B₀ shimming 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.