Application of random signal correlation techniques to ultrasonic flaw detection

Application of random signal correlation techniques to ultrasonic flaw detection E. S. FURGASON,

V. L. NEWHOUSE,

N. M. BILGUTAY

and G. R. COOPER

A system is described and analysed which applies random signal correlation techniques to ultrasonic flaw detection. The use of correlation and time integration techniques gives it a signal-to-noise ratio correlation gain of the order of 104, and the use of a random transmitted signal whose range resolution is independent of its duration permits a peak-to-average transmitted power ratio of the order of unity. It is shown that random signal correlation systems should therefore be capable of much greater range and/or resolution than is possible with current pulse-echo systems.

T Rmax Ppeakz->_

Introduction Conventronal ultrasonic pulse-echo systems are widely used to detect flaws produced during the manufacture or use of many types of metal and ceramic components. These systems transmit short bursts of radio frequency ultrasound mto the test object and display the echoes reflected from inhomogeneities on an oscilloscope. The time of occurrence and amplitude of these echoes can be related respectively to the locatron and magnitude of the sound reflectors. To avoid range ambigurtres m systems that transmit the same waveform in each burst, it is necessary to wait until the echo from the most distant target has returned before another burst can be transmitted. Therefore, the repetition period T of the rf bursts is limited by

T>-

Wru~x

(1)

where c is the velocity of sound and Rm range from whrch echoes can be detected.

the maximum

To obtain fine range resolution, it is necessary to transmit a correspondingly narrow burst of rf of length AT where CAT

Pavg

AT

Thus for pulse-echo systems the ratio of peak to average transmitted power has to be at least as large as the ratio of the maximum range to the desired range resolution. This ratio will usually be of the order of lo* or more in practical systems. Since transducers are limited in the peak power that they can handle by electrrcal breakdown effects, the large peak-to-average power ratio required can limit the maximum ratio of range to resolutron that can be obtained by pulse-echo systems. The other major problem faced by pulse-echo flaw detectors IS the fact that strongly sound absorbing material makes it necessary to use the largest possible average transmitted power rf the returning echoes are to be larger than the thermal receiver noise.’ Since ultrasonic transducers are strongly limited m average power handling capacity by overheating, this hmrts the range of pulse-echo systems when used in strongly sound absorbing or scattering materials. Since the thermal noise power of amplifiers is proportronal to their bandwidths, rt can be shown that the ratio of the signal-to-noise power at the output of a flaw detection system to that at the output of the echo receiving amplifier IS given by the expression

(2)

aR=l

SNRou t

B.In

SNRin

Bout

-_=-

From equations (1) and (2) the ratio of peak to average transmitted power can be written as

where Bin and Bout are respectively input and output of the system. The authors are III the School of Electrical University, West Lafayette, lndtana 47907, 10 September 1974.

ULTRASONICS.

(3)

AR

JANUARY

1975

Englneerlng, Purdue USA. Paper recewed

(4)

the bandwidths

at the

In the pulse-echo systems currently m use, the bandwidth of the signal received at the receiver and the bandwidth of

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the output signal emergmg from the detector mately the same. Thus these systems are not prove the signal-to-noise ratio of the received received echo must therefore be much larger thermal noise of the echo amplifier.

are approxlable to lmecho, and the than the

In current and as yet largely unpublished work, Woodmansee * et al and Seydell 3 use time-averaging techniques to improve the input signal-to-noise ratio of flaw detection systems by integrating the echo signals over relatively long time periods, thus effectively restricting the output bandwidth. Woodmansee achieves time mtegration by means of a lockm amplifier whereas Seydell digitizes the echo signal and uses a d@al computer. Both systems use conventional techniques transmitting short bursts of rf. Therefore they require high peak-to-average power ratios. The random signal flaw detection system described in the remamder of this paper falls into the class of correlation receivers, and like the systems of Woodmansee et al and Seydell, can produce a huge enhancement m the input signal-to-noise ratio by the use of time integration. In addition, it uses noise as the transmitted slgnal, so that the resolution along the ultrasonic beam is independent of signal duration as will be shown later. Consequently, the peak-to-average transmitted power can be kept close to unity, so that the maximum power that can be transmitted is no longer limited by transducer electrical breakdown effects. The transmission of long-duration signals is shown to lead to deterioration of system performance in the presence of distributed targets, due to incoherent ‘clutter’ type echoes from flaws outside the range cell from which the system 1s receiving coherent echoes. A technique involving the use of two focused transducers with overlapping radiation patterns is shown to overcome this problem. The use of a random signal or ‘noise’ in imaging systems has been considered by a number of authors 4-8 m connexlon with radar but appears to have been first demonstrated experimentally, again m radar, by McGillem, Cooper and Waltman. Additional random signal radar systems have been described by Poirier,lO Craig et al,‘l Carpentler l2 and Smlt and.Kneefel.13 Random signal systems do not seem to have been used previously for ultrasonic flaw detection, although a pseudo-random code ultrasomc Doppler system has been used for blood-flow measurement 14 and a random signal ultrasonic Doppler system is being developed for the same purpose.’ 5-17

System description

and theory

System operation

The block diagram of the basic random signal system 1s shown m Fig. 1. The noise or rf source produces electrical signals which are converted mto ultrasound and transmitted into the sample by the plezoelectric transducer. Echoes reflected from mhomogeneitles are picked up by an ldentical receiving transducer and are reconverted mto electrical signals. The received signal 1s then correlated with a sample of the transmitted signal which has been delayed by 7d, by means of a delay line.

form of a low-pass filter. In the presence of the target shown, the system will produce a non-zero output when the delay rd imposed on the reference signal by the delay line, roughly equals the time of flight 7s of the acoustic signal from the transmitting transducer to the target and back to the receiver. Under these circumstances the noise signals entering the two inputs of the correlator are identical, causing it to produce its maximum output. It 1s shown below that if 1~~- Tdl% l/r& where B is the bandwidth of the transmitted slgnal, the signals entering the correlator are uncorrelated, so that its time-averaged output 1s zero. If the length of the delay line is changed slowly with time, 7d varies, so that the system scans over a line in the test ObJect, producing an output on each occasion that the time varying delay 7d is nearly equal to the time of flight 7s. A practical version of the random signal flaw detection system 1s shown m Fig.2. The transrmtting and receiving transducers are arranged so that then ‘antenna patterns’ overlap in the crosshatched region. The variable delay line 1s made up of two identical ultrasonic transducers m a water bath whose separation can be varied by a micrometer. The amplified echo signal and the reference signal emergmg from the delay line are passed through Schmitt triggers actmg as clipping circuits. (The term ‘clipping’ 1s used here and subsequently to refer to the process m which a zero mean analogue signal is transformed into a bmaly signal having the same zero crossings). It can be shown that this w’ay of transforming a noisy multi-valued signal mto a signal which can only have one of two magnitudes, merely reduces the effective signal-to-noise ratio by a factor 2/7r if the signalto-noise ratio 1s much less than unity. The clipped echo and reference signals are fed mto a correlator whose gated output 1s displayed on one axis of a pen recorder. The other axis of the recorder 1s connected to a mlcLdmeter which controls the distance between the transducers of the delay line. Clipping the echo and reference signals allows digital processing of these signals, which will be advantageous m future, for more elaborate versions of the present system. In the present system it permits the correlation function to be performed by a simple exclusive-OR gate as shown in Fig.2. The purpose of the following NOR gate 1s to insure that only correlator outputs from the desired echoes reach the integrator and that thermal noise generated at other times, as well as signals leaking directly from the transmitter to the receivmg transducer, are excluded. This is achieved by allowing the NOR gate to pass signals only during the time that reference signals are emerging from the delay line. To scan a portion of a specimen, the separation between the transducers of the delay line is varied by means of a micrometer which causes the pen of the recorder to traverse. Whenever the reference delay Td is approximately equal to the time of flight 7s, the correlator produces an output which is displayed by the pen recorder. A typical output

Nouse source

Low-pass filter

4

The amplified echo signal together with the reference signal emerging from the delay line enter the correlator which consists of a multiplier followed by an integrator in the

12

Delay ~,j

output

i Correlotor

Fig 1

Basic random slgnal system

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1975

Pulse generator

ClIppIng crrcutts

I,

1

Amplrfrer Flaw srgnal

NOR

J

Rf or norse generator Range scan

< Water

Experrmental

Frg.2

random signal flaw detectron

>

j

delay I me

system employing

Correlator

srgnal clipping and polarity

coincrdence

correlator

Distance

oj

2

Noise

Comparrson of system operatron Frg.3 as the transmrtted srgnal

usrng perrodrc 4 ps bursts

showing the two surfaces of a 1 mm diameter wire in water is shown in Figs 3 and 4. Analysis

Under conditions where the thermal amplifier norse is negligible compared to the echo signal, where no signal clipping is used, and where the correlator is of the analogue type, the system can be modelled, as shown in Fig.5 where both the reference signal delay rd and the signal time of flight delay rs, are represented by delay lines. The correlator output corresponds to the time average of the product of the two delayed versions of the transmitted noise signal. This time average 1s the auto-correlation function R~(rs - rd) of the transmitted noise signal x(t).

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I Noise

Frg.4 Comparison transmitted srgnal

of system operatron

usrng 1 ps bursts as the

Fig.6 shows the time average of the correlator bell-shaped transmitted noise spectrum

output for a

WY

of system operation

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1975

Sxcf) = (B/2)2 + (/-

(5)

fe)2

where fe is the centre frequency and B 1sthe bandwidth. The correlator output RX(rS - rd) is the inverse Fourier transform of S,(j) and may be written as RX(rs - rd) = t eP”fB17d-7sl cos 2nfe(rd

- 5-s)

(6)

It can be seen that the time average of the correlator output reaches a maximum when rS = Ed. It IS also clear that the

13

Slgnal delay T* YNoise source Reference Flg.5

delay s,j

Model of the basic random slgnal system

correlator output falls to l/e of its maxlmum when 1~d - ~~1equals l/nB. From equation (2) it follows that the range resolution of this system is c

AR-2rrB If the bandwidth of the transmitted signal approaches the maximum signal frequency, then it can be shown that the range resolution approaches the theoretical minimum of one-quarter wavelength of the maximum transmitted frequency. Equation (7) 1s an extremely important result since it demonstrates that the resolving power of the random signal system depends purely on the bandwldth and not on the time duration of the transmitted signal as in pulse-echo systems. Furthermore, since every burst of transmitted noise is different, no range ambiguity results even if additional bursts are transmitted before the last echo of the previous burst has returned to the receiver. A random signal system can thus transmit noise almost contmuously so that the ratio of peak-to-average transmitted power can approach unity. It IS of interest to point out that the range resolution of the random signal system as given by equation (7) is the same as that of a pulse-echo transmitting a periodic signal having the same spectrum as that of the noise signal. For instance, if such a pulse-echo system transmits bursts of rf having the shape of Flg.6b, it can be seen that two targets will be resolved if their range differs by

System waveforms a - spectrum of the transmltted noise Flg.6 signal, b - correlator output voltage as the range cell is scanned through the target

trarily large by simply increasing the integration correlator.

time of the

As shown in equation (4), the signal-to-noise ratio enhancement is equal to the ratio of input to output bandwidths. For pulse-echo systems ths ratio was shown to be umty since input and output bandwidths are comparable. However, this is not the case for the random-signal system since the bandwidth at the receiver is determined by the transmitted bandwidth Bin while the output bandwidth Bout equals the reciprocal of the integration time T of the correlator which can be made arbitrarily long. Thus, the slgnalto-noise ratio improvement provided by the random signal system is given by the equation SNRout SNRin

Bin

--zBT

Bout

A more exact calculation for the clipped-signal system I8 actually used here predicts a correlation gain for small mput signal-to-noise ratios of

CAT AR-T-

(8)

where AT IS the time required for the rf burst to decline from its peak value to e-l of its maximum height. From Fig 6 clearly

Combining equations (8) and (9) gives the range resolution of the pulse-echo system as

AR -

which 1s seen to be identical to the resolution signal system, given by equation (7). ratio

of the random-

improvement

A ver:i important property of the random-signal flaw detection system IS that its sensitivity can be made almost arbi-

14

_ 2” BT

SNRin

71

(10)

where (Yis the mark-space ratio, B IS the bandwidth of a Gaussian noise transmitted, and T IS the integration time of the correlator. Smce this result was derived for a statlonary random transmitted signal, it must also be true for perlodlc transmitted signals, eg, short bursts of rf as used m conventional pulse-echo systems. Experimental results described below verify equation (10) and demonstrate that enhancement ratios of the order of thousands can be obtained. The clutter

$i

Signal-to-noise

SNRout

problem

and its solution

As explained earlier, if the ratio of peak to average transmitted power IS to be made small, the mark-space ratio (Y should be as close as possible to Its maximum value of unity If the transmitted signal simultaneously covers targets inside and outside the range cell, signals from both regions are received simultaneously. (The range cell IS that region of space from which the scattered slgnal will correlate with the delayed version of the transmitted signal). The echoes from targets outside the range cell are uncorrelated with

ULTRASONICS.

JANUARY 1975

those from inside the range cell, and therefore do not affect the mean value of the output signal corresponding to the target under observation. They do, however, increase the effective system input and output noise. These so called ‘clutter’ echoes can be minimized by shortening the transmitted signal until its length is no longer than the time of flight through the range cell, and lowering the transmitted burst repetition frequency to the value used in conventronal pulse-echo systems. However, this procedure completely negates the advantage of noise in lowering the peak-toaverage transmitted power ratio. There exists a technique,r5 illustrated in Fig.7, which avoids clutter even in the presence of continuous transmitted noise. Here focused transducers are positioned such that the beam pattern intersects the receiver beam pattern only m that regron in which the range cell is located. Any echoes from targets outside this small region approach the receiving transducer at angles at which it is extremely Insensitive. Therefore, the only echoes that can be recognized by the receiver are those from targets within the region where the transmitter and receiver beam patterns overlap. Clutter type signals can be avoided completely by making the overlap region comparable to the size of the target. This technique for clutter avoidance circumvents the only apparent disadvantage of the large mark-space ratio which can be used in random signal flaw detection systems. This should be particularly useful when examining materials with many grain boundaries, where clutter problems would normally be severe.

The correlator output waveform

The correlator output as displayed on the pen recorder for a given distribution of sound reflecting inhomogeneities may be calculated as follows. Let the acoustic reflection coefficient be a(~) for targets situated so that the time of flight to and from the transducers is T. The range R of a target for which the time of flight is r is

Ignoring absorption effects, the echo received from a transmitted signal x(t) which reaches the correlator will be

a(~) x(t - T) dr

YW = -cc

Since the reference channel input to the correlator is x(t - or) where rr corresponds to the delay line setting, the time averaged output of an analogue correlator will be

Z(Tr) =

%U’:n”’ region

Tronsmlt;er

Fig.7

Clutter

Receiver

avoidance

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system

JANUARY

1975

(11) _m

An acoustic reflector whose dimensions are large compared to the transmitted wavelengths may be treated as a reflecting surface for which a(r) becomes a delta function. Therefore, for a single reflecting surface with time of flight ra, equation (11) becomes

&(T- ~0) R~(T - 73) d7

= Rx(~a- Tr)

are assumed adlacent.

1

where E [] stands for expectation value. Since neither t nor rr are variables of integration, the expectation can be brought inside the integral giving the time average of the correlator output as

Z(Tr) =

rf the transducers

a(~) x(t - T) dr

z(Tr) = E

(12)

We see, therefore, that each reflecting surface will produce the auto-correlation function of the transmitted spectrum at the pen recorder output, wrth the central peak of the spectrum corresponding to the location of the reflecting surface. This result applies to both random and periodic transmitted waveforms. Examination of the form of the system output in Figs 3 and 4 shows that the experimental outputs correspond to the auto-correlation function of a long burst of rf in one case, and of Gaussian noise with a bell shaped spectrum in the other. To produce a system output which most closely resembles the inhomogeneity profiles, the correlator output could be subjected to some form of deconvolution process, either using a digital computer, as has been done by Seydell,13 or by some form of electrical network. However, this procedure tends to enhance any noise mixed m with the original echo, and therefore is only useful in the rather rare cases where the input signal-to-noise ratro is high.

15

Experimental

results

Range resolution

The system described above has been operated successfully to detect artificial flaws consrstmg of wires in water, both when transmitting ultrasound m bursts of 4.8 MHz sine waves (subsequently referred to as rf) or bursts of 2 MHz bandwidth random signals with a 4.8 MHz centre frequency (subsequently referred to as noise). In the experrments described here, the transmitting and receiving transducers of the system were arranged almost parallel so that their beams overlapped as shown in the shaded region in Frg.2. By moving one of the transducers in the water delay lme with a micrometer, the internal delay of the reference signal was changed, thus scanning the system across a series of targets. (For a clutter avoidance system in which the transducer beam patterns overlap at a small region only, they will have to be moved physically at the same time as the delay lure length 1s varied. Electronic techniques for moving the beam pattern intersectron point might also be possible). In the experiments described here, both the wire targets and the transducers were immersed m a water bath. The outputs of the system were recorded by a pen recorder which produces the type of display shown in Frg.3. Here the x-axis corresponds to the spatial coordinate and the y-axis to the strength of the echo from various targets. Fig.3 corresponds to the observation of a 1 mm copper wire; the upper half of the figure shows the output when a 4 ,us burst of 4.8 MHz rf is transmitted and the lower half shows the output for 2 MHz bandwidth noise of the same burst length. Notice that the rf signal can detect the presence of the wire but cannot resolve the front and back edges. The noise signal which has the same duration as the rf signal but larger bandwidth, is easily able to resolve these edges. However, when a 1 /.LSpulse is used, as shown m Frg 4, the rf 1s also able to resolve the edges but at a cost of higher peak-to-average power. Inspection of Fig.3 shows that thrs particular system, using noise, could have resolved targets wrth a separatton as low as 250 I_tm. Erg.8 shows the correlator output for a 1 ml1 (25.4 ,um) wire. It 1s clear from this plot that because of its great sensitivity the system is able to detect targets far smaller than rts resolution limit. It should be noted that the output pattern produced by the system when using noise agrees very closely with that pre-

drcted theoretically and illustrated m Frg.6. As predicted, it 1s clear from Figs 3 and 4 that for noise the resolution does not depend on signal duration but only on bandwidth. It is also clear from these figures that the range resolution of noise 1s much higher than that of pulsed rf of equal duration but lower bandwidth. It should be noted, however, that the improvement of the signal-to-noise ratio produced by our correlatron type system is evident not only when transmitting noise but also when transmitting pulsed rf. Signal-to-noise

enhancement

measurements

The correlatron gam of a system using clipped signals and a polarity coincidence correlator with input signal-to-noise ratios much less than unity was grven in equation (10) as SNRour _ 2” BT SNRln

71

For our low-pass filter pen recorder combmation with a measured time constant of T = 120 ms and transmitted bandwrdth of 2 MHz, equation (10) predicts a signal-tonoise ratio enhancement of approximately 8 x lo3 with the mark-space ratio of l/20 that was used. The measured signal-to-noise ratio enhancement agreed with the predicted value to well within the experimental uncertainty. It is believed that the signal-to-noue ratio enhancement quoted above could be increased by an order of magnitude simply by mcreasmg the mark-space ratio to l/2 from l/20. This was not experrmentally verified since it was already difficult to measure the enhancement ratio of 8 x 103. Further increase m enhancement would have been extremely drfficult to measure and probably inaccurate because of leakage effects at high attentuations. Fig.9 shows the correlator output for a 3 mrl(76.2 pm) gold wire target with superimposed outputs correspondmg to the transmitted signal attenuation of 20 dB over the previous larger output. It 1s clear that even after 40 dB attenuation of the input signal, the output signal-to-noise ratio is still greater than unity. It should also be noted that the received signal at the output of the amplifier was completely buried in thermal receiver noise after only 20 dB attenuation was imposed on the transmitted signal. These results clearly indicate that the random signal system can recover signals that are deeply burled m noise. Conclusion The results of this research have shown that a simple correlation type ultrasonic receiver can detect echoes which have at least 8 000 times less power than the thermal receiver noise without any noticeable problems due to mechanical vibration or electronic instabilities. Several further orders of magnitude improvement m sensitivity may be possible by simply lengthening the system mtegratron time above the 0.1 s used m this work and using a larger mark-space ratro for the transmitted signal.

Flg.8

16

Detection

of 1 md (25 4 pm) Au wire in water at 15 cm

Following the work orrgmally done for radar, rt has been established that when using noise as the transmitted signal, no range ambiguities exist, so that a signal can be transmitted almost continuously This lessens the peak-toaverage transmitted power to near unity, thus lessening the nsk of transducer electrical breakdown. It has also been shown that prezoelectrrc transducers transmitting ULTRASONICS.

JANUARY

1975

noise can produce the same range resolution as when transmitting periodic bursts of rf having the same spectral density. Furthermore, a clutter avoidance technique which has not been used m radar has been described which should make it possible to transmit noise almost continuously wrthout deterioration of the system response due to clutter type echoes from flaws situated outside the system range cell. It has also been established that the system can easily detect the presence of 25 pm wire flaws which are far smaller than its present lower resolution limit of 2.50 pm. When operating at hrgh resolutron, the system described can trade speed for sensitivity by varymg the integration time of the correlator output. When examining objects which are expected to contain a very small number of possible flaws, increased speed can be obtained without sacrificing sensitivity, by narrowmg the transmitted spectrum, thus enlarging the range cell. The fact that the system has much greater sensitrvity and uses much lower peak-to-average transmitted power than conventional systems should enable it to greatly extend the size of strongly absorbing objects that can be examined by ultrasound, or to use higher frequencies which are too strongly absorbed to be practical at present, and thus obtain greatly improved resolutron. The ability of a random signal correlatron system to provrde high sensitivity for a given transmitted power also makes it possible to reduce the peak and average power required for a given sensitivity. This feature and the absence of range ambrguity may make such systems of interest for organ scanning, particularly in the case of the brain, where the skull produces undesirably large absorption and reverberation. Other advantageous apphcatrons of random signal correlatron systems to medicine may exist m the case of blood-flow measurement. Acknowledgements

3

4

Krautkramer, J., Krautkramer, H. Ultrasonic Testmg of Materials, Springer, New York (1969) Kennedy, .I. C., Woodmansee, W. E. Signal processmg in non-destructrve testing, Boeing Pubhcatlon SAOPI-FOI RB2 (25 Apnl 1973) Seydell, J. A. Improved dtscontinurty detection m ceramic materral usmg computer-alded ultrasonic non-destructive techniques, Symp Proc 2nd Army Mat Tech Conference (November 197 3) Grant, M. P., Cooper, G. R., Kamal, A. K. A class of notse radar systems, ProcZEEE (July 1963) 1060

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12 13 14

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References

2

corresponds to the transmitted slgnal attenuation of 20 dB over the next largest output. Input slgnal-to-noise ratlo 1s approximately umty for the largest output

11

The authors are happy to acknowledge J. A. Seydell and W. E. Woodmansee for makmg their research results accessible to us before publication, P. J. Bendrck for many useful discussions and G. T. Mitchell for his help in the construction of circuits. This work is supported by ARPA no DA1573 Cl2 and the Purdue NSF-MRL programme.

1

Demonstratton of the large correlatron gatn of the random Flg.9 Each output signal system using 3 mil (76 2 pm) wtre as the target

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Srebert, W. M. A radars detectron philosophy, IRE Tram IT-2 (September 1966) 204 Carpentier, M. Radars, Concepts Nouveaux, Dunod, Pans (1966) Woodward, P. M. Radar ambrgutty analysrs, RRE technical note no 731 (February 1967) Berkowitz, R. S. Modern Radar, John Wrley (1965) McGillem, C. D., Cooper, G. R., Waltman, W.B. An experrmental random signal radar, Proc Nat Electr Conf 23 (October 1967) Poirier, J. L. Quasr-monochromatic scattering dnd some posstble radar apphcatlons, Radro Scwnce 3 (September 1968) 881 Craig, S. E., Fishbern, W., Rittenbach, 0. E. Contmuouswave radar wrth htgh range resolution and unambiguous velocrty determinatron,IRE Tram MIL-6 (April 1962) 153 Carpentier, M. H. Usmg random functrons rn radar apphcattons, De Zn,wmeur 46 (13 November 1970) E 166 Smit, J. A., KneefeJ, W. B. S. M. Notse radar experrment, Chrzst Huggenslab Rep (January 1965) Waag, R. C., Myklebust, J. B., Rhoads, W. L., Gramiak, R. lnstrumentatron for non-mvdstve cardtac chamber flow rate measurement, IEEE Ultrasonic Symp Proc (1972) 74 Newhouse, V. L., Cooper, G. R., Fergenbaum, H., Jethwa, C. P., Kaveh, M. Ultrasomc blood velocity measurement usmg random signal correlatton techmques, Dtgest of the ICMBE, Dresden, GDR Jethwa, C. P., Saggio HI, F. Pulsed random stgndl ultrasomc Doppler system and high-resolutton transducer m blood flow studtes, IEEE Ultrasomcs Symp Proc (1973) 199 Bendick, P. J., Newhouse, V. L. An ultrasonic random slgnal flow measurement system, Journal of the Acoustleal Socrety of America (August 1974) Newhouse, V. L., Furgason, E. S., Bdgutay, N. M. Random signal flaw detection, Proceedmgs of the 1974 IEEE Ultrasomcs Symposium

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