ABSTRACTS, ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM

and the other pulses with 20 ns risetimes. The tonebursts are received by the AET and their powers are measured to derive insertion loss of the specimen. The pulses are received by a focused ~-MHZ piezoelectric transducer and their t i mes-of - flight are measured to derive sound speed in the spec i men. Recorded data are temporal aver ages of eight successive interrogations under identical conditions. Spatial averaging within each pixel can also be invoked. The substitution technique is the basis for the measurements: insertion loss of the specimen is compared with those of a series of calibrated rubber blocks fabricated from RTV and 0 to 8 percent alumina powder; time-of-flight through the specimen is compared with that through an equal path of reference liquid, in our case 10 percent f ormalin solution. Initial results for breast tissue will be presented.

This work was supported by PHS Grant No. CA34398 awarded by the National Cancer Institute, DHHS. Cl1 L.J. Busse and J.G. Mi 1 f er , J--4~~~~-~...-~Q~~-._~~~.-. Z!, 1370-1376 (1981).

FREQUENCY-DEPENDENT ANGULAR SCCITTERING OF ULTRASOUND BY TISSUE-MIMICKING MATERIALS AND EXCISED TISSUE, W.J. Davros, J.A. Zagzebski , and E.L. Madsen, University of Wisconsin, 1300 University Avenue, Madison, WI 53706.

Know1 edge of the scattering properties of human soft tissue and t i ssue phantoms is fundamental to proposed quantitative diagnostic modalities, including gray scale imaging of backscattering strength and inverse scattering reconstruction imaging. FI computer-controlled apparatus has been developed that is capable of rapid, accurate measurements of the differential scattering cross section per unit volume in excised tissues and in tissue-mimicking phantom materials. These measurements are carried out as functions of ultrasonic frequency and scattering angle. The design of the apparatus facilitates straightf or-ward data analysis by minimizing effects of phase cancellation at the receiving transducer, uncertainties in the volume doing the scattering and any nonuniformity in the incident beam intensity.

The apparatus and data analysis scheme have been tested for precision and accuracy over the range from 2 MHz to 8 MHz

by conducting experiments on tissue-mimicking gel samp 1 es whose acoustic properties, including scattering strength, are known. Experimental and theoretical results on accuracy tests are in excellent agreement and will be presented. An important aspect of our measurements i 5 that coherent scattering, related to the surface of the spherical scattering vol umes employed, is negligible. This has been demonstrated experimentally. Results on the frequency-dependent angular scattering from $rr * female human breast t i ssue and e vitro dog liver tissue wi 11 be presented. Effects of tissue decomposition on the outcome of experiments is minimized by rapid data acquisition which is made possible having the experiment under computer control.

BACKSCATTER SYSTEM CALIBRATION USING CALIBRATED SCATTER PHANTOMS, Thomas M. Burke’, Martha P. Vargas’, and Dan N. Tidwell', %General Electric Medical System Group. 3920 Security Park Drive, Ranch0 Cordova, CCI 95670 and 'California State University at Sacramento, Sacramento, CA.

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ABSTRACTS, ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM

A simple procedure for obtaining estimates of the

frequency and range dependent transfer backscatter function for an experimental scatter measuring system was studied. The procedure relies on a compar i son of calculated scatter n-055 sections to corresponding estimates of the received scattered wave acoustic intensity measured for a small number of calibration phantoms. The best est i mate of the system

transfer f unct i on was found by averaging the results from all the calibration phantoms. An analysis of uncertainties contained in theoretically describing the scatter cross section for the calibration phantoms indicated that they should lie within 10 percent of the true cross section of the materials. Thus, 10 percent is the wper limit on the present accuracy of the system. The resultant transfer functions were used to estimate the scattering properties of eighteen well-characterized phantom materials of which scatter characteristics spanned the expected range of tissue in the frequency range from 1.0 to 7.0 MHz. The experimental results agreed to within +/- 20 percent of the theoretically predicted backscatter cross sect i on values. The experimental error in the measurement data was approx i mate1 y +/- 15 percent. This procedure represents a simple and accurate method of experimentally determining the frequency dependent scatter nature of tissue mimicking materials without having a detailed know1 edge of the spat i al nature of the interrogating ultrasound beam.

PROGRESS IN CONTRAST-DETAIL PHANTOMS, S. W. Smith', M.F. Insanal, K. Nel son”, and H. Lopez =, ICenter for Devices and Radiological Health, FDA, Rockville, MD 20857 and gladi ati on Measurements, Inc., Middleton, WI 53562.

At several previous meetings, we have described our contrast-detail (C-D) phantom for evaluating the detectability of lesions in ul trasound i mages as a function of lesion size and contrast. One limitation of our previous phantom designs has been the uncet-tai nty of the contrast-detail measurements due to a single sample of the speckle noise arising from the single acoustic scan window in the phantom. We have developed two new phantom designs to remove this source of measurement uncertainty.

In the first new version of the contrast-detail phantom, each conical gel target is positioned on the axis of an individual gelatin cylinder 15 cm in diameter and 12 cm in length. Thus, each conical target can now be examined from many independent orientations, effectively eliminating uncertainty due to limited speckle sampling. Of course, the cone is still fixed at a single depth, 7.5 cm, and the new version adds considerably to the weight of a complete 8 cone contrast-detai 3 phantom.

In our second new C-D design, we have used the RMIAJniversity of Wisconsin shot tower t i ssue mimicking material Cl1 which consists of a water-borne slurry of agar spheres approximate1 y 1 mm in diameter containing approximately 20 urn graphite scatterers as the background mater i al . Solid agar cones containing different concentrations of the same spheres serve as the contraet- detail targets suspended in the background material. The background material can be stirred and each cone can be rotated to provide many independent samp 1 es ( i mages 1 of the speckle noise. The cones can be fixed at any desired depth or

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