A Study of in vitro phase aberration measurements in ultrasonic imaging

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Title: A Study of in vitro phase aberration measurements in ultrasonic imaging
Author: Huck, Todd
Abstract: Phase aberrations in soft tissues have been shown in previous studies to create disturbing degradations in the point spread function (psf) of ultrasonic phased array imaging systems. These aberrations are caused by spatial velocity fluctuations due to the inhomogeneous nature of soft tissue. Certain time delay corrections are applied to the signal received by each element of a phased array based on the assumption of constant velocity. Small deviations from this assumption in a soft tissue can result in spatial fluctuation in time delay and have a negative effect on the corrected output. The solution is a two step process. First, the nature and magnitude of the phase aberrations has to be estimated and secondly, a correction procedure must be developed. This project dealt mainly with the first part of the process. A prototype experimental system to measure phase delays in in vitro, excised soft tissue samples (animal and human) was developed and tested. Our approach was slightly different from that found in recent literature in the following ways: (1) very high frequency sampling (1000 MHz) and direct cross correlation of the data without interpolation for delay estimates. (2) use of short pulses of different center frequencies and band spectrums for an identical soft tissue aberrating layer. By comparing our high frequency sampling method to another method where the signal was sampled at a lower frequency (10 MHz) followed by a data interpolation scheme, significant differences were shown to exist between the two. Also, it was shown that the tissue beam pattern in the presence of phase aberrations, was affected by the frequency spectrum of the short pulse used.
Record URI: http://hdl.handle.net/1850/12135
Date: 1994-01

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