Transmission of ultrasound beams through human tissue--focusing and attenuation studies.

Abstract The focussing of ultrasound through human tissue has been measured for a range of transducer diameters (11–50 mm). The defocussing effects appear to be due to two major mechanisms: (1) attenuation and (2) phase distortion of the converging wavefront due to refractive index variations in the tissue. Attenuation studies show that the first mechanism is responsible for a large proportion of defocussing and can be controlled by the frequency bandwidth of the system. The second mechanism is important for heterogeneous tissues such as breast, where large fractions of the ultrasound energy can be deflected off axis. Focussing properties were highly dependent on tissue type, however for all tissues, optimum focussing was achieved below a focal-number of 3.