Abstract :
[en] Contrary to conventional MRI (cMRI), quantitative MRI (qMRI) quantifies tissue physical microstructural properties and improves the characterization of cerebral damages in relation with various neurological diseases. With a multi-parameter mapping (MPM) protocol, 4 parameter maps are constructed: saturated magnetization transfer (MTsat), proton density (PD), longitudinal relaxation (R1) and effective transverse relaxation (R2*) rates, reflecting tissue physical properties associated with iron and myelin contents. Here, we used qMRI to investigate the microstructural changes happening over time in multiple sclerosis (MS).
Seventeen MS patients (age 25-65, 11 RRMS) were scanned on a 3T MRI, with at least one year separation between two acquisition sessions, and the evolution of their parameters was evaluated within several tissue classes: normal appearing white matter (NAWM), normal appearing cortical and deep gray matter (NACGM and NADGM) as well as focal white matter (WM) lesions. Brain tissue segmentation was performed using US-with-Lesion, an adapted version of the Unified Segmentation (US) algorithm accounting for the lesion tissue class, based on qMRI and FLAIR images. An individual annual rate of change for each qMRI parameter was computed, and its correlation to clinical status was evaluated. As for WM plaques, three areas were defined within them. A Generalized Linear Mixed Model (GLMM) tested the effect of area and time points, as well as their interaction on each median qMRI parameter value.
Patients with a better clinical evolution showed positive annual rate of change in MT and R2* within NAWM and NACGM, suggesting repair mechanisms in terms of increased myelin content and/or axonal density. When examining focal WM lesions, qMRI parameters within surrounding NAWM showed modification in terms of reduction in MT, R1 and R2* combined with increased of PD even before any focal lesion is visible on conventional FLAIR MRI.
The results illustrate the benefit of multiple qMRI data in monitoring subtle changes within normal appearing brain tissues and plaque dynamics in relation with tissue repair or disease progression.
