ABSTRACTS, ULTRASONIC IMAGING AND TISSUE CHARACTERIZATION SYMPOSIUM

Temperature sensing has many important applications such as materials processing, hyperthermia treatment of cancer and nuclear reactor safety monitoring. At the present time, invasive methods using thermistors and thermocouples are commonly employed. Temperature measurements by ultrasound have been investigated. The basis of this method is that the speed of acoustic wave in a material is a function of temperature. Therefore, a variation of temperature can be determined from the change of velocity of propagation. Typically, the change in temperature is minute and the corresponding change in acoustic velocity is extremely small. I f envelope detection is used, it is almost impossible to detect such a small velocity change with reasonable accuracy. The work reported here employs a technique we have developed recently to monitor the shift in echo position due to a temperature change in a material. The technique involves the use of a reference signal to minimize the range uncertainty and a high sampling rate circuit to give fine resolution in velocity change. An 0.2 mm range accuracy has been achieved with a 2.25 MHz operating frequency. Three- dimensional temperature profile imaging of an experimental model has been obtained. Techniques and experimental results will be presented.

SESSION 7: MYOCARDIAL IMAGING

TOWARD THE CALIBRATED CARDIAC IMAGE: USING THE ULTRASOUND EQUATION AND MAXIMUM LIKELIHOOD ESTIMATION OF BACKSCATTER, T.L. Rhyne,' K.B. Sagar,2 L. Pelc3 and D.C. Warltier3, 'Research and Development, Marquette Electronics Inc., Milwaukee, WI 53223, and Departments of 2Cardiology and 3Pharmacology, Medical College of Wisconsin, Milwaukee, WI 53226.

Mapping of ischemic myocardium through the intact chest requires sensitive measurement of the absolute magnitude and temporal modulation of myocardial backscatter. We have previously described an absolute measure of the backscatter per cm of tissue that is independent of transmitted signal, instrumentation, diffraction and bulk loss. We developed a maximum likelihood estimator for the magnitude of the Rayleigh-like backscatter spectrum, and named it the Integrated Backscatter Rayleigh magnitude at some

reference frequency (e.g., IBR5 at 5 MHz). In comparison to other integrated backscatter techniques, the IBR achieves minimum variance and zero bias by optimally weighing the spectral magnitude of the Rayleigh-like scatterers and summing the squared signals over a small region. Correct mechanization of the IBR dictates the system design of the ideal tissue characterization (TC) imager by removing the effects of diffraction and intervening tissue throughout the image field, and by selecting optimal choices for transmitted signal and regional smoothing.

The Ultrasound Equation expresses the round-trip response of a transducer, diffraction, bulk loss, and tissue target system to the transmitted waveform. Forming a TC image requires solving the Ultrasound Equation for the tissue properties over the entire region of the image, which requires precise knowledge of regional variation of all the elements in the equation. Of these elements, the transducer, diffraction, and Rayleigh-like scattering of the myocardium are the best known, and result in straightforward corrections to the received echoes. Most importantly, the optimal estimate of backscatter requires a resultant signal that is whitened before squaring and integrating. We will discuss combining spectrally- shaped transmitted signals with time-varying receiver filters to achieve whitened signals throughout the processed echo. Bulk loss maps are used to direct the dynamic filters. All the above combine to form a calibrated image independent of subject sonographer instrument settings.

PROBABILITY DENSITY OF ULTRASONIC BACKSCATTER FROM TISSUE-EQUIVALENT PHANTOM, IN-VIVO CANINE MYOCARDIUM AND IN-VIVO HUMAN MYOCARDIUM, 3.B. Hampshire II, J.W. Strohbehn, M.D. McDaniel, P.J. Fitzgerald, J.A. Power and

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