Axonal Integrity and Fractional Anisotropy in the Normal-Appearing White Matter of Patients with Multiple Sclerosis: Relationship to Cerebro-Functional Reorganization and Clinical Disability

Preliminary findings in 10 MS patients suggest that axonal integrity (as reflected in H-MRSI NA/Cr values) within normalappearing white matter (NAWM) is strongly – but not completely – related (r2=0.53) to the structural integrity of NAWM [as reflected in diffusion-tensor-imaging fractional anisotropy (FA)]. The relationship between NAWM-NA/Cr and both (i) the amount of cerebro-functional compensation (as reflected by degree of fMRI-activity lateralization during a hand-movement task) and (ii) the clinical disability (as reflected by EDSS scores) that is seen in these patients (r2=0.66, r2=0.72, respectively) is stronger than the relationship between these same measures and NAWM-FA (r2=0.35, r2=0.38, respectively). Introduction Proton MR spectroscopic imaging (MRSI) studies of cerebral N-acetylaspartate (NA; a marker of neuronal integrity) in patients with multiple sclerosis (MS) have shown that clinically-relevant axonal disturbances occur even in white matter that appears normal on conventional MRI (NAWM). Diffusion tensor imaging (DTI) studies have shown that fractional anisotropy [FA; a measure of (i) the degree of alignment of cellular structures within fiber tracts and (ii) the structural integrity of these tracts] is also affected in the NAWM of MS patients. In the present study, we examined: (i) the relationship between NA and FA in the NAWM of MS patients, (ii) the relationship of NAWM-NA and -FA to the cerebro-functional reorganization seen on functional MRI (fMRI) in such patients 4,5 and (iii) the relationship of NAWM-NA and FA to the clinical disability seen in these patients [as measured with Kurtzke’s Expanded Disability Scale (EDSS)]. Methods Subjects: We studied 10 patients (8 women and 2 men) with definite MS [8 relapsing-remitting MS and 2 secondary-progressive MS] from the Multiple Sclerosis Clinic of the Montreal Neurological Hospital. Proton MRI / MRSI: MRI and MRSI examinations were performed in the same scanning session using a 1.5T, Philips Gyroscan ACS II using a body coil transmitter and a quadrature head-coil receiver. MRI: Fifty 3mm thick, contiguous proton-density-weighted (PD) and T2-weighted images were acquired parallel to the AC-PC line using a dual turbo spinecho sequence (TR 2075 ms, TE 30/90 ms, 256×256 matrix, field of view (FOV) 250 mm). T1-weighted images were acquired using a 3D gradient-echo sequence (TR 35 ms, TE 10.2 ms, 40°excitation angle). MRSI: A 20-mm-thick MRSI volume-of-interest (VOI; a volume that contains multiple MRI voxels) that measured 90 mm x 90 mm in the axial plane was centered on the corpus callosum. MRSI data were acquired using a double spin-echo excitation method (TR 2000, TE 272 ms, 32×32 phase-encodes, FOV 250×250 mm) and post-processed as described previously. Metabolite resonance-intensities were determined automatically from fitted-peak areas using in-house software. For each voxel, signal intensities from NA were expressed as ratios to creatine (Cr) in the same MRSI voxel. Mean NA/Cr ratios for the entire VOI were also calculated for each patient. DTI / fMRI: DTI and fMRI examinations were performed in the same scanning session using a 1.5 T Siemens Magnetom Vision scanner. DTI: DTI data were acquired from each of 5 horizontal slices through the MRSI-VOI using an echo-planar sequence (TR 3000 ms, TE 98, voxel size 1.875 x 1.875 x 5.0 mm) with 5 signal averages performed following magnitude reconstruction. Diffusion-sensitizing gradients were applied in 6 directions isotropically spanning half the unit sphere with two nonzero b-values (300 s/mm and 600 s/mm) per direction. The data were processed to determine the diffusion tensor on a pixel-by-pixel basis for each of the slices. FA was calculated from the eigenvalues λi of the diffusion tensor as follows: