### ASA 124th Meeting New Orleans 1992 October

## 3pBV4. Time-shift compensation of ultrasonic pulse focus degradation using
least-mean-square error estimates of time delay.

**Dong-Lai Liu
**

**
Robert C. Waag
**

**
**
*Dept. of Elec. Eng., Univ. of Rochester, Rochester, NY 14627
*

*
*
Focus degradation produced by abdominal wall has been compensated using
least-mean-square error estimates of time shifts in measurements of ultrasonic
pulses from a curved transducer that emits a hemispheric wave and simulates a
point source. The pulse waveforms were measured in a two-dimensional aperture
after propagation through a water path and after propagation through different
specimens of human abdominal wall. Time histories of the virtual point source
were reconstructed by removing the time delay produced by geometric path
differences and including time shift produced by propagation inhomogeneities in
the case of compensation, finding the amplitudes of the Fourier harmonics
across the aperture, calculating the Fraunhofer diffraction pattern of each
harmonic, and summing the patterns. This process used a least-mean-square error
solution for the relative arrival time expressed in terms of the arrival time
differences between neighboring points. Comparisons of the time histories in
the source plane show that the -30 dB effective radius of the focus can be 90%
greater for propagation through abdominal wall than through a water path and
that this percentage is reduced to 37% by time-shift compensation.