MP2RAGE, a self bias-field corrected sequence for improved segmentation and T₁-mapping at high field

The large spatial inhomogeneity in transmit B₁ field (B₁⁺) observable in human MR images at high static magnetic fields (B₀) severely impairs image quality. To overcome this effect in brain T₁-weighted images, the MPRAGE sequence was modified to generate two different images at different inversion times, MP2RAGE. By combining the two images in a novel fashion, it was possible to create T₁-weigthed images where the result image was free of proton density contrast, T₂* contrast, reception bias field, and, to first order, transmit field inhomogeneity. MP2RAGE sequence parameters were optimized using Bloch equations to maximize contrast-to-noise ratio per unit of time between brain tissues and minimize the effect of B₁⁺ variations through space. Images of high anatomical quality and excellent brain tissue differentiation suitable for applications such as segmentation and voxel-based morphometry were obtained at 3 and 7 T. From such T₁-weighted images, acquired within 12 min, high-resolution 3D T₁ maps were routinely calculated at 7 T with sub-millimeter voxel resolution (0.65-0.85 mm isotropic). T₁ maps were validated in phantom experiments. In humans, the T₁ values obtained at 7 T were 1.15 ± 0.06 s for white matter (WM) and 1.92 ± 0.16 s for grey matter (GM), in good agreement with literature values obtained at lower spatial resolution. At 3 T, where whole-brain acquisitions with 1 mm isotropic voxels were acquired in 8 min, the T₁ values obtained (0.81 ± 0.03 s for WM and 1.35 ± 0.05 for GM) were once again found to be in very good agreement with values in the literature.