Patent Publication Number: US-2005131299-A1

Title: Differential partial beamforming

Description:
CROSS REFERENCE TO RELATED CASES  
      Applicants claim the benefit of Provisional Application Ser. No. 60/528,554, filed 10 Dec. 2003. 
    
    
      This invention relates to medical ultrasound imaging systems and, in particular, to the reduction of noise in analog delay lines of the pre-beamforming stage of a multiline beamformer.  
      Ultrasonic imaging systems use ultrasonic acoustic waves to observe the internal organs of subject. The frequency range of the ultrasonic acoustic waves can be described essentially by its lower limit: 20 kHz, roughly the highest frequency a human can hear, although medical ultrasound imaging systems tend to use frequencies in the 2 to 15 MHz range. Ultrasound systems emit ultrasonic pulses which, if not absorbed, echo (i.e., reflect), refract, or are scattered by structures in the body. These echoes, refractions, and back-scatterings are received by the ultrasound system, which translates them into images which can be seen and interpreted by medical personnel.  
      As shown in  FIG. 1 , a typical ultrasound system is comprised of an ultrasonic transducer (or scanhead)  100 , which is typically in the form of a probe which is held by the ultrasound system operator and moved over different portions of the subject&#39;s anatomy in order to obtain the desired image. Transducer  100  transmits the ultrasonic waves into the subject, and then receives the resulting signals. Transducer  100  is usually connected to the base ultrasound system  130  by a cable  120 , although the transducer can have a wireless connection with the rest of the ultrasound system (e.g., radio transmission; see U.S. Pat. No. 6,142,946 to Hwang et al.). The base ultrasound system  130  comprises processing and control equipment  132 , as well as the display  133 .  
      The components which transmit and receive the ultrasonic waves in the transducer may be implemented differently in various ultrasonic systems. In the ultrasound system of  FIG. 1 , the face  101  of the transducer  100  (which is placed against the flesh of the subject to perform the imaging) consists of an array  110  of piezoelectric elements, which both transmit and receive the ultrasonic waves. In ultrasound systems that use such arrays, the ultrasonic waves are created (and the resulting signals are interpreted) by a process called “beamforming” which is performed mostly in signal processing hardware and software. When transmitting, individual piezoelectric elements in the transducer array  110  are stimulated in particular patterns in order to form and focus one or more ultrasonic beams. When receiving, the signal information received by individual piezoelectric elements in the transducer array  110  are delayed, combined, and otherwise manipulated in order to form electronic representations of one or more ultrasonic beams.  
      In a “multiline” beamformer, the transducer array  110  transmits a single ultrasonic beam, but the receive beamformer electronics synthesizes several receive ultrasonic beams with different orientations. The oldest and most basic approach to multiline beamforming is to use multiple single line beamformers which are operated in parallel, such as described in U.S. Pat. No. 4,644,795 to Augustine, which is incorporated by reference. In such an arrangement, each element in the transducer array is connected to a channel of the beamformer. Each of these channels applies delays to the signals from its corresponding element which are appropriate to steer and focus the beam being formed by the beamformer. The signals delayed by each channel of the beamformer are combined to form a uniquely steered and focused beam, and the multiple beams produced simultaneously by parallel operated beamformers are used to form multiple lines of an ultrasound image.  
      An example of this is shown in  FIG. 2A , in which each of the elements  211  of transducer array  210  in transducer  200  has a channel on which its received signals are transmitted over cable  220  to the processing means  232  in the ultrasound system. The signals received by the elements  211  may, or may not, be conditioned in the transducer (e.g., impedance matching) and then transmitted over cable  220  to the ultrasound system. The processing means  232  takes the received signals, which are still in analog form, and converts them to digital signals using analog-to-digital converters (A/D)  233 . The resulting digital signals are then delayed by digital delays  234  and summed together by summer  235  to form an acoustic receive sensitivity profile focused at any desired point within the imaging plane.  
      This approach is sufficient if the number of elements  211  being sampled in the transducer array  210  remain fairly low, i.e., under 200 or so elements (traditional beamformers have 128 channels). However, if the transducer array  210  has thousands of acoustic elements  211 , and one desires to use samples from each of those elements, cable  220  would have to carry thousands of channels, which would require a prohibitively large cable and more power than is available from a standard electric outlet (the typical power source for most ultrasound systems). For these and other reasons (including the excessive cost of such a cable and the associated electronics), the approach shown in  FIG. 2A  is not feasible when fully sampling the ˜3000 elements which may be available in a transducer array.  
      One solution to this problem of complexity, called “sub-array beamforming”, is shown in  FIG. 2B  and is described in the paper entitled “Fully Sampled Matrix Transducer for Real Time 3D Ultrasonic Imaging” by Bernard Savord and Rod Solomon (Paper 3J-1, Proceedings of the 2003 IEEE Ultrasonics Symposium, Oct. 5-8, 2003 (IEEE Press)) and U.S. Pat. No. 5,318,033 to Savord, both of which are hereby incorporated by reference. As described therein, and shown in  FIG. 2B , in sub-array beamforming, the beamforming function is split into two stages, the first stage taking place in the transducer  200 , and the second stage taking place in the processing means  232  of the ultrasound system. By performing partial beamforming in the first stage inside transducer  200 , the number of channels required to be transmitted over cable  220  is drastically reduced.  
      In the example of sub-array beamforming shown in  FIG. 2B , individual elements  211  in transducer array  210  are grouped into sub-arrays  240 - 1  to  240 - n . Each element  211  in each sub array  240  has a pre-amplifier  241  and a low power analog delay  242 , and each sub-array  240  has a sub-array summer  245  for combining the appropriately delayed signals within the sub-array into one channel. Examples of low power analog delay technology which can be used in the first stage include mixers, phase shifters, charge coupled devices (CCD), analog random access memory (ARAM), sample-and-hold amplifiers, and analog filters. All these technologies have sufficient dynamic range and use sufficiently low power to allow their integration into application-specific integrated circuits (ASICs) capable of fitting inside transducer  200 .  
      When performing beamforming, the signals received by transducer elements at the rim of the array are typically delayed on the order of +/−10 microseconds relative to the signals received by transducer elements in the center of the array. Implementing such a delay in analog delay circuit takes up a lot of room on an integrated circuit; implementing such a delay for hundreds of transducer elements would require too much room, i.e., the ASIC would be too large to fit within transducer  200 . In order to keep the aggregate size of these analog delay circuits sufficiently small, it is necessary that each analog delay be implemented in a relatively short length. This required short delay length can be achieved by (1) applying a relative delay to each element, i.e., a delay length which is appropriate relative to the other elements in its sub-array, and (2) having each element within a sub-array located substantially contiguously. Because the relative delays between contiguously located elements are small, the analog delays  242  within each sub-array  240  will be sufficiently small. Preferably, the signals received by the elements within each sub-array  240  will be aligned to within a small fraction of a wavelength at the nominal receive frequency for steering angles of up to 45° or so (neglecting focal delays). After the sub-array signals are formed, different bulk delays may be applied to each sub-array signal, where each bulk delay imposes the appropriate delay on each sub-array relative to the other sub-arrays.  
      The partially beamformed analog signals from sub-arrays  240 - 1  to  240 - n  are transmitted on channels  222 - 1  to  222 - n  over cable  220  to processing means  232  in the ultrasound system, where the sub-array analog signals are converted to digital by A/Ds  233 , appropriately delayed by digital delays  234 , and then combined by final summer  235 . The bulk delays discussed in the paragraph above may be implemented by digital delays  234 .  
      Although contiguous, the transducer elements which comprise a sub-array may form a variety of shapes or patterns on the transducer array. For example, in a rectangularly shaped transducer array, each column of transducer elements may form a sub-array. Such a construction is described in U.S. Pat. No. 6,102,863 to Pflugrath et al., which is incorporated by reference, and in U.S. Pat. No. 5,997,479 to Savord et al. In the aforementioned application, “elevation” beamforming (i.e., combining the signals in each column of elements) is performed in the transducer, while “azimuth” beamforming (i.e., combining the row of previously combined columns) is performed by the processing means in the ultrasound system.  
      In U.S. Pat. No. 6,682,487 to Savord, filed Sep. 10, 2002, each sub-array forms an irregularly-shaped hexagonal “patch” of twelve transducer elements. As shown in  FIG. 3  (a reproduction of  FIGS. 6 and 7  of the &#39;291 application), the transducer array  210  is comprised of small boxes each representing a transducer element  211 . The overall transducer array  210  has a roughly dodecahedral circumference, in which the sub-array patches are shown as alternating light and dark groupings. One patch  240  is shown circled in the overall transducer array  210 , and this patch is shown magnified above and to the left of transducer array  210 . Although shown here spaced apart from each other, the transducer elements  211  in an actual patch  240  would be closely packed together in a repeating hexagonal pattern. In the transducer array of the &#39;291 application, the twelve element patch pattern is only used when receiving signals from the subject (i.e., during receive beamforming), whereas a three element pattern is used for transmitting the ultrasonic waves (i.e., during transmit beamforming).  
       FIG. 4A  is a schematic representation of a single analog delay line within a sub-array. As seen in  FIG. 4A , the signal received by individual element  211  within sub-array  240  is amplified by pre-amplifier  241  before being appropriately delayed by analog delay  242 , which is under the control of control  244 . The appropriately delayed signal from analog delay  242  is combined with the appropriately delayed signals from the other elements within sub-array  240  by sub-array summer  245  to form sub-array signal.  
       FIG. 4B  is an exemplary implementation of the single delay line shown in  FIG. 4A . As mentioned above, the analog delay may be implemented by any combination of mixers, phase shifters, charge coupled devices (CCD), analog random access memory (ARAM), sample-and-hold amplifiers, and analog filters. The specific implementation shown in  FIG. 4B  uses analog random access memory (ARAM) to implement analog delay  242 . Specifically, the signal received by element  211 , after being amplified by pre-amplifier  241 , is sampled, i.e., is latched onto one of the capacitors  420 . The sampled signal remains stored on the capacitor until it is latched out of the capacitor (thus applying the appropriate delay). The latched out signal is amplified by post-amplifier  450  before being combined with the other signals in the patch sub-array by sub-array summer  245 . The timing of latch-in gates  410  and latch-out gates  430  are under the control of two shift registers  460  and  462 , respectively, as part of control  244 . Each shift register  460 - 462  is arranged to continually circulate one bit, thereby operating as ring counters. Each bit within a shift register  460 - 462  is associated with a corresponding gate in the gates  410 - 430 . When the circulating one is shifted into a particular bit in the shift register, the gate corresponding to the particular bit latches (resulting in a signal sample either entering or leaving one of the capacitors  420 ).  
      Dynamic receive focus module  475  controls the relative timing between when the signals are sampled by latch-in gates  410  and when sampled signals are fed to sub-array summer  245  by latch-out gates  430  (this is used, for instance, to effect a “focal update”). Dyanamic receive focus module  475  is under the control of clock delay controller  470 , which, in turn, is fed control data for forming the current receive beam from clock command memory  480 . Although shown here in a particular configuration, dynamic receive focus module  475  can be placed, and/or implemented, in a number of ways.  
      As can be seen from the exemplary implementation of  FIG. 4B , although the signals themselves are stored in analog form, the gates which latch the analog signals are controlled by digital components, such as shift registers  460 - 462 , the clock signal, module  475 , clock delay controller  470 , etc. Because there are thousands of elements in the transducer array, a high level of integration is required of the components in the patch beamforming ASICs which must fit in the hand-held transducer.  
      Unfortunately, the close integration of the components in the ASICs brings the digital clock and control circuitry within close proximity of the analog circuitry. Because of this close proximity, digital noise from the various nearby digital components is inevitably coupled onto the analog signals which are either stored on or otherwise acted upon by the analog circuits, and this coupled digital noise reduces the effective dynamic range of the channel. This noise may be coupled capacitively, inductively, or electromagnetically. Because the analog signals have a relatively low voltage level at this stage of the receive signal path, any digital noise coupled to the analog signal can have a considerable effect. Furthermore, because the various analog signals are stored on different capacitors and focal updates occur intermittently, the coupled digital noise is not consistent from signal sample to signal sample and, therefore, the coupled noise does not alias to DC and cannot be completely removed by filtering.  
      In addition, the high level of integration makes it very difficult to adequately isolate the power and ground lines of the analog circuitry. Digital noise, which is inevitably present on the digital power and ground lines, is usually benign. However, when digital noise contaminates (i.e., is coupled onto) the analog power and ground lines, it is also coupled onto the analog signals being stored and transmitted by the analog circuitry.  
      In the field of ultrasound imaging, there have been prior art attempts to reduce “common mode signals” (i.e., noise) in general, i.e., noise caused by the general environment, or by large-scale active components in the ultrasound system itself (e.g., the display). For example, in U.S. Pat. No. 4,984,465 to Piel et al., a differential circuit is used to decrease the noise coupled onto the signals being transmitted over the cable from the transducer to the rest of the ultrasound system, where the signals are processed. In the &#39;465 patent, the signals from each transducer element are coupled through a transformer before they are transmitted to the rest of the ultrasound system. These transformers are located in the hand-held transducer. In order to perform the noise reduction, half of the transformers in the &#39;465 patent invert the signal received from its corresponding transducer element, while the other half do not. Thus, the signals on half of the channels transmitted over the cable are inverted, while the signals on the other half are not. Once these signals arrive at the base ultrasound system, the inverted signals are input to the inverted input of the differential circuit, and the non-inverted signals are input into the non-inverted input of the differential circuit. The output of the differential circuit is the sum of the received signals from the transducer elements, with the common mode signals that were coupled to the signals as they were transmitted over the cable suppressed.  
      In a similar patent from the same inventors, U.S. Pat. No. 5,062,429 to Smith et al., the transducers themselves are polarized in an alternating fashion so that half of the received signals are generated already inverted, thereby suppressing any common mode signals which were coupled anywhere on the receive path from the transducers to the differential amplifier.  
      However, it would be difficult to implement these prior art solutions on present-day transducer systems. For example, because each element in the transducer in the Piel et al. patent requires a transformer, several thousand transformers would be needed in a typical two-dimensional array transducer, which would not be practical. As another example, the Smith et al. patent requires a complicated connection fixture in order to poll the piezoelectric material after it has been diced into individual array elements, which makes the manufacturing process needlessly complex.  
      Even if the practical problems were solved, there are more fundamental problems when applying these prior art references to the problem of suppressing coupled digital noise (as described above). To start with, these prior art solutions suppress common mode signals which are affecting multiple transducer elements in the same manner. By contrast, the problem with a single analog delay line, as shown in  FIG. 4A , is that each of the signal samples from one single transducer element will be affected differently by the common mode signals. Because each sampled signal is stored on a different capacitor at a different time and each sampled signal travels a different path, the different sampled analog signals will have differing amounts of coupled noise. The prior art solutions will not suppress the time-varying noise effects on the individual delayed signal samples transmitted on a single channel (i.e., received from a single transducer element). Even the noise that is common between multiple channels will not be completely eliminated, because the different delays imparted to the signals on different channels results in the noise on the different channels being not exactly out of phase, and, therefore, the differential amplifier will not completely eliminate the noise.  
      Furthermore, the prior art solutions are directed to the problem of common mode signals along the entire transmission path from the transducer elements (or transformers) to the ultrasound system itself. By contrast, the problem discussed above in reference to  FIGS. 4A and 4B  is the noise caused by the tight integration of digital and analog components on an integrated circuit that is contained entirely within the hand-held transducer. In other words, the prior art solutions are directed towards the macro-environment of the ultrasound system, e.g., the effect of electromagnetic noise from the environment surrounding the transmission path of the received signals in the ultrasound system, whereas the problems discussed above concern the micro-environment of densely packed capacitors on an integrated chip in the hand-held transducer.  
      Therefore, there is a need for a system and method for substantially decreasing the effect of capacitively, inductively, or electromagnetically coupled digital noise, as well as the digital noise coupled via the analog power and ground lines, on the analog signals being processed in the partial beamforming stage within the transducer of an ultrasound imaging system.  
      One objective of the present invention is to provide a system and method for substantially decreasing the effect of digital noise coupled capacitively, inductively, electromagnetically, or through the analog power and ground lines on the analog signals in the partial beamforming stage in the transducer of an ultrasound imaging system.  
      Another objective of the present invention is to provide a system and method for decreasing the effect of charge injection on the analog signals in the partial beamforming stage in the transducer of an ultrasound imaging system.  
      Still another objective of the present invention is to provide a system and method for decreasing the general effect of noise on analog signal samples in an ultrasound imaging system.  
      These and other objectives are met by the present invention, in which each transducer element has two analog delay circuits, one of which is supplied with the signal received by the transducer element, the other of which is supplied with an inverted version of the signal received by the transducer element. After both signals have traversed their respective analog delay paths, one of them is inverted and then both of them are combined in order to substantially suppress the effects of the localized common mode signals. The combination of the two signals causes any noise which commonly affected both analog delay circuits to be substantially suppressed by cancellation. Preferably, the layouts of the analog delay circuits are substantially the same, and the analog delay circuits are positioned so that they have a substantially similar orientation relative to, and are a substantially similar distance from, other components on the integrated circuit. 
    
    
      Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.  
      In the drawings:  
       FIG. 1  shows the large-scale components of a conventional ultrasound imaging system;  
       FIG. 2A  shows a conventional implementation of multiline beamforming in an ultrasound imaging system;  
       FIG. 2B  shows sub-array mulitline beamforming according to the prior art;  
       FIG. 3  shows an exemplary embodiment of a transducer array with “patch” sub-arrays for multiline beamforming according to the prior art;  
       FIG. 4A  is a schematic representation of a single analog delay line within a sub-array according to the prior art;  
       FIG. 4B  is a specific implementation of the single analog delay line shown in  FIG. 4A  using analog random access memory (ARAM) according to the prior art;  
       FIG. 5A  is a schematic representation of a single analog delay line within a sub-array according to a preferred embodiment of the present invention;  
       FIG. 5B  is a schematic representation of a single analog delay line within a sub-array according to another preferred embodiment of the present invention;  
       FIG. 5C  is a schematic representation of a single analog delay line within a sub-array according to yet another preferred embodiment of the present invention; and  
       FIG. 6  is a specific implementation of a single analog delay line using analog random access memory (ARAM) according to the preferred embodiment of the present invention shown in  FIG. 5A . 
    
    
      The preferred embodiments of the present invention are directed to a system and method for ameliorating the effects of digital noise coupled onto analog signals in the tightly integrated environment of an ASIC used for partial beamforming in an ultrasound imaging system. In general, embodiments of the present invention use duplicate analog delay lines to substantially duplicate, on an inverted copy of the received signal traversing the duplicate analog delay, the effect of localized common mode signals on the original received signal traversing the original analog delay line. After both signals have traversed their respective analog delay paths, one of them is inverted and then both of them are combined in order to substantially suppress the effects of the localized common mode signals.  
      Because of the two different levels of “partial beamforming” before the “final beamforming” performed in the base ultrasound system, the following nomenclature has been adopted in the claims and in some parts of the specification: the first level of partial beamforming, where the signal from a single transducer element is sampled and appropriately delayed (i.e., by analog delay  242  in  FIG. 4A ), is called “element beamforming”. The second level of partial beamforming, where the element beamformed signals are combined together to form a sub-array beamformed signal (i.e., by sub-array summer  245 ), is called “sub-array beamforming”. The final level of beamforming, where the sub-array signals are appropriately delayed and summed together to form the acoustic profile (i.e., by digital delay  234  and final summer  235 ), may be called “final beamforming” or “beamforming”, as this is the end of the beamforming process.  
      A schematic representation of a single analog delay line according to a presently preferred embodiment of the present invention is shown in  FIG. 5A . The original delay line according to the prior art, comprised of pre-amplifier  241 , analog delay  242 , sub-array summer  245 , and control  244 , remains the same, and the combined sub-array signal  247  still travels over cable  220  on channel  222  to the ultrasound system, where it is converted to digital by A/D  233 , delayed by digital delay  234 , and then finally summed with the other digitally converted and delayed sub-array signals by final summer  235 .  
      However, in contrast to the prior art analog delay line shown in  FIG. 4A , the pre-amplifier  241  of the preferred embodiment in  FIG. 5A  produces two amplified signals, a non-inverted signal which is input into analog delay  242 , and an inverted signal which is input to duplicate analog delay  542 . Duplicate analog delay  542  is preferably as similar as possible to analog delay  242  in layout, as well as relative orientation to, and distance from, other components (in particular, active components, such as the digital clock and control lines, the analog power and ground lines, and the power and ground planes). Furthermore, the gains on both signal paths should match.  
      Analog delay  242  and duplicate analog delay  542  both receive the same control signals from control  244  so that the actions of both analog delays are the same. Both output signals (i.e., the non-inverted signal output by analog delay  242  and the inverted signal output by duplicate analog delay  542 ) are input into element summer  590 . However, as indicated by the minus sign next to the inverted signal input into element summer  590 , the inverted signal is inverted again when it is input into element summer  590 . Element summer  590  combines the non-inverted signal and the twice-inverted signal to output corrected element signal  593  which is input into sub-array summer  245 . In other embodiments, the non-inverted signal may be inverted, rather than the inverted signal (for an explanation of why either signal may be inverted, see discussion below). In the preferred embodiment of  FIG. 5A , each analog delay line has a duplicate analog delay and an element summer, so that sub-array summer  245  combines corrected element signals from each of the elements in the sub-array.  
      To explain how the system of  FIG. 5A  substantially suppresses the common mode signals picked up from digital noise and analog ground and power line noise, we will trace the paths of the non-inverted and twice-inverted signals. First, pre-amplifier  241  outputs a non-inverted signal and an identical, but inverted, signal. When output by pre-amplifier  241 , these signals are still fairly pristine representations of the originally received signal. The non-inverted signal has the value +s (represented by voltage), while the inverted signal has the value −s. Second, the non-inverted signal is processed by analog delay  242  while the inverted signal is similarly processed by duplicate analog delay  542 . Because both analog delays  242  and  542  have been built as similarly as possible, the effect of noise as the signals are processed is substantially the same on both signals. For purposes of explanation, it will be assumed that the noise coupled through analog delay  542  is identical to the noise coupled through analog delay  242  and, furthermore, that the gains of the two signals paths are identical. Thus, the non-inverted signal is output as +s+n (where n=noise), while the inverted signal is output as −s+n. Third, both delayed signals are input to element summer  590 ; however, the inverted signal, having a current value of (−s+n), is inverted before being combined with the non-inverted signal, and thus has the value −(−s+n), or (+s−n). When the non-inverted signal and the twice-inverted signal are combined in element summer  590 , the result is (+s+n)+(+s−n), or 2s, the noise signal n having been cancelled in the combination.  
      Even if the noise coupled onto the analog signals in the two analog delays  242  and  542  is not identical, there is still a substantial reduction in noise. Furthermore, the connections to element summer  590  can be reversed so that the non-inverted signal is inverted and the inverted signal is unchanged. In such a case, the output of element summer  590  is −(+s+n)+(−s+n), or −2s. Again, the noise is cancelled. The signal output by element summer  590  is inverted, but this is generally of no consequence for medical ultrasonography as the echo data are passed through a detection stage, which discards the phase, prior to processing the data for display.  
      The above paragraph is intended as a simplified explanation of the principle by which the present invention substantially reduces common mode signals in the individual analog delay lines of a ultrasound transducer sub-array, and is not intended to limit the invention in any way, shape, or form. For example, the number of times either signal is inverted and the precise location along the receive path for the inversions are not limited (as long as the noise component of one signal serves to substantially cancel the noise component in the other signal when they are combined together). As another example, although the explanation in the above paragraph indicates that the output of element summer  590  is 2s, the output of an element summer in an actual implementation of the preferred embodiment shown in  FIG. 5A  might be greater or lesser than 2s. As yet another example, because there are now two analog delays where, in the prior art, there was only one, it might be decided to reduce the size of the capacitors in the analog delays  242  and  542  by half in order to conserve space. Although such capacitors would hold only half the charge, when the non-inverted and twice-inverted signals were combined by the element summer  590 , the resulting corrected element signal  593  would be back up to full strength (i.e., s). Furthermore, the gain in the summers may be implemented using differential amplifiers, where the gain could differ from unity.  
      As one can see from the duplicated design elements of the preferred embodiment in  FIG. 5A , the analog delay lines of the present invention require greater space and greater power than prior art analog delay lines, such as the one shown in  FIG. 4A . The total space and power of the ASIC is not doubled however, because the clock drivers and control circuitry can be shared, and the requirements for the transmit circuitry remain unchanged. As an example, on an ASIC in which the analog delay circuitry takes up about ⅓ of the silicon area, the silicon area required by an embodiment of the present invention would increase the silicon area of the ASIC by a third. However, the increase in performance provided by the substantial diminution in coupled noise in the analog signals may balance the increased size and cost.  
       FIG. 5B  is a schematic representation of a single analog delay line according to another presently preferred embodiment of the present invention. Similarly to  FIG. 5A , the prior art delay line, comprised of pre-amplifier  241 , analog delay  242 , sub-array summer  245 , and control  244 , remains, and the combined sub-array signal  247  still travels over cable  220  on channel  222  to the ultrasound system, where it is converted to digital by A/D  233 , delayed by digital delay  234 , and then finally summed with the other digitally converted and delayed sub-array signals by final summer  235 . Also similarly to  FIG. 5A , the pre-amplifier  241  of the preferred embodiment in  FIG. 5B  produces two amplified signals, a non-inverted signal which is input into analog delay  242 , and an inverted signal which is input to duplicate analog delay  542 . Analog delay  242  and duplicate analog delay  542  both receive the same control signals from control  244  so that the actions of both analog delays are the same.  
      However, in contrast to the preferred embodiment of  FIG. 5A , the non-inverted delayed signal output by analog delay  242  and the inverted delayed signal output by duplicate analog delay  542  are not input into a single element summer, but rather input into two separate summers. Specifically, in the preferred embodiment of  FIG. 5B , the non-inverted signal output by analog delay  242  is input into pre-sub-array summer  595 A, while the inverted delayed signal output by duplicate analog delay  542  is input into the pre-sub-array summer  595 B. Pre-sub-array summer  595 A also receives the non-inverted signal output from each analog delay  242  within sub-array  240 ; similarly, pre-sub-array summer  595 B also receives the inverted delayed signal output from each duplicate analog delay  542  within sub-array  240 . Thus, the output of pre-sub-array summer  595 A in  FIG. 5B  is the same as the output of the sub-array summer  245  of the prior art analog delay line in  FIG. 4A . However, the output of pre-sub-array summer  595 A is input into sub-array summer  245  of the preferred embodiment of  FIG. 5B , as is the output of pre-sub-array summer  595 B (inverted, as indicated by the minus sign), and both outputs are combined by sub-array summer  245  to generate sub-array signal  247 .  
      The element summer  590  in  FIG. 5A  and the sub-array summer  245  in  FIGS. 5B and 5C  are preferably implemented with differential amplifiers.  
      As shown by  FIGS. 5A and 5B , the location where the non-inverted signal and the twice-inverted signal are combined can be anywhere along the receive beamforming path. In the preferred embodiment of the present invention shown in  FIG. 5C , the combination location is moved from inside the transducer downstream to the inside of the ultrasound system.  
      Similarly to the preferred embodiment shown in  FIG. 5B , the preferred embodiment shown in  FIG. 5C  has two pre-sub-array summers  595 A and  595 B which combine the non-inverted and inverted signals, respectively of sub-array  240 . However, unlike the preferred embodiment shown in  FIG. 5B , the sub-array summer  245  is not located within hand-held transducer  200 , but is rather located with the processing means  234  in the base ultrasound system  230 . Thus, the preferred embodiment in  FIG. 5C  transmits the two differential signals (output by the two pre-sub-array summers  595 A and  595 B) from hand-held transducer  200  to base ultrasound system  230  on two separate channels  222 A and  222 B on cable  220 . Because of this, the number of channels carried in cable  220  in  FIG. 5C  is twice the number of channels carried on cab le  220  in  FIG. 5B . Nevertheless, even though it must carry twice the channels, the cable  220  of the preferred embodiment shown in  FIG. 5C  need not be double the cost of cable  220  in  FIG. 5B . The typical prior art cable  220  is a coaxial cable, which is required in order to shield the channels from each other and which is fairly expensive. However, because of the improved noise rejection provided by the differential signaling, the cable  220  in  FIG. 5C  does not require the shielding provided by coaxial cable, and, thus, could be comprised of much less expensive and easier to terminate cable. For example, cables such as twisted pairs or ribbon cables (e.g., IMAGIN manufactured by W. L. Gore &amp; Associates, Inc.) could be used.  
      Even though three different preferred embodiments have been shown and described herein, it should be understood that the present invention is not limited to those three preferred embodiments, and embodiments which substitute different elements for some of the elements shown in any and all of the three preferred embodiments are intended to be covered by the scope of the present invention. For example, although all three preferred embodiments described herein use a cable  220  to connect the hand-held transducer  200  to the base ultrasound system  230 , an embodiment according to the present invention could replace cable  220  with some form of wireless communication connection between the transducer  200  and base ultrasound system  230 . As another example, although all three preferred embodiments described herein have the A/D converters  233 , digital delays  234 , and final summer  235  located within the base ultrasound system  230 , an embodiment according to the present invention could have one or more of these components located outside of the base ultrasound system  230 , perhaps in an additional component, or placed within the hand-held transducer  200 .  
      As can be determined from the above description, the present invention provides a system and method for substantially decreasing the effect of digital noise coupled capacitively, inductively, electromagnetically, or through the analog power and ground lines on the analog signals being processed in the sub-array beamforming stage within the transducer of an ultrasound imaging system. However, the present invention also provides a system and method for reducing the effects of other types of common mode signals and, as seen in the preferred embodiment of  FIG. 5C , can also be used to ameliorate the effects of common mode signals from the general environment. Furthermore, it is contemplated that other beneficial noise reduction effects are possible with the present invention. For example, there are indications that the present invention may also reduce the charge injection noise of the latch-in and latch-out gates.  
      As was mentioned above, the analog delay  242  (and duplicate analog delay  542 ) may be implemented by any combination of mixers, phase shifters, charge coupled devices (CCD), analog random access memory (ARAM), sample-and-hold amplifiers, and analog filters, as would be known to one skilled in the art (and as described in the various references cited in the Background section). In order to provide guidance for the reader,  FIG. 6 , which shows an exemplary specific implementation of the preferred embodiment in  FIG. 5A , is presented herein. Similarly to the specific implementation of a prior art analog delay line shown in  FIG. 4B , the exemplary specific implementation of  FIG. 6  uses analog random access memory (ARAM) to implement analog delays  242  and  542 . Analog delay  242  appears on the top half of  FIG. 6 , in much the same form as analog delay  242  appears in  FIG. 4B . Specifically, there is a bank of capacitors  420 , each of which stores a sample of the signal received by element  211  (after being amplified by pre-amplifier  241 ). A particular capacitor  420  stores the present signal when the latch-in gate  410  of that capacitor closes, thereby allowing the capacitor to store the amplified signal output by the pre-amplifier  241 . When the latch-out gate  430  of that particular capacitor  420  closes, the stored (sampled) signal is provided to post-amplifier  450 . The time between the latch-in and latch-out of the sampled signal provides the appropriate delay.  
      Also similarly to  FIG. 4A , the set of latch-in gates  410  and the set of latch-out gates  430  are controlled by control  244 , and, more particularly, by shift registers  460  and  462 , respectively. Shift register  460  controlling the latch-in gates  410  operates as a ring counter under control of the system clock. Shift register  462  controlling the latch-out gates  430  operates as a ring counter under control of module  475 . Clock delay controller  470  controls dynamic receive focus module  475 , and thereby controls the delay timing of the sampled signals. Clock command memory  480 , in turn, controls clock delay controller  470 .  
      However, in contrast to  FIG. 4A , the bottom half of  FIG. 6  contains duplicate analog delay  542 , a close replica of analog delay  242  on the top of the page. The latch-in gates  610  and  630  of duplicate analog delay  542  are also under the control of shift registers  460  and  462 , thus the delay timing of the sampled signals in duplicate analog delay  542  should be substantially the same as analog delay  242 . Rather than the normal amplified signal, pre-amplifier  241  provides an inverted amplified signal as the input to duplicate analog delay  542 . The delayed output of analog delay  242  and the delayed output of duplicate analog delay  542  (after being inverted) are combined by element summer  590  to form corrected element signal  593 . The corrected element signal  593  is input into sub-array summer  245 . Each analog delay line in sub-array  240  has a duplicate analog delay and an element summer, so that sub-array summer  245  is combining corrected element signals from each of the elements within the sub-array.  
      As stated above, the specific ARAM implementation shown in  FIG. 6  is presented as an example in order to provide general guidance as to how one may implement the preferred embodiment in  FIG. 5A , and it is not intended to limit the present invention in any way, shape, or form. The analog delay line according to the present invention may be implemented using any analog technology, including, but not limited to, any combination of mixers, phase shifters, charge coupled devices (CCD), analog random access memory (ARAM), sample-and-hold amplifiers, and analog filters, as would be known to one skilled in the art (and as described in the various references cited in the Background section).  
      While there have shown and described and pointed out fundamental novel features of the invention as applied to a preferred embodiment thereof, it will be understood that various omissions and substitutions and changes in the form and details of the devices illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. For example, it is expressly intended that all combinations of those elements and/or method steps which perform substantially the same function in substantially the same way to achieve the same results are within the scope of the invention. Moreover, it should be recognized that structures and/or elements and/or method steps shown and/or described in connection with any disclosed form or embodiment of the invention may be incorporated in any other disclosed or described or suggested form or embodiment as a general matter of design choice. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto.