Dynamic parallel transmit diffusion MRI at 7T

Abstract

AbstractDiffusion MRI (dMRI) is inherently limited by SNR. Scanning at 7T increases intrinsic SNR but 7T MRI scans suffer from regions of signal dropout, especially in the temporal lobes and cerebellum. We applied dynamic parallel transmit (pTx) to allow whole-brain 7T dMRI and compared with circularly polarized (CP) pulses in 6 subjects.Subject-specific 2-spoke dynamic pTx pulses were designed offline for 8 slabs covering the brain. We used vendor-provided B0and B1+mapping. Spokes positions were set using the Fourier difference approach, and RF coefficients optimized with a Jacobi-matrix high-flip-angle optimizer. Diffusion data were analyzed with FSL.Comparing whole-brain averages for pTx against CP scans: mean flip angle error improved by 15% for excitation (2-spoke-VERSE 15.7°vsCP 18.4°,P=0.012) and improved by 14% for refocusing (2-spoke-VERSE 39.7°vsCP 46.2°,P=0.008). Computed spin-echo signal standard deviation improved by 14% (2-spoke-VERSE 0.185 vs 0.214 CP,P=0.025). Temporal SNR increased by 5.4% (2-spoke-VERSE 8.47vsCP 8.04, P=0.004) especially in the inferior temporal lobes. Diffusion fitting uncertainty decreased by 6.2% for first fibres (2-spoke VERSE 0.0655 vs CP 0.0703,P<0.001) and 1.3% for second fibers (2-spoke VERSE 0.139vsCP 0.141,P=0.01). In conclusion, dynamic parallel transmit improves the uniformity of 7T diffusion-weighted imaging. In future, less restrictive SAR limits for parallel transmit scans are expected to allow further improvements.