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DISCUSSION Although there are no clearly defined hazards associated with pulse-echo ultrasound

as currently used in diagnostic medicine, there is now a gradual increase in interest in exposure parameters for clinical equipment due in part to experimental observations of effects of low-temporal-average-intensity, pulsed ultrasound like those described above. To encourage this trend, the American Institute of Ultrasound in Medicine awards commen- dations to manufactureres who provide specific information about the acoustical quantities which characterize their equipment. At present, the quantity which is recommended by AIUM/NEMA to characterize temporal peak intensity is the spatial peak, pulse average intensity. The performance standard for ultrasound therapy products administered by the National Center for Devices, and Radiological Health (NCDRH) requires that I, as well as the total power be indicated by the equipment. The NCDRH guide to be used by manufacturers of medical diagnostic ultrasound equipment in the preparation of product reports to be filed with NCDRH asks for I, as well as Ipa to characterize the peak intensity.

The example chosen for these tests admittedly is contrived to distinguish clearly be- tween the pulse average and maximum intensities. However, the difference in the usefulness of the two quantities as predictors of this biological effect is striking. In addition to its apparent biological relevance, the NCRP's maximum intensity has an advantage of simplic- ity of definition. With the advent of PVDF hydrophones which have uniform response over a wide range of frequencies and reasonably long term stability in calibration, it should be possible for measurements of maximum intensity to be made easily not only in the laboratory and factory but also in the clinical setting.

ACKNOWLEDGEMENTS. The authors are indebted to Dr. Harold Stewart of the National Center for Devices and

Radiological Health and Professor Paul Carson, University of Michigan Medical Center for helpful discussions and information.

This study was supported in part by U.S.P.S. Grant GMD9933.

Yours etc.,

Dept. of Electrical Engineering, University of Rochester, Rochester, N.Y.

E.L. Carstensen, R.B. Berg, S.Z. Child.

References American Institute of Ultrasound in Medicine/National Electrical Manufacturers Association

(AIUM/NEMA) (1983). Safety standard for diagnostic ultrasound equipment. J. Ultrasound Med. 2:Sl-S50.

Carson, P.L., Fischella, R. and Oughton,'T..V. (1978). Ultrasonic power and intensities produced by diagnostic ultrasound equipment. Ultrasound Med. Biol. 3:341-350.

Carson, P.L. (1983). In a personal communication, Prof. Carson reports that with the pulse shapes employed in pulse echo and pulse Doppler ultrasound the ratios of Im to I range from 1 to values as great as 6. pa

Carstensen, E.L., Parker, K.J. and Barbee, D.B. (1983). Temporal peak intensity. J. Acoust. Sot. Am. (In press).

Child, S.Z., Carstensen, E.L. and Lam, S.K. (1981). Effects of ultrasound on Drosophila: III. Exposure of larvae to low-temporal-average-intensity, pulsed irradiation. Ultrasound Med. Biol. 7:167-173.

N.C.R.P. (1983). Biological effects of ultrasound: Mechanisms and clinical applications. Scientific Committee No. 66, National Council for Radiation Protection and Measurements. Bethesda, Maryland.

A Moving Tissue-Equivalent Phantom For Ultrasonic Real-Time Scanning And Doppler Techniques.

Sir: Introduction Test phantoms which mimic the ultrasonic properties of tissue reasonably accurately

have been described in the literature (Madsen et al, 1978; McCarty and Stewart, 1982). These phantoms are of value for comparing machines and for checking their constancy of performance. As tissue-equivalent phantoms improve, their value for predicting the performance of machines in clinical imaging also increases. However,the phantoms described in the literature have all

L456 Letters to the Editor

been static ones containing no moving parts. Since in practice virtually no tissues examined by ultrasound are static this is a significant limitation of present phantoms. It is worth bearing in mind that the echo signals resulting from scattering at closely spaced targets are altered by small motions of the targets. For example, with 5 MHz ultrasound the wave- length in tissue is about 0.3 mm. A change in the component of the separation of the tar- gets along the ultrasound beam of as little as a tenth of a wave-length, i.e. 0.03 mm, could be expected to alter significantly the signal level from the targets.

Tissue motions are also of importance since echoes from structures move in a similar way in an image whereas noise signals change in a random fashion. The eye is also partic- ularly sensitive to moving edges. These factors contribute to the clarity of real-time images.

A moving tissue phantom has been constructed for the assessment of ultrasonic scanners, The device has also been used to acquire an increased understanding of the factors which contribute to image quality.

Moving tissue-equivalent phantom. To produce realistic images, the phantom was made from a tissue-equivalent material

comprising a mixture of gelatin, water, propanol and graphite powder (Madsen et al 1978). Other synthetic material can also be used which is often easier to work with, e.g. Bulpren reticulated foam (Lerski et al 1982). The material is contained in a cylindrical drum whose wall is a thin polycarbonate sheet of thickness 0.25 mm. A few drums constructed with 1 mm thick plastic walls had large refraction artifacts at the sides of their images (Fig. l), The drum is interchangeable to aid development of test phantoms which simulate different applications. Several have been constructed containing simulated cysts and tumours (Figs. 1 and 2). Arrays of nylon fibres or holes have been included for resolution and registration tests. The drum is rotated or oscillated about its axis by a variable-speed electric motor coupled to the drum by reduction gearing and a magnetic drive. The container, surr- ounding the drum, is filled with oil to permit easy use with linear or sector scanners (Fig. 3).

Fig. 1. Frozen cross-sectional image of the tissue phantom. The echoes from the wire arrays for resolution tests are located centrally and are saturated in this image. Three disc shaped structures of increased echogenicity represent tumours. One fluid filled cyst-like structure is also shown. The image of the cross section through the drum does not appear circular due to a refraction artifact in the 1 mm thick plastic wall of the drum.

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Fj Lg. 2. Cystic structures in the phantom material. These structures al :e used to determine the resolving power of a scanner and how it varies as the structures move throughout the field of view.

Fig. 3. Use of a test-rig with a rotating transducer real-time sector scanner.

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Uses of Moving Phantoms. The motion feature of the test phantom has been found to be relevant for a number

of reasons. 1. Study of the speed of scan effects. When a moving structure is imaged with a frame rate which is too low, the structure

appears to jump between positions rather than move in a smooth fashion. This is usually only a problem with fast moving structures if the true frame rate of the machine is re- latively low e.g. less than 20 frames/set. Moving phantoms can be used to check the pre- sentation of structures over a range of speeds. The unit can also be used to check on distortion in the presentation of heart valves due to the finite time taken for the ultra- sound beam to sweep across the field of view.

2. Test of M-mode recording. The quality and accuracy of M-mode facilities can be checked

phantom. 3. Assessment of lag . In old systems lag may be due to the response of a TV camera

new ones it is most likely due to signal processing techniques such or automatic TGC.

4. Resolution tests throughout field of view. With a moving tissue phantom, the presentation of structures

with a moving tissue

or display screen, in as signal averaging

can be observed as they move throughout the field of view. The resolving power of most ultrasonic imagers varies with position in the field of view.

5. Assessment of speckle pattern motion effects. Speckle pattern motion effects can either be detrimental or beneficial to image

quality. The speckle pattern in ultrasonic images of organs is largely an interference pattern resulting from the summation of echo signals from scattering centres in the paren- chyma and is not simply related to tissue structure. Motion of the organ is therefore not related in a simple way to motion of the speckle pattern. The moving tissue phantom has been used to compare the motion of the echo pattern and the motion of the scattering cen- tres. The diversity of these motions is often quite large and depends on the type of scanner and its beam characteristics (Morrison et al 1983). Observation of the speckle pattern motion is employed to see if a scanner is particularly susceptible to speckle pattern motion artifacts.

Since the speckle pattern changes rapidly with motion of the scattering centres, some signal averaging occurs when the eye observes the image on a display screen. This averaging can improve the contrast resolution in the image.

6. Assessment of signal processing. Many real-time scanners now have electronic smoothing and signal averaging techniques

in their signal processing circuitry. This processing may take place over several image frames. It is therefore most effectively evaluated using a moving test phantom.

7. Doppler tests. The moving test phantom has been found to be of value in setting up and testing

pulsed Doppler units. A moving target consisting of a very large number of scattering centres is a reasonable approximation to moving blood. A drum containing the scattering medium with a fluid-filled central core is used for the tests (FPg. 4). The registration of the Doppler sample volume is checked by noting the Doppler signal as the position of the sample volume is moved throughout the real-time image of the phantom. The signal level should drop quickly as the sample volume moves into the liquid core. This test was also used to confirm that side-lobes were not contributing significantly to the output from a Doppler unit used in cardiology. Reflectivity of muscle is much higher than that of blood so muscle movements may contribute to the total output signal if they are inter- cepted by weak side-lobe beams.

The Doppler output frequency can be calibrated by relating it to the velocity of the scattering material at different radii in the drum.

One of the more difficult aspects of Doppler instruments to set up is the direction sensing circuitry. This circuitry can be set up with the moving phantom by noting the output signal when the sample volume is located in regions of forward or reverse motion.

8. Tests during research and development. The moving phantom has been useful in research and development work. Automatic TGC

systems are being studied which often derive their compensating control signals from echoes gathered from 4 or 5 frames. They have therefore a finite response time. Synthetic aper- ture transducers for electronic focusing gather echo data over several transmit and receive cycles to generate one line in the image. They are therefore slower and more susceptible to motion than standard arrays. In both these areas of research and development, a moving phantom provides a more realistic test than a static one.

Conclusion Several of the tests and assessments described are qualitative. To make them more

quantitative it will be necessary to construct the phantom from materials of known attenuating, reflecting and scattering properties (McCarty and Stewart 1982). However,

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Fig. 4. A phantom structure employed for testing Doppler instruments. The position of the sample volume of a pulsed Doppler unit is shown by the line in the image of the central cavity.

even in its present form the moving tissue-equivalent phantom has proved to be very usefu in several areas of medical ultrasonics. As equipment becomes more complex, it is more necessary to include motion in test pieces which seek to simulate tissue.

Yours etc.,

W.N. McDicken, D.C. Morrison, D.S.A. Smith,

Dept. of Medical Physics and Medical Engineering, The Royal Infirmary, Edinburgh, Scotland.

References. Lerski, R.A., Duggan, T.C. and Christie, J. (1982). A simple tissue-like ultrasound

phantom material. Br. J. Radiol. 55:156-157. Madsen, E.L., Zagzebski, J.A., Banjavic, R.A. and Jutila, R.E. (1978). Tissue mimicking

materials for ultrasound phantoms. Med. Phys. 5:391-394. McCarty, K. and Stewart, M. (1982). A simple calibration and evaluation phantom for

ultrasound scanners. Ultrasound Med. Biol. 8:393-401. Morrison, D.C., McDicken, W.N. and Smith, D.S.A. (1983). Motion artifact in real-time

ultrasound images. Ultrasound Med. Biol. (In Press).