Laura M. Hinkelman
Robert C. Waag
Dept. of Elec. Eng., Univ. of Rochester, Rochester, NY 14627
Thomas L. Szabo
Hewlett--Packard Imaging Systems, Andover, MA 01810
Ultrasonic wavefront distortion produced by transmission through human chest wall specimens was measured. Pulses with a center frequency of 2.3 MHz were received over a 20.00x20.16 mm[sup 2] area with a measurement spot size of 0.21x0.40 mm[sup 2] after propagation through a chest wall specimen. Secondary wavefronts produced by interactions between transmitted pulses and structures of the rib cage sometimes disrupted the main wavefront and interfered with the determination of wavefront distortion caused by soft tissue inhomogeneities. Differences in arrival time and energy level between 11.60x14.28 mm[sup 2] regions of the measured waveforms that did not include secondary wavefronts and references that account for geometric delay and spreading were computed. For 16 different intercostal spaces, the average rms value of the arrival time fluctuations was 21.3 ns with a mean correlation length of 2.50 mm. The energy level fluctuations had an average rms value of 1.57 dB and a correlation length of 1.98 mm. The results indicate that soft tissue inhomogeneities in chest wall specimens significantly distort ultrasonic pulses in the low MHz frequency range, although the magnitude of this distortion is noticeably smaller than that measured for abdominal wall specimens at 3.75 MHz [Hinkelman et al., J. Acoust. Soc. Am. 95, 530--541 (1994)].