B₀ shimming of the human heart at 7T

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.

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.

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.

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.