B0 shimming of the human heart at 7T

Abstract

Purpose: Inhomogeneities of the static magnetic B0 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 B0-field inhomogeneities, and their homogenization (B0 shimming) is a prerequisite. Limitations of state-of-the-art shimming are described, regional B0 variations are measured, and a methodology for spherical harmonics shimming of the B0 field within the human myocardium is proposed. Methods: The spatial B0-field distribution in the heart was analyzed as well as temporal B0-field variations in the myocardium over the cardiac cycle. Different shim region-of-interest selections were compared, and hardware limitations of spherical harmonics B0 shimming were evaluated by calibration-based B0-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 B0-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 B0-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 B0-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 B0 shimming of the human heart at 7 T were described. Cardiac phase–specific shimming strategies were found to be superior to vendor-supplied shimming.