Abstract:
Methods are described for parallel beamforming with beam combination of formed beams from different transmit beams in an ultrasound imaging system. The methods provide advantages because the combination of receive beams that are produced from different transmit beams enables the production of an ultrasonic image at faster frame rates that does not suffer from the image quality degradation normally associated with parallel beamforming, and does not require process intensive interpolation.

Description:
RELATED APPLICATION  
       [0001]    This application claims the benefit of the filing date pursuant to 35 U.S.C. §119(e) of Provisional Application Serial No. 60/281,365 (Parallel Beamforming With Beam Combination), filed Apr. 4, 2001, the disclosure of which is incorporated herein by reference. 
     
    
     
       BACKGROUND  
         [0002]    The invention relates generally to the field of ultrasonic imaging. More particularly, the invention relates to multiple beam beamforming for ultrasonic imaging.  
           [0003]    Multiple beam beamforming techniques, sometimes called parallel beamforming, form two or more receive beams in response to a single transmit beam. For example, one broad beam is transmitted and two or more receive beams are simultaneously formed from responsive echo signals.  
           [0004]    Due to the parallel processing of data, image artifacts may be generated. The image artifacts are manifested in geometrical distortion, brightness variation, blockiness and/or degraded lateral resolution. One proposed solution to reduce the presence of these artifacts is provided in U.S. Pat. No. 5,667,373 to Wright et al. Coherent data along a same line is averaged, and data from two different lines are phase adjusted and interpolated coherently to form a synthized line of data. One problem with the Wright solution is the performance of process intensive interpolation to create the synthesized beam.  
         BRIEF SUMMARY  
         [0005]    The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiments described below include methods and systems for forming received ultrasound data. Multiple receive beams are formed in response to each transmit beam. Two or more receive beams responsive to different transmit beams are formed along a same line. The co-linear receive lines are combined to remove artifacts. The combination is responsive to the geometric distortion. For example, the receive lines are weighted as a function of the distance from any receive line to the associated transmit line. In one embodiment, four receive lines are obtained for each transmit line. Two of the four receive lines for one transmission are co-linear with a respective two of the four receive lines of another transmission. Data for each of the co-linear lines is combined. Unequal weights are used for combining each set of co-linear data due to the different distances between each receive line and the associated transmit lines.  
           [0006]    A first aspect is a method for forming received ultrasound beams for an ultrasound image. The method comprises transmitting at least two adjacent transmit beams; responsive to the transmitting, forming at least two adjacent receive beams for each of the transmit beams; and combining at least two of the received beams. The combined received beams are disposed at approximately the same location, and at least two of the combined received beams are formed from different transmit beams.  
           [0007]    A second aspect is a system for producing output frames in an ultrasound imaging system. The system comprises transducers, a beamformer, digital echo processors, and a scan converter. The transducers are adapted to transmit and receive ultrasound pulses. The beamformer is adapted to simultaneously generate at least four receive beams in response to a transmit beam. Each digital echo processor is adapted to combine data corresponding to at least two receive beams emanating from the same target location. The scan converter includes processing resources adapted to process the combined receive beam data to form an output frame.  
           [0008]    In a third aspect, a method for forming received ultrasound data is provided. At least first and second beams are transmitted along first and second different scan lines. At least first and second receive beams are formed in response to the first transmitted beam. At least third and fourth receive beams are formed in response to the second transmit beam. The third receive beam is co-linear with the second receive beam. The second receive beam is combined with the third receive beam in response to unequal weights. A system including a transducer, beamformer and processor for implementing the above method may also be provided.  
           [0009]    In a fourth aspect, another method for forming received ultrasound data is provided. Acoustic energy is transmitted along first and second lines. The first line is spatially different than the second line. A first set of data representing at least four receive lines is formed in response to the transmission along the first line, and a second set of data representing at least four receive lines is formed in response to the transmission along the second line. At least first and second receive lines of the first set are co-linear with third and fourth receive lines, respectively, of the second set. Data of the first receive line is combined with data of the third receive line, and data of the second receive line is combined with data of the fourth receive line. A system including a transducer, beamformer and processor for implementing the above method may also be provided.  
           [0010]    In a fifth aspect, yet another method for forming received ultrasound data is provided. Acoustic energy is transmitted along first and second transmit lines. The first transmit line is spatially different than the second transmit line. A first set of data representing at least two receive lines is formed in response to the transmission along the first transmit line. A second set of data representing at least two receive lines is formed in response to the transmission along the second transmit line. At least a first receive line of the first set is co-linear with a second receive line of the second set. Data of the first receive line is combined with data of the second receive line in response to first and second weights. The first weight is a function of a distance of the first receive line from the first transmit line, and the second weight is a function of a distance of the second receive line from the second transmit line. A system including a transducer, beamformer and processor for implementing the above method may also be provided.  
           [0011]    Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.  
       
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS  
       [0012]    The components and the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views.  
         [0013]    [0013]FIG. 1 is a block diagram of an ultrasound system of one embodiment for forming received ultrasound data.  
         [0014]    [0014]FIG. 2 is a flow chart diagram of one embodiment for forming received ultrasound data.  
         [0015]    [0015]FIG. 3 is a graphical representation of one embodiment for forming received ultrasound data.  
         [0016]    [0016]FIG. 4 is a flow chart diagram of another embodiment for forming received ultrasound data. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    At least two receive beams are formed from each of at least two separate transmit beams. In one embodiment, two co-linear or overlapping receive beams responsive to different transmit beams are combined. In another embodiment, two sets of two or more co-linear receive beams responsive to different transmit beams are combined. The combination accounts for the center shift associated with the distance between a receive beam and the associated transmit beam. Geometrical distortion occurs, in part, because the limited width of the transmit beam profile causes the receive beam centers to shift towards the respective transmit beam, geometrically distorting the formed beam.  
         [0018]    By combining or blending data for co-linear receive beams, beamforming is accomplished with no interpolation between formed beams to form synthetic beams. Some interpolation of synthetic lines may be provided. The received beams can be formed in a consistent symmetrical manner in the lateral direction, reducing geometrical distortion and other image artifacts. The blending of overlapping receive beams smoothes or filters the transition from one receive line to another in a resulting image.  
         [0019]    [0019]FIG. 1 shows a block diagram of an ultrasound system  10  for forming received ultrasound data. The ultrasound system  10  includes a transmit beamformer  12 , a transducer  14 , a receive beamformer  16 , a processor  18 , a scan converter  20  and a display  22 . Additional, different or fewer components may be provided. In various embodiments, the ultrasound system  10  is one of an Antares or other system manufactured by Siemens Medical Systems, a Sequoia® or other system manufactured by Acuson, a Siemens Company, or an other ultrasound system manufactured by these or other companies.  
         [0020]    The transmit beamformer  12 , in part, operates as a conventional transmit beamformer to generate a set of transmit signals for the individual transducer elements included in the transducer  14 . For example, the transmit beamformer  12  forms one or more a suitably shaped ultrasonic pulses for the various elements of the transducer  14 . The pulses are delayed relative to each other for steering by any suitable combination of delays, phase shifts and phase rotations. The focus delays are selected to cause ultrasonic signals from the transducer  14  to constructively interfere at a selected transmit focus along a selected transmit line  24 .  
         [0021]    In one embodiment, the transmit beamformer  12  uses delays and apodization to form transmit beams with a wide beamwidth. For example, a beamwidth with a −6 dB roll-off at spatial locations beyond a greatest distance from a center of the transmit beam to any receive beams formed in response to the transmit beam is used. Other beamwidths may be used. For example, the transmit beamwidth may be more narrow where the combination of receive beams is associated with beams different distances from the center of the transmit beam. The combination reduces or removes the amplitude difference associated with each receive beam due to the transmit beamwidth.  
         [0022]    The transducer  14  comprises a linear transducer array, a curved array, a phased array, a single element, a one-dimensional array, a 1.5 dimensional array or a two-dimensional array of elements. In response to the transmit pulses from the transmit beamformer  12 , the transducer  14  transmits a beam of ultrasound energy along a scan line  24 . These ultrasound transmit beams are sequentially scanned across a field of view, such as scanning in a linear, Vector®, sector or other format. The transmit beams are sequentially transmitted along spatially different scan lines  24 , such as parallel or adjacent scan lines  24  within the field of view of the transducer  14 .  
         [0023]    The receive beamformer  16  comprises a digital or analog beamformer for applying delays, apodization amplification and summing for forming receive beams responsive to each of the transmit beams. In one embodiment, the receive beamformer  16  is one of the receive beamformers disclosed in U.S. Pat. Nos. 5,544,128; 5,667,373; 5,718,230; 5,793,701 and 6,016,285, the disclosures of which are incorporated herein by reference. For example, the receive beamformer  16  includes separate channels for forming two or more receive beams from echo signals in response to one transmit beam. Separate delays and/or apodization are applied in each channel. As another example, echo signals from the transducer  14  are applied to both a single channel for common processing and then to separate channels for forming the two or more different receive beams. As yet another example, the echo signals responsive to one transmit beam are stored, and the two or more receive beams are formed sequentially. In another example, separate receive beamformers are provided for each receive beam or sub-sets of all of the receive beams formed in response to one transmit beam.  
         [0024]    The receive beamformer  16  forms the receive beams along two or more receive lines  26 . The receive lines  26  are spatially distinct from each other. The receive lines  26  are also spatially distinct from the transmit line  24 , but one of the receive lines  26  may be co-linear or along the same line as the transmit line  24 . In one embodiment, receive beams are formed along one, two or more receive lines  26  on each side of the transmit line  24  for the corresponding transmit beam. The receive lines  26  are symmetrical or asymmetrical in position relative to the transmit line  24 .  
         [0025]    The processor  18  comprises one or more of an application specific integrated circuit, a general processor, a digital signal processor, other processors now known or later developed, analog circuits, digital circuits or combinations thereof. In one embodiment, the processor  18  is part of a detector, such as a B-mode, Doppler or flow detector. In other embodiments, part or all of the processor  18  comprises one or more processors dedicated to combining data representing receive beams, processors used for scan conversion, processors in the receive beamformer  16 , control processors or other processors. The processor  18  combines data representing co-linear receive beams prior to detection (coherent data) or after detection. For coherent data (e.g. in-phase and quadrature data or radio frequency data), the combination is of magnitude information with or without phase alignment. In alternative embodiments, the processor  18  is positioned to combine scan converted data or data elsewhere between the receive beamformer  16  and the display  22 .  
         [0026]    The scan converter  20  comprises one or more analog or digital devices for formatting ultrasound data representing the field of view into a format for the display  22 . The display  22  generates an image based on the combined, scan converted data. A B-mode, Doppler or flow mode, combination B-mode and Doppler or flow mode or other image is displayed.  
         [0027]    [0027]FIG. 2 shows a method of one embodiment of the operation of the system of FIG. 1. Two different transmit beams are transmitted in acts  30  and  34 . In acts  32  and  36 , multiple receive beams are formed in response to each of the acts  30  and  34 , respectively. In act  38 , the data for co-linear receive beams responsive to the different transmit beams is combined.  
         [0028]    The transmit acts  30  and  34  are performed sequentially, but may be temporally separated by other transmissions of ultrasound energy. The transmit beams are transmitted along spatially different lines  24  (FIG. 1). The different lines  24  are adjacent or substantially parallel lines within the scan pattern, but may be spatially separated by transmission along one or more intervening lines  24 . In alternative embodiments, the spatially different lines  24  overlap or intersect within the field of view.  
         [0029]    In response to the transmit act  30 , two or more receive beams are formed in act  32 . For example, two, three, four or more spatially different adjacent receive beams are formed representing lines  26  near the transmit line  24 . The receive beams are each spatially different. One or none of the receive beams may be co-linear with the transmit beam (i.e. one receive line  26  is along the transmit line  24 ). Any of various spatial distributions of the receive beams may be used, such as equally spaced, substantially parallel receive beams positioned symmetrically about the transmit line  24  or center of the transmit beam. In other embodiments, unequal spacing of the receive beams, non-parallel receive beams, and/or unsymmetrical placement about the transmit line  24  is provided.  
         [0030]    In response to the subsequent transmit act  34 , two or more receive beams are formed in act  36 . These subsequent receive beams of act  36  are formed in a similar manner as discussed above for act  32 . As a result, two sets of data each representing a plurality of receive beams responsive to a single transmit beam are provided. A receive beam responsive to one transmitted beam may be co-linear with a different transmitted beam. In one embodiment, the transmit lines  24  are spaced such that the receive lines  26  for any given transmit line  24  are between transmit lines  24  adjacent to that given transmit line  24 . Other spacing with wider or narrower distribution of receive beams relative to transmit beams may be used.  
         [0031]    One or more receive beams responsive to different transmit beams are co-linear. Receive beams are considered co-linear if both receive beams represent substantially the same receive line  26 . Receive beams associated with different beam widths, different beam patterns due to different foci or focal patterns, different aberrations, or slightly divergent are also co-linear. A portion of an arcing or other non-linear receive beam may be co-linear with another different receive beam.  
         [0032]    In one embodiment with two receive beams for each transmit beam, one receive beam of one set is co-linear with one receive beam of another set. In an embodiment with three receive beams for each transmit beam, one or two receive beams of one set are co-linear with one or two receive beams of another set. In an embodiment with four receive beams for each transmit beam, one, two or three receive beams are co-linear with one, two or three receive beams of another set.  
         [0033]    For scanning a field of view, additional sets of data representing multiple receive beams are formed in response to additional sequential transmissions. The additional sequential transmissions are along spatially different transmit lines  24 , such as a transmission along a transmit line  24  adjacent to a most recently transmitted beam. One or more of the receive beams are co-linear with one or more receive beams responsive to a different transmit beam. In one embodiment, two, three or more receive beams responsive to two, three or more transmit beams are co-linear. All of the receive beams are co-linear with another receive beam in one embodiment, but one or more of the receive beams may not be co-linear with another receive beam.  
         [0034]    In act  38 , the co-linear receive beams are combined. In one embodiment, a weighted combination is used. The receive beams along a receive line are weighted relative to other receive beams along the same receive line. The weights are equal (i.e. an average) or unequal, such as applying one weight to one receive beam and applying a different weight to a different, co-linear receive beam. In one embodiment, the weights provide a unity gain, such as the weights for the receive beams adding to one. Non-unity gain may be provided. The weight is a same for an entire depth of the receive beam or varies as a function of depth. The data representing the receive beam is multiplied or divided by the weight. The weighted data for the co-linear receive beams is summed or otherwise combined. Unit gain may be provided by normalizing the combined data.  
         [0035]    In one embodiment, the weights are selected as a function of the distance from the receive beam or line  26  to the associated transmit beam or line  24 . Where the distance is the same for each co-linear receive beam, the weights are equal. Where the distance is different, unequal weights are provided. For example, data for a receive beam spaced closer to the receive beam&#39;s corresponding transmit line (i.e. a shorter distance) is weighted more heavily (i.e. a greater weight is applied) than data for another co-linear receive beam spaced further from this other receive beam&#39;s corresponding transmit line. In one embodiment using symmetrically spaced receive beams, the weights are linear blending coefficients. The weights are 1/(N+1), 2/(N+1), . . . N/(N+1) where N is the number of receive beams in the receive beam set responsive to one transmit beam that are co-linear with receive beams of the receive beam set responsive to an adjacent transmit beam. For the embodiment shown in FIG. 3, N is 2 and the weights are ⅓ and ⅔. The distance indicates an amount of center shift associated with a receive beam. Other indications of the amount of center shift may be used for adaptively selecting the weights. The weighting acts to move the receive beam position, reducing artifacts.  
         [0036]    The weighting and combination discussed above is repeated for multiple co-linear receive beams throughout the field of view. The combined data is used for calculations or to form an image for display. The combined data includes reduced artifacts from parallel receive beamforming. By using overlapping or co-linear receive beams, the frame rate is reduced from a frame rate provided by no overlapping receive beams. However, by provided a sufficient number of receive beams in response to each transmit beam, a high frame rate with reduced artifacts is maintained.  
         [0037]    [0037]FIG. 3 shows a graphical representation of one embodiment for providing a high frame rate with reduced parallel beamforming artifacts. Four receive beams  60 A-D,  62 A-D,  64 A-D are formed for each transmit beam  50 ,  52 ,  54 . Each set of four receive beams  60 A-D,  62 A-D,  64 A-D are spaced evenly and symmetrically around the corresponding transmit beam  50 ,  52 ,  54 , respectively. The receive beams  60 C-D,  62 A-D,  64 A-D are co-linear with receive beams of adjacent transmit beams  50 ,  52 ,  54 ,  56 , such as two receive beams  60 C and  60 D on one side of a corresponding transmit beam  50  being co-linear with a respective two receive beams  62 A and  62 B on one side of a different corresponding transmit beam  52 . All of the receive beams  60 C-D,  62 A-D,  64 A-D are co-linear with another receive beam  60 C-D,  62 A-D,  64 A-D except at the edges of a scan pattern (i.e.  60 A and  60 B). Other combinations of co-linear scanning with all or only a sub-set of all of the receive beams having a co-linear receive beam may be provided.  
         [0038]    Receive beam  60 C formed in response to transmit beam  60  tends to shift to the left towards the transmit beam  60 , while receive beam  62 A formed in response to the transmit beam  62  tends to shift to the right towards the transmit beam  62 . The amount of shift depends on the distance between the receive beam and the corresponding transmit beam. For example, a greater amount of shift is associated with the receive beams  62 A or  60 D than the shift associated with receive beams  60 C or  62 B. Receive beams  62 A and  60 D are more than twice are far from the transmit beams  52  and  50 , respectively, than the receive beams  62 B and  60 C, also respectively.  
         [0039]    Geometric distortion reduction is accomplished by combining receive beam  60 C and  62 A because the left shift of receive beam  60 C is canceled or reduced by the right shift of the receive beam  62 A. Because the shifts can differ in magnitude, a simple average may reduce, but not eliminate, the artifact. Reduction may be sufficient.  
         [0040]    Similarly, brightness variations can be ameliorated by combination of the received beams  60 C-D,  62 A-D,  64 A-D. For example, receive beams  60 B and  60 C closer to the corresponding transmit beam  50  may be brighter than receive beams  60 A and  60 D further from the corresponding transmit beam  50 . This brightness variation may be reduced or eliminated by combining brighter received beams (e.g. receive beam  60 C responsive to the corresponding transmit beam  50 ) with dimmer receive beams (e.g.  62 A responsive to the corresponding transmit beam Artifacts caused by beam-to-beam similarity for receive beams formed in response to the same transmit beam are reduced by combining receive beams responsive to different transmit beams. The beam-to-beam similarity is manifested in both thermal noise components and acoustic speckle patterns. In an image in which alternating pairs of receive beams are provided due to use of synthetic lines with actual lines (see FIG. 1A and col. 11, lines 10-45 of U.S. Pat. No. 5,667,373), a pattern of more similar-less similar-more similar-less similar, etc. is provided. The image may appear blocky and have degraded lateral resolution. The difference between receive beams or combined received beams is reduced by combining the receive beams as shown in FIG. 3.  
         [0041]    One or more of the artifacts discussed above may be further reduced by selection of weights applied to data representing co-linear receive beams. For example, the co-linear receive beams  60 C and  62 A are combined in a weighted sum to reduce the center shift artifact described above. Because the center shift artifact becomes more pronounced for receive beams disposed farther from their corresponding transmit beam (e.g. receive beam  62 A from transmit beam  52 ), the receive beams disposed closer to the corresponding transmit beam (e.g. receive beam  60 C from transmit beam  50 ) is altered by a higher or greater weight. In the example of FIG. 3, the receive beam  60 C is weighed at K%, where K is higher than 50% (e.g., approximately 70% or a weight of 0.70 or 0.667) and the receive beam  62 A is weighed at the lower (1−K)% (e.g., approximately 30% or a weight of 0.30 or 0.333). Similarly, receive beam  60 D has a lower weight than receive beam  62 B.  
         [0042]    This weighting process continues for other co-linear combinations of receive beams. Receive beam  62 C, weighed at K%, is then combined with receive beam  64 A, weighed at (1−K)%, while receive beam  64 B, weighed at K%, is combined with receive beam  62 D, weighed at (1−K)%. After the co-linear receive beams are combined, a resulting output frame of data represents the field of view with reduced or eliminate parallel beamforming artifacts.  
         [0043]    The receive beams  60 A-D,  62 A-D and  64 A-D are distributed along receive lines  26  across the field of view as represented by the beam index numbers 0-7. According to one embodiment, a transmit beam (e.g., first transmit beam  50 ) is centered between two receive beams (e.g.,  60 B and  60 C). Four receive beams (e.g.,  60 A-D) are formed simultaneously in response to the transmit beam  50 . Receive beams  60 A and  60 B may be output to the scan converter  20  or other processing device, while the receive beams  60 C and  60 D are stored in a memory for further processing or combining with subsequent receive beams (e.g.  62 A and  62 B). The second transmit beam  52  is disposed adjacent to the first transmit beam  50 , and is centered between the receive beams  52 B and  52 C. Four receive beams (e.g.,  62 A-D) are formed in response to the second transmit beam  50 . The received beams  62 A-D or a sub-set of the receive beams (e.g.  62 C and  62 C) are stored in memory for subsequent combination.  
         [0044]    In one embodiment, four digital echo processors (DEP) or combination channels combine data representing co-linear receive beams. Each digital echo processor combines the data for every fourth receive line  26  or beam index number. As the sequentially acquired data representing co-linear receive beams is available, the processors combine the data. The data pipeline disposed after each digital echo processor outputs combined data representing a receive line  26 . In alternative embodiments, one or other number of digital echo processors or other processors combine the data. The resulting output data represents beams corresponding to the beam index or receive lines shifted as a function of the weighted combination.  
         [0045]    To achieve a two-to-one frame rate improvement as compared to conventional reception of one receive beam for each transmit beam, four receive beams are formed simultaneously by the receive beamformer  16  (FIG. 1). The frame rate can be improved even further. For example, a four-to-one frame rate improvement is obtained for using a receive beamformer  16  having bandwidth sufficient to form eight receive beams in response to one transmit beam. As another example, only the two receive beams spaced furthest from the center of the transmit beam are co-linear with the outermost receive beams responsive to an adjacent transmit beam. The outermost receive beams are co-linear and combined, but the inner most of the four receive beams are used without cc-linear combination.  
         [0046]    [0046]FIG. 4 shows one method for forming the receive ultrasound data as graphically represented by FIG. 3. A first transmit beam is transmitted from a transducer and is centered between two or more receive beams in act  70 . In response to the first transmit beam, a beamformer forms four receives beams  10  in act  72 . In act  74 , the beamformer provides data corresponding to two received beams at an edge of the field of view without combination to corresponding digital echo processors. The digital echo processors output the data for these two receive beams to the scan converter. Alternatively, these two receive beams are discarded since parallel beamforming artifacts may not be reduced or eliminated in the first and last two receive beams of the scan format (i.e. receive beams at the edge of the field of view). The digital echo processor also stores data of the other receive beams for subsequent combination in a local memory in act  76 .  
         [0047]    A second beam is transmitted in act  78 , where the second transmit beam is centered between two receive beams. In responsive to the second transmit beam, a beamformer forms four receive beams in act  80  and stores the receive beams for subsequent and/or passes the receive beams for current combination in act  82 . The digital echo processors then generate data representing two adjacent receive lines as two weighted sums of co-linear receive beams responsive to the first and second transmit beams in act  84 . The combined data is output to the scan converter.  
         [0048]    The method then continues by transmitting a third beam centered between two receive beams in act  86 . In response to the third transmit beam, the beamformer forms four receive beams in act  88  and stores the receive beams for subsequent and/or passes the receive beams for current combination in act  90 . The digital echo processors then generate data representing two adjacent receive lines as two weighted sums of co-linear receive beams responsive to the first and second transmit beams in act  92 . The combined data is output to the scan converter. The process continues by transmitting additional beams centered between receive beams and processing the receive beams in a similar fashion, i.e., outputting two receive beams to the scan converter after each transmit, until the frame of data representing the field of view for an ultrasound image is complete in act  94 .  
         [0049]    While the invention has been described above by reference to various embodiments, it should be understood that many changes and modifications can be made without departing from the scope of the invention. For example, five or more receive beams are generated for each transmit beam. Different numbers of receive beams may be formed as a function of the transmit beam, such as forming three receive beams for a first transmit beam and four receive beams in response to a second transmit beam. A variety of received beam and transmitted beam sequences or combinations may be used. Different scan formats may also be used.  
         [0050]    It is therefore intended that the foregoing detailed description be understood as an illustration of the presently preferred embodiment of the invention, and not as a definition of the invention. It is only the following claims, including all equivalents, that are intended to define the scope of this invention.