One significant safety concern of MR imaging is radio-frequency (RF) induced tissue heating. This effect has been widely investigated for various body models, positions in the coils, and anatomies [1-3]. This work investigates the impact of patient orientation on specific absorption rate (SAR) for 3 Tesla (T) two-channel parallel transmit (pTx) pelvis imaging.

Simulations were performed using SEMCAD X v14.8.6 (SPEAG). Two human body models from the Virtual Family [4] (Duke, 72.4 kg male, and Ella, 59.6 kg female) were positioned in a supine position in a representative 70 cm-diameter, 16-rung, 3 T body coil in a pelvis landmark. The models were evaluated in both “head first” and “feet first” patient entry orientations (defined arbitrarily in the simulations), which is equivalent to a forward-ramped or a reversed-ramped magnet configuration (see Figure 1).

The body coil was excited at 128 MHz using unit voltage source excitations. The resultant electric and magnetic fields in the body for each channel were exported using a uniform (2 mm)^3 grid. The data was post-processed using MATLAB (Mathworks). Following the Q-matrix formulation [5], whole body SAR, peak spatial SAR, and SAR ratio (= peak SAR / whole body SAR) were computed for quadrature excitation, and 3-spokes (see Figure 2) and 5-spokes pTx pulses. The SAR values were scaled to an average B1rms of 1 µT in the central axial (x-y) slice of the body. Peak SAR was computed by averaging over a volume of 10x10x10 voxels.

SAR (at B1rms = 1 µT) is shown in Table 1. In all cases, the whole body SAR is approximately unchanged for quadrature and pTx. For Ella, the maximum peak SAR (and SAR ratio) for orientation 2 at quadrature are substantially higher than for orientation 1. In addition, the peak SAR (and SAR ratio) for orientation 1 are substantially increased (30-40%) for pTx as compared to quadrature. However, for orientation 2, the peak SAR and SAR ratios for pTx are within ~10% of quadrature. For Duke, in both orientations, all three SAR values are within 10% of quadrature (peak SAR and SAR ratio for pTx are slightly less than quadrature).

Coronal cross-sectional slices for the SAR distributions at the location of the maximum peak spatial SAR for quadrature excitation (left panel), 3-spokes pTx (middle panel) and 5-spokes pTx (right panel) are shown in Figure 3 for Ella for both patient entry orientations. The SAR distributions in the body are changed for pTx excitation as compared to quadrature. The intent of pTx pulses is to improve B1 uniformity within a particular anatomy (e.g., abdomen or pelvis) as compared to quadrature; this generally leads to a more uniform SAR distribution.

For Ella, the location of maximum peak SAR is in the wrist for all cases. For orientation 1, the location of the peak SAR flips from the left wrist to the right wrist when comparing quadrature to pTx. However, for orientation 2, the location of peak SAR does not change. Since one of Ella's wrists is closer to the body than the other wrist, the field rotation and distribution for different excitation schemes significantly affect local SAR. For Duke, the location of peak SAR is either in the wrist or hip/thigh (not shown).

When evaluating the performance of pTx excitations, pTx SAR is generally compared to quadrature SAR. However, patient orientation is often not considered. This work shows that patient position and orientation are significant factors in evaluating pTx excitation, particularly when comparing pTx to quadrature excitation, since peak SAR and SAR ratio can vary drastically (up to 40%). This has important implications for evaluating patient safety and system performance. Although only one body coil was evaluated, it is expected that these results will generalize to any birdcage-type body coil, and will extend to more than two excitation channels. Future work will consider additional body models, pTx pulses, and patient positions. Peak SAR and SAR ratio for two-channel pTx excitations are highly dependent on patient position and orientation (head-first vs feet-first) within the body coil.