Deaberration of incoherent wavefront distortion: An approach toward inverse filtering

Techniques for the correction of ultrasonic wavefront distortion are compared using measured pulse transmission through human breast specimens. The measured data were obtained by recording the pulse signals on each element of a linear array that was moved in elevation to synthesize a two-dimensional aperture. The one-way point spread functions were reconstructed using matched filtering technique, phase conjugation, time-delay compensation, and backpropagation followed by phase conjugation, each with and without amplitude compression. Two measures of performance were compared, the mainlobe diffraction shape and the ratio of the energy outside the mainlobe to the energy inside the mainlobe. Matched filtering, which compensates the phase distortion but also increases the variation in the modulus, performs more poorly than other techniques with respect to both measures. Phase conjugation and time-delay compensation, which leave the magnitude of the wavefront unchanged, have similar beamwidths but phase conjugation is consistently superior with respect to energy ratio. The backpropagation method, which models wavefront distortion using a phase screen at a computed position between the source and aperture, is shown to perform better. The use of a novel amplitude compression that approaches inverse filtering improves the performance of the compensation techniques significantly. This is because inverse filtering optimizes image fidelity, in contrast, for example, to matched filtering, which optimizes SNR. With the amplitude compression method, the results of the one-way experiments show that the mainlobe shape can be recovered down to -30 dB.