Introduction

Metachromatic leukodystrophy (MLD) is a dysmyelinating autosomal recessive disease characterized by accumulation of sulfatide in the white matter (WM) due to the deficiency of the arylsulfatase A enzyme^{1 2} Conventional MRI is usually used in diagnosis and follow-up of these patients despite clinical indiscrepancies with hyperintensity on T2 weighted imaging (WI). Because the patients with milder or no involvement of the brain on T2WI can be a candidate for a bone marrow transplant (BMT), advanced imaging techniques which better reflect myelin abnormality have the potential to improve clinical care.
We aimed to analyse WM in MLD patients using a multiecho T2 mapping analysis technique, Myelin Water fraction (MWF)⁴, which enables the quantitative evaluation of myelin based on T2 relaxometry data and DTI. We also searched for correlations betwen MWF and DTI metrics in both groups.

Methods

The study protocol was approved by IRB and all participants’ parents gave informed consent.
Subjects
Eight patients with MLD (M/F:2/6) and 8 healthy control(HC)s (M/F;1/7) (Mean age (range): 7.8 (4-17) and 9.5 (5-15) respectively) participated in the study. The groups were similar in gender and age (p >.05).
Image Acquisition
All participants underwent MR imaging on a 1.5 T scanner (Magnetom Area, Siemens, Germany) with a 20 channel head coil.
The imaging protocol included whole-brain 3D sagittal T1 MPRAGE (TR/TE: 1900/2.7, ST: 1 mm, NEX: 1, FA: 150), axial T2WI (TR/TE: 3240/99, ST: 5 mm, NEX: 2, FA: 150), axial T2 multiecho CPMG (TR/TE: 2280/9.0, ST: 4 mm, NEX: 1, 32 echoes with 9ms echo spacing, 4 concatenations), and EPI-DTI with 64 independent direction data set (TR/TE: 4300/89, ST:1.8 mm, NEX: 1).
Data Processing and Analysis
MWF maps were calculated by fitting a three-compartment model to the T2 decay curve of each voxel, where each compartment is represented by an inverse Gaussian distribution⁵. MWF was calculated as the ratio of the integral of the first compartment to the total integral of the T2 spectrum. The amount of myelin in different regions of the brain was collected by drawing symmetrical 22 ROIs (6*6 mm^2) on the brainstem; from middle cerebellar peduncle(MCP)s, corticospinal tracts, transverse pontine fibers, medial lemniscus, and on both hemispheres from temporal WM, posterior limb of internal capsules, posterior and frontal periventricular areas, anterior and posterior parts of corona radiata and genu and splenium of corpus callosum (CC) on MWF maps. All the processing was done on MATLAB with in-house developed software.
FA (fractional anisotropy), MD (mean diffusivity), AD (axial diffusivity), RD (radial diffusivity) values were also collected by drawing the same 22 ROIs by using DSI_Studio⁶ software.
To investigate whole-brain voxel-based WM diffusion alterations, tract-based spatial statistics (TBSS)⁷ included in the FSL v.6.0 package was used. After preprocessing, diffusion tensor fitting (FSL DTIFit⁸), and calculation of FA, MD, AD, RD maps, FA maps were registered and aligned to the average space as input for TBSS, and the mean FA skeleton was computed. A permutation-based inference with 500 permutations was performed. Threshold-free cluster enhancement output was obtained and corrected for multiple comparisons. Family-wise error (FWE)-corrected maps were obtained with p <.05. Cluster-based thresholding was performed, which included Gaussian smoothing, application of a threshold (t: 1.5), and forming clusters from 26 neighboring suprathreshold voxels. Due to group analysis for MD, AD, RD images, original non-linear registration was applied to them. To enhance skeletonized MD, AD, RD images, skeletonized mean FA was projected on these images. According to p <.05, FEW-corrected maps achieved.
Statistical analysis for group differences and correlation of imaging data with clinical parameters was performed using R programming using RStudio⁹ The t-test analyses were applied to our data for significance at p <.05. Pearson product-moment correlation coefficient test was used to test whether a correlation was present between myelin maps and DTI metrics.

Results

Structural Changes
The patients showed lower myelin values than HCs in all regions except right medial lemniscus and the differences were statistically significant bilaterally in temporal WM, periventricular areas, corona radiata, CC, bilateral MCPs, and in the left part of transverse pontine fibers (p < .05) (Table 1 and Figure 1).
TBSS-DTI maps showed a diffuse reduction in FA, widespread elevation in RD and less extensive elevation in MD, and limited reduction in AD (Figure 3). As compared with HCs, patients showed significantly lower AD in all regions without statistical significance in any region (p >.05) but a single ROI in CC splenium showing higher (p=.008) AD. Significantly higher RD values in 15 regions (all, p <.05), higher MD values in 6 regions (all, p < .02). Most extensive significant alterations were shown in FA with lower values in all regions of patients than HCs except medial lemniscus and anterior part of corona radiata (p < .05).
Among DTI parameters, the most extensive correlations of myelin were with FA, in 10/22 ROIs (p < .05). Also, there was a correlation between myelin and AD in the internal capsule, MD in genu of corpus callosum, and RD in corpus callosum (p < .05).

Conclusion

Compared to HCs, MLD patients showed a widespread reduction in myelin and FA of white matter. MWF and DTI can provide better surrogate markers of the disease activity.

Acknowledgements

No acknowledgement found.