Finite-amplitude effects on ultrasound beam patterns in attenuating media.

Some problems relevant to medical ultrasonics are addressed through experimental measurements of focused, pure-tone beam patterns under quasilinear conditions where significant nonlinearities are manifested. First, measurements in water provide a comparison of the beam patterns of the fundamental and nonlinearly generated harmonics against recent theoretical predictions of others. The radial beamwidths, presence and spacing of sidelobes, axial distances to peak pressures, focal shock parameter, time-domain waveform asymmetry, and post-focal falloff of the fundamental through fifth harmonics are discussed relative to various models under preshock conditions (sigma less than 1). Second, the focused sources are placed in a more attenuating fluid to mimic the behavior of these fields in tissue. The changes in beam characteristics are examined relative to measurements at the same intensities in water, and relative to theoretical predictions. The results suggest that, given a known linear (low-intensity) focused beam pattern in water, guidelines can be followed to predict the beam pattern of the fundamental and higher harmonics at higher intensities in water, and then in attenuating media such as tissue.