Ultimate Intrinsic SNR Depends on B₀ Orientation

This work aims to investigate the effect of $B_0$ orientation on the uiSNR in MRI.

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.

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.

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.