A novel and rapid approach to estimate patient-specific distortions based on mDIXON MRI

While MRI-only radiation treatment planning (RTP) is becoming more widespread, a robust clinical solution for patient-specific distortion corrections is not available. This work explores B ₀ mapping based on mDIXON imaging, often performed for MR-only RTP, as an alternative to separate dual-acquisition gradient-recalled echo imaging, with the overarching goal of developing an efficient and robust approach for patient-specific distortion correction. Initial benchmarking was conducted by scanning a phantom and generating B ₀ field maps with two approaches: (1) conventional B ₀ mapping and (2) experimental mDIXON imaging. Distortion maps were derived from the field maps and compared. The head and neck regions, including brain, of ten healthy volunteers were then evaluated at 1.5 T and 3 T. Distortion maps were again compared between approaches, using difference maps and histogram analysis. Overall, conventional B ₀ mapping was well approximated by mDIXON imaging: The distortions of 95% of the voxels in the phantom estimated by mDIXON and conventional B ₀ mapping differed by  <0.02 mm (1.5 T) and  <0.04 mm (3 T), while the 95-percentiles of the distortions estimated by conventional B ₀ mapping were  <0.06 mm (1.5 T) and  <0.12 mm (3 T). In head and neck the distortions of 99% of the voxels were within  ±0.2 mm at 1.5 T for both approaches and within  ±0.4 mm and  ±0.5 mm at 3 T for mDIXON imaging and conventional B ₀ mapping, respectively. The majority of differences in vivo were confined to regions with high spatial variation of the B ₀ field, mostly around internal air cavities. For 1.5 T, the mDIXON imaging-based correction alone reduced the 95-percentile of distortions from 0.15 mm to 0.03 mm and within the brain from 0.06 mm to 0.02 mm. Slightly lower reductions were observed at 3 T. In conclusion, mDIXON imaging closely approximated conventional B ₀ mapping for patient-specific distortion assessment. Estimates in the brain were in good agreement, and slight differences were observed near air/tissue interfaces in the head and neck. Overall, mDIXON imaging-based B ₀ field maps may be advantageous for rapid patient-specific distortion correction without additional imaging.