Abstract:
A method and apparatus for acquiring multi-line acquisition ultrasound data. Ultrasound signals are transmitted into an area of interest. Echo signals are acquired and analyzed to produce a first data stream associated with a first receive beam. The first data stream is decimated by removing at least two consecutive data samples therefrom, while passing at least two consecutive data samples to form a first decimated data stream. Using this decimation pattern, a bandwidth of one-half to three-quarters of the data sampling rate after decimation may be achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   The application relates to and claims priority from provisional patent application Ser. No. 60/477,826, titled “Ultrasound Method and Apparatus for Multi-Line Acquisition”, filed Jun. 12, 2003, the complete subject matter of which is expressly hereby incorporated herein in its entirety. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to diagnostic ultrasound systems. In particular, the present invention relates to method and apparatus for acquiring and processing ultrasound data streams to reduce the bandwidth of the data without deteriorating the performance of the beamformer, and to share the bandwidth between two or more data streams belonging to different beams in multi-line acquisition. 
     FIG. 3  illustrates a typical configuration of a receive beamformer  200 . Digitized channel signals are input to a number of identical signal processing devices (beamformer ASICs)  202 - 206 , that are interconnected in a beamsum pipeline. The ASIC  202  performs a partial beam formation of channels  1 - 8 . The partial beamsum  208  is then input to ASIC  204  and added to the partial beamsum of channels  9 - 18  within ASIC  204 . This process is continued down the beamsum pipeline until the final beamsum  210  is coming out of the last ASIC  206  in the chain. 
   For a time-delay beamformer, such as receive beamformer  200 , the data may typically be 20 bit wide and have a data rate of 40 MHz per beam, i.e. 800 Mbit/s data bandwidth. With simultaneous reception of parallel beams, or multi-line acquisition (MLA), this data rate is multiplied by the number of parallel beams. The cost of the interconnect infrastructure within the ultrasound system increases with data bandwidth, as more pins are required on integrated circuits for signal processing, more pins on circuit board connectors, or a faster clock rate of the data paths. 
     FIG. 4  illustrates the contents of the partial beamformer ASICs  202 - 206  of  FIG. 3 . An input data rate (sampling rate) of 40 MHz is assumed, although other data rates may be used. Each channel  1 - 8  is processed through a per-channel beamformer  212 - 216 . The per-channel beamformers  212 - 216  perform per-channel beamforming (time delay and optionally per-channel amplitude weighting). 
   The output from the beamsummer  218 , the partial beamsum  208 , is then passed through a low-pass anti-aliasing filter  220  that cuts off frequencies above 10 MHz, reducing the bandwidth of the signal. Data output from the low-pass anti-aliasing filter  220  is represented as data stream A  224 , following a sequence of samples A1 A2 A3 A4 A5 . . . and so on. A decimator  222  then reduces the data rate by throwing away every other sample of the data stream A  224  to produce data stream B  226 , giving the sample sequence of A1 X A3 X A5 X . . . and so on. The X&#39;s in data stream B  266 , and all subsequent data streams discussed herein, represent data samples which have been thrown away. Data stream B  226  is then summed by summer  228  with a cascading input  230 . The cascading input  230  may be supplied through an optional delay line  232  to allow for summing with a subsequent device. The delay line  232  may not be required for certain beamformer architectures. 
   Alternatively, the cutoff-rate of the anti-aliasing filter  220  may be 20 MHz/n, wherein n=1,2,3,4 . . . , allowing for a data output rate of 40 MHz/n by throwing away (n-1) samples for every sample that is retained. A larger value of n results in a greater reduction of the data rate. Unfortunately, the maximum usable frequency of the receive beamformer  200  becomes reduced by a factor n, for example, from 20 MHz to 20 MHz/n. 
   Thus, a system and method are desired to acquire data with a maximum frequency which is not limited to one-half the data sampling rate of the output stream that addresses the problems noted above and others previously experienced. 
   BRIEF DESCRIPTION OF THE INVENTION 
   A method for acquiring ultrasound data comprising acquiring echo signals from an area of interest, analyzing the echo signals to produce a first data stream associated with a first receive beam, and decimating the first data stream by removing at least two consecutive data samples therefrom to form a first decimated data stream. 
   An ultrasound system comprising a transmitter transmitting ultrasound signals into an area of interest, a receiver receiving echo signals from transmitted ultrasound signals, and a beamformer processing the echo signals to simultaneously form first and second data streams associated with different first and second receive beams. The beamformer includes a decimator removing from at least one of the first and second data streams at least two consecutive data samples, and an output outputting information based on an output of the decimator. 
   A decimation subsystem comprising an input receiving a first data stream comprising data samples. The decimation subsystem further comprises a first decimator receiving the first data stream and removing at least two consecutive data samples therefrom, while passing at least two consecutive data samples to output a decimated subset of the first data stream. 
   A method for acquiring ultrasound data comprising acquiring echo signals from an area of interest, producing first and second data streams associated with first and second receive beams based on the echo signals, and filtering the first and second data streams to form first and second filtered data sets having partially overlapping frequency bands. The method also comprises decimating the first and second filtered data sets to form first and second decimated data sets. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a block diagram of an ultrasound system formed in accordance with an embodiment of the present invention. 
       FIG. 2  illustrates an ultrasound system formed in accordance with one embodiment of the present invention. 
       FIG. 3  illustrates a typical configuration of a receive beamformer. 
       FIG. 4  illustrates the contents of the partial beamformer ASIC of  FIG. 3 . 
       FIG. 5  illustrates the contents of a partial beamformer ASIC comprising a programmable anti-aliasing (a-a) filter in accordance with one embodiment. 
       FIG. 6  illustrates multiplexing two data streams into a single data stream in accordance with one embodiment 
       FIG. 7  illustrates a complex demodulator which may be used in connection with the systems of  FIGS. 4 and 5  in accordance with one embodiment. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  illustrates a block diagram of an ultrasound system  100  formed in accordance with an embodiment of the present invention. The ultrasound system  100  includes a transmitter  102  which drives an array of elements  104  within a transducer  106  to emit pulsed ultrasonic signals into a body. A variety of geometries may be used. The ultrasonic signals are back-scattered from structures in the body, like blood cells or muscular tissue, to produce echoes which return to the elements  104 . The echoes are received by a receiver  108 . The received echoes are passed through a beamformer  110 , which performs beamforning and outputs an RF signal. The RF signal then passes through an RF processor  112 . Alternatively, the RF processor  112  may include a complex demodulator (not shown) that demodulates the RF signal to form IQ data pairs representative of the echo signals. The RF or IQ signal data may then be routed directly to RF/IQ buffer  114  for temporary storage. A system controller  120  controls the operation of the components of the ultrasound system  100 . 
   The ultrasound system  100  also includes a signal processor  116  to process the acquired ultrasound information (i.e., RF signal data or IQ data pairs) and prepare frames of ultrasound information for display on display system  118 . The signal processor  116  is adapted to perform one or more processing operations according to a plurality of selectable ultrasound modalities on the acquired ultrasound information. Acquired ultrasound information may be processed in real-time during a scanning session as the echo signals are received. Additionally or alternatively, the ultrasound information may be stored temporarily in RF/IQ buffer  114  during a scanning session and processed in less than real-time in a live or off-line operation. 
   The ultrasound system  100  may continuously acquire ultrasound information at a frame rate that exceeds  50  frames per second—the approximate perception rate of the human eye. The acquired ultrasound information is displayed on the display system  118  at a frame-rate that may be different than that of the acquired data. An image buffer  122  is included for storing processed frames of acquired ultrasound information that are not scheduled to be displayed immediately. Preferably, the image buffer  122  is of sufficient capacity to store at least several seconds worth of frames of ultrasound information. The frames of ultrasound information are stored in a manner to facilitate retrieval thereof according to its order or time of acquisition. The image buffer  122  may comprise any known data storage medium. 
     FIG. 2  illustrates an ultrasound system formed in accordance with one embodiment of the present invention. The system includes a transducer  10  connected to a transmitter  12  and a receiver  14 . The transducer  10  transmits ultrasonic pulses and receives echoes from structures inside of a scanned ultrasound volume  16 . Memory  20  stores ultrasound data from the receiver  14  derived from the scanned ultrasound volume  16 . The volume  16  may be obtained by various techniques (e.g., 3D scanning, real-time 3D imaging, volume scanning, 2D scanning with an array of elements having positioning sensors, freehand scanning using a Voxel correlation technique, 2D or matrix array transducers and the like). 
   The transducer  10  is moved, such as along a linear or arcuate path, while scanning a region of interest (ROI). At each linear or arcuate position, the transducer  10  obtains scan planes  18 . The scan planes  18  are collected for a thickness, such as from a group or set of adjacent scan planes  18 . The scan planes  18  are stored in the memory  20 , and then passed to a volume scan converter  42 . In some embodiments, the transducer  10  may obtain lines instead of the scan planes  18 , and the memory  20  may store lines obtained by the transducer  10  rather than the scan planes  18 . The volume scan converter  20  may store lines obtained by the transducer  10  rather than the scan planes  18 . The volume scan converter  42  receives a slice thickness setting from a control input  40 , which identifies the thickness of a slice to be created from the scan planes  18 . The volume scan converter  42  creates a data slice from multiple adjacent scan planes  18 . The number of adjacent scan planes  18  that are obtained to form each data slice is dependent upon the thickness selected by slice thickness control input  40 . The data slice is stored in slice memory  44  and is accessed by a volume rendering processor  46 . The volume rendering processor  46  performs volume rendering upon the data slice. The output of the volume rendering processor  46  is passed to the video processor  50  and display  67 . 
   The position of each echo signal sample (Voxel) is defined in terms of geometrical accuracy (i.e., the distance from one Voxel to the next) and ultrasonic response (and derived values from the ultrasonic response). Suitable ultrasonic responses include gray scale values, color Doppler values, and angio or power Doppler information. 
     FIG. 5  illustrates the contents of a partial beamformer for a single beam in accordance with the present invention. ASIC  250  comprises a programmable anti-aliasing (a-a) filter  252 . Elements corresponding to the elements in  FIG. 4  have the same item numbers. Their function is similar to the previous discussion regarding  FIG. 4 , and thus will not be further discussed. The a-a filter  252  has a pass-band that is either “low-pass” or low-band  260 , i.e. from 0-10 MHz, “band-pass” or mid-band  262  from 5-15 MHz, or “high-pass” or high-band  264  from 10-20 MHz. The a-a filter  252  provides three discrete modes of operation for the beamformer  110 , with a useable frequency range corresponding to the selected pass-band of the associated a-a filter  252 . The data stream A  256  output by the a-a filter  252  follows a sequence of A1 A2 A3 A4 A5 . . . and so on. 
   A decimator  254  functions differently for the different modes of the a-a filter  252 . For the low-band  260  and high-band  264  modes, the decimator  254  decimates every other data sample in the data stream A  256 , and outputs data stream C  258  following a sequence of A1 X A3 X A5 X . . . and so on. Mathematically, the data stream C  258  corresponds to multiplication of the sample stream [A1 A2 A3 A4 A5 A6 . . . ] with the sequence [101010101010. . . ]. The spectrum of the resultant waveform has frequency components centered around 0 Hz and 20 MHz, so the spectra of the a-a filtered signals in the low-band  260  and high-band  264  modes will not overlap after decimation. Those skilled in the art will recognize that all of the information in the a-a filtered signal (data stream A  256 ) will be fully preserved through the decimation process even for the high-band  264  filtered signal.  FIG. 5  illustrates the applicable data stream C  258  for the associated a-a filter  252  mode. Data stream C  266  is associated with the low-band  260  mode, and data stream C  270  is associated with the high-band  264  mode. 
   When the a-a filter  252  is in the mid-band  262  mode, the a-a filter  252  outputs the data stream A  256  following the pattern of A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 . . . and so on. The decimator  254  passes two consecutive data samples of the data stream A  256 , and decimates two consecutive data samples of the data stream A  256 . The decimator  254  therefore outputs the data stream C  258  following a sequence of A1 A2 X X A5 A6 X X A9 A10 . . . and so on. Mathematically, the data stream C  258  corresponds to multiplication of the sample stream [A1 A2 A3 A4 A5 A6 . . . ] with the sequence [110011001100110 0 . . . ]. The spectrum of the resultant waveform has frequency components centered around 0 Hz and 10 MHz only, so even in this case the spectrum of the a-a filtered signals will not overlap after decimation. Those skilled in the art will recognize that all of the information in the a-a filtered signal (data stream A  256 ) will be fully preserved through the decimation process.  FIG. 5  illustrates the data stream C  268  for the associated mid-band  262  mode. 
   From the previous description, it is clear that the entire frequency range from 0 to 20 MHz is now covered with three overlapping frequency bands with a (real) data rate of only 20 MHz. Although  FIG. 5  illustrates a 40 MHz sampling frequency, it should be understood that sampling frequencies other than 40 MHz may be used. 
   Reducing the data rate by decimation does not affect the delay or phase of different data streams. Therefore, the decimation process may be performed at any stage of the beamforming process. Alternatively, the a-a filter  252  and decimator  254  may be moved up within the process and replicated as a part of the per-channel beamformer  212 . 
     FIG. 6  illustrates how the data streams from two partial beamformers can be multiplexed into a single 40 MHz data stream. Each single beam partial beamformer  270  and  272  comprises a set of per-channel beamformers  212 - 216 , such as for 8 or 16 channels. The number of per-channel beamformers  212 - 216  in each single beam partial beamformer  270  and  272  is determined by the hardware implementation. Therefore, single beam partial beamformers  270  and  272  receive signals through channels as discussed previously in relation to  FIG. 4 . The single beam partial beamformers  270  and  272 , however, receive signals from the same set of channels, and are typically receiving beams that are spatially different. The single-beam partial beamformers  270  and  272  output data stream A  276  and data stream B  278 , respectively. Data stream A  276  follows a sequence of A1 A2 A3 A4 A5 A6 A7 A8 A9 A10 . . . and so on. Data stream B  278  follows a sequence of B1 B2 B3 B4 B5 A6 B7 B8 B9 B10 . . . and soon. 
   The function of the a-a filter  252  and decimator  254  remain the same as previously discussed for  FIG. 5 , and have been illustrated in  FIG. 6  as a-a filter/decimators  282  and  284 . A mode select line  280  provides an input to the a-a filter/decimators  282  and  284  from the system controller  120  that selects the band-pass mode, either low-band  260 , mid-band  262 , or high-band  264 . The input indicates to the ultrasound system  100  which band to operate for a given beam. 
   When either low-band  260  or high-band  264  are selected, the data streams C  286  and D  288  follow the patterns: C=A1 X A3 X A5 X A7 X A9 X . . . and so on, and D=B1 X B3 X B5 X B7 X B9 X . . . and so on. The data streams C  286  and D  288  are input to a MUX/FIFO  290 . The MUX/FIFO  290  multiplexes the two inputs and outputs data stream E  292 , which follows the pattern of A1 B1 A3 B3 A5 B5 A7 B7 A9 B9 . . . and so on. 
   If the mid-band  262  mode is selected, the data streams C  286  and D  288  follow the sequences: C=A1 A2 X X A5 A6 X X A9 A10 . . . and so on, and D=B1 B2 X X A5 A6 X X A9 A10 . . . and so on. The data streams C  286  and D  288  are input to the MUX/FIFO  290 , which multiplexes the two inputs and outputs data stream E  292 , which follows the sequence of E=A1 A2 B1 B2 A5 A6 B5 B6 A9 A10. and so on. The data streams A  276 , B  278 , C  286 , D  288 , and E  292  for the mid-band  262  mode are illustrated on  FIG. 6  for clarity. It should be understood that other forms of multiplexing that provide equivalent function also exist, for example data stream E  292  may be E=A1 B1 A2 B2 A5 B5 A6 B6 A9 B9 A10 B10 . . . and so on. 
   The data stream E  292  is sent to the summer  228 , which sums the data stream E  292  with the cascading input  230 . As previously discussed, the delay line  232  is optional. Typically, a single ASIC  274  may be used to perform the function of one or more of the single-beam partial beamformers  270 - 272 , a-a filter/decimators  282 - 284 , and MUX/FIFO  290 . The complete multi-beam beamformer topology comprises a plurality of partial beamformers  270 - 272 , or ASICS  274 , in cascade, interconnected with a single 40 MHz data stream in the same way as shown in  FIG. 3 , i.e. using a single data stream connecting the partial beamformers  270 - 272  or ASICS  274 . Therefore, a 2-factor reduction of the interconnect data rate between the partial beamformers  270 - 272 , or ASICS  274 , may be accomplished without compromising the operating frequency range of the beamformer  110 , as two (or N) single-beam partial beamformers  270 - 272  or ASICs  274 , each having a data rate of 20 MHz, may be multiplexed into the 40 MHz output data stream. 
     FIG. 7  illustrates a complex demodulator  300  which may be used in connection with the systems of  FIGS. 5 and 6 . The complex demodulator  300  may be included in the RF processor  112 . The beamsum output data streams  308  from the last partial beamformer in the cascade topology is input to the multiplier  304 , such as data stream C  258  ( FIG. 5 ) or data stream E  292  ( FIG. 6 ). 
   In the case of low-band  260  or high-band  264 , data demodulation can be done in the following way. The input data stream  308  may be either low-band  260  or high-band  264  data and comprise data samples from data streams C  286  and D  288  ( FIG. 6 ). A RAM table  302  provides a complex, time dependent demodulation waveform to a multiplier  304 . For example, the waveform may be M=exp (−j*2*pi*f*k/fs/2), k=0,1,2 . . . , where a constant demodulation frequency f and a demodulation amplitude of 1 has been assumed for simplicity. The sampling frequency fs=40 MHz and k is the running time index. Therefore, assume that the desired demodulation data stream for the decimated data stream C  286  is Mc1 Mc2 Mc3 Mc4, and that the desired demodulation data stream for data stream D  288  is Md1 Md2 Md3 Md4. The multiplier  304  interleaves M, input from the RAM table  302 , with data streams C  286  and D  288 , and outputs data stream F  314 , where F=Mc1 Md1 Mc2 Md2 Mc3 Md3 Mc4 Md4 . . . and so on. 
   The data stream F  314  is filtered by a FIR filter  306 , typically with real coefficients. For simplicity, assume that the desired impulse response of the FIR filter  306  is the same for both data streams, such as h1 h2 h3 h4 . . . , h(N). The desired operations if the demodulator in this case are, therefore, c=h conv (C*Mc) and d=h conv (D*Md) where “conv” means “convolved with”. By selecting the FIR filter  306  coefficients as the desired impulse response interleaved with zeros, H={h1 0 h2 0 h3 0 . . . h(N)}, one who is skilled in the art will realize that the output demodulation data stream G  310  then becomes G=c1 d1 c2 d2 c2 d3 c3 d3 . . . and so on, as desired. 
   The following applies if the beamsum input data stream  308  is mid-band  262  data, such as data stream C  268  ( FIG. 5 ). Mathematically demodulation can be done by inserting two zero samples between the mid-band  262  sample pairs, such that input data stream  308  may be represented by Cz=A1 A2 0 0 A5 A6 0 0 A9 A10 0 0 . . . and so on. The RAM table  302  provides the complex, time dependent signal to the multiplier, for example Mz=exp (−j*2*pi*f*k/fs), k=0,1,2, . . . where a constant demodulation frequency f and a demodulation amplitude of 1 has been assumed for simplicity. The demodulation waveform Mz has been sampled at the original sampling frequency, such as fs=40 MHz. The data stream F  314  output by the multiplier  304  is then filtered by the FIR filter  306  with the desired coefficients {h(k)}, k=1,2,3, . . . N. The output of the filter is decimated by two by throwing away every other sample to give a data output rate that is the same as the input data rate of 20 MHz. The 20 MHz output demodulation data stream G  310  will be G=g1 g2 g3 g4 g5 . . . . 
   It may be noted in the above example that one-half of the samples of the sequence Mz*Cz (or data stream F  314 ) are zeros that don&#39;t contribute to the output sum. Moreover, because the bandwidth of the input data stream  308  is limited to 10 MHz, the sampling rate of the complex data output can be reduced to 10 MHz without loss of information. 
   Therefore, an alternative demodulation sequence requiring only half the data rate in the FIR filter  306  /multiplier  304  may be accomplished even when the beamsum input data stream  308  is mid-band  262  data. The input data stream  308  may be represented by Cz=A1 A2 A5 A6 A9 A10 . . . and so on. The input data stream  308  is input at a  20  MHz rate. The multiplier  304  interleaves the input data stream  308  and the samples of Mz from the RAM table  302  that correspond to the non-zero samples of Cz, i.e. {Mz(k)}, k=1, 2, 5, 6, 9, 10, . . . and so on. 
   The FIR filter  306  filters the data stream F  314  with a coefficient set that is different for the odd and even numbered time index. Assuming for simplicity that N=4*m−2, where m is an integer, the odd-sample coefficients are {ho}={h1 h2 h5 h6 h9 h10 . . . h(N−1) h(N)} and the even-sample {he}={h7 h8 h11 . . . h(N−2) 0} Thus, the coefficients of the FIR filter  306  toggle back and forth between {ho} and {he} for every other sample. The output becomes G=g1 g2 g3 g4 g5 . . . as in the previous example. If the output rate is limited to 10 MHz, the complication of using time dependent coefficients can be avoided. It is then possible to use only one coefficient set, such as {ho}, and use only the odd numbered output data from the FIR filter  306 . The even samples need to be thrown away by the decimator  312 , so the output becomes G=g1 x g3 x g5 x . . . . 
   The following example illustrates demodulating multiplexed mid-band  262  data, such as data stream E  292  ( FIG. 6 ), wherein the data are interleaved two and two as discussed previously. For simplicity, assume that the desired demodulation waveform for data stream C  286  is Mc1 Mc2 Mc3 Mc4 . . . and so on, and that the desired demodulation waveform for data stream D  288  is Md1 Md2 Md3 Md4 . . . and so on. Moreover, assume that the desired impulse response of the FIR filter  306 , {h1 h2 h3 h4 . . . , h(N)} is the same for both data streams C  286  and D  288 . The desired operations are, in other words, c=h conv (C*Mc) and d=h conv (D*Md) where conv means “convolved with”. 
   Complex demodulation will result by making M an interleaved version of data streams C  286  and D  288 , taking two samples of each at a time, M=Mc1 Mc2 Md1 Md2 Mc3 Mc4 Md3 Md4 . . . and so on, and selecting the FIR filter  306  coefficients as the desired impulse response interleaved with zeros. Assuming a 10 MHz output rate (decimation by two on the output) only one set of coefficients is necessary. These coefficients become Ho={ho(1) ho(2) 0 0 ho(3) ho(4) 0 0 ho(5) ho(6) 0 0 . . . ho(N−1) ho(N))} The demodulation data stream G  310  output from the FIR filter  306  will be {x c1 x d1 x c3 x d3 x c5 x d5 x c7 x d3 . . . }. For clarity, x indicates that a data sample is thrown away by the decimator  312 , and the output data are {c1 c3 c5 . . . }and {d1 d3 d5 . . . }, and G=c1 d1 c3 d3 . . . and so on. 
   It should be noted that the data streams C  258  ( FIG. 5 ) and E  292  ( FIG. 6 ) may be sent to a common complex demodulator, such as the complex demodulator as shown in  FIG. 7 . Therefore, the same FIR filter  306  may be used for data streams having every other data sample removed, and for data streams having two or more consecutive data samples removed by changing the coefficients of the FIR filter  306  through software, without modifying any hardware within the beamformer  110 . 
   Thus, by using the decimation and multiplexing system as illustrated in  FIGS. 5 and 6 , namely, by decimating at least two consecutive data samples, an additional bandwidth (mid-band  262  of 5-15 MHz), or data sampling rate of one-quarter to three-quarters of the data sampling rate after decimation, may be achieved. Therefore, the maximum usable frequency range of the beamformer in a decimating mode may be expanded while maintaining the data rate at 20 MHz, for example. As overlapping frequency bands may be obtained, the image performance of transducers  10  having a center frequency close to 10 MHz is improved. As stated previously, transducers  10  having other center frequencies may also be used and achieve improved image performance. 
   While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.