Absorption and attenuation in soft tissues. II. Experimental results

Abstract

For pt.I see ibid., vol.35, no.2, p.242 (1988). To determine the relative contributions of ultrasonic loss mechanisms in tissues, independent measurements of total attenuation and local absorption were obtained at discrete frequencies within the range of medical diagnostic equipment, 1-13 MHz. Automation techniques were applied to aspects of experimentation where extensive averaging or curve fitting could be used to improve accuracy. Novel approaches were also implemented to calibrate focused beams for use in pulse-decay absorption measurements which covered a wider frequency range, utilizing smaller sample volumes than were previously practical. These approaches enable comparisons of the magnitude and frequency dependence of attenuation and absorption in biological media, and permit some inferences to be made as to the relative contribution of scattering to total attenuation. The results of studies on agar-gelatin phantoms, calf liver, and collagenous pig liver indicate that absorption comprises 90-100% of total attenuation in these materials. Studies on bovine brain matter and leg muscle are less definitive because complications include the inhomogeneous grey and white matter composition of brain, and fiber anisotropy in muscle. However, average results from these tissues also show a major contribution of absorption to attenuation.