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

Abstract

Quantitative magnetization transfer imaging provides in vivo estimates of liquid and semisolid constituents of tissue, while estimates of the liquid subpopulations, including myelin water, can be obtained from multicomponent T 2 analysis. Both methods have been suggested to provide improved myelin specificity compared to conventional MRI. The goal of this study was to investigate the sensitivity of each technique to the progression of acute, gadolinium-enhancing regions of multiple sclerosis. Magnetization transfer and T 2 relaxometry data were acquired longitudinally over the course of 1 year in five relapsing-remitting multiple sclerosis patients and in five healthy controls. Parametric maps were analyzed in enhancing lesions and normal-appearing white matter regions. Quantitative magnetization transfer parameters in lesions were most abnormal at the time of enhancement and followed a pattern of recovery over subsequent months. Lesion myelin water fraction was abnormal but did not show a significant trend over time. Quantitative magnetization transfer was able to track the degree and timing of the partial recovery in enhancing multiple sclerosis lesions in a small group of patients, while the recovery was not detected in myelin water estimates, possibly due to their large variability. Our data suggest the recovery is characterized by quick resolution of inflammation and a slower remyelination process. Key words: quantitative; magnetization transfer; multicomponent T 2 ; multiple sclerosis; acute lesions A few MRI methods have been suggested to provide improved pathologic specificity in human white matter (WM) over conventional MRI sequences. Two techniques proposed to be more specific for myelin are quantitative magnetization transfer imaging (QMTI) and multicomponent T 2 analysis of spin echo data (QT2). In this longitudinal study, both techniques were used to follow the progression of the gadolinium (Gd)-enhancing region of acute lesions of multiple sclerosis (MS).The magnetization transfer (MT) effect can be exploited to produce contrast in MRI (for a review see Henkelman et al. (1) and Tofts (2)) and is sensitive to the principal constituents of myelin in human WM (3). MT can be described using a two-component model, grouping spins into the free (or liquid) pool, consisting of water hydrogen nuclei with long T 2 (T 2f > 10 ms), and the restricted (or semisolid) pool, consisting of hydrogen nuclei of large lipids with T 2 too short to be imaged directly using MRI (T 2r < 100 ls). Detailed information about these spin populations can be mapped using offresonance QMTI techniques, with an appropriate twopool model of MT in tissue (4). QMTI yields the relative size of the restricted proton pool (F), the first-order forward magnetization exchange rate (k f ), and most relaxation parameters of the free and restricted pools (R 1f , T 2f , and T 2r ) (5).While QMTI measurements do not provide absolute specificity to the molecular constituents of tissue, there is convergent evidence that MT changes in WM reflect cha...

Quantitative Magnetic Resonance Imaging of the Brain : Applications for Tissue Segmentation and Multiple Sclerosis

“…Over time, T2 decreases to that of chronic lesions (84,139,170), similar to the behaviour of T1. One study detected monoexponential relaxation in acute WM lesions, which became bi-exponential during remission, and finally returned to mono-exponential as the lesion turned into a chronic lesion (168).…”

“…The initial increase in T2 of the lesion has also been attributed to increased water content due to the oedema (84,168). Over time, T2 decreases to that of chronic lesions (84,139,170), similar to the behaviour of T1.…”

“…In acute WM lesions T1, T2 and PD are all substantially increased (73,84,(167)(168)(169) which, during the following months, decreases towards slightly elevated T1, T2 and PD seen in chronic lesions (84,170). The main pathological process in the initial increase is attributed to inflammatory oedema, causing increases in both T1 and T2, as well as PD.…”

“…Technological developments have therefore been suggested to characterise and quantify DAWM, as well as to detect changes in NAWM. These techniques include diffusion tensor imaging (DTI) (157,158,160), quantitative magnetisation transfer imaging (qMTI) (84,155,158), quantitative MR spectroscopy (qMRS) (161,162), and qMRI (84, 157, 158).…”

Clinical Applications of Synthetic MRI of the Brain

“…[16,92] Previous reports of relaxation measurements in MS lesions have shown variable result. Some findings are in line with our study, with shorter T2 in enhancing lesions and longer T2 in non-enhancing lesions [93], and longer T2 in enhancing lesions compared to NAWM [94]. An early study reported a decrease in T1 and T2 over time in acute brain stem lesions.…”