Regional variations in normal brain shown by quantitative magnetization transfer imaging

Abstract

A quantitative magnetization transfer imaging (qMTI) study, based on a two-pool model of magnetization transfer, was performed on seven normal subjects to determine, on a regional basis, normal values for the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon. Regions were identified on high-resolution anatomical scans using a combination of manual and automatic methods. Only voxels identified as pure tissue at the resolution of the quantitative scans were considered for analysis. While no left/right differences were observed, significant differences were found among white-matter regions and gray-matter regions. These regional differences were compared with existing cytoarchitectural data. In addition, the pattern and magnitude of the regional differences observed in white matter was found to be different from that reported previously for an alternative putative MRI measure of myelination, the 10 -50-ms T 2 component described as myelin water.Magn Quantitative magnetization transfer imaging (qMTI) is a new methodology that yields in vivo parametric images of the pool sizes, exchange, and relaxation parameters that characterize the MT phenomenon (1,2). In the past, MT ratios (MTRs) have been used extensively in the study of white-matter disease, particularly multiple sclerosis. Recent studies have also reported MTR changes in diseases affecting the gray matter, such as dementia (3,4). However, the interpretation and comparison of these results has been hampered by the reduction of the MT phenomenon to a single parameter.Unlike MTRs, the parametric images determined by qMTI reflect intrinsic MR properties of the tissue, which are independent of the details of the acquisition pulse sequence. This facilitates the comparison of MT parameters between groups and individuals. As a first step toward subsequent population comparisons, we report here the results of a study of normal individuals to determine normative values for the MT parameters and assess variations in these parameters among brain regions.The qMTI protocol used here is based on a two-pool model of MT in which the nuclei of hydrogen atoms bound to free-moving water molecules exchange magnetization with a pool of hydrogen bound to large molecules with relatively restricted motion. By selectively modifying the magnetization state of this latter restricted-motion pool using off-resonance RF pulses, one can infer this pool's relaxation properties and exchange rate by using conventional MRI to observe changes in the free-moving proton pool. The qMTI protocol yields the relative size of the restricted pool (F), the forward exchange rate (k f ), the T 2 of the free and restricted pools (T 2,f and T 2,r ), the T 1 of the free pool (T 1,f ), and the apparent T 1 and T 2 (T 1,obs , T 2,obs ) of the system as a whole. In addition, a reference solution was scanned for each subject, which allowed the proton density of each pool (PD f and PD r ) to be computed relative to water.While the MT phenomenon is not specific to a particular mole...

Characterizing healthy and diseased white matter using quantitative magnetization transfer and multicomponent T2 relaxometry: A unified view via a four‐pool model

“…The restricted pool spin-lattice relaxation constant R 1r , to which the fit of the two-pool model is relatively insensitive, was fixed to a value of 1 sec -1 . This is identical to the analysis we perform on in vivo imaging data (8,(26)(27)(28). In the MT experiments, gaussian noise was added to the simulated data to produce an SNR of 75 in the data without MT saturation, to match roughly what we obtain in routine QMTI experiments at 1.5 T. Again, results from 10,000 noise iterations were averaged.…”

Optimal acquisition schemes for in vivo quantitative magnetization transfer MRI

“…Second, we use Eq. [7] to account for the variability of MT parameters within the brain (18,35). The optimizations for the single set and the 15 ROIs return very similar schemes (schemes 2 and 3), with almost identical values of V and V max , which suggests that the optimum sampling depends only weakly on the exact parameter settings in the optimization, at least over the range of combinations observed in healthy brain tissue.…”

Quantitative magnetization transfer and myelin water imaging of the evolution of acute multiple sclerosis lesions

“…Strong correlations have been observed between F and the myelin lipid content observed with the Luxol fast blue stain in fixed (6) and fresh (7) postmortem tissue. QMTI parameter variations in the WM of healthy controls are consistent with tissue myelination (8), and densely myelinated WM fiber tracts are distinguishable in maps of the restricted pool fraction (9). In MS, F is substantially reduced in chronic lesions (5,(10)(11)(12), and small but significant decreases in F have also been reported in the normal-appearing WM (NAWM) of patients (11,13).…”