The large spatial inhomogeneity in transmit B 1 field (B 1 + ) observable in human MR images at high static magnetic fields (B 0 ) severely impairs image quality. To overcome this effect in brain T 1 -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 T 1 -weigthed images where the result image was free of proton density contrast, T 2 ⁎ 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 B 1 + 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 T 1 -weighted images, acquired within 12 min, high-resolution 3D T 1 maps were routinely calculated at 7 T with sub-millimeter voxel resolution (0.65-0.85 mm isotropic). T 1 maps were validated in phantom experiments. In humans, the T 1 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 T 1 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. © 2009 Elsevier Inc. All rights reserved.
IntroductionIn the past decade, the magnetization-prepared rapid gradient echo, MPRAGE (Mugler and Brookeman, 1990), sequence has become one of the most commonly used sequences to obtain T 1 -weighted anatomical images of the human brain, in particular at high magnetic field. MPRAGE images are routinely used as anatomical reference for fMRI or for brain tissue classification in voxel-based morphometry (Ashburner and Friston, 2000). However, at high static magnetic fields (≥ 3 T), the increased inhomogeneity of the transmit B 1 + and receive B 1 − fields creates intensity variations throughout the image (bias field). Bias fields not only render segmentation and quantitative analysis difficult but also severely affect image quality at ultra-high fields (≥7 T). The use of adiabatic pulses to perform the inversion in the MPRAGE is only partially able to mitigate the effects of inhomogeneous B 1 . A number of strategies have been proposed to minimize or to correct bias fields generated by the inhomogeneity of the B 1 fields. Most correction strategies aim at correcting the combined (transmit and receive) bias field via post-processing techniques. This can be done either by low-pass filtering (Cohen et al., 2000;Wald et al., 1995) or by fitting slowly varying functions such as Gaussians or low order polynomials (Styner et al., 2000). The result from these low pass filters or fits is then su...