Mean sound speed estimation with focusing quality evaluation for medical ultrasound imaging

In ultrasound receive beamforming, a fixed sound speed (e.g., 1540 m/s) is assumed for calculating focusing time delays. However, due to the variations of sound speeds incurring in soft tissues, phase distortions are introduced, leading to defocusing and consequent degradation in image quality. The phase distortion becomes severe when imaging fatty tissues and obese patients. The various sound speed estimation methods have been proposed, but these methods require the substantial amount of the computer complexity. In this paper, a new mean sound speed estimation method, in which focusing quality is examined by computing the minimum average sum of the absolute difference (MASAD) of raw radio-frequency (RF) data during receive beamforming, is presented to enhance spatial and contrast resolution. The proposed MASAD method was evaluated with the in vitro data from a tissue mimicking phantom by using a 7.5-MHZ linear array transducer with the Ultrasonix's SonixTouch research platform connected the SonixDAQ parallel data acquisition system. For quantitative evaluation, the lateral resolution was measured with contour maps of -3 dB, -6 dB and -12 dB.