Introduction

MS is characterized by demyelination, axonal loss and iron deposition in white matter (WM) lesions1. Yet, to date, it is unclear to what extent these pathological processes are present in MS WMLs. In fact, MS lesions undergo multiple waves of de- and re-myelination, which lead to the final lesion phenotype of either demyelinated or partly/fully remyelinated2. Iron content is also extremely heterogeneous across MS WMLs and many chronic active MS lesions harbor iron-laden macrophages3. QSM is a field-to-source inversion method to map the local susceptibility sources in the biological tissue4. QSM has been used to classify MS lesions with respect to their myelin and iron content5 and it has been shown that most of MS lesions appears hyperintense in QSM, which might be due to loss of diamagnetic myelin or to accumulation of paramagnetic iron, or both. Very recently, novel method was proposed (magnetic susceptibility source separation- ‘chi-separation’), which disentangles the iron and the myelin contribution to the QSM signal 6. In the present study, we applied this method to study the comparative contribution of myelin loss and iron deposit to QSM hyperintensity in MS WMLs.

Methods

Fifty MS patients underwent multi-parametric MRI in a 3T Prisma system (Siemens Healthcare, Germany) using a 64-channel head coil. The MRI protocol included: (i) 3D FLAIR (TR/TE/TI=5000/386/1800 ms), T1 map and MP2RAGE (TR/TI1/ TI2=5000/700/2500 ms) with resolution 1 mm3; (ii) FastT2-prep for myelin water imaging (TR/TE/resolution = 7.5/0.5 ms/1.25x1.25x5 mm3)7 ; (iii) (III) multi-echo gradient echo images (ME-GRE; spatial resolution 0.75 x 0.75 x 3mm3, TR 49ms, number of echoes 10 and TEs=6.7, 10.8, 14.8, 18.9, 22.9, 27.0, 31.1, 35.1, 39.2, 43.2ms). WML were automatically segmented 8 and manually corrected (n = 485). WM masks were obtained using Freesurfer9. Hyperintense QSM lesions were identified on 3D EPI QSM (n = 286). Three subjects and a total of 22 lesions were excluded because of being affected by artifacts. A 4-voxel peri-plaque (PP) layer of normal-appearing WM surrounding the lesions was automatically computed. For all included lesions the relative negative and positive susceptibility changes (hereinafter called delta^neg, delta^pos), as measured in negative and positive chi-separation maps, were calculated as: (PP-WMLs)/PP. And the lesions were classified into four groups: (i) Both delta^neg & delta^pos are positive (i.e. their values are not less than 0). (ii) Both delta^neg & delta^pos are negative. (i.e. their values are less than 0). (iii) delta^neg is positive & delta^pos is negative. (iv) delta^neg is negative & delta^pos is positive.

Results

286/485 (58.96%) of MS WMLs showed QSM hyperintensity. The lesion-wise distribution of positive and negative susceptibility inside QSM hyperintense WMLs shows a wide distribution (figure 1). Out of 264 included lesions, in 168 lesions (63.63%) both delta^neg & delta^pos were positive (group i, Figure 2-B), showing therefore lower negative and positive susceptibility inside WMLs compared with PP tissue. In 15/264 WMLs (5.68%), both delta^neg (small negative values) and delta^pos were negative (group ii, Figure 2-C). In 78/264 WMLs (29.54%), delta^neg was positive and delta^pos was negative (group iii, Figure 2-A). Three remaining lesions were included in group iv and, where delta^neg was negative and delta^pos was positive. However, they were located at the boundary between WM and gray matter or close to WM bundles were the delta calculation is not reliable.

Discussion

Our results showed that most QSM hyperintense lesions exhibited positive delta^neg & delta^pos, revealing that those lesions exhibit low myelin and iron content compared to PP tissue. We have previously suggested that some QSM lesions may correspond to chronic inactive lesions, which are characterized by extensive demyelination and low activated-cells content (that are rich in iron): the current findings support therefore this previous hypothesis2. In 29.88% of WML QSM hyperintensity was due to both demyelination and iron deposition, which is a characteristic of iron-laden macrophages in demyelinated areas10. In a minority of QSM hyperintense lesions (5.68 %), both delta^neg and delta^pos were negative, suggesting that those lesions may correspond to actively remyelinating focal plaques 11.

Conclusion

Our study disentangles first the contribution of myelin loss and iron deposition in MS WMLs which appears hyperintense in QSM. Our results show that QSM hyperintensity encompasses three categories of MS WMLs and mostly represents co-occurrence of iron and myelin loss in chronic inactive MS WMLs. A small proportion of this lesions, appear to be actively remyelinating.

Acknowledgements

We acknowledge all the study participants.

References

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