Physical Principles of the Ultrasonic Attenuation and Backscatter of Soft Tissues: Dependence on the Angle of Propagation and the Physiologic State

Quantitative measurements of the interactions of ultrasound with inhomogeneous media serve the dual purposes of elucidating the physics of such interactions and making possible the noninvasive assessment of the state or quality of those materials. One objective of the research presented in this thesis was to provide a physical basis for the application of quantitative techniques to measurements of interactions of ultrasound with an intrinsically interesting inhomogeneous material, soft tissue. A survey of theoretical techniques relevant to the investigation of ultrasonic propagation in inhomogeneous media with both symmetric and non-symmetric inhomogeneities is presented, and the results of calculations based on these theories are compared to experimental measurements. Interactions of ultrasound with several types of soft tissues were investigated under a variety of conditions. Canine myocardium was studied in the normal state, during ischemic insult, after prolonged ischemia, and during reperfusion following transient ischemia. In addition, isolated bullfrog gastrocnemius muscle was studied during relaxation and tetany. The ultrasonic attenuation and backscatter of soft tissues were found to be dependent upon several physiologic and morphologic parameters. In tissues which exhibit regular organization into longitudinal fibers, such as muscles, the attenuation and backscatter were found to be dependent on the direction of propagation relative to the direction of the dominant orientation of the fibers. Specifically, the attenuation was greatest for propagation parallel to the dominant fiber direction, while backscatter was maximum for propagation perpendicular to the fibers. In contrast, these parameters were found to be independent of the angle of insonification in liver, a soft tissue that does not contain regularly oriented fibers. The attenuation and backscatter were affected by the state of contraction of skeletal muscle, with both attenuation and backscatter increasing during tetany. The backscatter of myocardium was observed to vary systematically throughout the cardiac cycle, with the backscatter decreasing at the onset of systole and increasing during diastole. This variation was obliterated during transient ischemia and was partially restored with reperfusion.