B0shimming of the human heart at 7T

Abstract

Purpose Inhomogeneities of the static magnetic B(0)field are a major limiting factor in cardiac MRI at ultrahigh field (>= 7T), as they result in signal loss and image distortions. Different magnetic susceptibilities of the myocardium and surrounding tissue in combination with cardiac motion lead to strong spatio-temporal B-0-field inhomogeneities, and their homogenization (B(0)shimming) is a prerequisite. Limitations of state-of-the-art shimming are described, regional B(0)variations are measured, and a methodology for spherical harmonics shimming of the B(0)field within the human myocardium is proposed. Methods The spatial B-0-field distribution in the heart was analyzed as well as temporal B-0-field variations in the myocardium over the cardiac cycle. Different shim region-of-interest selections were compared, and hardware limitations of spherical harmonics B(0)shimming were evaluated by calibration-based B-0-field modeling. The role of third-order spherical harmonics terms was analyzed as well as potential benefits from cardiac phase-specific shimming. Results The strongest B-0-field inhomogeneities were observed in localized spots within the left-ventricular and right-ventricular myocardium and varied between systolic and diastolic cardiac phases. An anatomy-driven shim region-of-interest selection allowed for improved B-0-field homogeneity compared with a standard shim region-of-interest cuboid. Third-order spherical harmonics terms were demonstrated to be beneficial for shimming of these myocardial B-0-field inhomogeneities. Initial results from the in vivo implementation of a potential shim strategy were obtained. Simulated cardiac phase-specific shimming was performed, and a shim term-by-term analysis revealed periodic variations of required currents. Conclusion Challenges in state-of-the-art B(0)shimming of the human heart at 7 T were described. Cardiac phase-specific shimming strategies were found to be superior to vendor-supplied shimming.