CROSS-SECTIONAL MEASUREMENTS OF ULTRASONIC WAVE-FRONT DISTORTION CAUSED BY MODEL RANDOM-MEDIA

A curved array of receivers and a hemispheric emitter were used to mea sure amplitude and phase distortion caused by propagation through rand om medium phantoms at frequencies of 0.7 MHz, 1.2 MHz, and 2.2 MHz. Th e receiver array contained twelve modules each with sixteen elements 0 .68 mm wide and 0.88 mm high spaced 0.10 mm apart and resulted in 192 measurements over the cap of a sphere at polar angle increments of 0.1 76-degrees spanning a total angle of 33.8-degrees. Azimuthal rotation of the array in 2.04-degrees increments produced measurements along 88 longitudinal segments. Field measurements were made for propagation t hrough a homogeneous water path to characterize receiver element sensi tivity variations and geometric positioning errors as well as inherent nonuniformities in the emitted field. These meaSurements were used as a reference for comparison of data collected when each of two phantom s, one comprised of graphite particles in a gel and the other containi ng 1 cm diameter agar spheres in gel, were introduced between the rece iving array and the emitter to distort the ultrasonic beam. In all the measurements, phase was unwrapped and a second-degree polynominal fit was subtracted to obtain the phase fluctuations. Phase fluctuation po wer spectra were then calculated by averaging the square magnitudes of Fourier transforms which were taken over intervals that correspond to modules comprising the receiver array. The phantom containing the aga r spheres was found to introduce significantly more distortion in the ultrasonic beam than that produced by the graphite-gel random medium t hat was found to produce a distortion only slightly above the system n oise. The data collection and processing steps of this investigation m ay now be applied to characterize ultrasonic wavefront distortion prod uced by other model media and tissues.