High-Resolution Ultrasonic Imaging via Estimation and Correction of Aberration
Robert C. Waag, PhD
Arthur Gould Yates Professor of Engineering
Departments of Electrical and Computer Engineering and Radiology, and Rochester Center for Biomedical Ultrasound, University of Rochester, Rochester, NY
The use of ultrasound to produce images for diagnosis has become common because sound is nonionizing at low levels, soft tissues have different mechanical properties, and technologic advances have enabled practical implementation of sophisticated apparatus. Although many ultrasonic imaging instruments employ one-dimensional arrays to focus dynamicaly and to scan the beam electronically using an assumed sound speed and geometrically determined delays, instrument performance is now limited by present apertures and waveform distortion. Large apertures using two-dimensional arrays with aberration correction, specialized electronics, and new algorithms for beamformation are required to scan volumes of tissue with resolution determined only by aperture size, pulse length, and frequency (wavelength) and not by wavefront distortion. To achieve this goal, wavefront distortion produced by abdominal wall, breast, and chest wall has been measured and the abdominal and chest wall measurements have been compared to distortion simulated using sound speed and density determined from stained cross sections. The results show that waveform distortion varies with tissue type, scattering contributes significantly to distortion, and arrival time and energy level fluctuations can be large compared to adjustment increments in current beamformers. Pulse-echo studies using different f-number transmitters have shown that wavefront distortion is significantly underestimated with a broad incident beam and demonstrated that a sharply focussed incident beam is necessary for good estimation of time shifts and for effective compensation. With a novel 80x80 element two-dimensional array system, point-spread functions have been found for propagation through a water path and through a tissue-mimicking aberration path without and with compensation for aberration. Also, pulse-echo images have been formed through a water path, through a tissue-mimicking aberrator, and through the aberrator using aberration correction that consisted of time-shift compensation in the transmit-receive aperture or backpropagation followed by time-shift compensation. Backpropagation followed by time-shift compensation was found to be more effective than time-shift compensation in the aperture. The results demonstrate the possibility of high-resolution ultrasonic image formation using aberration correction and provide a basis for further development of ultrasonic imaging technology for improved medical diagnosis.