The general objective of this project is to achieve up to an order of magnitude improvement in point and contrast resolution of clinical ultrasonic imaging systems through the use of enlarged apertures and adaptive focussing techniques.
The specific aims of the research are to make pulse-echo measurements of ultrasonic propagation through tissue paths, to determine from these measurements parameters that describe wavefront distortion, to calculate propagation through tissue paths based on known ultrasonic and morphologic properties of tissue, to form high-resolution b-scan images using adaptive focussing that compensates for wavefront distortion, and to evaluate focus characteristics and image resolution in view of propagation path and scattering object morphology. Pulse-echo measurements will be made using a unique two- dimensional array with 6400 elements and a special experimental apparatus with programmable transmit waveforms. Fresh specimens of abdominal wall, chest wall, fat, liver, muscle, and appropriate layers thereof obtained during autopsy as well as breast obtained from surgery will be employed for tissue paths. Point reflectors, tissue mimicking scattering phantoms, and fresh tissues such as liver will be used for scattering objects. From the measurements, arrival time fluctuations, waveform shape changes, and statistics of distortion will be determined. Calculations of wavefront distortion caused by a tissue path will use a finite-difference, time-domain technique with cross-sectional maps of tissue properties constructed from gross and histologic analysis. Imaging will be accomplished with new adaptive beamformation techniques based on backpropagation, space-time processing, and random input filter models of propagation that all employ information derived from measured pulse-echo data. Focus characteristics and image resolution will be carefully evaluated and critically compared to available geometric and adaptive focussing techniques. Quantitative conclusions about wavefront the effects of distortion in ultrasonic imaging and the performance of correction algorithms will be developed. Improved resolution from expanded useful apertures and adaptive focussing techniques resulting from success in this research will significantly increase the capability of ultrasound to distinguish between normal and diseased tissue and to determine the severity of disease in circumstances not currently possible.
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