Optimization of White-Matter-Nulled Magnetization Prepared Rapid Gradient Echo (MP-RAGE) Imaging

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Abstract

To optimize the white-matter-nulled (WMn) Magnetization Prepared Rapid Gradient Echo (MP-RAGE) sequence at 7 Tesla (T), with comparisons to 3T.Optimal parameters for maximizing signal-to-noise ratio (SNR) efficiency were derived. The effect of flip angle and repetition time (TR) on image blurring was modeled using simulations and validated in vivo. A novel two-dimensional (2D) -centric radial fan beam (RFB) k-space segmentation scheme was used to shorten scan times and improve parallel imaging. Healthy subjects as well as patients with multiple sclerosis and tremor were scanned using the optimized protocols.Inversion repetition times (TS) of 4.5 s and 6 s were found to yield the highest SNR efficiency for WMn MP-RAGE at 3T and 7T, respectively. Blurring was more sensitive to flip in WMn than in CSFn MP-RAGE and relatively insensitive to TR for both regimes. The 2D RFB scheme had 19% and 47% higher thalamic SNR and SNR efficiency than the 1D centric scheme for WMn MP-RAGE. Compared with 3T, SNR and SNR efficiency were higher for the 7T WMn regime by 56% and 41%, respectively. MS lesions in the cortex and thalamus as well as thalamic subnuclei in tremor patients were clearly delineated using WMn MP-RAGE.Optimization and new view ordering enabled MP-RAGE imaging with 0.8-1 mm(3) isotropic spatial resolution in scan times of 5 min with whole brain coverage.

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