A new ultrasonic imaging system using time delay spectrometry

Ultrasound in Med. & Biol., Vol. 1, pp. 119-131. PergamonPress, 1974. Printedin Great Britain.

A NEW ULTRASONIC IMAGING SYSTEM USING TIME DELAY SPECTROMETRY R. C. HEYSER and D. H. LE CROISSETTE Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Ca. 91103, U.S.A. (Receired 30 April 1973; and in final form 2 August 1973)

Almtract--A new method of forming a visual image by ultrasound is described. A shadowgraphic transmission image similar to an X-ray radiograph is produced by the application of a technique known as Time Delay Spectrometry. The system uses a repetitive frequency sweep with a linear relationship between frequency and time and the transmitting and receiving crystal are scanned in raster fashion about the subject. By electronic processing, an image may be built up which represents the energy transmitted through the specimen with a given time delay. An intensity modulated picture encompassing the full shades-of-gray capability of the recording system can be produced. A second type of image showing transmission time through the specimen may also be formed. Brightness changes in the displayed image in this case correspond to changes in the ultrasonic transmission time through the specimen. There is no analog for this typeofimage in current X-ray or ultrasonic practice. Examples of both types of images of specimens both in vitro and in vivo are shown. The advantages and potentials of this method for biomedical ultrasonic imaging and analysis are discussed. Key words: Acoustics, Ultrasonics.

INTRODUCTION

THIS paper describes a new method for forming a visual image using ultrasonic energy. The method allows a transmission image to be produced which is analogous to a conventional X-ray radiograph. Cathode-ray tube pictures showing the transmission characteristics of both soft tissue and bones over a 100 cm 2 area have been made. Two distinct types of image may be produced; in both cases an intensity modulated picture is obtained on a cathode ray tube encompassing the full shades-of-gray capability of the recording system. In the first type of image, brightness (shades-of-gray) at any point is a function of the energy received by a transducer as it is moved over a predetermined raster scan. In a medical diagnostic system, this scanned raster would be located so that a shadowgraphic image is obtained of the region of interest. An image will then be formed that is a representation of the absorption of the ultrasound beam transmitted through soft tissue or bone with the minimum

transmission time. This facility allows an anechoic transmission image of soft tissue to be obtained. The second type of image that can be produced is a transmission time picture. In this case, brightness changes in the displayed image correspond to variations in the time taken for the ultrasound beam to travel from transmitter to receiver. There is no analog for this type of image in current X-ray or ultrasonic practice. This system is capable of operating over the complete spectrum of acoustic and ultrasonic frequencies. In addition to its imaging properties, the system possesses the capability of being used for making anechoic transducer and system measurements. Furthermore, the method employs control over both the frequency and time domain signals. The output used to produce the image may be displayed simultaneously as a frequency or time-varying signal on a cathode ray tube or recorder. In this way, amplitude and phase of the received signal are directly obtained as a function of both frequency and time. 119

120

R. C. HEYSERand D. H. LE CROISSETTE TIME DELAY SPECTROMETRY

IIIIIII

TRANSMITTER

This medical imaging system is a direct application of Heyser's Time Delay Spectrometry (TDS)(Heyser, 1967, 1969a, 1969b, 1971) at ultrasonic frequencies. The basic technique will be briefly reviewed here for convenience. The system uses an ultrasonic transmission repetitively swept in a linear manner from frequency F1 to F2 as shown in Fig. 1. At the receiver, the signal has the same format, delayed by the transmission time through the subject. The first signal, received at time T1, is normally that via the direct path (line-of-sight) from transmitter to receiver. Any other signal arriving at the receiver, such as the reflected path signal shown in Fig. 2 will arrive at a later time T2. A selection of incoming signals is made by a narrow bandpass filter in the equipment in the following manner. The ultrasonic sweep generator output is heterodyned with a selectable stable frequency source as shown in Fig. 2. This subsidiary source may then be set so that the narrow passband of the filter accepts only the signals which arrive via the direct path; all other signals outside of the passband of the filter will be rejected. For example, the reflected path signal, which will incur a greater delay (T2) on its longer path to the receiver, will be received by the transducer but will produce a heterodyned signal outside of the narrow limits of the filter and will not be accepted. If the frequency of the FORA LINEARSWEEP FI X° = F

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Fig. 1. Time Delay Spectrometer frequency sweep.

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\

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FREQUENCY OFFSET GENERATOR

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NARROW PASSBANDFILTER I OUTPUT

Fig. 2. Block diagram of system.

offset generator is decreased, it is possible to selectively accept the reflected path signal and reject the direct transmission. Therefore, by a simple setting of the offset generator frequency, the operator can determine whether he wishes to accept the direct path transmission or reject this and accept only those signals that are delayed by a preset time delay (such as the reflected path signal shown in Fig. 2). The minimum path difference which can be resolved (AX) is given by A X -= c / a f

(I)

where Af is the total swept frequency, and c is the velocity of propagation. When a linear frequency-vs-time sweep is generated, it is easy to transfer the data between the frequency and time domains. Using oscillators with suitable phase stability, a repetitive output spectrum including both amplitude and phase is obtained. Furthermore, the frequency range over which the data is obtained is fixed by the operator and direct measurements of the characteristics of the medium may be obtained as the sweep progresses. The transmitted signal has a predetermined frequency spectrum with the equivalent of a time tag to each frequency component. In the simplest case, this consists of a linear frequency sweep with time in which the tag is the moment of occurrence of each frequency. Upon emergence from the specimen, the frequency components with a given time delay are reassembled to yield the frequency spectrum. This is the time-delayed spectrum for which the process is named.

A new ultrasonic imaging system

This method not only provides the signal complex spectrum with an amplitude and phase, but signal components due to longer path lengths, such as those caused by scattering, are suppressed because they lie outside of the passband of the filter. The signal displayed at this point is the anechoic frequency response of the combined transducers and tissue path in a medical ultrasound system. The specific time interval represented by this frequency spectrum is separately selectable. Thus, if a multipath situation is encountered in which the desired signal is closely followed by an undesired signal which traveled a slightly different path, it is possible to accept the signal only at the specific arrival time of interest and narrow the time win~low to the extent necessary to assure that the appropriate path is selected. In most cases the minimum arrival time will be chosen since this will normally coincide with the direct path between transmitter and receiver. ANECHOIC PROPERTIES OF TDS

Separation of incoming signals arriving at different times is a distinct advantage of TDS. The separation resolution in the time domain is given by equation (1) above. For ultrasonic waves travelling in water, c = 1500 m s -a and, assuming a 1 MHz total sweep, c

AX = - - = 1-5 mm.

af

(2)

In the frequency domain, the distance resolution is determined by

Bc (dF/dt) where B is the spectrum bandwidth of the filter in Hz, and dF/dt is the sweep rate in Hz s- 1. A typical set of parameters might be B = 100 Hz sweep rate of 20 MHz s- 1. For a water path, the distance resolution is then 7-5 mm. In this case, the time domain resolution of 1-5 mm given in the previous paragraph would dominate. In the transmission system shown in Fig. 2 it can be seen that the direct path signal will arrive with a transmission time (7"1)which is less than the time taken by any other arriving signal, such

121

as the reflected signal arriving with a transmission time, T2. Thus by setting the offset generator to correspond to the first arrival of signals from the transmitter the equipment will accept only the signals travelling in a straight line through the medium and will reject any reflected or refracted signals. The only exception to this might occur when the medium is grossly inhomogeneous such as the rare instance when a substantial fraction of a non-direct path is through bone. Some of the difficulties experienced with previous continuous wave transmission systems in producing images have been associated with scattering and refraction of the ultrasound beam in its passage through tissue (Guttner et al., 1952). In these systems there was no way to reject late-arriving signals. By contrast the TDS system enables the operator to set the offset frequency to reject all signals not arriving by the most direct path. This produces a clearer image and gives a shadowgraph picture which can be directly related to absorption and losses in the direct path. TIME AND FREQUENCY MEASUREMENTS

When the relationship between time and frequency in the sweep is linear, there is an easy transformation between the two domains. It should be noted that the frequency range over which the measurement is carried out is under the control of the operator. The significance of this to medical ultrasound measurements is that time delay spectrometry simultaneously yields data on the time-domain vector and frequencydomain vector that represent wave propagation through the body. In so doing, an unprecedented range and sensitivity of measurement is available for time-of-arrival measurements and their spectrum measure. For ultrasonic imaging, the information available over the frequency sweep is integrated and plotted as a single scalar for each position where the sweep is carried out. The image is constructed out of a large number of elements. The brightness on the screen of the CRT for each element is a function of the integrated energy received across the sweep, as processed by the TDS equipment. Examples of images produced

122

R . C . HEYSERand D. H. LE CROISSETTE

from a system operating with a frequency sweep from 3.0 to 2-0 MHz are given later. The amplitude or energy of the accepted signal as a function of frequency is immediately available in this system. This information may be used to measure the atteiauation in soft tissue as a function of frequency. Direct comparison of the curve of received energy vs frequency and the curve through a substantially loss-free material gives the relative attenuation as a function of frequency. It is possible to optimize the system by normalizing the power-frequency curve at the receiver. By this technique, the energy which is fed to the transmitter is made to be proportional to the absorption at any frequency so that the received power is approximately constant across the sweep. COHERENT PROCESSING

This system may be operated in a coherent mode. If a source and coherent offset generator are used, it is possible to make phase (transmission-time) measurements and images as well as intensity measurements and images. In the TDS system, it is possible to plot a vector on a

CRT which shows the instantaneous phase of the signal at the receiver as it occurs through the sweep. Figure 3 shows an example of this. The frequency sweep was from 3 to 2 MHz and the sweep started at the center of the screen. As the sweep frequency was reduced downwards from 3 MHz, the line drawn on the screen represented the instantaneous phase of the received signal relative to the generated sweep but with the appropriate time delay inserted. The final point of the sweep (at 2 MHz) represents the value of all these values of phase summed throughout the duration of the sweep. It is possible to plot a phase image by, for example, plotting an intensity on the CRT screen which is proportional to, or a function of, sin ~bwhere ~bis the angle made by a straight line drawn from the origin on Fig. 3 to the final (2 MHz) point and the vertical. Images plotted in this manner are shown later in Figs. 13, 14 and 15. The use of phase offers the possibility of increasing the resolution of TDS measurements by one to two orders of magnitude. For example, if it is possible to detect a change in phase angle of 3.6° this increases the resolution of the system

Fig. 3. TDS phase plot.

A new ultrasonic imaging system

by a factor of 360/3•6 or 100 over a typical amplitude-sensitive system. BEAM SHARPENING

An ultrasonic transducer beam shape is normally defined by the shape and regularity of the transducer and by the ratio of the transducer diameter (or dimension) to the wavelength• For a regular, circular disc transducer the beam has the shape shown in Fig. 4 in the far field• This is of the form

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intensity at angle 0 is a Bessel function of the first kind is the diameter of the transducer is the wavelength . is the angle from the transducer axis.

The first minimum occurs at an angle of beam divergence given by: Jl.22 2/ •

-- arc s , n l - w

I

For a typical transducer, D = 10 mm, f = 1 MHz, 2 = 1.5 mm (in water) and so 0M = 10.5°. The total beam divergency between minima is therefore 21 ° . The half-intensity level on the beam (3 dB down from the peak) due to the combined effect of transmitting and receiving probes occurs at an angle of about 3.2 ° in this case. The 3 dB total beam width is thus 6.4°•

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123

Polar diagrams are usually drawn as if the transducer acted as a point source. Since the transducer must be an extended source relative to the wavelength to produce a beam of finite angle, as shown in equation (2), a further spreading of the beam occurs. Focusing of the beam by using an acoustic lens close to the transducer can reduce the beam spreading to a limited extent but a beam which is narrower than the transducer can only be obtained over an extended distance with difficulty. Time delay spectrometry has the effect of bounding the beam width• Since the signal is swept across a preset frequency range, the time domain value can be obtained by a direct Fourier transformation where the only active integral occurs across the swept frequencies• This results in a time domain resolution curve sin X of the Iorm --~-- as given in Fig. 4. The distance _

to the first null is c/Af where c is the velocity of ultrasound in the medium and Af is frequency sweep• Figure 5 shows the geometrical relationship when the transmitting and receiving probes are separated by a distance A. When the path length from transmitter to receiver is A + c/Af, the first null in the intensity vs distance curve is reached. This corresponds to the upper line marked 'zero intensity' in Fig. 5. There is another null corresponding to the lower path on the other side of the minimum path transmission shown. In a time delay spectrometry transmission system as shown diagrammatically in Fig. 5, the beam width is determined by two factors: (a) the physical characteristics of the transducer and the wavelength; (b) the reduction in signal from paths which take a time differing from the preset transmission time such that the incoming signal arrives outside of the passband of the TDS ZERO INTENSITY

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124

R. C. HEYSERand D. H. LE CROISSETTE

system filter. For the transducer discussed above, assuming Af = 0.5 M H z and A = 20 cm, the first null occurs at 9.9 ° on each side of the axis. The 3 dB total width in this case can be shown to be 8-8° . The use of TDS, therefore, is responsible for additional shaping of the beam since the equipment will severely attenuate signals arriving outside the time gate. In the case quoted, the use of TDS will sharpen the beam so that there is a combined drop of 6 dB from the two limiting phenomena at about 7.6 ° total beam width. The combined 3 dB total beam width can be shown to be about 6.0 ° . REFLECTION AND TRANSMISSION SYSTEMS

This system is equally capable of being operated in a reflection or transmission mode and both have been demonstrated experimentally. The arguments which have been presented here for the transmission case may be used with little modification for a reflecting system. Current pulse-echo equipment has reached such an advanced state of evolutionary development that it was thought to be advantageous to demonstrate the TDS transmission system first. EXPERIMENTAL EQUIPMENT

An experimental ultrasonic unit was built to verify the feasibility of the method. The electronic design was based on that used previously for acoustical testing (Heyser, 1967). A block diagram of the assembled ultrasound instrumentation is shown in Fig. 6. A Tektronix spectrum analyzer (plug-in type 1L5) was used as the primary swept-frequency source. This unit was modified to accept a phase coherent frequency reference as well as to provide an intermediate frequency output. The physical movement of scanning probes in a precise raster pattern about a test specimen was obtained by positioning an X - Y chart recorder drive-oriented for vertical/lateral motion above a water tank which contained the specimens. The recorder chosen had a particularly robust mechanical servo. It was thus possible to mount the ultrasonic crystals on a yoke suspended in the water and rigidly fixed to

the recorder pen carriage. Figure 7 shows the general mechanical arrangement. Simple unbacked barium titanate probes in the form of discs of 5 mm dia. were used as shown in Fig. 8. The probes were protected by an epoxy coating. Since the system was tuned to accept only those signals which had the lowest transmission time between the transmitter and receiver, it was not necessary to heavily attenuate the signal at the rear of the transmitting probe. The crystals had a natural resonant frequency of 2.5 MHz. The display raster which was adopted for this initial work is illustrated in Fig. 9. The linear format was chosen to minimize the slew rate on the scanner. A typical scan was completed in about 13 min. A Tektronix Model 603 display storage monitor was used as the primary display device after initial experimentation with a Model 535A oscilloscope. In the Model 603 it is possible to view the image as it is forming and to photograph it at a later time. Although the display storage monitor was designed for computer graphics display without half-tone capability, successful half-tone imaging has been obtained for ultrasound by the technique of area modulation. This is accomplished by taking advantage of the fact that the stored spot diameter is a function of the electron beam intensity. The resultant image dynamic range is less than that of a straight monitor without storage but the picture is quite good. To produce a transmission image, the time delay offset was adjusted for the direct signal path through the image. The scanning yoke in the water path was then moved in a raster scanning pattern keeping the spacing between the transmitter and receiver fixed at all times. The scanning spot on the display monitor moved in synchronism with the probes. The time domain vector shown in Fig. 3 was quantified for length by integrating over the frequency sweep and the spot intensity was controlled to correspond to this value. The convention chosen for the display was that an increase in signal strength brightened the spot. This caused shadows to appear dark, in analogy to human visual experience. A simple inversion was pro-

A new ultrasonic imaging system

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Fig. 6. T D S ultrasound instrumentation.

vided in case one wishes to use the X-ray con-~ vention, in which increasing opacity of the subject corresponds to a brighter image. The equipment was built to include the option of employing a modulating voltage which could be made proportional to the vector length squared, the logarithm of vector length, or the logarithm of the logarithm of vector length. In this way, it was possible to use the degree of contrast that gave the greatest amount of information in the picture regardless of the dynamic range of intensity obtained at the receiving transducer. IMAGES

Over 250 transmission images have so far been made. The specimens used have varied

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from simple geometrical shapes formed by metals and plastics to the in vivo human forearm. Experimental measurements have verified the theoretical calculations of spatial and temporal resolution. A few examples of the images produced are given here. Most of the pictures were taken with a sweep frequency (Af) of 1 MHz and with the system operating between 2 and 3 MHz. Figure 10 is the ultrasonic transmission image of an excised lamb's kidney. The specimen was immersed in a sealed plastic bag filled with saline solution. The kidney "was thoroughly massaged to expel any air bubbles from exposed arteries. The visible structure, as seen ultrasonically, eroded as the specimen aged. The

126

R. C. HEYSER and D. H. LE CROISSETTE I

41

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VERTICAL 1

HORIZOIqTALI~ SCAN FORMAT ASPECT RATIO: HORIZ. LINES: LINE TIME: FRAME TIME:

1:1 140 6 SEC (APPROX) 800 SEC (APPROX)

Fig. 9. Display raster format.

Fig. 7. Mechanical scanning mechanism.

kidney was not fixed and within several days all of the internal structure in the transmission ultrasonogram had disappeared. The scan covered is approximately 7.5 by 7 cm showing an apparent lateral resolution of the order of 2 ram. Extremely high contrast between stronger and weaker signal components is utilized here in order to reveal subtle detail of internal soft tissue. Figure 11 is an in vivo picture of the long flexor group of muscles of the human forearm. The region shown is from the ulna (shown as the black top right-hand portion of the picture) to the interface between the water and the skin

COAXIAL CABLE

SHIELD & INNER CONDUCTOR SOLDERED TO OPPOSITE SIDES

~".J THIN EPOXY COATING SIDE VIEW

FRONT VIEW

Fig. 8. Ultrasonic probe configuration.

(shown in the bottom left-hand side). The equipment was optimized for the velocity of sound propagation through muscle which is higher than that through water. The sound which was transmitted through the muscles was therefore processed to produce a brighter image than that which arrived through the water path. The muscles appear brighter even though the level of the sound transmitted through the water was several orders of magnitude higher. The longest dimension of the tissue region illuminated is approximately 8 cm. A transmission image of the lower forearm in vivo is shown in Fig. 12. The forearm runs diagonally across the illustration with the wrist towards the bottom left corner, the radius being represented by the gray upper region immediately below the bright band, and the ulna is contained in the lower lighter region. The dynamic range of the signal was in excess of 60 dB. This range was compressed to about 20 dB to allow the limited brightness capability of the monitor to display information across the picture. It has thus been established that images can be produced through bone. The intent of preserving the entire dynamic range was to allow visualizing structure through the bone as well as soft tissue in one picture. The resulting image appears to be of low contrast when compared with conventional high contrast techniques where the bone would appear as a dark shadow. The superimposed rectangular grid is a calibration scale with 1 cm per major division further subdivided with 2 mm marks. The lateral resolution is well under 2 mm.

A new ultrasonic imaging system

Fig. 10. Image of an excised lamb's kidney.

Fig. 11. Human flexor dig!torum profundus muscle.

127

128

R.C. HEYSERand D. H. LE CROISSETTE

Fig. 12. Lower forearm.

Three transmission time, or phase, images are shown in Figs. 13, 14 and 15. The picture of the lamb's kidney shown in Fig. 13 was made by modulating the screen brightness proportional to the amplitude of sin ~b where' ~b is the angle

made by a straight line drawn from the origin on Fig. 3 to the termination of the vector relative to the vertical or any other fixed line. There is no signal amplitude information in the image. This is therefore a contour plot of transmission time

Fig. 13. Transmission time contour plot of lamb's kidney.

A new ultrasonic imaging system

129

Fig. 14. Transmission time contour plot of lower forearm.

through the specimen. The difference in time between successive dark bands is that necessary to cause a change in phase of 180° over the complete frequency sweep. This corresponds to a transmission time difference of about 200 nsec.

Figure 14 is a transmission contour plot of the forearm in the same region as Fig. 12. The amplitude information has again been suppressed and the black bands denote contours separated by a difference in transmission time of about 200 nsec.

Fig. 15. Transmission time contour plot of excised liver.

R. C. HEYSERand D. H. LECROISSETTE

130

Figure 15 is a transmission contour plot through an excised liver. The difference in thickness between adjacent contours assuming the liver were completely homogeneous, is approximately 0.2 ram. EDGE EFFECT

In transmission ultrasound an edge effect often occurs which helps to outline boundaries. When a simple boundary is encountered a portion of the sound is normally transmitted and a portion is reflected. In the case of a rounded boundary, such as would be found in a spherical, cylindrical or ellipsoidal shape or one which has a three-dimensional rounded form, total reflection will occur at some angle determined by the relative refractive indices on the two sides of the boundary. Above this critical angle total reflection takes place and the transmitted signal drops to zero. In practice because of the finite size of the transducer polar diagrams this condition does not occur over the whole of the incident wave front. It has been observed to occur in rounded objects to a sufficient degree to produce an enhancing edge effect in the picture. DIRECT ATTENUATION MEASUREMENTS

It is possible to make a direct measurement of attenuation as a function of frequency by a substitution method. A direct plot of amplitude vs frequency can be obtained on an oscilloscope (see Fig. 6). If a low-loss fluid such as water is used as a coupling medium between the transmitter and receiver, the substitution of a lossy medium of any type will reduce the received signal as viewed on the oscilloscope. A direct comparison of the attenuation of the lossy medium and the low-loss fluid is therefore obtained across the frequency sweep. Measurements of this type are now in progress and will be reported at a later date. SUMMARY AND

CONCLUSIONS

The feasibility of this new system has been demonstrated in a series of measurements in vitro and by more limited in vivo tests. In the frequency range of 2-3 MHz and with a frequency sweep of 1 MHz the theoretical resolu-

tion limit of approximately 1.5mm in the amplitude mode has been reached experimentally. It has been shown that the shadowgraph images produced by ultrasonic transmission have some features which can be correlated with known tissue structure. Using electronic dynamic range compression it has been demonstrated that images showing a shades-of-gray variation may be displayed on a storage tube and photographed. The transmission time images exhibit a series of contours which join points of equal transmission time through the specimen. When the specimen is completely homogeneous, these are also equal-thickness contours. This type of imagery might find a use in locating discontinuities in tissue where the inhomogeneity causes either a change in thickness or velocity through the tissue. These transmission, or phase, images may be formed with a resolution considerably in excess of the amplitude images. A specific feature of this application of Time Delay Spectrometry is that the relationship between time and frequency throughout the duration of the sweep is linear. The information being processed through the apparatus, therefore, is immediately available as a function of both time and frequency. This allows a measurement of the absorption vs frequency to be made from a cathode ray tube display. The experiments have been conducted so far with a single set of transducers mounted on a rigid yoke and scanned by an X - Y recorder. The time fo make one picture consisting of about 120 lines per frame is approximately 13 min. This system was limited by the mechanical properties of the scanning X - Y recorder; a single transducer pair could be driven at a rate at least an order of magnitude faster and produce a comparable image. By the use of multiple sweep frequency systems, mosaics or sets of transducers, an image could be produced in a time of less than a second. The frequency range of 2-3 MHz and a sweep frequency of 1 MHz was chosen because of the availability of equipment. A wider sweep frequency will enhance the resolution so that resolutions in the sub-millimeter range are feasible in the amplitude mode.

A new ultrasonic imaging system

Although the transducers have been operated in a water bath for convenience, the fluid is used only to provide adequate energy coupling. Coupling using water bags will operate with this system. The power levels which have been used in this system have been from 20 mW down to 15 #W per square cm. This is two orders of magnitude below the region designated as safe by Ulrich (1971). This is a continuous wave system where the sweep is operating almost continuously. There are no periods of time when bursts of ultrasonic energy occur; consequently, the peak power and the average power are approximately equal. Acknowledgement--This paper presents the results of one phase of research carried out at the Jet Propulsion Labora-

131

tory, California Institute of Technology, under Contract No. NAS 7-100, sponsored by the National Aeronautics and Space Administration.

REFERENCES

Guttner, von W., Fiedler, G. and Patzold, J. (1952) Ultraschallabbildungen am Menschlichen Schadel. Acustica 2, 148-156. Heyser, R. C. (1967) Acoustical measurements by time delay spectrometry. J. Audio Engng Soc. 15, 370-382. Heyser, R. C. (1969a) Time Delay Spectrometer. U.S. Patent 3,466,652. Heyser, R. C. (1969b) Loudspeaker phase characteristics and time delay distortion. J . Audio Engng Soc. 17, 30-42, 130-138. Heyser, R. C. (1971) Determination of loudspeaker arrival times, J. Audio Engng Soc. 19, 734-742, 829-834, 902-905. Ulrich, W. D. (1971) Ultrasound Dosage for Experimental Use on Human Beings, Research Report No. 2, Project M 4306.01-1010BXK9, U.S. Naval Medical Research Inst.