Robust Minimmun Variance Beamformer using Phase Aberration Correction Methods

摘要

The minimum variance (MV) beamformer is an adaptive beamforming method that has the potentialto enhance the resolution and contrast of ultrasound images. Although the sensitivity of the MVbeamformer to steering vector errors and array calibration errors is well-documented in other fields, inultrasound it has been tested only under gross sound speed errors. Several robust MV beamformershave been proposed, but have mainly reported robustness only in the presence of sound speed mismatches.Additionally the impact of PAC methods in mitigating the effects of phase aberration in MVbeamformed images has not been observedAccordingly, this thesis report consists on two parts. On the first part, a more complete analysisof the effects of different types of aberrators on conventional MV beamforming and on a robust MVbeamformer from the literature (Eigenspace-based Minimum Variance (ESMV) beamformer) is carriedout, and the effects of three PAC algorithms and their impact on the performance of the MV beamformerare analyzed (MV-PC). The comparison is carried out on Field II simulations and phantom experimentswith electronic aberration and tissue aberrators. We conclude that the sensitivity to speed of sounderrors and aberration limit the use of the MV beamformer in clinical applications, and that the effectof aberration is stronger than previously reported in the literature. Additionally it is shown that undermoderate and strong aberrating conditions, MV-PC is a preferable option to ESMV.On the second part, we propose a new, locally-adaptive, phase aberration correction method (LAPAC)able to improve both DAS and MV beamformers that integrates aberration correction for each point inthe image domain into the formulation of the MV beamformer. The new method is tested using fullwavesimulations of models of human abdominal wall, experiments with tissue aberrators, and in vivocarotid images. The LAPAC method is compared with conventional phase aberration correction withdelay-and-sum beamforming (DAS-PC) and MV-PC. The proposed method showed between 1-4 dBhigher contrast than DAS-PC and MV-PC in all cases, and LAPAC-MV showed better performancethan LAPAC-DAS. We conclude that LAPAC may be a viable option to enhance ultrasound imagequality of both DAS and MV in the presence of clinically-relevant aberrating conditions.