Patent Publication Number: US-2005124887-A1

Title: Three dimensional scan conversion of data from mechanically scanned array probes

Description:
This invention claims the benefit of Provisional U.S. Patent Application Ser. No. 60/524,404, filed Nov. 21, 2003. 
    
    
      This invention relates to ultrasonic diagnostic imaging and, more particularly, to three dimensional ultrasonic imaging with a mechanically oscillating array.  
      Real time three dimensional ultrasonic diagnostic imaging systems have been constructed with both electronically steered and mechanically steered probes. Electronic beam steering is highly advantageous when scanning rapidly moving objects such as the heart. Real time three dimensional scanning probes with two dimensional arrays for cardiac scanning are described in U.S. Pat. Nos. 5,993,390 (Savord), 6,013,032 (Savord), 6,102,860 (Mooney), 6,126,602 (Savord), and 6,375,617 (Fraser), for example. Mechanical beam steering is advantageous for 3D abdominal scanning when a large aperture is desired. U.S. Pat. No. 5,460,179 (Okunuki et al.) shows a 3D imaging probe which mechanically sweeps a curved one-dimensional array within the probe. As the ID array is swept, it scans image planes in the normal manner and those planes can then be processed to form a three dimensional image over the volume through which the image plane of the probe is swept.  
      However, mechanically sweeping an array probe as it is scanning presents problems due to the mechanical motion. When the probe is scanning as it is moving the scan planes will not be orthogonal to the direction of transducer motion but will be at a slight angle to that direction. This is because the probe is at a slightly different position along its path of travel with each transmitted and received beam. If the probe is scanned in both directions of travel the planes on the return sweep will be canted at a different angle than those of the forward sweep. This difficulty often manifests itself as a scintillating or shimmering effect in the image as the speckle pattern changes from one sweep to another. This problem can be eliminated by stepping the transducer array between discrete scanning positions, but the starting and stopping of the transducer array sweep will result in unacceptable sweep rates and hence less than acceptable real time imaging. Accordingly it is desirable to be able to sweep the transducer at a speed which provides real time 3D frame rates but without the creation of disturbing image artifacts.  
      In order to provide smooth real time three dimensional imaging it is desirable to mechanically scan the array transducer over the image volume at a relatively high scanning rate. However, a high scanning rate will mean that the volumetric region is scanned with fewer beams than would occur during a slower rate of scanning, which results in a decrease in spatial resolution in the 3D image. It would be desirable to be able to scan the transducer array at a high scan rate for smooth real time scanning, particularly in the presence of motion in the body, while still retaining the greater beam density and higher spatial resolution of a slower rate of scanning.  
      In accordance with the principles of the present invention, a three dimensional ultrasonic imaging probe includes an array transducer which is swept over a volumetric region being imaged. As the transducer is swept its beam scanning direction is periodically reversed. In one illustrated embodiment the beam scanning direction is reversed each time that the direction of travel of the array is reversed. In another embodiment the beam scanning direction is reversed with each successive scan plane.  
      In accordance with another aspect of the present invention the volumetric image is produced from the echo data acquired during multiple sweeps of the array transducer as it is scanned. Thus, the 3D image can exhibit greater spatial resolution due to the use of a greater number of received beams to produce the image. In accordance with a further aspect of the invention the scan conversion of echo data from multiple sweeps utilizes the relative temporal and spatial characteristics of the received echoes in the production of 3D image data. 
    
    
      In the drawings:  
       FIG. 1  illustrates in block diagram form an ultrasonic diagnostic imaging system constructed in accordance with the principles of the present invention;  
       FIG. 2  illustrates the mechanical oscillation of an array transducer for three dimensional scanning;  
       FIG. 3  illustrates a mechanism which mechanically oscillates an array transducer for three dimensional scanning;  
       FIGS. 4   a - 4   d  illustrate scan planes resulting from different beam scanning directions by a three dimensional imaging probe of the present invention;  
       FIG. 5  illustrates the scanning of a volumetric region with reversal of the beam scanning direction with successive scan planes;  
       FIG. 6  illustrates the receiver of a three dimensional imaging system with a scan converter for three dimensional imaging;  
       FIG. 7  illustrates the scan conversion of a three dimensional image value from surrounding acquired data values; and  
       FIG. 8  illustrates the scan conversion of three dimensional echo data to form image display values, taking into consideration the spatial and temporal characteristics of the data. 
    
    
      Referring first to  FIG. 1 , an ultrasonic diagnostic imaging system  10  constructed in accordance with the principles of the present invention is shown in block diagram form. The system  10  includes an ultrasound processor  12  that is coupled to a probe or scan head  14  by a connecting cable  16 . The ultrasonic processor  12  includes a transmitter  18  that generates signals at ultrasonic frequencies for emission by the scan head  14 , and a receiver  36  to detect signals received by the scan head  14 . In order to isolate the transmitter  18  from the scan head  14  while the receiver  36  is in operation, a transmitter isolation unit  22  decouples the transmitter  18  from the cable  16 . Correspondingly, when the transmitter  18  is in operation, a receiver protection unit  24  decouples the receiver  36  from the cable  16 . A controller  26  interacts with the transmitter  18 , the receiver  36 , the transmitter isolation unit  22  and the receiver protection unit  24  to coordinate the operation of these components. The controller  26  similarly interacts with a display system  28  to permit signals received by the processor  12  to be visually displayed.  
      The scan head  14  includes a transducer assembly  30  that is comprised of one or more piezoelectric elements that are configured to emit ultrasonic pulses in a desired direction when excited by signals generated by the transmitter  18 , and to convert the reflected portions of the pulses into electrical signals that may be detected by the receiver  36 . The transducer assembly  30  may include a one-dimensional array of transducer elements arranged in a planar, convex or even a concave arrangement of elements. In addition, the transducer assembly  30  may include other higher dimensional arrays of elements, such as a 1.5 or even a two-dimensional array.  
      Still referring to  FIG. 1 , the scan head  14  further includes a positional actuator  32  that is coupled to the transducer assembly  30  to position the transducer assembly  30  in a desired direction, and further to repetitively scan an anatomical region in the desired direction so that a real-time image of the region may be formed. The positional actuator  32  is coupled to the controller  26  through the cable  16  to transmit control inputs from the controller  26  to the actuator  32  so that the movement of the transducer assembly  30  may be controlled. The actuator  32  may be controlled, for example, by controlling a voltage or a current transferred to the actuator  32 . Alternatively, the actuator  32  may be controlled by transferring a control signal from the controller  26  to a separate controller located within the scan head  14  that further controls a current or a voltage transferred to the actuator  32 . The scan head  14  also includes a positional sensor  34  that is coupled to the transducer assembly  30 . The positional sensor  34  determines the directional orientation of the transducer assembly  30  as it is moved by the positional actuator  32 , and is similarly coupled to the controller  26  by the cable  16  to provide positional input signals to the controller  26 .  
      Turning now to  FIG. 2 , a partial side view of a probe with a mechanically oscillating array transducer is shown. In  FIG. 2 , an axis  102  projects upwardly from  FIG. 2 , so that the transducer assembly  30  scans through a scanning angle  106 . The scanning angle  106  may be centered about the axis  102 , so that the transducer assembly  30  sweeps from the axis  102  to sweep angle limits that correspond to a complete rotation of a drive shaft  48  (discussed in conjunction with  FIG. 3 ). Alternatively, the transducer assembly  30  may be swept through a scanning angle that is less than the scanning angle  106  by controlling a positional actuator  42  to rotate in a first direction less that a full revolution of the drive shaft  48 , then rotating the drive shaft  48  in a second direction opposite to the first direction. Accordingly, scanning angles that are less than the scanning angle  106 , which is the maximum obtainable scanning angle, may be conveniently obtained.  
      Still referring to  FIG. 2 , a positional actuator  42  (see  FIG. 3 ) may also be controlled to sweep the transducer assembly  30  about an angle that is centered on another axis that is oriented at an angle with respect to the axis  102  so that the transducer assembly  30  may scan into anatomical regions that cannot be adequately scanned when the transducer assembly  30  is scanned through angles centered about the axis  102 . For example, in performing an ultrasound scan in an upper abdominal or thoracic region, it is often difficult to properly position a scan head so that interfering reflections from ribs or other tissues are avoided. The ability to scan about an axis that is not aligned with a longitudinal axis of the support structure  46  of the scan head is therefore regarded as particularly advantageous.  
       FIG. 3  is a cross sectional isometric view of the 3D mechanical probe of  FIG. 2  which is suitable for use in a constructed embodiment of the present invention. The probe  40  includes a positional actuator  42  that is mechanically coupled to the transducer assembly  30  and a positional sensor  44 . The transducer assembly  30 , the positional actuator  42  and the positional sensor  44  are positioned within a supporting structure  46 . The positional actuator  42  includes a drive shaft  48  that extends upwardly from the positional sensor  44  along a longitudinal axis of the probe  40 . The drive shaft  48  is rotationally supported within the supporting structure  46  of the probe  40  by bearings  50  positioned near respective ends of the drive shaft  48 . The positional actuator  42  also includes an armature structure  52  that is stationary with respect to the supporting structure  46 , and a permanent magnet field structure  54  coupled to the drive shaft  48 . When the armature structure  52  is selectively energized, a torque is developed that rotates the drive shaft  48  in a desired rotational direction so that the drive shaft  48  and the field structure  54  form a driven member. The armature structure  52  may also be selectively energized to rotate the drive shaft  48  in increments of less than a full rotation, and/or at different rotational rates during the rotation of the drive shaft  48 .  
      The positional actuator  42  further includes a crank member  56  that is coupled to the drive shaft  48 , which rotatably couples to a lower, cylindrical-shaped portion of a connecting member  58 . The relative position of the crank member  56  with respect to the supporting structure  46  allows adjustment of the mechanical sweeping range of the transducer array assembly  30 . An upper end of the connecting member  58  is hingeably coupled to a pivot member  60  that is axially supported on the structure  46  by a pair of bearings  62 . The pivot member  60  further supports a cradle  64  that retains the transducer assembly  30 . Although not shown in  FIG. 3 , the cradle  64  may also include electrical contacts so that individual elements in the transducer assembly  30  may transmit and receive ultrasonic signals, as more fully described above. The contacts may further be coupled to a conductive assembly, such as a flex circuit, that is coupled to the processor  12 , as shown in  FIG. 1 . Briefly, and in general terms, rotational motion imparted to the crank member  56  by the drive shaft  48  produces an oscillatory motion in the pivot member  60 , which permits the transducer assembly  30  to be moved through a selected scan angle.  
      The positional sensor  44  includes a counter  66  that is stationary with respect to the supporting structure  46 , and an encoding disk  68  that is fixedly coupled to the drive shaft  48 , so that the encoding disk  68  and the drive shaft  48  rotate in unison. The encoding disk  68  includes a plurality of radially-positioned targets that the counter  66  may detect as the encoding disk  68  rotates through a gap in the counter  66 , thus generating a positional signal for the shaft  48 . Since the angular position of the array  30  may be correlated with the rotational position of the shaft  48 , the encoding disk  68  and the counter  66  therefore cooperatively form a sensor capable of indicating the angular orientation of the array  30 . In one particular embodiment, the encoding disk  68  and the counter  66  are configured to detect the rotational position of the drive shaft  48  by optical means. The disk  68  and the counter  66  may also be configured to detect the rotational position of the drive shaft  48  by magnetic means, although still other means for detecting the rotational position of the drive shaft  48  may also be used. In still another particular embodiment, the encoding disk  68  and the counter  66  are configured to have an angular resolution of at least 1000 counts per revolution.  
      Still referring to  FIG. 3 , the probe  40  further includes a cover  70  that is coupled to the supporting structure  46 . The cover  70  is formed from a material that is acoustically transparent at ultrasonic frequencies. The cover  70  further partially defines an internal volume  72  that sealably retains an acoustic coupling fluid (not shown) that permits ultrasonic signals to be exchanged between the transducer assembly  30  and the cover  70  by providing a suitable acoustic impedance match. In one aspect, a silicone-based fluid may be used that also provides lubrication to the mechanical elements positioned within the volume  72 . A shaft seal  74  is positioned within the supporting structure  46  that surrounds the drive shaft  48  to substantially retain the acoustic coupling fluid within the volume  72 . The internal volume  72  further includes an expandable bladder  76  that is positioned below the crank member  56  to permit the fluid retained within the volume  72  to expand as the fluid is heated or exposed to low pressure, thus preventing leakage of the fluid from the volume  72  that may result from excessive fluid pressures developed within the probe  40 .  
      In use, a mechanical scanning array probe such as that of  FIGS. 2 and 3  will transmit and receive beams as the array is moved back and forth in opposition to the region of the body being scanned. It is well known that the speckle pattern generated by adjacent coherent beams is established by the relationship of the transmit and receive apertures to the scatterer field of the underlying tissue. If the aperture/scatterer relationship is changing, a scintillating or shimmering effect is produced in the image as the speckle pattern continually changes its appearance. One way to stabilize the speckle pattern against such artifacts is to ensure that the transmit/receive aperture is continually in the same spatial location during scanning.  FIG. 4   a  illustrates a scanning pattern which accomplishes this stabilization. In this drawing each horizontal line represents the beams of a scan plane as viewed axially, that is, from the perspective of the array transducer. In this embodiment the array is stepped from one scanning position to another. The illustrated sequence begins by transmitting a first scan plane  86 , followed by a second scan plane  87  and so forth, and ending with scan planes  88  and  89 . The arrow  82  indicates the direction of travel of the array transducer from one scan plane location to another. After scan plane  89  has been transmitted and received the array transducer either returns to its starting position (scan plane  86 ), or reverses its sweep direction and scans scan plane  88  and then back to scan plane  86 . When the array is halted at each new scanning position an image plane is scanned by a series of beams  1 ,  2 ,  3 , . . .  126 , 127 , 128  which are transmitted from left to right as indicated by the arrow  84 . However, the time required to start and stop the array transducer at each scan plane location is considerable. Thus, the time needed to acquire echo signals from the complete volume being scanned is excessive and the volume frame rate will be extremely low.  
      To improve the volume frame rate to at or near real time, it is necessary to transmit and receive beams for the scan planes as the array transducer is continually moving. The array only stops momentarily at the end of a sweep when its scanning direction is changed. This results in a parallelogram-shaped scanning pattern as shown in  FIG. 4   b,  rather than the rectangular pattern of  FIG. 4   a.  This is due to the fact that the array transducer is ever so slightly advanced in its direction of travel  82  as each successive beam  1 , 2 , 3 , . . .  126 , 127 , 128  is transmitted and received. However this scanning sequence gives rise to a problem when the sweep direction  82  of the array transducer is reversed, as shown in  FIG. 4   c.  In this drawing the sequence of grey-shaded scan planes  86 , 87  . . .  88 , 89  are those acquired when the transducer array is moved in the direction  82 . The blackened scan plane sequence  96 , 97 , . . .  98 , 99  are those acquired when the sweep direction of the transducer array is reversed, as indicated by the direction of travel arrow  92 . As this drawing illustrates, the scan planes are tilted at an inverse angle when the scanning direction of the array transducer is reversed. This causes the scan planes of the respective scanning directions to intersect but never to overlap. Hence the apertures on the forward and reverse scanning directions will be different, giving rise to the shimmering artifact.  
      In accordance with a first aspect of the present invention, the order of beam firing is reversed with the array sweep direction is reversed, as shown in  FIG. 4   d.  When the array transducer is moving in the forward direction  82  the transducer beams are fired from left to right as shown by arrow  84 . When the array transducer is moving back in the reverse direction  92 , the beams are fired from right to left as indicated by arrow  94 . As a consequence, the transducer array on its return sweep will overlie the same points and fire any given beam into the same tissue region as it did on the forward sweep. This ensures that the scatterer field seen by the beam pattern in the forward sweep direction is identical to that of the reverse sweep direction. This causes the speckle pattern from sweep to sweep to be stable without the blurring effect of methods that combine data from sequential volumes.  
      When the movement of the aperture is very fast relative to the beams firing time, the beams can be axially “bent” in appearance, a problem which can be corrected by “hose” correction.  
      In accordance with another aspect of the present invention, the beam firing direction is reversed, not with each array sweep direction change, but with each scan plane. The beams of successive scan planes will thereby take on a zigzag appearance as shown in  FIG. 5 . In this example a first scan plane  86  is scanned from left to right as indicated by the small circles  1 , 2 , 3 , 4 , which represent successive beams which are transmitted and received from the left side of the array transducer to the right, as indicated by the arrowheads drawn on the scan plane  86 . As the last beam of scan plane  86  is transmitted and received at the end  86   e  of the scan plane, the direction of beam transmission is reversed for the next scan plane  87 . This scan plane  87  is then scanned from right to left starting with the beams shown as small circles  1 , 2 , 3 , 4 . This scan plane  87  is scanned until the plane has been fully scanned with the last beam transmitted and received at the end  87   e  of the scan plane  87 . The direction of beam scanning by the array transducer is again reversed and the next scan plane  88  is scanned from left to right, and the succeeding scan plane  89  is scanned from right to left, both as indicated by the arrowheads drawn on the scan planes.  
      When the array transducer has reached the end of its sweep in the direction  82 , it reverses its sweep direction as indicated by the dashed arrow  92 . A series of scan planes  96  . . .  99 , shown in dashed lines, is then scanned as the array transducer sweeps back to its initial position. It is seen that the volume scanned by the scan planes is thereby scanned with a series of angled scan planes covering the volume in a zigzag pattern of scan planes. For some applications this scanning pattern may provide more complete spatial scanning and hence better images than the series of parallel, fully overlapping scan planes of  FIG. 4   d.    
      In accordance with another aspect of the present invention, more detailed 3D images are produced by using the data acquired during two successive sweeps of the array transducer to form the images. In the example of  FIG. 5 , this would mean that the echo data of planes  86  . . .  89  of the first (direction  82 ) sweep of the array and the echo data of planes  96  . . .  99  of the second (direction  92 ) sweep of the array are used to form one image. When the array transducer completes a third sweep in the direction  82 , the data from this third sweep and the data from the second sweep of the array are used to form the next 3D image in the sequence. The older data of the first sweep in direction  82  is replaced by the new data of the subsequent sweep in direction  82  to form the new 3D image. The third 3D image in the sequence would be formed by the data of the third sweep in direction  82  and the data of a fourth sweep in direction  92 . In this way, detailed 3D images are formed with a relatively high frame rate of display.  
      The details of a receiver  36  ( FIG. 1 ) for receiving and processing this scan data is shown in  FIG. 6 . A beamformer  120  receives echo signals from the elements of the transducer assembly  30  and forms coherent receive beams. The coherent echo data is coupled to a signal processor  122  which processes the echo data as by filtering, harmonic separation, B mode detection or Doppler detection in accordance with the imaging mode being used. The received beams are then stored in a FIFO frame buffer  124 .  
      When all of the scan planes needed to form a 3D image have been stored in the FIFO frame buffer  124  the echo data is coupled to a 3D scan converter  130 , the operation of which will be discussed more fully below. The scan converted data is stored in a display image memory  126 , which may typically store the data in an x,y,z three dimensional format. The data needed to produce a display frame is coupled to a volume renderer  128  which renders a three dimensional image by any of a variety of known rendering techniques. The volume rendered image is then coupled to the display  28  for display of the three dimensional image.  
      Returning to  FIG. 5 , it can be seen that the data acquired by scan planes  86 - 99  exhibits a number of characteristics. For instance, at the ¼ and ¾ positions in the lateral (left-right) dimension, the scan planes are relatively uniformly separated (below the ¼ and ¾ arrows), providing relatively uniform spatial sampling of the volume being imaged. However, at the lateral edges and in the center, the spatial sampling is less uniform in the elevation (sweep) direction. In addition, the data at these lateral-most and central locations exhibit different temporal characteristics, examples of which are circled by ovals  102  and  104 . All of the echo data from beams within oval  102  are acquired at the end of scan plane  96  and at the beginning of scan plane  97  of the reverse sweep when the array transducer is moving in direction  92 . Thus, motion artifacts would not be a serious problem in this region of the volumetric display.  
      However in the center of the image there are different temporal characteristics. In oval  108  the scan plane data is in close spatial identity as the image planes intersect in this region. But the image plane data is from image planes which are relatively greatly separated in time, as the data of scan plane  88  was acquired during the first (direction  82 ) sweep, whereas the data of scan plane  97  was acquired during the second (direction  92 ) sweep. The scan plane data of oval  106  is even more temporally disparate, as the data of scan plane  86  was acquired at the beginning of the first sweep in direction  82 , whereas the data of scan plane  99  was acquired at the end of the second sweep in direction  92 . The possibilities of motion artifact are therefore the greatest in this region. To guard against these motion artifacts, more temporal interpolation will be used when combining data in this region. However, when the third sweep begins and the data within the oval  106  consists of the data from scan plane  99  at the end of the second sweep and the data from the first scan plane ( 86 ′) of the third sweep, the great temporal disparity is no longer present. When this scan plane data is combined, little temporal interpolation is needed as motion artifacts will be relatively low.  
      To take these disparities into consideration, in accordance with a further aspect of the present invention, 3D scan conversion is performed by spatial and temporal weighting of data values being combined which varies with the different spatial and temporal characteristics of the data which is being combined.  
      This may be appreciated by considering the type of signal combination which is performed in scan conversion. One common type of scan conversion is four-point interpolation as described in U.S. Pat. No. 4,468,747 (Leavitt)(see  FIG. 7A ) and U.S. Pat. No. 4,581,636 (Blaker et al.)(see  FIG. 2 ), which show this technique as applied to scan conversion for two dimensional images. In general, four-point interpolation locates the four acquired data values at corners of a quadrilateral area in which the image point to be determined is located. The image point is produced by combining the four data values with weights which are a function of their spatial distance from the image point being determined. This technique may be applied to three dimensional scan conversion as shown by  FIG. 7 . In this example a center image point S c  is to be determined. The value of image value S c  is found by considering the eight acquired data values S 1 -S 8  at the corners of a volume enclosing the image point S c . The value of S c  would be determined by combining the values of data points S 1 -S 8  as a function of the distance from image point S c . In practice the number of data points being combined can vary. It can be as large as a cluster of data values in the vicinity of the image point being calculated. Data point clusters of sixteen, thirty-two and sixty-four values have been used in constructed embodiments of the present invention, and greater or lesser numbers of values may also be used.  
      A simple example of spatial and temporal weighting which may be used in an embodiment of the present invention is illustrated in  FIG. 8 , in which four acquired data values are combined to form an image value. In this example data values T 1  and T 2  are virtually identical spatially, as are data values T 1 ′ and T 2 ′. This is indicated in the drawing by the almost complete overlap of the circles of T 1  and T 2  on one hand, and the circles of T 1 ′ and T 2 ′, on the other. The data values T 1  and T 1 ′ are nearly identical temporally, with both being acquired during the same sweep of the array transducer. Likewise, in this example the data values T 2  and T 2 ′ are nearly identical temporally. The data values T 1  and T 2 , which are substantially identical spatially, are spatially offset from the similarly spatially identical data values T 1 ′ and T 2 ′. These conditions could arise, for instance, when data values are acquired at the intersection of scan planes  86  and  99 , where the acquired data values of the two scan planes could be spatially identical yet temporally separate, as each was acquired during a different sweep of the array transducer.  
      If these four data values were being combined to determine a scan converted image value in the region of oval  106 , temporal interpolation may be emphasized to reduce potential motion artifacts, due to the large temporal difference between scan planes  86  and  99 . For example, the temporal weighting may be emphasized by a temporal weight of 60% as compared with a 40% spatial weighting. A scan converted value between these data points would thus be of the form:  
         T   1     =       0.6   ⁢           ⁢     (         T   2     +     T   2   ′       2     )       +     0.4   ⁢           ⁢     (         T   1     +     T   1   ′       2     )             
 
 If these data values were from the region of oval  108  where less temporal interpolation would be needed, as scan planes  88  and  97  are more closely acquired in time and present a lesser possibility of spatial artifacts, then spatial weighting could be emphasized more greatly than temporal weighting. Again exemplary weights of 60% and 40% are used, and the scan conversion formula would be of the form:  
         T   1     =       0.6   ⁢           ⁢     (         T   1     +     T   2       2     )       +     0.4   ⁢           ⁢     (         T   1   ′     +     T   2   ′       2     )             
 
 By varying the weighting of scan conversion to take into consideration the spatial and temporal aspects of the data being combined, the high spatial line density resulting from combining frames of two or more sweeps can be provided with temporal resolution which is relatively low in artifacts. 
 
      It will be appreciated that the same zigzag coverage of the volume being scanned can be accomplished by reversing the beam scanning direction from frame to frame, as shown in  FIG. 5 , or by maintaining the same beam scanning direction through both sweeps as shown in  FIG. 4   c.  In either case, the selective spatial and temporal weighting described above can be employed to produce high quality images that are low in temporal artifacts.  
      It will also be appreciated that the scan conversion techniques of the present invention may also be used to scan convert volumetric echo data acquired by electronically steered (2D matrix array) probes in addition to mechanically scanned array probes.