiments thereof will be further elucidated with reference to the figures, where 
           [0025]      FIGS. 1A and 1B  show flowcharts of embodiments of the method according to the invention; 
           [0026]      FIG. 2  schematically illustrates part of an ultrasound imaging apparatus according to the invention; and 
           [0027]      FIGS. 3A ,  3 B and  3 C illustrate how the various images are obtained according to an embodiment of the invention 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]      FIG. 1A  illustrates an embodiment  10  of the method according to the invention. The embodiment  10  comprises a tissue imaging step  11  of transmitting first sound waves into a volumetric region  22 , of receiving echoes of said first sound waves from a target plane  21  (said target plane including the target anatomy), and of processing said received echoes to produce a tissue image. In this tissue imaging step  11   a  tissue mode specific set of parameters is used, thereby ensuring the optimal imaging of the target anatomy in the tissue. The embodiment  10  further comprises a specular object imaging step  12  of transmitting second sound waves into said volumetric region  22 , of receiving echoes of said second sound waves from a plurality of image planes  24 ,  25  separated in an elevation direction at selected elevations around the target plane  21 , and of processing the echoes received from the target plane and from the plurality of image planes to produce a specular object image. In this specular object imaging step  12  a specular object mode specific set of parameters is used, thereby ensuring the optimal imaging of the specular object, such as for example a needle. In a display step  13  of the embodiment  10  both the tissue image and the specular object image (often referred to as a so-called needle-specific image) are combined into a single image (often referred to as a so-called needle-enhanced image) for display. 
         [0029]    In a further embodiment the specular object imaging step  12  comprises a segmentation sub-step  121 . In this segmentation sub-step the specular object is segmented out of each individual plane of the plurality of image planes. In the specular object imaging step  12  the specular object image is than created out of the segmented individual planes. 
         [0030]      FIG. 1B  illustrates an alternative embodiment  101  of the method according to the invention. This embodiment  101  comprises the tissue imaging step  11 , the specular object imaging step  12  and the display step  13  as described above with reference to  FIG. 1A . However, the tissue imaging step  11  and the specular object imaging step  12  are performed multiple times and in an interleaved manner, as is indicated by loop  111 . First a tissue image of the target plane  21  is imaged. Next a first plurality of image planes is imaged. This first plurality of images planes consist, for example, of the three image planes around the target plane  21 . Next a subsequent tissue image of the target plane  21  is imaged, after which a second plurality of image planes is imaged. This second plurality of images planes consisting, for example, of three image planes adjacent to the first plurality of image planes. This loop  111  continues until all of the planes in the plurality of images planes are imaged. It is noted that the plurality of image planes may be subdivide in alternative ways and that it is even not necessary that the planes in the plurality of images planes are imaged sequentially. In a further embodiment the tissue image of the target plane  21  is combined in display step  13  with the last available specular object image each time such a tissue image is imaged in one of the tissue imaging steps  11 . 
         [0031]      FIG. 2  illustrates part of an ultrasound imaging apparatus according to the invention implementing an embodiment of the above described method according to the invention. An array transducer  31  insonifies a volumetric region  22  including a target anatomy with ultrasound. The volumetric region  22  comprises a two dimensional target plane  21  and a plurality of image planes, here illustrated by two image planes  24 ,  25  located immediately adjacent to and on opposite sides of the target plane  21 . The incident ultrasound from the array transducer  31  is shaped and controlled by a beamformer  26 . The beamformer  26  provides capability for generating a 2-dimensional tissue image of the target plane  21  by using parameters from a tissue mode specific set of parameters and capability for generating a 3-dimensional plurality of image planes  24 ,  25  by using parameters from a specular object mode specific set of parameters. The beamformer comprises a module  27  which provides the capability to image elevation planes, such as planes  24  and  25 , at different angular and/or lateral elevations. Ultrasound returned and received by the ultrasound array transducer  31  is fed to a data processor  28  for producing a tissue image and for producing a specular object image. These two images are fed to an image processor  29  for displaying a combined tissue image and specular object image. 
         [0032]    In an embodiment the data processor  28  is adapted to segment the specular object out of the plurality of image planes to produce a specular object image only showing that object. To this end the data processor  28  may include dedicated hardware tailored to this segmentation process. 
         [0033]      FIG. 3A  shows an array transducer  31  and the target plane  21  and the plurality of image planes  24 ,  25 ,  34  and  35 . The target plane  21  is imaged in the tissue imaging step  11  to produce the tissue image. The plurality of image planes  24 ,  25 ,  34  and  35  are imaged in the specular object imaging step  12 . According to this embodiment the target plane  21  is also imaged in the specular object imaging step, now using the specular object specific set of parameters. This is represented by the further image plane  36 , which coincident with the target plane  21 . 
         [0034]      FIG. 3B  shows the array transducer  31  and the target plane  21  in relation to a needle  37 . It can be seen from the figure that the needle is not aligned with the target plane  21  and does not lie completely along the target plane  21 , but sits at an angle relative to it. The solid line of the needle  37  is indicative of the part of the needle in front of the target plane  21 , while the dashed line is indicative of the part of the needle lying behind the target plane  21 . 
         [0035]      FIG. 3C  illustrates the images obtained in the various steps according to the invention when visualizing such an out-of-plane needle as is depicted in  FIG. 2B . In the tissue imaging step  11   a  tissue image  39  of the target plane  21  is obtained. Next a plurality  40  of image planes is obtained for several elevations in the specular object imaging step  12 . Here it has been chosen to have the target plane itself (represented by the further image plane  36 ) and three elevations in the positive angular direction and three in the negative angular direction with respect to the target plane  21 . The target plane location represented by the further image plane  36  is labeled with a zero in the figure and the elevated image planes with a positive or negative one, two or three. It is noted that this number of elevated image planes is merely an example and should not be construed as limiting. In display step  13  the tissue image  39  and the plurality  40  of image planes are combined into a final image  38  to be displayed. This combination can be done by overlaying the 3-dimensional image information from the plurality  40  of image planes on the 2-dimensional tissue image. Alternatively, this combination can be done by first combining the 3-dimensional image information from the plurality  40  of image planes into a specular object image and subsequently overlaying this specular object image on the tissue image. The information from the individual planes in the plurality  40  of image planes may, for example, be combined by averaging to form the specular object image. It is noted that the combination of the images into a final image  38  to be displayed is not limited to overlay techniques only, but that other techniques, including showing the tissue image and the specular object image alongside each other, may be used as well. 
         [0036]    An embodiment of the invention involves an optional segmentation sub-step  121 . In this sub-step a plurality  41  of segmentation images is made available over the same range as for the plurality  40  of image planes. Known segmentation algorithms can be used to segment the specular object (such as the needle in this example) out of the plurality  40  of image planes to form the plurality  41  of segmentation images. In the display step  13  the tissue image  39  and the plurality  41  of segmentation image are now combined into the final image  38 . Alternatively, the plurality  41  of segmentation images are combined into a specular object image only showing that object (such as the needle in this example). 
         [0037]    As can be seen in  FIG. 3B  the needle does not lie completely along the target plane  21 , but sits at an angle relative to it. Part of the needle is in front of the target plane  21 , while part of the needle is behind the target plane  21 . This results in each individual image plane in the plurality  40  of image planes showing (part of) the needle in a different location in said individual image plane, as is shown in  FIG. 3C . 
         [0038]    In an embodiment of the invention the needle (more in general, the specular object) as shown in the combined final image  38  is coded depending on the position deviation of the individual image planes from the target plane  21 . Each part of the needle originating from a different individual image plane has a different coding in the reconstructed needle shown in the combined final image  38 . A preferred coding technique is color coding the needle, since the use of color is easy to absorb and understand by an operator. A an example, the part of the needle shown in the combined final image  38  originating from the individual image planes in front of the target plane  21  may be colored in red while the part of the needle originating from the individual image planes behind the target plane  21  may be colored in blue.