- Email: [email protected]
Ultrasonic reflex transmission imaging
ULTRASONIC
IMAGING
7,
201-214
(1985)
ULTRASONIC REFLEX TRANSMISSION Philip
S. Green
Bioengineering SRI International,
and Marcel
IMAGING
Arditil
Research Laboratory Menlo Park, CA 94025
Reflex Transmission Imaging (RTI) is a new imaging method by which orthographic transmission images can be made using augmented B-mode Conventional transmission imaging requires acoustic coupling equipment. to large areas on both sides of the body, whereas RTI can be performed from one side with a single, small transducer probe. In this mode, transmission images in a plane normal to the beam are made by integrating the reverberations from beyond the focal zone of the transducer. These reverberations provide, in essence, a source of incoherent insonification from behind the focal plane. Preliminary in-vitro images have been made using a computer-~interfaced rectilinear scanner with a l-inch diameter f/2.8 transducer. The images have good resolution and signal-to-noise ratio, and a short Backscatterer inhomogeneity is well smoothed. depth-of-field. Transmission images provide information that is complementary to B-scans. RTI will allow both to be made with the same instrument and A time-gated reflection C-scan could be presented on the same display. generated simultaneously. Other RTI modes, including an attenuation Ba 1985 Academic Press, Inc. mode, also are discussed. Key words:
Attenuation; backscatter; C-scan; reflex; transmission; ultrasonic; I.
imaging; multimodal; ultrasound.
INTRODUCTION
Although orthographic ultrasonic transmission imaging has a long history of developent, its clinical potential has only recently begun to be realized [l-5], while B-scan has found extensive clinical application. One reason for transmission imaging's slow entry is that it requires acoustic coupling to large areas on both sides of the body. This has restricted its potential use to those diagnostic applications in which water immersion is practical. Moreover, the masking effect of ultrasonic opacities (e.g. bone, gas in the lung and gut) further restricts the role of transmission imaging. In contrast, B-scan imaging is usually performed with a transducer of small dimensions, which allows flexible application and good access to most body organs. Reflex transmission imaging combines the characteristics of both methods, permitting transmission images to be made with a single Bscan-size transducer probe. RTI is totally compatible with B-mode equipment, and affords the opportunity to generate orthographic transmission images in a selected plane that can be directly correlated with the B-scan on the same display screen. The transmission image provides information complementary to that of the B-scan. Its orientation IPresent
address:
Battelle
Memorial
Institute,
201
Geneva,
Switzerland.
0161-7346/85 $3.00 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
GREEN AND ARDITI
relative to the body is similar to that operative parameter is attenuation (rather displayed by the B-scan). The increased characterization has centered about the tifying to the perceived importance this tic ultrasound.
of a conventional X-ray; its than reflectivity, as interest in ultrasound tissue measurement of attenuation, tesparameter will have in diagnos-
A reflection C-scan can be made simultaneously with expanding the potential diagnostic capability. Moreover, cess can also be used to form attenuation B-mode images.
RTI, further the RTI pro-
Transmission imaging studies at SRI [6-91 and elsewhere [lo] have shown that this mode can provide effective demonstration of vasculature, soft tissues of the musculoskeletal system, normal and pathologic breast tissues, and internal organs. RTI should be effective in these applications as well, even more so for internal organs, owing to factors discussed below. Because RTI can be performed in conjunction with routine B-scanning, it should find its role quickly once such bimodal equipment is available in the clinic. II.
PRINCIPLES
OF REFLEX TRANSMISSION
IMAGING
The development of orthographic transmission imaging over the past 15 years has proceeded along several paths. Many lens-focused concepts have been advanced, and numerous image-plane scan-conversion methods to detect the focused image have been demonstrated. Holography was the subject of intense investigation during the early 197Os, but failed to realize its anticipated potential for a variety of reasons. One of these--the deleterious effects of coherent interference--also plagued early lens-focused systems, producing diffraction artifacts from outof-focus structures. This phenomenon lead to the development of spatially and temporally incoherent methods of insonification [ll], which eliminated diffraction artifacts and provided smooth blurring of out-of-focus structures while maintaining sharp focus within the focal plane. Reflex transmission imaging also requires incoherence, which it derives from the random reflectivity of the tissues themselves, as described below. The reflex transmission imaging process is initiated in the same A brief ultrasonic pulse is launched into the manner as is a B-scan: body from a well-focused transducer. In B-scanning, reverberations from within the body are amplified, detected, and displayed as a function of time-of-arrival, which corresponds to depth; mechanical or electronic scanning provides the image's other dimension. However, the RTI signal processing is significantly different. With reference to figure 1, wave reflection signals emanating from within the region between the transducer and the distal end of the focal zone are gated off. Reflections from a selected range zone beyond the focal region are amplified with time-gain control, detected, and integrated. The value of this integral is strongly dependent on the attenuation in the focal zone. (Although it is also dependent on the attenuation in the entire focal cone and on the reflectivity of the tissues within the selected integration zone, the latter factors need not inhibit the formation of an image of high resolution and diagnostic significance.) The derived integral value is stored as one pixel of the RTI image. is scanned in To produce a complete orthographic image, the focal point using either a rectilinear scan, a sector a two-dimensional pattern, scan, or one of several combination scanning patterns, as depicted in
202
REFLEX TRANSMISSION IMAGING
Fig.
1
Geometry of reflex transmission imaging (RTI). Reflections from a zone beyond the transducer's focus serve as an insonification source for transmission imaging. The reflex insonfication source can be envisioned as a set of concentric semispherical laminae, one-half of the pulselength in thickness. Each one contributes a reverberation that is of Rayleighdistributed amplitude, and uncorrelated with the amplitudes from the other laminae.
figure 2. A variety of combinations of mechanical, linear-array, and phased-array scanning can be used. The resulting focal plane will be flat in the rectilinear case, but cylindrical or spherical in the others, unless dynamic control of the focal range and range-gate onset are used. Only with the rectilinear scan does the area imaged remain constant in both size and aspect ratio at all depths. A two-dimensional sector scan probe could closely resemble the B-scan probes now in use, as illustrated in figure 3, which also depicts the relationship between B-scan and RTI planes. By reference to the previous discussion of transmission imaging, we can consider the waves reverberated from beyond the focal zone to be, in effect, a source of incoherent insonification for transmission imaging of the focal plane. Because the transmitted pulse passes through the focal zone, the reverberations are already proportional to the attenuation at the focus. Owing again to the transducer's focal properties, only the reverberated waves returning through the focal zone are received with high sensitivity. Thus, focusing is provided both on transmission and reception, doubling the sensitivity to focal-zone attenuation as compared with conventional transmission imaging. Because focusing is invoked twice, the resolution also is superior to that of a conventional transmission image made using the same numerical aperture. The deleterious effects of tissues on either side of the focal plane can be minimized by increasing the numerical aperture. At f/numbers in the range of two to three, there seems to be (at least in the preliminary studies) adequate averaging of the attenuation and scatter properties of the out-of-focus tissues. Generally, the artifact these tissues contribute is of low spatial frequency. RTI does require the presence of a reasonably uniform body of tissue beyond the focal
203
GREEN AND ARDITI
FOCUSED
(a)
/OR
LINEAR
FOCUSED TRANSDUCER PHASED ARRAY
(c)
Fig.
Fig.
SECTOR
2
TRANSDUCER
FOCUSED TRANSDUCER LINEAR ARRAY, OR PHASED ARRAY
- LINEAR
(b)
LINEAR
\ -SECTOR
_
- SECTOR
(d)
SPIRAL
(e)
STAR
C-scanning patterns suitable for RTI.Patterns (c), (d),and (e) are compatible with ConventionalB-scan sector probes. Rectilinear pattern (a) provides the best near-field viewing area. The image planes formed by sector scanners would be flat,as shown,only with dynamic control of focus and range gate.
3
Relationship duced with probe.
of RTI and B-scan image planes that would be proa two-dimensional, dynamically focused sector-scan
204
REFLEX TRANSMISSION IMAGING
TRANSMIT/RECEIVE TRANSDUCER RASTER OR SECTOR SCANNED)
FOCAL
ZONE
PLANE
L Fig.
4
plane to degraded bone or hand and water-filled
VOLUME FROM WHICH REVERBERATIONS ARE INTEGRATED
Depiction of reflex insonification. The reflex insonification source can be envisioned as a set of concentric semispherical laminae, one-half of the pulse length in thickness. Each one contributes a reverberation that is of Rayleigh-distributed amplitude, and uncorrelated with the amplitudes from the other laminae. serve as a source if this tissue is gas. Transmission testicle could be sponge) behind
of reverberation. The image quality will be substantially inhomogeneous or abbreviated by images of appendicular structures such as the made using a scattering material (e.g. a the structure.
Like that of the B-scan, the noise in an RT image has both additive and multiplicative components. In RTI, additive noise (usually from the preamplifier) is suppressed by the signal-integration process; the longer the useful integration interval, the better the signal-to-noise ratio. Time/gain control can be used within the integration interval to optimize this ratio. In the B-scan, multiplicative noise of high modulation index is always present. Referred to as "speckle," it arises from the interference of pulse reflections from adjacent scatterers, and has become accepted as an integral part of the image. This pulse-interference phenomenon also produces noise in RTI, but of a substantially lower modulation index. With reference to figure 4, we can consider the reflex insonification source to be composed of concentric scattererfilled "isochronous" laminae [12,13] or shells of roughly spherical shape, lying beyond the focal zone. If the shell thickness is taken to be about half the pulse length, we can consider each shell to be a single composite source with a Rayleigh-distributed random amplitude, uncorrelated with the amplitudes of reverberation from all other shells. To estimate the signal-to-noise ratio, we first recall that the variance of a Rayleigh-distributed variable is approximately one-quarter of the square of its mean. Because the integration process sums the contributions of independent sources, we can apply the central limit theorem to arrive at a normally distributed reflex insonification signal of mean nA and variance nA2/4, where n is the number of independent shells and A is the average backscatter amplitude from each shell
205
GREEN AND ARDITI
(presumed is given integration noise is
to be set equal by time/gain control.) The number of shells by n = tR/tP, where tp is the pulse duration and tR the total time. Thus, the signal-to-noise ratio for multiplicative S/N = 10 log(4tR/tp)
For example, if the pulse the integration zone 25 ~6 sue), the signal-to-noise results generally support
(dB)
.
duration tp were 0.5 ps and the duration (equivalent to just under 2 cm of soft ratio would be 23 dB. The experimental this analysis.
tR of tis-
For low spatial frequencies, the contrast sensitivity (smallest detectable variation) of RTI will be limited by out-of-focus images of other planes. At the upper limit of resolution (one pixel), multiplicative noise will limit contrast sensitivity. In the example cited above, the 23-dB signal-to-noise ratio is equivalent of an amplitude contrast sensitivity of -20 log (1 - 10-23/20) x 0.6 dB. For example, if the length of the focal zone is 0.5 cm (traversed twice) and the frequency is 5 MHz, then the sensitivity in terms of the salient tissue attenuation parameter (dB/cm-MHz) would be 0.12. The range of this parameter for soft tissues is from 0.2 for blood up to 3.3 for muscle (transverse to the fiber direction). Thus, RTI should be able to detect variations in focal-plane attenuation at the highest spatial frequency of about 3.8 percent of the expected range of values, in dB. Although the signal-to-noise schemes can be devised to improve beneficial. Also, the scattering or arcuate segments--each of these by using segmented transducers and measures would reduce either scan would be an unwarranted complication.
ratio is probably adequate, various it. Analytic-signal processing may be laminae could be divided into annular would be statistically independent-parallel signal processing. Such speed or resolution, and probably
Because the RT image requires a two-dimensional raster scan, it would typically take 3 to 15 seconds to produce. We envision that the ultrasonographer first would perform a B-scan on the patient, using the RTI probe scanning in one direction only. With the aid of a screen cursor, a plane of interest would be selected in the B-scan; it need not be at constant depth if the probe can be focused dynamically. The instrument would then be switched to the RTI mode and the RT image acquired and displayed--alongside the B-scan, if desired. A reflection C-scan of the focal plane could be generated simultaneously and presented along with the B-scan and RTI image. III.
EXPERIMENTAL
APPARATUS and METHODS
The primary experimental facilities used in the preliminary studies were SRI's mechanical raster scanner (the Slow Scanner) and a PDP-11/40 computer system. The latter is equipped with a mini-MAP array processor (CSPI model MM-III) and a DeAnza color image display. SRI's Slow Scanner, which recently has undergone considerable modification for use in transmission imaging experiments2 is shown schematically in figure 5. the
The interface takes advantage of two devices already available on PDP-11: a 16-bit parallel input/output interface (DR-11K) and a
2NIH Grant
GM30890
206
REFLEX
TRANSMISSION
IMAGING
COMPUTER ROOM * ::.::.::::.::,.:::..:::,,“~,.-~,.~r::-:::: ‘“““‘y’.‘.: ......,..,.. :.... . .:.:.:; .. ...:.; . . . :~.~~~:~~::i:~::::~~~::~~:~ w
w
I
150 FEET !,, * _.
m:*
I
.,.s.‘,9s
ULTRASOUND LABORATORY VIDEO MONITOR
X-AXIS ENCDDER
-
SELECT LOGIC
ENCODER PROCESSING LOGIC
CONTROL LOGIC
-
Reset
1
TRANSMISSION IMAGING SUBUNIT
PVDF
Pig.
5
Block
diagram
of slow
MEI\ REC
TRANSMITTER
scanner
and its
computer
interface.
multichannel lo-bit A/D converter interface (AR-11). However, because these two computer interfaces were not entirely suited for direct control of the the slow scanner's digital and analog functions, interface electronics were designed and installed at the scanner, more than 150 feet from the computer. digital
When operated in the RTI mode, the interface functions: l Scanner X and Y motor control .
X-axis Pulse
encoder generator
signal timing
l
Analog
receiver
logic
l
Interaction
l
with
provides
processing control control
the PDP-11's
207
parallel
interface.
the
following
GREEN AND ARDITI
TRANSDUCER
BACKSCATTER $@y/
FOCAL
Fig.
6
Arrangement preliminary
When operated l l
l
of object, experiments.
in the RTI mode,
excitation. range-gating, of the received
BACKSCATTER TISSUE
PLANE
backscatterer,
the analog
RANGE ,
and transducer
electronics
in
the
provide:
Transducer Amplification, integration
pulse
Generation with gains bits.
of three-channel outputs to the lo-bit selected to provide an overall dynamic
time/gain signal.
ING
control,
detection,
and
A/D converter, range of 16
The transducer used in these experiments was a Panametrics S-MHz, f/2.5, heavily backed ceramic, focused at 71 mm. To produce RT images, the transducer was positioned so that its focus was at the selected plane within the object, as depicted in figure 6. The range gate was positioned to lie within the backscatter material behind the object, and the time/gain control was adjusted to give essentially equal amplitude returns from all ranges within the gate (distal gain was reduced someFigure 7 what if the additive noise was significant at that range). shows a typical ensemble of the electrical signals at various stages in the RTI signal-processing cycle.
TIME/GAIN SIGNAL
CONTROL
I1-GCl
RECEIVED SIC iN AL. AFTER TGC INTEGRATOR OUTPUT
-FOCAL PLANE REFLECTIONS
\ TRANSMITTED Fig.
7
Ensemble
PULSE
of electrical
signals
208
produced
during
RTI.
REFLEX TRANSMISSION
IV.
IMAGING
PRELIMINARY EXPERIMENTS
Several simple objects were used in the first imaging experiments. To provide adequate control over the process to facilitate evaluation of the results, we first used as a backscatter material a uniform gel phantom with scatter, attenuation, and velocity similar to that of tissue. The first object imaged was a 2.5-cm-long, thin plastic paper clip 8(a), has a suspended on a rubber band. The image, shown in figure field-of-view about 3.3 cm square. The resolution appears to be about 0.5 mm. The multiplicative noise in the background is well depicted as a fine, granular pattern, a result of the lack of correlation between the reflex insonification at adjacent pixels. Its amplitude is about that predicted. Because the attenuation of the thin plastic clip is rather low, the image contrast was electronically increased, emphasizing the background noise. If the range gate is made very narrow and moved into the focal plane, a conventional reflection C-scan is produced, as shown in figure 8(b). Figure 8(c) is the RT image of part of a plastic transistor radio case, using the same backscatter material as for figure 8(a), without increased contrast. The second series of experiments used biological material for both object and backscatterer. The object was a lamb kidney; the backscatterer was a contiguous beef kidney. Roth were suspended in a saline-filled plastic envelope, and care was taken to remove any trapped air. The objectives of this experiment were to determine whether biological structures could be imaged well in this mode, and what effect a complex backscatterer would have on the image. Figure 9 shows a series of 3-mm-spaced planes imaged within the lamb kidney, using a 35-mm-thick
Fig.
8
Reflex transmission (and reflectionc-scan) images of simpleobjects, with a gel phantom backscatter. (a) RTI of a 1" long, thin plastic paper clip; contrast increased electronically to show multiplicative (b) Reflection C-scan of same object. (c) RTI of plastic noise. transistor-radio case.
209
GREEN AND ARDITI
Fig.
9
Reflex tiguous
transmission beef kidney
images of a lamb as a backscatterer.
210
kidney (in-vitro) Image planes
are
with 3mm
a conapart.
REFLEX TRANSMISSION
backscatter zone within the beef Several conclusions can be drawn
IMAGING
kidney. The field-of-view immediately on inspecting
was 6.6 cm. these images:
s
Anatomical structures (especially vessels) in the lamb kidney are well resolved. Whether the vessels contained coagulated blood or autolysis gas was not known. Care was taken to not entrap air.
0
The depth of focus are differentiable. of the transducer's
s
The multiplicative moderate contrast larger field-of-view.
l
The nonuniformity of the backscatter from the inhomogeneous beef kidney is evident only as low-amplitude, low-spatial-frequency modulation of the image; it does not seriously diminish image quality.
is
shallow; planes This is a result focal properties.
noise setting,
is almost a longer
a few millimeters apart of the double application
undetectable, integration
The results of these tests suggest that reflex transmission have significant diagnostic potential, especially because formed as a convenient adjunct to B-mode imaging. V.
SIMULTANEOUS MULTIPLANE
owing to a more zone, and a
it
imaging may can be per-
RTI
The multiplane image set of figure 9 was produced by moving the transducer and range gate after each image was made, then rescanning. In clinical use, this sequential collection of the multiplane image would take about one minute. A multipulse scheme, illustrated in figure 10, has been devised to collect several image planes at once. This on the fact that very short method requires dynamic focus, and relies A series of brief pulses, while beneficial, are not essential to RTI. bursts of ultrasound of different center frequencies and substantially nonoverlapping spectra would be transmitted. If each one is of lower center frequency than the previous one, their reverberations will be associated with increasing range and thus greater high-frequency attenuation. During transmit, the dynamically focused transducer would be focused for each pulse to the desired depth of image to be formed for Similarly, on reception, either the array focus will be that pulse. switched for each pulse or (if the several integration zones overlapped) parallel sets of delay lines will be required, as shown in figure 10(b). In all cases, filters that respond only to the reverberations associated with the individual planes will be required. VI.
THE COMPOUND RTI B-SCAN
the reflex transmission concept also can be applied In principle, to produce attenuation images in the B-scan plane. This would be accomplished by displaying, as a function of range, the integrated RT signal from a range zone just beyond each point in range. This will require both dynamic focus and a continuous time-shifting of the zone of integration (a process that could be easily implemented in the digital domain). However, although such an image will have good lateral resolution, its range resolution will be poor compared to the conventional reflection B-scan, because it will be determined by the length of the transducer's focal zone, rather than by the pulse length. This calls to mind the early reflection B-scans, where the use of unfocused transducers resulted in good range resolution but poor lateral resolution.
211
GREEN AND ARDITI
TRANSMIT PULSES, AT DlFFfRfNT
FOCUSED RANGES
1 RECEIVER
INTEGRATION
fi ”
/ t I I , I 1
I, ,I FOCAL
I I I
I I
GATES
2
PLANES
I
I I I
I I n
2’
a
FOCAL ANNULAR ARRAY
ZONE
FOCAL
FOCAL
ZONE
n
RING
1
RING
m
1 ZONE
2
/
FILTERS-
b Fi .g.lO
SEPARATELY GATE0 RECEIVER CHANNELS
-
Method for simultaneous acquisition of multiplane RTI. (a) Transmit pulse sequence and multiple integration range gates associated with several focal planes. (b) Parallelprocessing receiver; each channel focuses the annular array at a separate range and filters the received signals to respond only to the reverberations of the pulse focused at that depth.
The compound scan was devised to overcome this limitation (and the problems of specularity, but that is beside the point). Similarly, RTI B-scans could be compounded and averaged or, more rigorously, backprojected and spatially filtered to provide good spatial resolution in all directions in the plane. RTI B-scans would bear a similarity to ultrasound computed-tomography attenuation images. They would not have the inherent quantitative accuracy, but would be more easily produced using modified B-mode equipment.
212
REFLEX
TRANSMISSION
VII.
IMAGING
DISCUSSION
Reflex transmission imaging provides the viewing perspective and the sensitivity to attenuation of orthographic transmission imaging and Transmission images provide does so using modified B-mode equipment. information that is complementary to B-scans. RTI will allow both to be made with the same instrument and presented on the same display. The range of clinical applications for RTI remains to be determined. In general, it should provide a more assured diagnosis than Bscan alone, especially because the images can be precisely correlated with electronic screen cursors. For the breast, RTI should be very complementary to B-scan, especially because transmission imaging is more effective than B-scan for visualizing tumors surrounded by fatty tissue. The potential application to monitoring kidney-stone lithotripsy is apparent; gall stones also should be well demonstrated. Peripheralanatomy and intraoperative imaging are also likely applications. As yet, little effort has gone into optimizing RTI performance. Selection of the best numerical aperture and range-gate position remain to be explored, and dynamic control of the focus and range-gate must be implemented in a clinically testable instrument.
REFERENCES
[II
Woltering, H., Matthias, Ultraschalltransmission,
H.H., Green, P.S., _Munch. -med. --Wschr.
and Klein, D., 126, 1431-1434
(1984).
[21 Matthias,
H.H., Woltering, H., and Guth, V., Die Ultraschalltransmissionskamera bei der diagnostlk von instabilen dysplastischen und dislozierten hueftgelenken bei neugeborenen kindern im 1. lebensjahr, Klinische Paediatrie 196 (1984).
t31
Hentz, V.R., Marich, K.W., and Dev, P., Preliminary upper limb with the use of ultrasound transmission Hand Surgery 9A, 188-193 (1984). -____-
[41
Woltering, Taenzer,
151
"Bilder (1984).
161 Green, logies n-87
H., J.,
Matthias, unpublished
aus der badewanne
H.H., work.
Green,
[Pictures
P.S., from
the
und
study of the imaging, 2.
Edmonds, bathtub],”
P.,
and Stern
P.S., Ultrasonic Transmission Imaging, in Emerging Technoin Surgery, L. Angelini, G. Fegiz, and P.N.T. Wells, eds., (Masson Italia Editori, Milano, 1984).
t71
Green, P.S., Orthographic Transmission Imaging, Diagnostic Medicine, P.N.T. Wells and M. Ziskin, (Churchill Livingstone, New York, 1980).
[81
Green, P.S., Schaefer, L.F., Jones, E.D., and Suarez, J-R., A New High-Performance Ultrasonic Camera, in Acoustical Holography, Vol. 5, P.S. Green, ed., pp. 493-504 (Plenum Press, New York, 1973).
[91 Marich,
in Clinics edE38-143
in
K.W., Zatz, L.M., Green, P.S., Suarez, J.R., and Macovski, A Real-time imaging with a new ultrasonic camera: part I, J. &&I. Ultrasound 3, 5-16 (1975).
213
GREEN AND ARDITI
[lo]
Scherg C., Brettel, H., Roeder, U., and Waidelich, W., Transmission Imaging with Incoherent Ultrasound, in _____ Optics and Biomedical Sciences, Vol. 31, G. von Bally and P. Greguss, eds. (SpringerVerlag, 1982).
[ll]
Havlice, J.F., val of Spurious porally Varying White and R.E. 1977).
[12]
Foster, D.R., Arditi, M., J.., Computer simulations Imaging 5, 308-330 (1983). ____-
[13]
Fink, M.A., measurement,
Green, P.S., Taenzer, J.C., and Mullen, W.F., RemoDetail in Acoustic Images Using Spatially and TemInsonification, in Ultrasound in Medicine 2, D.N. Brown, eds., pp. 1827-1828 (Ple&m Press, New York,
and Cardoso, IEEE Trans. __--
Foster, F.S., Patterson, M.S., and Hunt, of speckle in B-scan images, Ultrasonic Diffraction J.F., Sonics Ultrasonics -
214
effects --SU-31,
in pulse-echo 313-329 (1984).
IMAGING
7,
201-214
(1985)
ULTRASONIC REFLEX TRANSMISSION Philip
S. Green
Bioengineering SRI International,
and Marcel
IMAGING
Arditil
Research Laboratory Menlo Park, CA 94025
Reflex Transmission Imaging (RTI) is a new imaging method by which orthographic transmission images can be made using augmented B-mode Conventional transmission imaging requires acoustic coupling equipment. to large areas on both sides of the body, whereas RTI can be performed from one side with a single, small transducer probe. In this mode, transmission images in a plane normal to the beam are made by integrating the reverberations from beyond the focal zone of the transducer. These reverberations provide, in essence, a source of incoherent insonification from behind the focal plane. Preliminary in-vitro images have been made using a computer-~interfaced rectilinear scanner with a l-inch diameter f/2.8 transducer. The images have good resolution and signal-to-noise ratio, and a short Backscatterer inhomogeneity is well smoothed. depth-of-field. Transmission images provide information that is complementary to B-scans. RTI will allow both to be made with the same instrument and A time-gated reflection C-scan could be presented on the same display. generated simultaneously. Other RTI modes, including an attenuation Ba 1985 Academic Press, Inc. mode, also are discussed. Key words:
Attenuation; backscatter; C-scan; reflex; transmission; ultrasonic; I.
imaging; multimodal; ultrasound.
INTRODUCTION
Although orthographic ultrasonic transmission imaging has a long history of developent, its clinical potential has only recently begun to be realized [l-5], while B-scan has found extensive clinical application. One reason for transmission imaging's slow entry is that it requires acoustic coupling to large areas on both sides of the body. This has restricted its potential use to those diagnostic applications in which water immersion is practical. Moreover, the masking effect of ultrasonic opacities (e.g. bone, gas in the lung and gut) further restricts the role of transmission imaging. In contrast, B-scan imaging is usually performed with a transducer of small dimensions, which allows flexible application and good access to most body organs. Reflex transmission imaging combines the characteristics of both methods, permitting transmission images to be made with a single Bscan-size transducer probe. RTI is totally compatible with B-mode equipment, and affords the opportunity to generate orthographic transmission images in a selected plane that can be directly correlated with the B-scan on the same display screen. The transmission image provides information complementary to that of the B-scan. Its orientation IPresent
address:
Battelle
Memorial
Institute,
201
Geneva,
Switzerland.
0161-7346/85 $3.00 Copyright 0 1985 by Academic Press, Inc. All rights of reproduction in any form reserved.
GREEN AND ARDITI
relative to the body is similar to that operative parameter is attenuation (rather displayed by the B-scan). The increased characterization has centered about the tifying to the perceived importance this tic ultrasound.
of a conventional X-ray; its than reflectivity, as interest in ultrasound tissue measurement of attenuation, tesparameter will have in diagnos-
A reflection C-scan can be made simultaneously with expanding the potential diagnostic capability. Moreover, cess can also be used to form attenuation B-mode images.
RTI, further the RTI pro-
Transmission imaging studies at SRI [6-91 and elsewhere [lo] have shown that this mode can provide effective demonstration of vasculature, soft tissues of the musculoskeletal system, normal and pathologic breast tissues, and internal organs. RTI should be effective in these applications as well, even more so for internal organs, owing to factors discussed below. Because RTI can be performed in conjunction with routine B-scanning, it should find its role quickly once such bimodal equipment is available in the clinic. II.
PRINCIPLES
OF REFLEX TRANSMISSION
IMAGING
The development of orthographic transmission imaging over the past 15 years has proceeded along several paths. Many lens-focused concepts have been advanced, and numerous image-plane scan-conversion methods to detect the focused image have been demonstrated. Holography was the subject of intense investigation during the early 197Os, but failed to realize its anticipated potential for a variety of reasons. One of these--the deleterious effects of coherent interference--also plagued early lens-focused systems, producing diffraction artifacts from outof-focus structures. This phenomenon lead to the development of spatially and temporally incoherent methods of insonification [ll], which eliminated diffraction artifacts and provided smooth blurring of out-of-focus structures while maintaining sharp focus within the focal plane. Reflex transmission imaging also requires incoherence, which it derives from the random reflectivity of the tissues themselves, as described below. The reflex transmission imaging process is initiated in the same A brief ultrasonic pulse is launched into the manner as is a B-scan: body from a well-focused transducer. In B-scanning, reverberations from within the body are amplified, detected, and displayed as a function of time-of-arrival, which corresponds to depth; mechanical or electronic scanning provides the image's other dimension. However, the RTI signal processing is significantly different. With reference to figure 1, wave reflection signals emanating from within the region between the transducer and the distal end of the focal zone are gated off. Reflections from a selected range zone beyond the focal region are amplified with time-gain control, detected, and integrated. The value of this integral is strongly dependent on the attenuation in the focal zone. (Although it is also dependent on the attenuation in the entire focal cone and on the reflectivity of the tissues within the selected integration zone, the latter factors need not inhibit the formation of an image of high resolution and diagnostic significance.) The derived integral value is stored as one pixel of the RTI image. is scanned in To produce a complete orthographic image, the focal point using either a rectilinear scan, a sector a two-dimensional pattern, scan, or one of several combination scanning patterns, as depicted in
202
REFLEX TRANSMISSION IMAGING
Fig.
1
Geometry of reflex transmission imaging (RTI). Reflections from a zone beyond the transducer's focus serve as an insonification source for transmission imaging. The reflex insonfication source can be envisioned as a set of concentric semispherical laminae, one-half of the pulselength in thickness. Each one contributes a reverberation that is of Rayleighdistributed amplitude, and uncorrelated with the amplitudes from the other laminae.
figure 2. A variety of combinations of mechanical, linear-array, and phased-array scanning can be used. The resulting focal plane will be flat in the rectilinear case, but cylindrical or spherical in the others, unless dynamic control of the focal range and range-gate onset are used. Only with the rectilinear scan does the area imaged remain constant in both size and aspect ratio at all depths. A two-dimensional sector scan probe could closely resemble the B-scan probes now in use, as illustrated in figure 3, which also depicts the relationship between B-scan and RTI planes. By reference to the previous discussion of transmission imaging, we can consider the waves reverberated from beyond the focal zone to be, in effect, a source of incoherent insonification for transmission imaging of the focal plane. Because the transmitted pulse passes through the focal zone, the reverberations are already proportional to the attenuation at the focus. Owing again to the transducer's focal properties, only the reverberated waves returning through the focal zone are received with high sensitivity. Thus, focusing is provided both on transmission and reception, doubling the sensitivity to focal-zone attenuation as compared with conventional transmission imaging. Because focusing is invoked twice, the resolution also is superior to that of a conventional transmission image made using the same numerical aperture. The deleterious effects of tissues on either side of the focal plane can be minimized by increasing the numerical aperture. At f/numbers in the range of two to three, there seems to be (at least in the preliminary studies) adequate averaging of the attenuation and scatter properties of the out-of-focus tissues. Generally, the artifact these tissues contribute is of low spatial frequency. RTI does require the presence of a reasonably uniform body of tissue beyond the focal
203
GREEN AND ARDITI
FOCUSED
(a)
/OR
LINEAR
FOCUSED TRANSDUCER PHASED ARRAY
(c)
Fig.
Fig.
SECTOR
2
TRANSDUCER
FOCUSED TRANSDUCER LINEAR ARRAY, OR PHASED ARRAY
- LINEAR
(b)
LINEAR
\ -SECTOR
_
- SECTOR
(d)
SPIRAL
(e)
STAR
C-scanning patterns suitable for RTI.Patterns (c), (d),and (e) are compatible with ConventionalB-scan sector probes. Rectilinear pattern (a) provides the best near-field viewing area. The image planes formed by sector scanners would be flat,as shown,only with dynamic control of focus and range gate.
3
Relationship duced with probe.
of RTI and B-scan image planes that would be proa two-dimensional, dynamically focused sector-scan
204
REFLEX TRANSMISSION IMAGING
TRANSMIT/RECEIVE TRANSDUCER RASTER OR SECTOR SCANNED)
FOCAL
ZONE
PLANE
L Fig.
4
plane to degraded bone or hand and water-filled
VOLUME FROM WHICH REVERBERATIONS ARE INTEGRATED
Depiction of reflex insonification. The reflex insonification source can be envisioned as a set of concentric semispherical laminae, one-half of the pulse length in thickness. Each one contributes a reverberation that is of Rayleigh-distributed amplitude, and uncorrelated with the amplitudes from the other laminae. serve as a source if this tissue is gas. Transmission testicle could be sponge) behind
of reverberation. The image quality will be substantially inhomogeneous or abbreviated by images of appendicular structures such as the made using a scattering material (e.g. a the structure.
Like that of the B-scan, the noise in an RT image has both additive and multiplicative components. In RTI, additive noise (usually from the preamplifier) is suppressed by the signal-integration process; the longer the useful integration interval, the better the signal-to-noise ratio. Time/gain control can be used within the integration interval to optimize this ratio. In the B-scan, multiplicative noise of high modulation index is always present. Referred to as "speckle," it arises from the interference of pulse reflections from adjacent scatterers, and has become accepted as an integral part of the image. This pulse-interference phenomenon also produces noise in RTI, but of a substantially lower modulation index. With reference to figure 4, we can consider the reflex insonification source to be composed of concentric scattererfilled "isochronous" laminae [12,13] or shells of roughly spherical shape, lying beyond the focal zone. If the shell thickness is taken to be about half the pulse length, we can consider each shell to be a single composite source with a Rayleigh-distributed random amplitude, uncorrelated with the amplitudes of reverberation from all other shells. To estimate the signal-to-noise ratio, we first recall that the variance of a Rayleigh-distributed variable is approximately one-quarter of the square of its mean. Because the integration process sums the contributions of independent sources, we can apply the central limit theorem to arrive at a normally distributed reflex insonification signal of mean nA and variance nA2/4, where n is the number of independent shells and A is the average backscatter amplitude from each shell
205
GREEN AND ARDITI
(presumed is given integration noise is
to be set equal by time/gain control.) The number of shells by n = tR/tP, where tp is the pulse duration and tR the total time. Thus, the signal-to-noise ratio for multiplicative S/N = 10 log(4tR/tp)
For example, if the pulse the integration zone 25 ~6 sue), the signal-to-noise results generally support
(dB)
.
duration tp were 0.5 ps and the duration (equivalent to just under 2 cm of soft ratio would be 23 dB. The experimental this analysis.
tR of tis-
For low spatial frequencies, the contrast sensitivity (smallest detectable variation) of RTI will be limited by out-of-focus images of other planes. At the upper limit of resolution (one pixel), multiplicative noise will limit contrast sensitivity. In the example cited above, the 23-dB signal-to-noise ratio is equivalent of an amplitude contrast sensitivity of -20 log (1 - 10-23/20) x 0.6 dB. For example, if the length of the focal zone is 0.5 cm (traversed twice) and the frequency is 5 MHz, then the sensitivity in terms of the salient tissue attenuation parameter (dB/cm-MHz) would be 0.12. The range of this parameter for soft tissues is from 0.2 for blood up to 3.3 for muscle (transverse to the fiber direction). Thus, RTI should be able to detect variations in focal-plane attenuation at the highest spatial frequency of about 3.8 percent of the expected range of values, in dB. Although the signal-to-noise schemes can be devised to improve beneficial. Also, the scattering or arcuate segments--each of these by using segmented transducers and measures would reduce either scan would be an unwarranted complication.
ratio is probably adequate, various it. Analytic-signal processing may be laminae could be divided into annular would be statistically independent-parallel signal processing. Such speed or resolution, and probably
Because the RT image requires a two-dimensional raster scan, it would typically take 3 to 15 seconds to produce. We envision that the ultrasonographer first would perform a B-scan on the patient, using the RTI probe scanning in one direction only. With the aid of a screen cursor, a plane of interest would be selected in the B-scan; it need not be at constant depth if the probe can be focused dynamically. The instrument would then be switched to the RTI mode and the RT image acquired and displayed--alongside the B-scan, if desired. A reflection C-scan of the focal plane could be generated simultaneously and presented along with the B-scan and RTI image. III.
EXPERIMENTAL
APPARATUS and METHODS
The primary experimental facilities used in the preliminary studies were SRI's mechanical raster scanner (the Slow Scanner) and a PDP-11/40 computer system. The latter is equipped with a mini-MAP array processor (CSPI model MM-III) and a DeAnza color image display. SRI's Slow Scanner, which recently has undergone considerable modification for use in transmission imaging experiments2 is shown schematically in figure 5. the
The interface takes advantage of two devices already available on PDP-11: a 16-bit parallel input/output interface (DR-11K) and a
2NIH Grant
GM30890
206
REFLEX
TRANSMISSION
IMAGING
COMPUTER ROOM * ::.::.::::.::,.:::..:::,,“~,.-~,.~r::-:::: ‘“““‘y’.‘.: ......,..,.. :.... . .:.:.:; .. ...:.; . . . :~.~~~:~~::i:~::::~~~::~~:~ w
w
I
150 FEET !,, * _.
m:*
I
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ULTRASOUND LABORATORY VIDEO MONITOR
X-AXIS ENCDDER
-
SELECT LOGIC
ENCODER PROCESSING LOGIC
CONTROL LOGIC
-
Reset
1
TRANSMISSION IMAGING SUBUNIT
PVDF
Pig.
5
Block
diagram
of slow
MEI\ REC
TRANSMITTER
scanner
and its
computer
interface.
multichannel lo-bit A/D converter interface (AR-11). However, because these two computer interfaces were not entirely suited for direct control of the the slow scanner's digital and analog functions, interface electronics were designed and installed at the scanner, more than 150 feet from the computer. digital
When operated in the RTI mode, the interface functions: l Scanner X and Y motor control .
X-axis Pulse
encoder generator
signal timing
l
Analog
receiver
logic
l
Interaction
l
with
provides
processing control control
the PDP-11's
207
parallel
interface.
the
following
GREEN AND ARDITI
TRANSDUCER
BACKSCATTER $@y/
FOCAL
Fig.
6
Arrangement preliminary
When operated l l
l
of object, experiments.
in the RTI mode,
excitation. range-gating, of the received
BACKSCATTER TISSUE
PLANE
backscatterer,
the analog
RANGE ,
and transducer
electronics
in
the
provide:
Transducer Amplification, integration
pulse
Generation with gains bits.
of three-channel outputs to the lo-bit selected to provide an overall dynamic
time/gain signal.
ING
control,
detection,
and
A/D converter, range of 16
The transducer used in these experiments was a Panametrics S-MHz, f/2.5, heavily backed ceramic, focused at 71 mm. To produce RT images, the transducer was positioned so that its focus was at the selected plane within the object, as depicted in figure 6. The range gate was positioned to lie within the backscatter material behind the object, and the time/gain control was adjusted to give essentially equal amplitude returns from all ranges within the gate (distal gain was reduced someFigure 7 what if the additive noise was significant at that range). shows a typical ensemble of the electrical signals at various stages in the RTI signal-processing cycle.
TIME/GAIN SIGNAL
CONTROL
I1-GCl
RECEIVED SIC iN AL. AFTER TGC INTEGRATOR OUTPUT
-FOCAL PLANE REFLECTIONS
\ TRANSMITTED Fig.
7
Ensemble
PULSE
of electrical
signals
208
produced
during
RTI.
REFLEX TRANSMISSION
IV.
IMAGING
PRELIMINARY EXPERIMENTS
Several simple objects were used in the first imaging experiments. To provide adequate control over the process to facilitate evaluation of the results, we first used as a backscatter material a uniform gel phantom with scatter, attenuation, and velocity similar to that of tissue. The first object imaged was a 2.5-cm-long, thin plastic paper clip 8(a), has a suspended on a rubber band. The image, shown in figure field-of-view about 3.3 cm square. The resolution appears to be about 0.5 mm. The multiplicative noise in the background is well depicted as a fine, granular pattern, a result of the lack of correlation between the reflex insonification at adjacent pixels. Its amplitude is about that predicted. Because the attenuation of the thin plastic clip is rather low, the image contrast was electronically increased, emphasizing the background noise. If the range gate is made very narrow and moved into the focal plane, a conventional reflection C-scan is produced, as shown in figure 8(b). Figure 8(c) is the RT image of part of a plastic transistor radio case, using the same backscatter material as for figure 8(a), without increased contrast. The second series of experiments used biological material for both object and backscatterer. The object was a lamb kidney; the backscatterer was a contiguous beef kidney. Roth were suspended in a saline-filled plastic envelope, and care was taken to remove any trapped air. The objectives of this experiment were to determine whether biological structures could be imaged well in this mode, and what effect a complex backscatterer would have on the image. Figure 9 shows a series of 3-mm-spaced planes imaged within the lamb kidney, using a 35-mm-thick
Fig.
8
Reflex transmission (and reflectionc-scan) images of simpleobjects, with a gel phantom backscatter. (a) RTI of a 1" long, thin plastic paper clip; contrast increased electronically to show multiplicative (b) Reflection C-scan of same object. (c) RTI of plastic noise. transistor-radio case.
209
GREEN AND ARDITI
Fig.
9
Reflex tiguous
transmission beef kidney
images of a lamb as a backscatterer.
210
kidney (in-vitro) Image planes
are
with 3mm
a conapart.
REFLEX TRANSMISSION
backscatter zone within the beef Several conclusions can be drawn
IMAGING
kidney. The field-of-view immediately on inspecting
was 6.6 cm. these images:
s
Anatomical structures (especially vessels) in the lamb kidney are well resolved. Whether the vessels contained coagulated blood or autolysis gas was not known. Care was taken to not entrap air.
0
The depth of focus are differentiable. of the transducer's
s
The multiplicative moderate contrast larger field-of-view.
l
The nonuniformity of the backscatter from the inhomogeneous beef kidney is evident only as low-amplitude, low-spatial-frequency modulation of the image; it does not seriously diminish image quality.
is
shallow; planes This is a result focal properties.
noise setting,
is almost a longer
a few millimeters apart of the double application
undetectable, integration
The results of these tests suggest that reflex transmission have significant diagnostic potential, especially because formed as a convenient adjunct to B-mode imaging. V.
SIMULTANEOUS MULTIPLANE
owing to a more zone, and a
it
imaging may can be per-
RTI
The multiplane image set of figure 9 was produced by moving the transducer and range gate after each image was made, then rescanning. In clinical use, this sequential collection of the multiplane image would take about one minute. A multipulse scheme, illustrated in figure 10, has been devised to collect several image planes at once. This on the fact that very short method requires dynamic focus, and relies A series of brief pulses, while beneficial, are not essential to RTI. bursts of ultrasound of different center frequencies and substantially nonoverlapping spectra would be transmitted. If each one is of lower center frequency than the previous one, their reverberations will be associated with increasing range and thus greater high-frequency attenuation. During transmit, the dynamically focused transducer would be focused for each pulse to the desired depth of image to be formed for Similarly, on reception, either the array focus will be that pulse. switched for each pulse or (if the several integration zones overlapped) parallel sets of delay lines will be required, as shown in figure 10(b). In all cases, filters that respond only to the reverberations associated with the individual planes will be required. VI.
THE COMPOUND RTI B-SCAN
the reflex transmission concept also can be applied In principle, to produce attenuation images in the B-scan plane. This would be accomplished by displaying, as a function of range, the integrated RT signal from a range zone just beyond each point in range. This will require both dynamic focus and a continuous time-shifting of the zone of integration (a process that could be easily implemented in the digital domain). However, although such an image will have good lateral resolution, its range resolution will be poor compared to the conventional reflection B-scan, because it will be determined by the length of the transducer's focal zone, rather than by the pulse length. This calls to mind the early reflection B-scans, where the use of unfocused transducers resulted in good range resolution but poor lateral resolution.
211
GREEN AND ARDITI
TRANSMIT PULSES, AT DlFFfRfNT
FOCUSED RANGES
1 RECEIVER
INTEGRATION
fi ”
/ t I I , I 1
I, ,I FOCAL
I I I
I I
GATES
2
PLANES
I
I I I
I I n
2’
a
FOCAL ANNULAR ARRAY
ZONE
FOCAL
FOCAL
ZONE
n
RING
1
RING
m
1 ZONE
2
/
FILTERS-
b Fi .g.lO
SEPARATELY GATE0 RECEIVER CHANNELS
-
Method for simultaneous acquisition of multiplane RTI. (a) Transmit pulse sequence and multiple integration range gates associated with several focal planes. (b) Parallelprocessing receiver; each channel focuses the annular array at a separate range and filters the received signals to respond only to the reverberations of the pulse focused at that depth.
The compound scan was devised to overcome this limitation (and the problems of specularity, but that is beside the point). Similarly, RTI B-scans could be compounded and averaged or, more rigorously, backprojected and spatially filtered to provide good spatial resolution in all directions in the plane. RTI B-scans would bear a similarity to ultrasound computed-tomography attenuation images. They would not have the inherent quantitative accuracy, but would be more easily produced using modified B-mode equipment.
212
REFLEX
TRANSMISSION
VII.
IMAGING
DISCUSSION
Reflex transmission imaging provides the viewing perspective and the sensitivity to attenuation of orthographic transmission imaging and Transmission images provide does so using modified B-mode equipment. information that is complementary to B-scans. RTI will allow both to be made with the same instrument and presented on the same display. The range of clinical applications for RTI remains to be determined. In general, it should provide a more assured diagnosis than Bscan alone, especially because the images can be precisely correlated with electronic screen cursors. For the breast, RTI should be very complementary to B-scan, especially because transmission imaging is more effective than B-scan for visualizing tumors surrounded by fatty tissue. The potential application to monitoring kidney-stone lithotripsy is apparent; gall stones also should be well demonstrated. Peripheralanatomy and intraoperative imaging are also likely applications. As yet, little effort has gone into optimizing RTI performance. Selection of the best numerical aperture and range-gate position remain to be explored, and dynamic control of the focus and range-gate must be implemented in a clinically testable instrument.
REFERENCES
[II
Woltering, H., Matthias, Ultraschalltransmission,
H.H., Green, P.S., _Munch. -med. --Wschr.
and Klein, D., 126, 1431-1434
(1984).
[21 Matthias,
H.H., Woltering, H., and Guth, V., Die Ultraschalltransmissionskamera bei der diagnostlk von instabilen dysplastischen und dislozierten hueftgelenken bei neugeborenen kindern im 1. lebensjahr, Klinische Paediatrie 196 (1984).
t31
Hentz, V.R., Marich, K.W., and Dev, P., Preliminary upper limb with the use of ultrasound transmission Hand Surgery 9A, 188-193 (1984). -____-
[41
Woltering, Taenzer,
151
"Bilder (1984).
161 Green, logies n-87
H., J.,
Matthias, unpublished
aus der badewanne
H.H., work.
Green,
[Pictures
P.S., from
the
und
study of the imaging, 2.
Edmonds, bathtub],”
P.,
and Stern
P.S., Ultrasonic Transmission Imaging, in Emerging Technoin Surgery, L. Angelini, G. Fegiz, and P.N.T. Wells, eds., (Masson Italia Editori, Milano, 1984).
t71
Green, P.S., Orthographic Transmission Imaging, Diagnostic Medicine, P.N.T. Wells and M. Ziskin, (Churchill Livingstone, New York, 1980).
[81
Green, P.S., Schaefer, L.F., Jones, E.D., and Suarez, J-R., A New High-Performance Ultrasonic Camera, in Acoustical Holography, Vol. 5, P.S. Green, ed., pp. 493-504 (Plenum Press, New York, 1973).
[91 Marich,
in Clinics edE38-143
in
K.W., Zatz, L.M., Green, P.S., Suarez, J.R., and Macovski, A Real-time imaging with a new ultrasonic camera: part I, J. &&I. Ultrasound 3, 5-16 (1975).
213
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[lo]
Scherg C., Brettel, H., Roeder, U., and Waidelich, W., Transmission Imaging with Incoherent Ultrasound, in _____ Optics and Biomedical Sciences, Vol. 31, G. von Bally and P. Greguss, eds. (SpringerVerlag, 1982).
[ll]
Havlice, J.F., val of Spurious porally Varying White and R.E. 1977).
[12]
Foster, D.R., Arditi, M., J.., Computer simulations Imaging 5, 308-330 (1983). ____-
[13]
Fink, M.A., measurement,
Green, P.S., Taenzer, J.C., and Mullen, W.F., RemoDetail in Acoustic Images Using Spatially and TemInsonification, in Ultrasound in Medicine 2, D.N. Brown, eds., pp. 1827-1828 (Ple&m Press, New York,
and Cardoso, IEEE Trans. __--
Foster, F.S., Patterson, M.S., and Hunt, of speckle in B-scan images, Ultrasonic Diffraction J.F., Sonics Ultrasonics -
214
effects --SU-31,
in pulse-echo 313-329 (1984).