Phantom and in vivo evaluation of sound speed estimation methods: Preliminary results

For medical ultrasound imaging, the dynamic receive beamforming is important for improving image quality, i.e., spatial and contrast resolution. In current dynamic receive beamforming, a constant sound speed (e.g., 1540m/s) is assumed. However, the sound speed dispersed in soft tissues leads to defocusing and degradation of image quality. Various methods have been proposed to estimate the proper sound speed with received data, but these methods have not been verified their performance in clinical cases (e.g., breast tissue). In this paper, the five different sound speed estimation methods (i.e., coherent factor (CF), minimum average phase variance (MAPV), minimum average sum-of-absolute difference (MASAD), focus quality spectra (FQS), and modified nonlinear anisotropic difference (MNAD)) are evaluated with the tissue mimicking phantom and the in vivo breast data under the same condition. The pre-beamformed radio-frequency data (RF) for the tissue mimicking phantom and in vivo breast data are acquired using a 7.5-MHz linear array transducer with the SonixTouch research platform connected to the SonixDAQ parallel data acquisition system. In the phantom study, the five methods show considerable performance in estimating the optimal sound speed (i.e., 1450 ± 25 m/s). The CF and FQS methods also show the low errors in the in vivo breast study, but the MAPV, MASAD and MNAD methods have difficulty in estimating the optimal sound speed (1530 m/s) i.e., 25.0 ± 12.9 and 20.0 ± 8.2 vs. 72.5 ± 45.0, 72.5 ± 41.9, 52.5 ± 28.7, respectively. These results indicated that the CF and FQS methods can robustly estimate the optimal sound speed in the homogenous phantom and heterogeneous soft tissues (e.g., breast).