Patent Publication Number: US-2005131295-A1

Title: Volumetric ultrasound imaging system using two-dimensional array transducer

Description:
This invention claims the benefit of Provisional U.S. patent Application Ser. No. 60/528,797, filed Dec. 11, 2003. 
    
    
     TECHNICAL FIELD  
      This invention relates to ultrasound imaging systems, and, more particularly, to a system and method for performing volumetric imaging using a two-dimensional transducer that scans using multiple fan-shaped beams.  
     BACKGROUND OF THE INVENTION  
      Various noninvasive diagnostic imaging modalities are capable of producing cross-sectional images of organs or vessels inside the body. An imaging modality that is well suited for such real-time noninvasive imaging is ultrasound. Ultrasound diagnostic imaging systems are in widespread use by cardiologists, obstetricians, radiologists and others for examinations of the heart, a developing fetus, internal abdominal organs and other anatomical structures. These systems operate by transmitting waves of ultrasound energy into the body, receiving ultrasound echoes reflected from tissue interfaces upon which the waves impinge, and translating the received echoes into structural representations of portions of the body through which the ultrasound waves are directed.  
      In conventional ultrasound imaging, objects of interest, such as internal tissues and blood, are scanned using planar ultrasound beams or slices, which are preferably as thin as possible to provide good resolution of such objects accompanied by minimal clutter. A linear array transducer is conventionally used to scan a thin slice by narrowly focusing the transmitted and received ultrasound in an elevational direction and steering the transmitted and received ultrasound throughout a range of angles in an azimuthal direction. A linear array transducer operating in this manner can provide a two-dimensional image representing a cross-section through either a plane that is perpendicular to a face of the transducer for B-mode imaging or parallel to the face of the transducer for C-mode imaging.  
      Although B-mode and C-mode images are two-dimensional images, it is also possible to generate three-dimensional ultrasound images by either physically moving a linear array or by using a two-dimensional array transducer to steer the transmitted and received ultrasound about two orthogonal axes. Although two-dimensional B-mode or C-mode images can conventionally be generated at a sufficient rate to allow essentially real-time imaging (i.e., at least about 30 frames per second), it is generally not possible at the present time to generate three-dimensional ultrasound images at a rate that is sufficient to permit real-time imaging. Three-dimensional real-time imaging poses two major challenges: first, acquiring echoes from a volume in a sufficiently short time to maintain a real-time image frame rate, and, second, reducing volumetric data obtained from these echoes to a suitable two-dimensional image format with sufficient speed to provide real-time display.  
      One technique that has been developed to create ultrasound images providing information about anatomical structures in a three-dimensional volume is volumetric imaging, as disclosed in U.S. Pat. No. 5,305,756, which is incorporated herein by reference. Volumetric imaging can generally be accomplished at a sufficient speed to permit real time imaging. With reference to  FIG. 1 , volumetric imaging is accomplished using a transducer  10  having linear array elements  12 . The transmitted and received ultrasound is focused in the azimuthal direction AZ. However, lenses placed on the surface of the elements  12  or the surface geometry of the element  12  themselves cause the ultrasound to diverge in the elevation direction EL to generate a series of fan-shaped beams, collectively shown as  14 . The transducer  10  is scanned in a linear array format whereby the ultrasound is sequentially transmitted and received from each array element  12  to form the sequence of fan-shaped beams  14 . The beams  14  are orthogonal to the longitudinal surface of the transducer  10  to insonify a volumetric region. In the center of the insonified volumetric region is a plane of projection  18  that bisects each of the fan-shaped beams  14 . The plane of projection  18  is spatially represented by the ultrasound image produced by the transducer  10  and is a plane that typically is normal to the surface of the transducer  10  in the azimuthal direction. The resulting ultrasound image provides information about the entire three-dimensional volumetric region because the transducer  10  acoustically integrates all echoes at each range across the entire volumetric region. These echoes are then projected or collapsed onto the plane of projection  18 . Since the fan-shaped beams  14  diverge radially in the elevation direction, each constant range locus is a radial line as indicated by a constant range locus  20 . Each echo along the constant range locus  20  is projected to a point  22  of intersection of the locus  20  and the plane of projection  18 . Since this projection occurs at every range and azimuthal location throughout the volumetric region  16 , the image of the plane of projection  18  presents a two-dimensional projection of the entire volume. The resulting image is similar to the two-dimensional projection of a volume obtained using conventional x-ray imaging.  
      The volumetric image can be obtained as shown in  FIG. 1  in essentially real time because all of the echoes at each range across the entire volumetric region isonified by each beam  14  are processed as a single point on the plane of projection  18 . As a result, relatively little processing power is required, particularly compared to true three-dimensional ultrasound imaging.  
      While the transducer  10  may be scanned in a linear array format as shown in  FIG. 1  to form a sequence of fan-shaped beams, the transducer  10  may alternatively used by transmitting and receiving properly phased ultrasound signals to and from the array elements  12 . By operating the array elements as a phased array, the transducer  10  can electronically steer and focus the ultrasound as shown in  FIG. 2 . The ultrasound is therefore transmitted and received in a fan-shaped beam  30  that diverges in both the elevational and azimuthal directions. The electronic steering of the beam  30  enable the isonification of a pyramidal shaped volumetric region adjacent the transducer  10 . Ultrasound echoes from within this volumetric region are projected onto a triangular shaped plane of projection  36  and used to display a volumetric image.  
       FIG. 3  illustrates another technique that is described in U.S. Pat. No. 5,305,756 to produce of a fan-shaped beam in the elevational direction. As shown in  FIG. 3 , a transducer  40  has array elements  42  arranged in two dimensions. As in the transducer  10  of  FIGS. 1 and 2 , the array elements  42  are aligned in the azimuthal direction. However, each array element  42  is sub-diced in the elevational direction to form sub-elements  46   a,b,c . The sub-elements  46   a,b,c  aligned in the elevational direction allows a series of fan-shaped beams  48  that diverge in the elevational direction to be electronically generated rather than relying upon lenses or the geometry of the element surface to generate a fan-shaped beam. The sub-elements  46   a,b,c  generate the fan-shaped beams  48  by controlling the time that signals are sent to or received from the sub-elements  46   a,b,c . For example, the sub-element  46   b  could be actuated first, followed in rapid succession by the simultaneous actuation of the sub-elements  46   a  and  46   c . However, it is important to note that the sub-elements  46   a,b,c  are not used as a phased array in which properly phased ultrasound signals are transmitted from and received by the sub-elements  46   a,b,c . Thus, the beams  48  are not steered in the elevational direction. As with the previously described embodiments, the ultrasound echoes in the volumetric region isonified by the beams  48  are projected onto a plane  49  from which the volumetric image is created.  
      Although the conventional volumetric imaging technique described above represents a significant advance because it allows real time imaging of a three-dimensional volumetric space, it is not without its limitations. For example, as illustrated in  FIG. 4A , a transducer  50  shown when viewed in the azimuthal direction scans using a diverging beam  52  as illustrated in  FIGS. 1-3 . When the transducer  50  is scanning to a range of distances  56  from the transducer  50 , all of the points at that range  56  from the transducer  50  will be projected onto a plane of projection  60  as a set of points within a range of depths  62 . Therefore, all of the points in that range of distances  56  from the transducer  50  will appear to be in the range of depths  62  on the projection  60  even though the actual depths of the points vary throughout a substantially larger range  66 . As a result, viewed in the elevational direction as shown in  FIG. 4B , a set of points in the range of depths  62  will be erroneously projected to be within the range of depths  66 . Conversely, an anatomical structure that spans a range of depths can appear to be at a single depth because it is a constant distance from the transducer  50 .  
      The problem exemplified by  FIGS. 4A, 4B  is exacerbated when the elevational divergence angle of the beam  52  is large. Under such circumstances, the volumetric image can fail to clearly show the true configuration of anatomical structures.  
      Another problem with the conventional three-dimensional volumetric imaging technique shown in  FIGS. 1-3  can be explained with reference to  FIG. 5 .  FIG. 5  shows a transducer  80  viewed in the azimuthal direction that is transmitting a beam  82  that diverges in the elevational direction, in the same manner as shown in  FIGS. 1-3 . The diverging nature of the beam  82  inherently means that the beam  82  will isonify an area of interest beneath the transducer  80  that varies from a relatively small width near the transducer  80  to a relatively large width away from the transducer  80 . For example, the beam  82  will isonify a width W 1  at a distance D 1  from the transducer  80 , and will isonify a width W 2  at a distance D 2  from the transducer  80 . Therefore, the resulting volumetric image will be relatively narrow and show relatively little at the top of the image and will be relatively wide and show substantially more at the bottom of the image. The width of the image can be made equal by cropping the image, such as along lines  86 ,  88 , but doing so wastes image information that would otherwise be viewable.  
      Still another potential problem that may be encountered in using the three-dimensional volumetric imaging technique shown in  FIGS. 1-3  is that certain regions of the image may not be shown in the image with sufficient clarity. For example, since the image does not resolve anatomical structures that lie along the same constant range locus from the transducer, a structure that occupies only a small portion of the constant range locus may be obscured by other anatomical structures that also lie on the constant range locus.  
      There is therefore a need for a volumetric imaging system and method that clearly shows anatomical structures being imaged without geometric distortion, and does so in a manner that can generate an image having a substantially constant width throughout a range of depths.  
     SUMMARY OF THE INVENTION  
      A system and method of producing volumetric ultrasound images uses a two-dimensional array transducer to scan a region of interest. According to one aspect of the invention, the two-dimensional array transducer scans the region of interest in an azimuthal direction using a plurality of beams that diverge in an elevational direction and are positioned adjacent each other in the elevational direction. Ultrasound reflections in each beam are projection onto a respective plane of projection, and a volumetric ultrasound image is then created by combining the projections on the planes of projection for all of the beams into a common plane of projection.  
      According to another aspect of the invention, the two-dimensional array transducer scans the region of interest in an azimuthal direction using a plurality of beams that have a common center axis. The beams diverge in an elevational direction in respective divergence angles that are different for each beam. The beams scan respective ranges of scanning depths that are ordered inversely to an order of divergence angles of the beams. As a result, a beam scanning the shallowest range of scanning depths has the largest divergence angle and a beam scanning the deepest range of scanning depths has the smallest divergence angle. The ultrasound reflections in each beam are projected onto a common plane of projection, and the volumetric ultrasound image is created from the ultrasound reflections projected onto the common plane of projection for all of the beams.  
      In still another aspect of the invention, the two-dimensional array transducer scans the region of interest in an azimuthal direction using a pair of beams. A first beam diverges in a first direction and is used to scan the region of interest in a second direction that is perpendicular to the first direction. Similarly, a second beam diverges in a third direction and is used to scan the region of interest in a fourth direction that is perpendicular to the third direction. Ultrasound reflections in the first beam are projected onto a plane of projection that is perpendicular to the first direction, and ultrasound reflections in the second beam are projected onto a plane of projection that is perpendicular to the third direction. A volumetric ultrasound image is then created from the first and second planes of projection. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic isometric view illustrating one conventional technique for generating volumetric images.  
       FIG. 2  is a schematic isometric view illustrating another conventional technique for generating volumetric images.  
       FIG. 3  is a schematic isometric view illustrating still another conventional technique for generating volumetric images.  
       FIGS. 4A and 4B  are schematic elevational and azimuthal cross-section views, respectively, illustrating a limitation of the conventional volumetric imaging techniques shown in  FIGS. 1-3 .  
       FIG. 5  is a schematic elevational cross-section view illustrating another limitation of the conventional volumetric imaging techniques shown in  FIGS. 1-3 .  
       FIGS. 6A and 6B  are schematic elevational and azimuthal cross-section views, respectively, illustrating a technique for generating volumetric images according to one embodiment of the invention.  
       FIG. 7  is a schematic elevational cross-section view illustrating a technique for generating volumetric images according to another embodiment of the invention.  
       FIGS. 8A, 8B ,  8 C and  8 D are schematic views illustrating techniques for generating volumetric images according to still another embodiment of the invention.  
       FIG. 9  is a block diagram of an ultrasound imaging system that can be used to perform volumetric imaging according to the embodiments shown in  FIGS. 6-8 . 
    
    
     DETAILED DESCRIPTION  
      One aspect of the present invention and will now be explained with reference to  FIGS. 6A and 6B , which shows views of a two-dimensional array transducer  100  viewed in the azimuthal and elevational directions, respectively. As shown in  FIG. 6A , the transducer  100  scans using a diverging center beam  102  and a separate pair of diverging side beams  104 ,  106 . Ultrasound echoes scanned by each of these beams  102 ,  104 ,  106  are projected onto respective planes of projection  112 ,  114 ,  116 . Points at corresponding depths in the planes of projection are then combined to create a single plane of projection that is used to create the volumetric image. The plane of projection  112  can be used as the single plane of projection by transferring points on the planes of projection  114 ,  116  to the plane of projection  112  at the corresponding depth.  
      Significantly, the side beams  104 ,  106  scan to a ranges of distances  120  from the transducer  100  that is greater than a ranges of distances  122  that is scanned using the center beam  102 . The difference between the scan distance of the center beam  102  and the scan distance of the side beams  104 ,  106  is selected so that both scan distances are at substantially the same depth beneath the transducer  100 . As a result, the side beams  104 ,  106  and the center beam  102  scan to substantially the same depth. More specifically, as shown in  FIG. 6A , when the transducer  100  causes the center beam  102  to scan in the range of distances  122  from the transducer  100 , all of the points in that range of distances  122  will be projected onto the plane of projection  112  within a range of depths  126  that is only slightly smaller than the actual range of depths  128 . At the same time, when the transducer  100  causes the side beams  104 ,  106  to scan at the range of distances  120  from the transducer  100 , all of the points in that range  120  will be projected onto the planes of projection  114 ,  116  as points falling within the range although the actual locations of the points are in a range of depths  124 . However, this range of depths  124  differs from the range of distances at which points are projected onto the planes  114 ,  116  substantially less than in the conventional technique shown in  FIGS. 4A and 4B . As a result, when viewed in the elevational direction as shown in  FIG. 6B , the depth of anatomical structures will be correctly viewed with substantially less geometric distortion present using the conventional technique shown in  FIGS. 4A and 4B . The advantage of using side beams  104 ,  106  focused to a greater depth than the center beam  102  will be apparent by comparing  FIG. 6B  with  FIG. 4B .  
      Although the embodiment shown in  FIGS. 6A and 6B  uses only two side beams  104 ,  106 , it will be understood that a larger number of side beams could be used. Using a larger number of side beams further reduces the geometric distortion that would otherwise be present, but it increases the processing that is required to display an image and may therefore preclude real-time volumetric imaging. Alternatively, volumetric imaging could be accomplished using two side-by-side diverging beams (not shown), but doing so would result in greater geometric distortion but less processing compared to the technique shown in  FIGS. 6A and 6B . In general, scanning over a wider area or obtaining an image of greater clarity makes it desirable to use a larger number of beams, particularly if the processing power is available. Regardless of the number of beams that are used, the points on each plane of projection  112 ,  114 ,  116  are preferably projected onto a single plane of projection with a weight corresponding to the width of the respective beam. As a result, each ultrasound echo will be projected onto the plane of projection with the same weight regardless of the beam  102 - 106  that obtained the echo.  
      The diverging beams  102 ,  104 ,  106  can be generated by the two-dimensional transducer  100  using a variety of techniques. The beams  102 - 106  can be generated by operating array elements of the transducer  100  in a phase-arrayed manner either in respective sub-arrays to form the beams  102 - 106  at the same time or using all of the array elements of the transducer  100  to sequentially form each individual beam  102 - 106  at different times. Also, the array elements can be arranged in sub-arrays, each of which is provided with a lens or other mechanical structure to cause a respective beam  102 - 106  to be generated from the sub-arrays.  
      One embodiment of another aspect of the present invention is illustrated in  FIG. 7 , which shows a two-dimensional array transducer  140  that transmits and receives ultrasound and a plurality of sequentially generated beams  142 ,  144 ,  146  for scanning within a respective range of depths. The angle of divergence of each beam  142 - 146  is inversely related to the depth of its scanning range. Thus, the angle of divergence of the beam  142 , which scans to a relatively shallow depth, is relatively wide, and the angle of divergence of the beam  146 , which scans to a relatively large depth, is relatively narrow. As a result, the width of each beam  142 - 146  at the furthest extent of its scan depth is substantially the same for all beams  142 - 146 .  
      After ultrasound echoes have been obtained using the beams  142 - 146 , a volumetric image is generated by using the echoes within the scan range of each beam  142 - 146 . Thus, the image is generated from relatively shallow echoes using the beam  142 , moderately deep echoes using the beam  144 , and relatively deep echoes using the beam  146 . The resulting image can encompass a width shown by the dotted lines  150 ,  152 , which has a substantially larger width than the image area encompassed by the cropping lines  86 ,  88  shown in  FIG. 5 .  
      A variety of techniques can be used to generate the beams  142 - 146  with differing divergence angles. However, the beams  142 - 146  are preferably generated by controlling the array elements of the transducer  140  using phased-array techniques.  
      The technique shown in  FIG. 7  can, of course, be used with a single beam scanning within the each range, or multiple beams can be used to scan within each range using the technique shown in  FIGS. 6A and 6B .  
      One embodiment of still another aspect of the invention is shown in  FIGS. 8A-8D . In this embodiment, the two-dimensional array elements of a transducer (not shown) are used to scan in relatively narrow beams in which all of the points at each range are projected onto a central plane of projection. For example, as shown in  FIG. 8A , one volumetric scanning beam  150  is used that is perpendicular to a second volumetric scanning beam  152 . The resulting projections  154 ,  156 , respectively, show a vessel in transverse cross-section  160  and longitudinal cross-section  162 , respectively.  
      As shown in  FIG. 8B , two parallel scanning beams  170 ,  172  may be used to generate respective transverse cross sectional projections  174 ,  176  of a volumetric region of a vessel  178  that are parallel to each other and spaced apart a predetermined distance.  
      Although the scaling of the projections  154 ,  156  and  174 ,  176  is uniform in the embodiments of  FIGS. 8A and 8B , volumetric projections of an anatomical structure obtained using the same volumetric scanning beam may be shown with two different degrees of scaling, as shown in  FIG. 8C  more specifically, a single volumetric scanning beam  180  is used to generate a first projection  182  showing a vessel  184  to actual scale and a second projection  186  showing the vessel  184  in expanded form. This embodiment can allow anatomical structures to be shown with greater clarity.  
      Finally,  FIG. 8D  shows two volumetric scanning beams  190 ,  192  intersecting each other at substantially the same angle that an anatomical structure  194  would be viewed by respective eyes. The beams  190 ,  192  are used to generate a pair of image projections  196 ,  198  of the anatomical structure  194 , which are viewed by respective eyes so that the depth features of the anatomical structure can be visualized.  
      Although volumetric scanning beams having a variety of specific geometric relationships have been illustrated in  FIGS. 8A-8D , it will be understood that the use of a two-dimensional array transducer allows a great deal of flexibility in the geometric relationships of scanning beams that can be formed. Further, although  FIGS. 8A-8D  show only one or two volumetric scanning beams being used, it will be understood that a greater number of volumetric scanning beams can be used to create a correspondingly greater number of projected images.  
      One potential limitation of the various embodiments of the inventive volumetric scanning techniques may be the lack of resolution achievable at a specific depth. As mentioned above, all of the anatomical structures at the same depth are projected onto the same area of a plane of projection. Therefore, an anatomical structure occupying a relatively small width of the scanning beam may be masked or otherwise obscured by other anatomical structures at that same depth. To alleviate this potential problem, three-dimensional scanning can be used to resolve specific anatomical structures. The resulting image of such structures can be overlaid onto the volumetric image. Significantly, the relatively little amount of processing power required to perform volumetric scanning in accordance with the various embodiments of the invention may leave processing power available to perform three-dimensional scanning of limited areas without reducing the acquisition frame rate significantly. As a result, real-time imaging can still be achieved with this limited amount of three-dimensional scanning to overlay volumetric scanning of a larger area.  
      One embodiment of an ultrasound imaging system  200  that can be used to perform volumetric imaging in accordance with the present invention is shown  FIG. 9 . The imaging system includes a probe  210  having a two-dimensional array of transducer elements  212 . The probe  210  is coupled to through a cable  218  to a scanner  230 .  
      The scanner  230  includes a transmitter  232 , which generates high frequency signals that are applied to the transducer elements  212  to cause the transducer elements  212  to transmit ultrasound into tissues or blood. Ultrasound echoes of the transmitted ultrasound are received by the transducer elements  212 , which generate corresponding analog signals. These analog signals are applied to a preamplifier  234 , which amplifies the analog signals. The preamplifier  234  also includes internal TGC (time gain control) circuitry to compensate for attenuation of the transmitted and received ultrasound at greater depths. The amplified and depth compensated signals from the preamplifier  234  are applied to an analog-to-digital (A/D) converter  238  where they are digitized. The digitized echo signals are then formed into beams by a beamformer  244 . The beamformer  244  is controlled by a controller  246 , which is responsive to a user control. The controller  246  provides control signals to the transmitter  232  instructing the probe  210  as to the timing, frequency, direction and focusing of transmit beams. The controller  246  also controls the beamforming of the digitized echo signals received by the beamformer  244 . The output of the beamformer  244  is applied to an image processor  248 , which performs digital filtering, B mode detection, and Doppler processing on the beamformed digital signals. The image processor  248  can also perform other signal processing such as harmonic separation, speckle reduction through frequency compounding, and other desired image processing.  
      Scanning to produce the volumetric images as explained with reference to  FIGS. 6-8  is accomplished by the controller  246  controlling the beamformer  244  so that it scans ultrasound echoes having the configurations of the beams shown in  FIGS. 6-8 . The controller  246  may also control the transmitter  232  so that it transmits ultrasound in beams having the configuration shown in  FIGS. 6-8 . Since the two-dimensional array of transducer elements  214  has the ability to steer transmitted and received beams in any direction and at any inclination in front of the transducer  212 , the beams can have any orientation with respect to the transducer  212  and to each other.  
      The echo signals produced by the scanner  230  are coupled to the digital display subsystem  250 , which processes the echo signals for display in the desired image format. The digital display system  250  includes an image line processor  252 , which is samples the echo signals and splices segments of beams into complete line signals. The image line processor also averages line signals for signal-to-noise improvement or flow persistence. The image line signals from the image line processor  252  are applied to a scan converter  254 , where they are converted into the desired image format. For example, the scan converter  254  may perform Rho-theta conversion as is known in the art. The image is then stored in an image memory  258  from which it can be displayed on a display  260 . The image in the image memory  258  may also be overlaid with graphics to be displayed with the image. The graphics are generated by a graphics generator  264 , which is responsive to a user control. Individual images or image sequences can be stored in a cine memory  268  during capture of image loops.  
      For real-time volumetric imaging, the display subsystem  250  also includes a three-dimensional image rendering processor  270 , which receives image lines from the image line processor  252 . The three-dimensional image rendering processor  270  renders of a real-time three dimensional image, which is displayed on the display  260 .  
      Although the present invention has been described with reference to preferred embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.