Ultimate Intrinsic SNR Depends on B0 Orientation.

PURPOSE: This work aims to investigate the effect of B 0 orientation on the uiSNR in MRI. METHODS: Electromagnetic simulations were performed on both realistic body models and simplified geometric models. Comparisons between (1) opposite B 0 directions along the head-foot (HF) axis, corresponding to head-first and feet-first patient orientations, at various field strengths, and (2) B 0 along the HF, anterior-posterior (AP), and left-right (LR) directions at 0.55 T, corresponding to closed-bore (HF) and open (AP/LR) MR systems. The voxel-wise ratio of uiSNR between different B 0 orientations was calculated to characterize spatial variations throughout the models. RESULTS: For closed-bore MRI systems, flipping the patient orientation results in a spatial redistribution of uiSNR, and the magnitude of this redistribution increases with B 0 field strength. At 0.55 T, open MR systems with either AP- or LR-oriented B 0 yield higher uiSNR than closed-bore systems in the center of cylindrical-shaped structures. In realistic body models, LR-oriented B 0 yields higher uiSNR than AP-oriented B 0 in most regions at 0.55 T, with both orientations outperforming HF-oriented B 0 in the center of the body. CONCLUSION: The uiSNR distribution varies with B 0 orientation in realistic body models. At around 0.55 T, the flexibility in B 0 orientation enabled by different magnet configurations may serve as an additional degree of freedom for improving imaging performance, as uiSNR is strongly influenced by the orientation of the main field.