The general objective of this project is to achieve up to an order of magnitude improvement in point and contrast resolution of clinical ultrasound imaging systems by using enlarged apertures and adaptive focusing.
The specific aims of the research are to make pulse-echo measurements of ultrasonic propagation through tissue, to determine from these measurements parameters that describe aberration, to calculate propagation through tissue based on known ultrasonic and morphologic properties of tissue, to form high-resolution b-scan images using adaptive focusing that compensates for aberration, 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 large aperture, two-dimensional array with programmable transmit waveforms. Newly developed aberration correction methods based on a general filter-bank model for aberration will be refined and thoroughly evaluated. 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. Also, the size of the region over which aberration can be satisfactorily compensated with a single set of parameters will be measured as will pulse-echo sensitivity to off-axis scatterers. Calculations of wavefront distortion caused by a tissue path will use a k-space computational technique with cross-sectional maps of tissue properties constructed from gross and histologic analysis. Imaging will be accomplished with adaptive beamformation using estimated filter-bank parameters. Focus characteristics and image resolution will be carefully evaluated and critically compared to available geometric and adaptive focusing techniques. Quantitative conclusions about the effects of aberration in ultrasonic imaging and the performance of correction algorithms will be developed. Improved resolution from expanded useful apertures and adaptive focusing 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 possible now because resolution is limited by aberration.
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