Aberration correction with OFF: the overdetermined, fan-filtering algorithm

As medical ultrasound imaging moves to larger apertures and higher frequencies, tissue sound-speed variations continue to limit resolution. In geophysical imaging, a standard approach for estimating near-surface aberrating delays is to analyze the time shifts between common-midpoint signals. This requires the collection of complete data - echoes from every combination of an individual source and receiver. Unfocused, common-midpoint signals remain highly correlated in the presence of aberration; there is also tremendous redundancy in the data. In medical ultrasound, this technique has been severely impaired by the wide-angle, random scattering nature of tissue. Until now, it has been difficult to estimate azimuth dependent aberration profiles or to harness the full redundancy in the complete data. Prefiltering the data with two-dimensional fan filters largely solves these problems, permitting highly overdetermined, least-squares solutions for the aberration profiles at many steering angles. In experiments with a tissue-mimicking phantom target and silicone rubber aberrators at non-zero stand-off distances from a 1-D array transducer, this overdetermined, fan-filtering algorithm (OFF) significantly outperformed other published algorithms.