Method and apparatus for delaying ultrasound signals

An ultrasound beamformer that processes the signals of an array transducer includes a plurality of processing channels, one for each element of the active transducer array. Each channel includes a digitizing element for converting the received signal into digital samples and a delay element for delaying the digitized signal. The delays are chosen so that when the signals from the individual channels are combined, a beam forms in a particular direction. The invention implements sub-sampling period delays in the individual channels with low-complexity digital filters having superior delay characteristics with respect to frequency, but having undesirable attenuation characteristics with respect to frequency. The invention corrects for the undesirable attenuation characteristics via a single digital filter after the signals from the individual channels have been combined.

CROSS-REFERENCE TO RELATED APPLICATIONS 
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STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
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REFERENCE TO MICROFICHE APPENDIX 
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FIELD OF THE INVENTION 
The present invention relates to ultrasound imaging systems which utilize 
phased array beam steering and focusing, and more particularly, to 
ultrasound imaging systems which utilize digital filters to implement 
delays in each of a plurality of ultrasound signal paths to affect phased 
array beam steering and focusing. 
BACKGROUND OF THE INVENTION 
In a phased array ultrasound imaging system, the ultrasound transducer 
includes an array of transducer elements. To support this array of 
transducer elements, the system includes a plurality of parallel channels, 
wherein each channel includes a transmitter and a receiver connected to 
one of the transducer elements in the array. Each transmitter outputs an 
ultrasound pulse through a transducer element into an object to be imaged, 
typically a human body. The transmitted ultrasound energy is steered and 
focused by applying appropriate delays to the pulses transmitted by each 
element in the array so that the transmitted energy arrives at a desired 
point in-phase, thus the energy adds constructively at that point. This 
causes a portion of the pulse to be reflected back to the transducer array 
by various structures and tissues in the body. As the pulse of ultrasound 
energy passes through the object to be imaged, a continuous reflection 
signal returns to the transducer array. The portions of the reflected 
signal received earliest by the transducer array are representative of 
those portions of the object closest to the transducer array. In general, 
the amount of elapsed time from when the pulse is transmitted until the 
signal is received by the transducer is representative of the distance 
from the transducer. 
Steering and focusing of the received ultrasound energy is affected in 
similar manner. In a receive beamformer, the signal received from each of 
the transducers is processed and delayed, and then the signals from all of 
the transducer channels are summed in a signal summation element. The 
delay for each element is selected such that the reflected energy received 
by each transducer from the desired point is input into the summing 
element in phase (at the same time), thus creating a received beam that is 
focused at the desired point. The delays may be varied dynamically so that 
the transmitted beam can be scanned over a region of the body, and the 
signals generated by the beamformer can be processed to produce an image 
of the region. 
Ideally, the delay means will not affect the signal in any way other than 
to delay it. The attenuation and the phase of the frequency components of 
the signal being delayed should not vary with the amount of delay 
selected; otherwise, the signal summation of the several channels will be 
unevenly weighted and will not produce the desired results. Also, the 
preservation of the signal should remain constant over a relatively broad 
frequency range so that shorter, wide-band ultrasound transmission pulses 
may be used. 
In many prior art systems, the ultrasound signals remain in an analogue 
state until after signal summing element. In such systems, the delay means 
are usually limited to implementations such as fixed lengths of 
transmission line and all pass, constant group delay filters. 
In other prior art systems, the ultrasound signals are digitized prior to 
being delayed and summed. In such systems, the means for creating the 
delays are necessarily digital. A common method of delaying the digitized 
ultrasound signal is to pass the digital samples through a series of 
hardware registers which are clocked at the sampling frequency f.sub.s. 
For delays equal to an integer number of digitization intervals, each 
digital sample may be stored in a digital data storage device such as a 
Random Access Memory (hereinafter referred to as RAM); then the digital 
samples to be summed are properly aligned when extracted from the RAM. 
With either the hardware register or the digital storage device delay 
methods, the amount of signal delay is limited to an integer number of 
sampling intervals .tau., where .tau. is typically equal to .lambda./(4c), 
.lambda. is the wavelength of the transmitted signal and c is the velocity 
of propagation of the transmitted signal. However, for precise beam 
steering, a smaller amount of delay for each channel is often required 
(typically as small as .lambda./32c!). Passing the digitized ultrasound 
signal through a digital filter can provide the desired sub-sample period 
delay, as long as the original signal has been properly sampled. A 
continuous, band-limited signal which has been properly sampled can be 
completely reconstructed in the continuous domain. For this reason, 
digital filters can exhibit group delays (or equivalently, time delays) on 
signals which are less than the sampling period. The coefficients of the 
digital filter can be dynamically modified so that a range of delays can 
be selected. A relatively high order digital filter with a corresponding 
large number of coefficients is necessary to achieve an amplitude and 
phase response with respect to frequency which is independent of the 
selected sub-sample period delay. Because of the large number of channels, 
(e.g., 64 to 128 typical), there is a practical need to simplify 
components within the channels. Lower order digital filters exist that 
preserve the phase of the signal frequency components of an amount 
independent of the sub-sample period delay, over a wide frequency range, 
but such filters attenuate the amplitude of the signals. 
Accordingly, it is an object of this invention to provide an improved 
ultrasound signal delay means for processing received signals from an 
ultrasound transducer array. 
It is another object of this invention to provide an improved ultrasound 
signal delay means for processing received signals from an ultrasound 
transducer array which applies an independent delay to each of a plurality 
of ultrasound signal channels. 
It is yet another object of this invention to provide an improved 
ultrasound signal delay means for processing received signals from an 
ultrasound transducer array which applies an independent delay to each of 
a plurality of ultrasound signal channels, and each of the channel delay 
means incorporates a low order filter. 
It is a further object of this invention to provide an improved ultrasound 
signal delay means for processing received signals from an ultrasound 
transducer array which applies an independent delay to each of a plurality 
of ultrasound signal channels, each of the channel delay means 
incorporates a low order filter, and any undesirable signal 
characteristics caused by the low order filters are compensated by a 
filter following the channel signal summation element. 
SUMMARY OF THE INVENTION 
The present invention is directed to a method and apparatus for delaying 
ultrasound signals which in one aspect comprises a plurality of signal 
processing channels with each channel receiving an ultrasound signal from 
an element of a transducer array. Each channel includes a digitizing unit 
which converts the ultrasound signal into a series of digital data 
elements at a fixed sample rate. Each channel further includes a delay 
unit for selectably delaying the series of digital data elements from the 
digitizing means by one of a plurality of time increments. The series of 
delayed digital data elements from each of the signal processing channels 
are received by a summation unit, and in-phase samples from each of the 
signal processing channels are summed to form a composite signal 
comprising a series of composite digital data elements. The invention also 
includes a correction unit which receives the series of composite digital 
data elements and corrects them for one or more distortions introduced by 
the delay means. In one embodiment of the invention, the delay unit 
further includes a coarse delay unit which delays the channel signal by an 
integer number of sample period intervals, and a fine delay unit that 
delays the channel signal by one of a plurality of sub-sample period 
intervals. In another embodiment of the invention, the fine delay unit 
includes a low order digital filter which preserves the phase 
characteristics of the channel signal and attenuates the amplitude of the 
channel signal by an amount independent of the sub-sample interval delay 
selected.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention is directed to an ultrasound imaging system with an 
array transducer for producing images representing cross sections through 
the human body. 
FIG. 1A shows a simplified block diagram of one embodiment of an ultrasound 
beamformer 100 in accordance with the present invention. The ultrasound 
beamformer 100 includes a plurality of ultrasound transducers which form 
an active transducer array, a switching network 101, a plurality of signal 
processing elements 102, and a summation and post-processing element 104. 
Each signal processing element 102 includes a pre-conditioning element 
106, a digitizing element 108, and a gain, delay and apodization element 
110. The switching network 101 receives an ultrasound signal from each of 
the ultrasound transducers and selectively directs the signals to the 
signal processing elements. The switching network 101 allows the system to 
have fewer processing elements 102 than transducers, so that a set of 
processing elements 102 can sequentially process signals of transducers 
from multiple regions of the transducer array. 
Each pre-conditioning element 106 receives an ultrasound signal from the 
switching network 101, and the gain, delay and apodization element 110 
produces a processed signal to be combined with the processed signals from 
the other signal processing elements 102. The summation and 
post-processing element 104 receives the signals produced by the several 
signal processing elements 102. The summation and post-processing element 
104 produces a composite signal as a function of the signals received from 
the signal processing elements 102. FIG. 1B shows another embodiment of an 
ultrasound beamformer 100 in accordance with the present invention. In the 
embodiment of FIG. 1B, the summation portion of the summation and 
post-processing element 104 from FIG. 1A is distributed among the signal 
processing elements as distributed summation elements 112. Each 
distributed summation element receives the sum output for the next 
previous distributed summation element 112 and adds to it the signal from 
its own channel. The resulting sum is provided as the sum output for the 
next distributed summation element. 
The pre-conditioning element 106 receives the ultrasound signal from the 
ultrasound transducer in order to prepare the signal to be digitized by 
digitizing unit 108. The pre-conditioning unit can include elements that 
perform gain control and equalization functions, signal limiting functions 
and/or signal filtering functions to remove noise and other undesirable 
characteristics from the ultrasound signal. 
The digitizing unit 108 receives the pre-conditioned ultrasound signal from 
the pre-conditioning element 106 and samples the pre-conditioned 
ultrasound signal at a sampling frequency f.sub.s. To prevent aliasing, 
the sampling frequency f.sub.s must be at least twice the frequency 
bandwidth of the preconditioned ultrasound signal, and, if the A/D 
converter is not used as a mixer, twice the frequency of the highest 
frequency component of the preconditioned ultrasound signal. In one 
embodiment of the invention, the sampling frequency f.sub.s is chosen to 
be four times the frequency of the central frequency component of the 
pre-conditioned ultrasound signal. Each time the pre-conditioned 
ultrasound signal is sampled, the digitizing element 108 produces a 
digital data element representative of the amplitude of the 
pre-conditioned ultrasound signal at the instant in time that the sample 
occurred. The digitizing element 108 produces a series of digital data 
elements representative of the pre-conditioned ultrasound signal at a data 
rate equal to the sampling frequency f.sub.s. 
The gain, delay and apodization element 110 receives the series of digital 
data elements from the digitizing element 108. The gain portion of the 
gain, delay and apodization element 110 modifies the amplitude of the 
digitized ultrasound signal so that an apodization of the signals of the 
different channels can be obtained. An exemplary use of apodization is the 
reduction of the side lobes of the formed beam. The delay portion of the 
gain, delay and apodization element 110 time-shifts the digitized 
ultrasound signal. 
FIG. 2 shows an alternate view of the ultrasound beamformer 100 shown in 
FIG. 1. FIG. 2 conceptually presents all of the elements of the signal 
processing element 102 as a general processing element 202 and a delay 
element 204. The summation and post-processing element 104 is conceptually 
presented as a summing element 206, a correction element 208 and a general 
post-processing element 210. As described herein for FIG. 1B, the parallel 
summing element 206 may, in other embodiments of the invention, be 
distributed among the signal processing elements 102. In general, the 
invention includes a delay element 204 having desirable delay 
characteristics with respect to frequency and relatively simple, low 
complexity architecture, that may result in other undesirable 
characteristics that are substantially independent of the amount of the 
delay selected. An example of such an undesirable characteristic is the 
attenuation of the signal amplitude as a function of the signal frequency. 
The undesirable characteristics, if any, are mitigated or even completely 
corrected by the correction element 208 after summation. The low 
complexity of the delay element 204 is important because any component 
included in the signal processing element 102 must be duplicated a 
relatively large number of times (i.e., once for each channel). The 
correction element 208 can be more complex, since it is only instantiated 
once, after the signals from the signal processing elements 102 are 
combined via the summing element 206 (or, in other embodiments, via 
summing elements distributed among the signal processing elements). 
In one form of the invention, the delay element 204 is a digital 
Bell-Spline FIR filter having four coefficients. For more information 
regarding the Bell-Spline filter, see The Bell-Spline, a digital 
filtering/interpolation algorithm, by Enrico Dolazza, Proceedings of 
SPIE-The International Society for Optical Engineering, Vol 1092, Jan. 
31-Feb. 3, 1989 (hereinafter referred to as "the Dolazza paper"). A 
Bell-Spline filter is generally characterized by a .beta. value (a shape 
parameter) and a D value (a truncation parameter), as is more completely 
described in the Dolazza paper. FIGS. 3A, 4A and 5A show the magnitude 
response verses frequency, and FIGS. 3B, 4B and 5B show the delay response 
verses frequency of three typical four-coefficient Bell-Spline filters. In 
FIGS. 3, 4 and 5, the associated Bell-Spline filters have .beta. values of 
1.5, 2.0 and 4.0, respectively and D values of 1.22, 1.20 and 1.10, 
respectively. FIGS. 3C, 4C and 5C show the relationship between the 
filter's coefficient settings and amount of sub-sampling period delay 
generated by the filter. FIGS. 3A, 4A and 5A show the magnitude response 
verses frequency of the corresponding filter for the various coefficient 
settings. These graphs show that the signal attenuation as a function of 
the frequency is substantially independent of the filter delay. FIGS. 3B, 
4B and 5B show the amount of delay applied to a signal passing through the 
filter as a function of the signal frequency. The horizontal frequency 
axis is normalized to the sampling frequency of the ultrasound signal, 
which in all three cases is four times the central frequency of the 
ultrasound signal. Thus the 0.5 point on the horizontal axis represents 
f.sub.s /2. For broadband ultrasound signal processing, the frequency 
components of the signal exist in a frequency range centered on the 
central frequency and having a bandwidth approximately equal to the 
central frequency itself. In terms of normalized frequencies, this means 
that only the frequencies of interest are will between 0.1 f.sub.s and 0.4 
f.sub.s when the signal is sampled at f.sub.s, where f.sub.s is equal to 
four times the central frequency. All three filters provide 16 equal 
increments of the sub-sampling period delay, ranging from 0 to 15/16 of a 
sample period, although those skilled in the art will recognize that other 
sets of delay increments, wherein the delay elements are not necessarily 
equal, may be used. The particular amount of delay through the filter is 
selected by setting the four coefficients to the corresponding values. 
A common characteristic of the three Bell-Spline filters shown in FIGS. 3A, 
4A and 5A (and of Bell-Spline filters in general) is a magnitude verses 
frequency response which is substantially independent of the amount of 
delay through the filter. For example, the 16 delays which are traced in 
FIG. 3B (signal delay verses frequency) are nearly collinear in FIG. 3A 
(magnitude verses frequency); only near the f.sub.s /2 do the traces begin 
to diverge. Ideally, the magnitude response of the filter will be constant 
within the frequency range of interest, so that all signals received 
within that frequency range will be passed equally. However, the magnitude 
response for the three Bell-Spline filters, shown in FIGS. 3A, 4A and 5A, 
indicate variable attenuation characteristics verses frequency. Such 
frequency dependent signal shaping detrimentally effects subsequent signal 
processing of the ultrasound signals so that the resulting image is 
degraded. 
The invention utilizes a correction element 208 to compensate for the 
frequency dependent effect of the delay element 204. It is well known that 
the magnitude verses frequency response of the cascade of two systems is 
the product of the individual magnitude responses of the systems. 
Therefore, if the magnitude response of the correction element 208 is 
chosen to be proportional to the mathematical inverse of the magnitude 
response of the delay element 204, the overall magnitude response of the 
two filters will be substantially constant. Since the correction element 
208 is located after the summation element 206 in the signal processing 
chain and only needs to be instantiated once, the complexity of the 
correction element can be high relative to the complexity of the delay 
element without burdening the overall complexity of the system. 
When analog restoration filters are cascaded to compensate for the 
attenuation of some frequency component of the signal as described herein, 
the resulting signal-to-noise ratio generally decreases. This is because 
the restoration filter adds to the attenuated input signal and to its 
attenuated noise, the white noise of the filter itself. When digital 
filters are cascaded, the signal-to-noise level remains constant because 
while the sampled signal and its associated noise are attenuated together, 
the only source of additional noise is quantization noise, which can be 
arbitrarily reduced by increasing the number of bits into which the signal 
is digitally encoded. If sufficient dynamic range is designed into the 
system so that no loss of information occurs when the signal is 
attenuated, no further loss of information is caused by the digital 
filter. The invention provides a sufficient dynamic range margin 
throughout the digital processing chain to ensure that no loss of 
information takes place when the signal is attenuated, thereby eliminating 
additional noise contribution and maintaining the signal-to-noise ratio as 
the delay element 204 and the correction element 208 attenuate the 
digitized signal. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in respects as illustrative and 
not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all changes 
which come within the meaning and range of the equivalency of the claims 
are therefore intended to be embraced therein.