fig3 is a schematic pictorial view of the ultrasound imaging system 10 of the present invention . the system includes a hand - held scan head 12 coupled to a portable data processing and display unit 14 which can be a lap - top computer . the scan head 12 is connected to the data processor 14 by a cable 16 . in an alternative embodiment , the system 10 includes an interface unit 13 ( shown in phantom ) coupled between the scan head 12 and the data processing and display unit 14 . the interface unit 13 preferably contains controller and processing circuitry including a digital signal processor ( dsp ). the interface unit 13 performs required signal processing tasks and provides signal outputs to the data processing unit 14 and / or scan head 12 . fig4 is a schematic functional block diagram of the ultrasound imaging system 10 of the invention . as shown in fig4 the scan head 12 includes an ultrasonic transducer array 18 which transmits ultrasonic signals into a region of interest or image target 11 such as a region of human tissue and receives reflected ultrasonic signals returning from the image target . the scan head 12 also includes transducer driver circuitry 20 and pulse synchronization circuitry 22 . the pulse synchronizer forwards a series of precisely timed and delayed pulses to high voltage driver circuits in the drivers 20 . as each pulse is received by the drivers 20 , the high - voltage driver circuits are activated to forward a high - voltage drive signal to each transducer in the transducer array 18 to activate the transducer to transmit an ultrasonic signal into the image target 11 . ultrasonic echoes reflected by the image target 11 are detected by the ultrasonic transducers in the array 18 . each transducer converts the received ultrasonic signal into a representative electrical signal which is forwarded to preamplification circuits 24 and time - varying gain control ( tgc ) circuitry 25 . the preamp circuitry 24 sets the level of the electrical signals from the transducer array 18 at a level suitable for subsequent processing , and the tgc circuitry 25 is used to compensate for attenuation of the sound pulse as it penetrates through human tissue and also drives the beam forming circuits 26 ( described below ) to produce a line image . the conditioned electrical signals are forwarded to the beam forming circuitry 26 which introduces appropriate differential delay into each of the received signals to dynamically focus the signals such that an accurate image can be created . the signals delayed by the beam forming circuitry 26 are summed to generate a single signal which is forwarded over the cable 16 to the data processor and display unit 14 . the details of the beam forming circuitry 26 and the delay circuits used to introduce differential delay into received signals and the pulses generated by the pulse synchronizer 22 will be described below in detail . in one preferred embodiment , the dynamically focused and summed signal is forwarded to an a / d converter 27 which digitizes the summed signal . digital signal data is then forwarded from the a / d 27 over the cable 16 to buffer memories 29 and 31 . it should be noted that the a / d converter 27 is not used in an alternative embodiment in which the analog summed signal is sent directly over the system cable 16 . the a / d converter 27 is omitted from further illustrations for simplicity . data from buffer memory 31 is forwarded to scan conversion circuitry 28 in the data processing unit 14 . the scan conversion circuitry 28 converts the digitized signal data from the beam forming circuitry 26 from polar coordinates ( r , θ ) to rectangular coordinates ( x , y ). after the conversion , the rectangular coordinate data is forwarded to post signal processing stage 30 where it is formatted for display on the display 32 or for compression in the video compression circuitry 34 . the video compression circuitry 34 will be described below in detail . digital signal data is forwarded from buffer memory 29 to a pulsed doppler processor 36 in the data processor unit 14 . the pulsed doppler processor 36 generates data used to image moving target tissue 11 such as flowing blood . in the preferred embodiment , with pulsed doppler processing , a color flow map is generated . the pulsed doppler processor 36 forwards its processed data to the scan conversion circuitry 28 where the polar coordinates of the data are translated to rectangular coordinates suitable for display or video compression . a control circuit preferably in the form of a microprocessor 38 controls the operation of the ultrasound imaging system 10 . the control circuit 38 controls the differential delays introduced in both the pulsed synchronizer 22 and the beam forming circuitry 26 via a memory 42 and a control line 33 . in one embodiment , the differential delays are introduced by programmable tapped ccd delay lines to be described below in detail . the delay lines are tapped as dictated by data stored in the memory 42 . the microprocessor 38 controls downloading the coarse and fine delay line tap data from memory 42 to on - chip memories in both the pulsed synchronizer 22 and the beam forming circuitry 26 . the microprocessor 38 also controls a memory 40 which stores data used by the pulsed doppler processor 36 and the scan conversion circuitry 28 . it will be understood that memories 40 and 42 can be a single memory or can be multiple memory circuits . the microprocessor 38 also interfaces with the post signal processing circuitry 30 and the video compression circuitry 34 to control their individual functions . the video compression circuitry 34 as described below in detail compresses data to permit transmission of the image data to remote stations for display and analysis via a transmission channel . the transmission channel can be a modem or wireless cellular communication channel or other known communication means . the portable ultrasound imaging system 10 of the invention can preferably be powered by a battery 44 . the raw battery voltage out of the battery 44 drives a regulated power supply 46 which provides regulated power to all of the subsystems in the imaging system 10 including those subsystems located in the scan head 12 . thus , power to the scan head is provided from the data processing and display unit 14 over the cable 16 . fig5 is a detailed schematic functional block diagram of the scan head 12 used in the ultrasound imaging system 10 of the invention . as described above , the scan head 12 includes an array of ultrasonic transducers labeled in fig3 as 18 -( 1 ), 18 -( 2 ), . . . , 18 -( n ), where n is the total number of transducers in the array , typically 128 . each transducer 18 ( 1 )- 18 ( n ) is coupled to a respective processing channel 17 ( 1 )- 17 ( n ). each processing channel 17 ( 1 )- 17 ( n ) includes a respective pulsed synchronizer 22 ( 1 )- 22 ( n ) which provides timed activation pulses to a respective high voltage driver circuit 20 ( 1 )- 20 ( n ) which in turn provides a driving signal to a respective transducer 18 ( 1 )- 18 ( n ) in the transmit mode . each processing channel 17 ( 1 )- 17 ( n ) also includes respective filtered preamplification circuits 24 ( 1 )- 24 ( n ) which include voltage clamping circuits which , in the receive mode , amplify and clamp signals from the transducers 18 ( 1 )- 18 ( n ) at an appropriate voltage level . the time varying gain control circuitry ( tgc ) 25 ( 1 )- 25 ( n ) controls the level of the signals , and the beam forming circuitry 26 ( 1 )- 26 ( n ) performs dynamic focusing of the signals by introducing differential delays into each of the signals as described below in detail . the outputs from beam forming circuits 26 ( 1 )- 26 ( n ) are summed at a summing node 19 to generate the final focused signal which is transmitted over the cable 16 to the data processor and display unit 14 for subsequent processing . in the present invention , one embodiment of the beam forming and focusing circuit 26 can be integrated on a single microchip and utilizes cascaded charge - coupled device ( ccd ) tapped delay lines to provide individual coarse and fine delays resulting in a wide range of delays with fine time resolution . this embodiment of the beam forming system of the invention , referred to herein as charge domain processing ( cdp ) circuitry , includes a plurality of processing circuits which , in a receiving mode , differentially delay signals representative of image waveforms received as reflected ultrasonic energy from the target object in order to generate a focused image . in a transmitting mode , the processing circuits differentially delay signals , which are to be transmitted as ultrasonic energy to a target object by the array 18 of transducers 18 ( 1 )- 18 ( n ), in order to generate a focused directional beam . each of the processing circuits includes a first delay line having a plurality of delay units operable in the receiving mode for receiving an image waveform and converting same into sampled data such as charge packets . in the transmitting mode , the first delay line receives the imaging signals and converts same into sampled data such as charge packets . a selection control circuit is operable for reading the sampled data from a selected first delay unit of the first delay line so as to correspond to a selected first time delay to accommodate fine delay resolution of the image waveform or imaging signals . a second delay line having a plurality of delay units is operable for sensing the sampled data from the selected first delay unit . the control circuit is further operable for reading the sampled data from a selected second delay unit of said second delay line so as to correspond to a selected second delay time to accommodate coarse delay resolution of the image waveform or imaging signals . in the receiving mode , a summation circuit is provided for summing the sampled data from each of the selected second delay units in each of the processing circuits in order to produce a focused image . in the transmitting mode , an output circuit is provided for converting the sampled data from each of the selected second delay units in each of the processing circuits into signals representative of the focused directional beam . the beam forming and focusing operations involve forming a summation of the waveforms as observed by all of the transducer elements . however , in this summation , the waveforms must be differentially delayed so that they all arrive in phase at a summation circuit 19 ( see fig5 ). accordingly , each beam forming circuit 26 in accordance with the present invention provides a different time delay on each processing channel , and further varies that delay with time . the signals which are added in phase to produce a focused signal are then forwarded to the data processor and display unit 14 . for each nominal scanning direction , the differential delay required for information received by a transducer element 18 ( k ) in the array , relative to the first element 18 ( 1 ), varies predominantly with k , with a small correction as a function of time to correct focus for depth . the overall control of delay can involve very fine time resolution as well as a large range of delays . however , for a selected beam forming direction , this set of delays is achieved by a combination of a coarse delay in each channel to approximately compensate for direction , and a fine delay for each channel which combines the functions of focusing and refining the original coarse correction . according to one preferred embodiment of the beam forming circuitry 26 shown in operational block diagram form in fig6 each of the beam forming circuits 26 is respectively arranged in a predetermined one of the n - parallel processing channels 17 ( 1 )- 17 ( n ), one for each of the array of transducer elements 18 ( 1 )- 18 ( n ). each beam forming circuit 26 includes two cascading tapped delay lines 56 ( 1 )- 56 ( n ), 58 ( 1 )- 58 ( n ). each circuit 26 receives as an input a signal from a tgc circuit 25 ( see fig3 ). the first delay line 56 in each channel provides a fine time delay for its received signal , while the cascaded second delay line 58 provides a coarse time delay . each fine delay line has an associated programmable tap - select circuit 57 ( 1 )- 57 ( n ), and each coarse delay line has a programmable tap - select circuit 59 ( 1 )- 59 ( n ), both of which will be described in more detail hereinafter . the tap - select circuits are operable for effecting a variable delay time as a function of tap location . during the operation of the circuits 26 , signals which are received by each transducer element 18 are applied continuously to the input of its corresponding processing channel 17 . the input signals to each processing channel are converted into a sequence of sampled data for initial propagation through the respective fine tapped delay lines 56 . in accordance with a preferred embodiment of the present invention , both the fine 56 and coarse 58 tapped delay lines are charge - coupled device ( ccd ) tapped delay lines . exemplary programmable ccd tapped delay lines are described in , for example , beynon et al ., charge - coupled devices and their applications , mcgraw - hill ( 1980 ), incorporated herein by reference . accordingly , in the exemplary configuration of the processing circuit 26 using ccd delay lines , the input signals to each of the processing channels are converted to a sequence of charge packets for subsequent propagation through the fine and coarse delay lines . at a predetermined time , which is dependent on the tap location selected by the system 10 , a delayed sample is either destructively or nondestructively sensed from the selected tap of the fine delay line 56 . the delayed sample is in turn input to the front end of the corresponding coarse delay line 58 . the selected delay samples thereafter propagate through the coarse delay line , and are again destructively or nondestructively sensed at a properly selected tap location corresponding to a predetermined time delay designated in accordance with the operation of the ultrasound imaging system 10 . the sensed sampled data from the coarse delay line of each processing channel is simultaneously summed by a summation circuit 19 to form the output beam . with reference now to fig7 a more detailed operational block diagram of the beam forming circuits 26 ( 1 )- 26 ( n ) of the present invention is shown . as illustrated , the programmable tap - select circuits 57 ( 1 )- 57 ( n ) for the fine delay lines each include respective fine tap select circuits 60 ( 1 )- 60 ( n ) and fine tap select memory units 62 ( 1 )- 62 ( n ). in turn , the programmable tap - select circuits 59 ( 1 )- 59 ( n ) for the coarse delay lines each include respective coarse tap select circuits 64 ( 1 )- 64 ( n ) and coarse tap select memory units 66 ( 1 )- 66 ( n ). in accordance with a preferred embodiment of the beam forming circuits , the fine and coarse delay lines have differing clock rates . the fine delay line is clocked at a higher rate than the coarse delay line and is therefore capable of providing a much finer delay time than that of the coarse delay line . for instance , in an exemplary configuration , each circuit 26 has a 32 - stage fine tapped delay line clocked at 40 mhz and a 32 - stage coarse - tapped delay line clocked at 2 mhz . such a configured circuit can provide up to a 16 μs delay with a programmable 25 ns delay resolution . in contrast , it will be appreciated that if a single delay line were used , it would require approximately 640 stages of delays . furthermore , due to the cascaded delay line structure of the beam forming circuits of the present invention , a local memory of 5 - bit wide by 64 - stage is adequate for providing the dynamic focusing function for a depth up to 15 cm . however , if a single delay structure were used , it would require a local memory of 640 - bit wide by 1280 - stage long . during operation of an individual beam forming circuit 26 , the fine delay line taps are changed continuously by the microprocessor 38 via the memory 42 ( see fig4 ) during each echo receiving time to provide dynamic focusing . the fine tap select circuit 60 , in the form of a digital decoder , and the local fine tap select memory 62 are used to select the desired tap position of the fine delay line 56 . for example , the microprocessor instructs the memory 42 to download a data word to memory 62 to provide a digital address representative of the selected tap position to the select circuit 60 for decoding . in turn , the select circuit 60 effects the sampling of data from the selected tap . in an exemplary embodiment , a 5 - bit decoder is used to provide a 32 - tap selection . the tap position of the coarse delay line 58 is set once before each echo return and is not changed during each azimuth view direction . as with the operation of the fine delay line , the coarse tap select circuit 64 , in the form of a digital decoder , is used in conjunction with the local coarse tap select memory 66 to select the desired tap position of the coarse delay line . fig8 shows an operational block diagram of an alternative embodiment of the beam forming circuitry 26 of the present invention in which each circuit 26 includes a respective latching circuit 70 ( 1 )- 70 ( n ) that generates a tap setting signal to each of the fine tap select circuits 60 ( 1 )- 60 ( n ). when the tap setting signal is provided to the fine tap select circuits , the tap selection will be fixed at the last tap of the fine tap delay lines ( i . e . focusing point ), thus the dynamic focusing function is not operable . this operation is controlled by the imaging system in situations where , for example , the imaging point is at a distance from the transducer elements which does not require a precise fine delay time . in this manner , the size of the fine tap select memory 62 is reduced . an exemplary embodiment of the latching circuit 70 in accordance with the present invention is shown in fig9 . in operation , when the latch is set high by the microprocessor 38 , digital data from the memory 62 will pass through the cmos passing transistors , and the defined transistor inverter provides an input to the appropriate tap select circuit ( decoder ) 60 so as to implement the dynamic focusing function . in contrast , when the latch is set low , the passing transistors are disabled , and thus the inverter output will be latched to the last data address in the memory , i . e ., the last tap select position . using a 1 . 2 - μm ccd / cmos fabrication process provided by a conventionally known silicon foundry , orbit semiconductor , inc ., a prototype 10 - channel beam forming microchip based on the fine / coarse delay architecture described above has been designed and fabricated . due to the compactness of each fine and coarse delay line , and the simplification of its corresponding control circuits , this approach accommodates configuring the beam forming electronics of a 64 - element receiver array to be integrated on one single microchip . in the prototypical beam forming microchip , each processing circuit includes two cascaded programmable tapped delay lines ( each 16 - stages long ), two 4 - bit cmos decoders and a 4 × 64 - bit local memory for storing the tap locations . the prototype is configured with 10 processing channels , each of which includes the processing circuit of the present invention fabricated on a single silicon microchip . each processing circuit can provide up to 10 μs of programmable delay with a 25 ns delay resolution . the beam forming chip operates such that at each azimuth viewing angle , echo return signals from an image point at a given range resolution received by a transducer element are sampled by the corresponding processing channel . each processing circuit provides ideally compensated delays to each received return signal . all of the delayed outputs are then summed together to form a single beam or a focused image point . the chip area associated with each processing channel is only 500 × 2000 μm 2 . it follows that the dynamic beam forming electronics for a 64 - element receiver array can be integrated in a single microchip with chip area as small as 64 mm 2 , which corresponds to at least three to four order of magnitude size reduction compared to conventional devices . the fine - coarse tapping architecture of the present invention accommodates a 12 μs delay with a 25 ns resolution with the two cascaded ccd tapped delay lines . specifically , the architecture includes a first 16 - stage long delay line clocked at 40 mhz and a second 32 - stage long delay line clocked at 2 mhz . the shorter delay lines and the simplicity of the tapping circuit associated with these shorter delay lines allows all of the image - generating electronics to be integrated on a single chip . a single chip performs the electronic focus function for a 128 - element array with more than two orders of magnitude reduction in chip area , power consumption and weight when compared with conventional implementations . an operational block diagram of another alternative embodiment of the beam forming circuitry 26 of the present invention is shown in fig1 , in which the selected outputs of each coarse delay line 58 ( 1 )- 58 ( n ) are applied to respective multiplier circuits 80 ( 1 )- 80 ( n ) prior to being provided to the summation circuit 19 . an exemplary multiplier for use in the aforementioned embodiment of the beam forming circuits is described in co - pending u . s . patent application ser . no . 08 / 388 , 170 , entitled &# 34 ; single - chip adaptive filter utilizing updatable weighting techniques ,&# 34 ; filed feb . 10 , 1995 by alice m . chiang , which is incorporated herein by reference . the configuration of the multipliers 80 will accommodate the use of apodization techniques , such as incorporating a conventionally known hamming weighting or code at the receiving array to reduce the sidelobe level and generate better quality imagery . similar to the embodiment shown in fig8 latch circuits 70 ( 1 )- 70 ( n ) may be included in association with each of the beam forming circuits 26 ( 1 )- 26 ( n ) in order to control the latching of the tap select position for the fine delay lines 56 ( 1 )- 56 ( n ). conventional apodization and hamming weighting techniques are described in , for example , gordon s . kino , acoustic waves : devices , imaging , and analog signal processing , prentice - hall , inc . ( 1987 ), which is incorporated herein by reference . fig1 shows an operational block diagram of the cascaded dual tapped ccd delay lines used in pulse synchronizers 22 ( 1 )- 22 ( n ) to introduce delay into individual transmitted signals in the transmit mode of the ultrasound system 10 of the present invention . each pulse synchronizer circuit 22 ( 1 )- 22 ( n ) includes two cascading tapped delay lines 56 ( 1 )&# 39 ;- 56 ( n )&# 39 ; and 58 ( 1 )&# 39 ;- 58 ( n )&# 39 ;. the first delay line 56 &# 39 ; in each processing channel provides a fine time delay for the signals to be transmitted , while the cascaded second delay line 58 &# 39 ; provides a coarse time delay . each fine delay line has an associated programmable fine tap select circuit 60 ( 1 )&# 39 ;- 60 ( n )&# 39 ;, which receive tap select addresses from respective fine tap select memory units 62 ( 1 )&# 39 ;- 62 ( n )&# 39 ;. each coarse delay line has an associated programmable coarse tap select circuit 64 ( 1 )&# 39 ;- 64 ( n )&# 39 ;, which receive tap select addresses from respective fine tap select memory units 66 ( 1 )&# 39 ;- 66 ( n )&# 39 ;. the tap - select circuits are operable for effecting a variable delay time as a function of tap location . during the operation of the pulse synchronizers 22 in the transmission mode , signals which are provided from the microprocessor 38 via the memory 42 ( see fig4 ), are applied continuously to the inputs of each processing channel 17 ( 1 )- 17 ( n ). the input signals to each processing channel are converted into a sequence of sampled data for initial propagation through the respective fine tapped delay line 56 . in an exemplary configuration of the pulse synchronizer circuits 22 ( 1 )- 22 ( n ) using ccd delay lines , the input signals to each of the processing channels are converted to a sequence of charge packets for subsequent propagation through the fine and coarse delay lines . at a predetermined time which is dependent on the tap location selected by the imaging system , a delayed sample is either destructively or nondestructively sensed from the selected tap of the fine delay line 56 . the delayed sample is in turn input to the front end of the corresponding coarse delay line 58 . the selected delay samples thereafter propagate through the coarse delay line , and are again sensed at a properly selected tap location corresponding to a predetermined time delay designated in accordance with the operation of the microprocessor 38 of the ultrasound imaging system 10 . the sensed sampled data from each of the coarse delay lines 58 ( 1 )- 58 ( n ) are then converted and transmitted as ultrasonic pulse signals by the corresponding transducer elements 18 ( 1 )- 18 ( n ). in accordance with a preferred embodiment of the present invention , the fine and coarse delay lines of each pulse synchronizer circuit have differing clock rates . in the transmission mode , the fine delay line can be clocked at either a higher or lower rate than that of the coarse delay line in order to accomplish the desired beam forming and focusing . in another embodiment of the invention , an adaptive beam forming imaging ( abi ) technique is used in both the beam forming circuits 26 and the pulse synchronizer circuits 22 to introduce the appropriate delays to produce a focused image . the adaptive beam forming technique improves image quality and spatial resolution by suppressing artifacts due to scattering sources and clutter in the sidelobes of the transducer array response . this adaptive beam forming circuitry can also be implemented on a single chip . abi is a model - based approach to image reconstruction derived from superresolution techniques . abi offers improvements in resolution and reduction in sidelobes , clutter , and speckle . superresolution algorithms modified for imaging include the two - dimensional maximum likelihood method ( mlm ) and two - dimensional multiple - signal classification ( music ). abi incorporates models for the desired backscatter ( amplitude and phase ), providing better detection performance than conventional imaging methods . fig1 is a schematic functional block diagram depicting one embodiment of adaptive beam forming circuits 426 located in the scan head 412 in accordance with the present invention . in the adaptive beam forming circuits 426 , individual multiplier weights of the finite impulse response ( fir ) filter are controlled by a feedback loop , in such a way as to reduce clutter and interference or finite impulse response ( fir ) filters . in either case , the adaptive circuits are used to remove clutter and interference such as that caused by ultrasonic signal in the sidelobes of the array pattern to produce an image with much higher accuracy and resolution . each processing channel 428 ( 1 )- 428 ( n ) of the beam forming circuits 426 receives a signal from a respective time - varying gain control ( tgc ) circuit 25 at a respective tapped delay line 430 . the beam forming circuits 426 includes n processing channels 428 , one for each transducer in the array 18 . signals tapped off of each tapped delay line 430 are received by a set of weighted multiplying d / a converters 432 . each processing channel k includes m weighted multipliers 432 , labelled 432 k1 - 432 km . the weights of the multipliers 432 are set to generate an output signal from each processing channel which is summed at a summing node 419 . the summed signal is forwarded over the system cable 416 to the system control circuit such as the microprocessor 438 in the data processing and display unit 414 . the microprocessor 438 analyzes the signal for known characteristics of such effects as clutter , sidelobes and interference . in response to detecting such effects , the microprocessor 438 generates control signals used to drive the multiplier weights 432 to adjust the signals to eliminate these effects from the output signal and forwards the control signals to the multipliers via the system cable 416 on lines 440 . thus , the adaptive beam forming circuitry comprises a feedback circuit which alters received signals from a tapped delay line of each channel prior to summation of the signals . the summed signal is sensed and correction signals based on the sensing are forwarded in the feedback loop to the multipliers to correct the summed signal . the abi results in an image of much higher resolution and overall quality than is obtainable in prior systems . the abi technique results in at least two to three times better resolution than that provided by conventional imaging techniques . as an example , in conventional ultrasound , at a frequency of 5 mhz , a resolution of about 1 mm can be obtained . using abi techniques , a lateral resolution of approximately 300 μm is obtained . as described above , the ultrasound signal is received and digitized in its natural polar ( r , θ ) form . for display , this representation is inconvenient , so it is converted into a rectangular ( x , y ) representation for further processing . the rectangular representation is digitally corrected for the dynamic range and brightness of various displays and hard - copy devices . the data can also be stored and retrieved for redisplay . in making the conversion between polar and rectangular coordinates , the ( x , y ) values must be computed from the ( r , θ ) values since the points on the ( r , θ ) array and the rectangular ( x , y ) grid are not coincident . in prior scan conversion systems , each point on the ( x , y ) grid is visited and its value is computed from the values of the four nearest neighbors on the ( r , θ ) array by simple linear interpolation . this is accomplished by use of a finite state machine to generate the ( x , y ) traversal pattern , a bidirectional shift register to hold the ( r , θ ) data samples in a large number of digital logic and memory units to control the process and ensure that the correct asynchronously received samples of ( r , θ ) data arrive for interpolation at the right time for each ( x , y ) point . this prior implementation can be both inflexible and unnecessarily complex . despite the extensive control hardware , only a single path through the ( x , y ) array is possible . this means that full advantage of different ultrasound scan frequencies and , hence , imaging depth , cannot be taken . that is , different data are forced into the same format regardless of physical reality . in the scan conversion circuitry 28 of the present invention ( see fig4 ), hardware complexity and cost are drastically reduced through the use of a number - theoretic scheme for reliably generating the ( x , y ) grid traversal path in natural order , i . e ., using the ( r , θ ) samples as they are acquired . this approach provides greater flexibility and better fidelity to the actual medical data , as it permits the array traversals to be designed so that they do not impose an unnatural image reconstruction scheme . this scan conversion circuitry 28 of the present invention uses a farey - sequence generator process , which generates the ( x , y ) coordinates in the order in which they are encountered in the scanning . assume that the system received the first two scan rays ; it is desired to identify all the ( x , y ) integer pairs situated within the wedge for 0 & lt ; y ≦ l . a process which uses a farey sequences to generate all ( x , y ) pairs within two successive arrays with 0 & lt ; y ≦ l in the order of increasing angle is described here . the process exploits the fact that certain ( x , y ) pairs lie along the same angle , so it generates only ( a , b ) pairs which are mutually prime and then sets the rest of ( x , y ) pairs by ( x , y )= n ( a , b ) for n = 1 , 2 , . . . until ( n + 1 ) b & gt ; l . to better understand how this is accomplished , let us define a farey sequence . definition : the sequence of rational numbers whose denominator does not exceed l , arranged in increasing numerical order , is called the farey sequence of order l . if u / v is a fraction in lowest terms and v ≦ l , we will call u / v a farey fraction of order l . therefore , farey fraction is in lowest terms ; thus , its numerator and denominator are mutually prime . the theory of farey series is described in detail in g . h . hardy and e . m . wright , an introduction to the theory of numbers , oxford university press , london 1938 , pp . 23 - 24 , which is incorporated herein by reference . let a / b , c / d , e / f be three successive farey fractions of order l and let ## equ1 ## where equations 1 and 2 permit us to begin with any two successive farey fractions and interate through all the rest within the slice . a simple example of using farey fractions of order 10 to generate all the ( x , y ) display points within the 46 °- 54 ° arc on a 10 × 10 grid is shown in fig1 . substituting the values for the first two successive farey fractions of the order l = 10 , a = 1 , b = 1 , and c = l - 1 = 9 , d = l = 10 into equations 1 and 2 , one obtains the next farey fraction with e = 8 , f = 9 . now , repeating the same calculation with a = 9 , b = 10 , and c = 8 , d = 9 , yields the next farey fractions with e = 7 , f = 8 . it is straightforward to generate all the ( x , y ) pairs within the given arc . if the user wants to map the same rays into a finer display grid ( for example , onto a 20 × 20 display points ), we use the same routine but with l = 20 i . e ., use the farey function of order 20 to generate all the ( x , y ) display points . simple arithmetic will show that the ( x , y )- pairs are ( 19 , 20 ), ( 18 , 19 ), ( 17 , 18 ), . . . . as can be seen in fig1 , all the grid points within the two successive scan lines are generated in natural order of increasing angle , i . e ., ## equ2 ## this characteristic allows a scan conversion system that automatically adapts to variation in scan angle φ o . systems with programmable , non - uniformly spaced scan arrays are possible with the farey sequence implementation . in one embodiment of the invention , the data processing and display unit 14 is programmed to carry out the scan conversion process . as mentioned above , the ultrasound imaging system 10 of the present invention also includes a pulsed doppler processor 36 which allows for generation of color flow maps . thus , moving targets ( particularly flowing blood ) can be displayed , letting physicians see the body &# 39 ; s inner functions without surgery . the generic waveform 111 for pulsed doppler ultrasound imaging is shown in fig1 . the waveform consists of a burst of n pulses with as many as j depth samples collected for each pulse in the burst . fig1 also shows a block diagram of the pulsed doppler signal processor 36 for this imaging technique , where the returned echoes received by each transducer are sampled and coherently summed prior to in - phase and quadrature demodulation at 113 . the demodulated returns are converted to a digital representation at sample - and - hold circuits 115 and a / d converters 117 , and then stored in a buffer memory 119 until all the pulse returns comprising a coherent interval are received . the n pulse returns collected for each depth are then read from memory , a weighting sequence , v ( n ), is applied to control doppler sidelobes , and a n - point fft is computed at 121 . during the time the depth samples from one coherent interval are being processed through the doppler filter , returns from the next coherent interval are arriving and are stored in a second input buffer . the integrated doppler processing device described herein performs all of the functions indicated in the dotted box of fig1 , except for a / d conversion , which is not necessary because the device provides the analog sampled data function . the remaining circuitry and the operation thereof is described in u . s . pat . no . 4 , 464 , 726 to alice m . chiang , issued aug . 7 , 1984 , entitled &# 34 ; charge domain parallel processing network ,&# 34 ; which is incorporated herein by reference . this pulsed - doppler processor ( pdp ) device has the capability to compute a matrix - matrix product , and therefore has a broad range of capabilities . the device computes the product of two real valued matrices by summing the outer products formed by pairing columns of the first matrix with corresponding rows of the second matrix . in order to describe the application of the pdp to the doppler filtering problem , we first cast the doppler filtering equation into a sum of real - valued matrix operations . the doppler filtering is accomplished by computing a discrete fourier transform ( dft ) of the weighted pulse returns for each depth of interest . if we denote the depth - doppler samples g ( k , j ), where k is the doppler index , 0 ≦ k ≦ n - 1 , and j is the depth index , then ## equ3 ## the weighting function can be combined with the dft kernel to obtain a matrix of doppler filter transform coefficients with elements given by the real and imaginary components of the doppler filtered signal can now be written as ## equ4 ## in equations 5 and 6 , the indices of the double - indexed variables may all be viewed as matrix indices . therefore , in matrix representation , the doppler filtering can be expressed as matrix product operation . it can be seen that the pdp device can be used to perform each of the four matrix multiplications thereby implementing the doppler filtering operation . the pdp device 36 of the invention includes a j - stage ccd tapped delay line 110 , j ccd multiplying d / a converters ( mdacs ) 112 , j × k accumulators 114 , j × k doppler sample buffer 517 , and a parallel - in - serial out ( piso ) output shift register 118 . the mdacs share a common 8 - bit digital input on which elements from the coefficient matrix are supplied . the tapped delay line 110 performs the function of a sample - and - hold , converting the continuous - time analog input signal to a sampled analog signal . in operation , the device 36 functions as follows : either the real or imaginary component of the returned echo is applied to the input of the tapped delay line 110 . at the start of the depth window , the video is sampled at the appropriate rate and the successive depth samples are shifted into the tapped delay line 110 . once the depth samples from the first pulse return interval ( pri ) are loaded , each element in the first column of the transform coefficient matrix w is sequentially applied to the common input of the mdacs 112 . the products formed at the output of each mdac 112 are loaded into a serial - in - parallel - out ( sipo ) shift register 521 . the collection of j × k products computed in this fashion represent an outer product matrix . these products are transferred from the sipos to ccd summing wells which will accumulate the outer product elements from subsequent pris . the process is repeated until all pulse returns ( rows of f ) have been processed . at this point , each group of k accumulators 114 holds the k doppler samples for a specific depth cell . the doppler samples are simultaneously clocked into the accumulator output piso shift registers 519 . these registers act as a buffer to hold the j × k depth - doppler samples , so processing can immediately begin on the next coherent interval of data . finally , the accumulator shift registers 521 are clocked in parallel transferring all the depth samples for a given doppler cell into the device output piso shift register 118 . samples are serially read out of the pdp device in range order , which is the desired order for flow - map display . a prototype pdp - a device for 16 - depth samples has been fabricated . the pdp - a can be used to process returns of a burst waveform with as many as 16 range samples collected for each pulse in the burst . the capability of detecting weak moving targets in the presence of a strong dc clutter has been successfully demonstrated by the prototype pdp device . a two - pdp implementation for color flow mapping in an ultrasound imaging system is shown in fig1 . in this device , during one pri the top pdp component 120 computes all the terms of the form w r f r and w i f r as shown in equations 5 and 6 , while the bottom component 122 computes terms of the form - w i f i and w r f i . the outputs of each component are then summed to alternately obtain g r and g i . as mentioned above , the imaging system of the invention also includes video compression circuitry 34 which conditions the data and transforms it into a compressed format to permit it to be transferred to a remote location . in a preferred embodiment , the video data compression circuitry is of the type described in u . s . pat . nos . 5 , 126 , 962 to alice m . chiang , issued jun . 30 , 1992 , entitled &# 34 ; discrete cosine transform processing system ,&# 34 ; and 5 , 030 , 953 to alice m . chiang , issued jul . 9 , 1991 , entitled &# 34 ; charge domain block matching processor ,&# 34 ; both of which are incorporated herein by reference . fig1 is a schematic functional block diagram of an alternative preferred embodiment of the ultrasound imaging system of the invention . in the embodiment of fig1 , a multiplexer 319 is added to the scan head 312 between the ultrasonic transducer array 318 and the drivers 20 and preamplification circuitry 24 . in this embodiment , signals are processed from only a portion of the transducer array 318 at any given time . for example , with a 128 - element array 318 , in one embodiment , only 64 elements will be processed at a time . the multiplexer 319 is used to route the 64 signals to the preamplification 24 and subsequent circuits . the multiplexer 319 is also used to route the driver pulses from the drivers 20 to the 64 elements of the array 318 currently being driven . in this embodiment , referred to herein as the sub - aperture scanning embodiment , circuit complexity is substantially reduced since processing channels need only be provided for the number of elements which are being processed , in this example , 64 . images are formed in this embodiment by scanning across the transducer array 318 and selectively activating groups of adjacent elements to transmit and receive ultrasonic signals . fig1 a and 17b are schematic pictorial views of display formats which can be presented on the display 32 of the invention . rather than storing a single display format as is done in prior ultrasound imaging systems , the system of the present invention has multiple window display formats which can be selected by the user . fig1 a shows a selectable multi - window display in which three information windows are presented simultaneously on the display . window a shows the standard b - scan image , while window b shows an m - scan image of a doppler two - dimensional color flow map . window c is a user information window which communicates command selections to the user and facilitates the user &# 39 ; s manual selections . fig1 b is a single - window optional display in which the entire display is used to present only a b - scan image . while this invention has been particularly shown and described with references to preferred embodiments thereof , it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims .