ULTRASONIC WAVE DISTORTION MEASURMENT AND CORRECTION

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The general objective of this project is to improve the point and contrast resolution of clinical ultrasonic imaging systems by the development of adaptive focussing techniques. The specific aims of the research are to measure wavefront distortion produced by propagation through tissue paths, to characterize focus degradation associated with this distortion, to calculate propagation through tissue paths based on known ultrasonic and morphologic properties of tissue, and to evaluate existing methods and develop improved methods of adaptive focussing to remove pulse distortion. Pulse-transmission and pulse-echo measurements will be performed using a two-dimensional aperture and a specialized custom experimental apparatus to image a source or a region of scatterers. Time delay differences, energy level variations, waveform shape changes, and first-order as well as second-order statistics of wavefront distortion will be determined from the measurements. The measurements will also be used to characterize the focus and image degradation produced by propagation through a tissue path. All of the measurements will employ fresh specimens of abdominal wall, chest wall, fat, liver, muscle, and appropriate layers thereof obtained during autopsy as well as fat and breast obtained from surgery. Calculations of the wavefront distortion caused by propagation through a tissue path in transmission and pulse-echo configurations will be made using a parabolic approximation. In these calculations, the effect of tissue inhomogeneities will be represented by phase screens that will be constructed from gross and histologic analysis of tissue specimens as well as from ultrasonic measurements. Available adaptive focussing techniques for the removal of pulse distortion will be investigated and critically evaluated. Improved adaptive focussing techniques will be developed through the use of information obtained from the wavefront distortion measurements and calculations. A thorough analysis, statistical summary, and comparison of data obtained in studies comprising each of the specific aims will be made to develop quantitative conclusions about the influence of wavefront distortion in clinical ultrasonic imaging systems and the potential for correction of these distortions. The data analysis will provide information that enables improved resolution from expanded useful apertures. Further improved resolution will be obtained from adaptive focussing techniques developed in this research. The results are expected to provide a foundation for a significant increase in the capability of ultrasound to distinguish between normal and diseased tissue and to determine the severity of disease in circumstances not currently possible.