The role of gray and white matter segmentation in quantitative proton MR spectroscopic imaging

Abstract

Since the brain's gray matter (GM) and white matter (WM) metabolite concentrations differ, their partial volumes can vary the voxel's 1H MR spectroscopy (1H-MRS) signal, reducing the sensitivity to changes. While single-voxel 1H-MRS cannot differentiate the WM from GM signals, partial volume correction is feasible in MR spectroscopic imaging (MRSI), using segmentation of the MRI that is always acquired for VOI placement. To determine the magnitude of this effect on metabolic quantification, we segmented the 1 mm3 resolution MRI into GM, WM and CSF masks that were co-registered with the MRSI grid to yield their partial volumes in every ~1 cm3 spectroscopic voxel. Each voxel then provided one equation with two unknowns – its i- metabolite's GM and WM concentrations: CiGM, CiWM. With the voxels' GM and WM volumes as independent coefficients, that over-determined system of equations can be solved for the global, averaged CiGM and CiWM. Trading off local concentrations differences offers three advantages: (i) higher sensitivity due to combined data from many voxels; (ii) improved specificity to WM versus GM changes; (iii) reduced susceptibility to partial volume effects. These improvements make no additional demands of the protocol, measurement time or hardware. Applying the approach to 18 volunteers' 3D MRSI sets, 480 voxels each, yielded N-acetylaspartate, creatine, choline and myo-inositol CiGM of 8.5±0.7, 6.9±0.6, 1.2±0.2, 5.3±0.6 mM; and CiWM of 7.7±0.6, 4.9±0.5, 1.4±0.1 and 4.4±0.6 mM. We show that unaccounted voxel WM or GM partial volume can vary absolute quantification by 5–10% (more for ratios) that can often as much as double the sample size required to establish statistical significance.

Metabolic Abnormalities in the Hippocampus of Patients with Schizophrenia: A 3D Multivoxel MR Spectroscopic Imaging Study at 3T

“…2 b – d . An in-house program (MATLAB 14, Mathworks, Framingham, MA) calculated the fraction of each tissue mask inside each voxel (36), from which F jk = (GM volume+ WM volume)/ (voxel volume) for Eq. [1] was obtained.…”

“…2. The software then estimated the global GM and WM concentrations of each metabolite in the retained N ≥ 2 hippocampus voxels using linear regression, as described previously (36). This approach overcomes the GM/WM partial volume issue encountered by single-voxel MRS.…”

Enhanced Axonal Metabolism during Early Natalizumab Treatment in Relapsing-Remitting Multiple Sclerosis

“…A segmentation based on existing methods 21,22 was then applied to determine the specific metabolite concentrations for each tissue type and region. This approach accounts for the partial volume effects of GM, WM, and CSF by using the respective PVE values of the MRSI voxels and accounts for change in brain volume with time.…”

“…Moreover, inaccurate repositioning of the MRSI slab is partially compensated by the method, correcting for the change in the tissue content of every voxel. [21][22][23] On the basis of the PVE, we determined 3 tissue types: GM, NAWM, and LWM. NAWM was defined as the WM content of voxels that contain Ͻ1% lesion PVE, whereas LWM was defined as the WM volume of voxels that contain Ͼ5% lesion PVE.…”

Metabolic Imaging in Humans

“…On the other hand, metabolic differences between white matter and grey matter have also been estimated to account for partial volumes effects when the other matter is diffused into the matter of interest 58 . After studying with 18 volunteers, it was found that NAA, Cr, Cho, and myo -Ino concentrations in grey matter were 8.5 ± 0.7, 6.9 ± 0.6, 1.2 ± 0.2, and 5.3 ± 06 mM, while those in white matter were 7.7 ± 0.6, 4.9 ± 0.5, 1.4 ± 0.1, and 4.4 ± 0.6 mM, respectively.…”