Improved detection of focal cortical lesions using 7T magnetisation transfer imaging in patients with multiple sclerosis.

Background: Cortical lesions account for a larger proportion of brain demyelination than white matter (WM) lesions. They are often missed on conventional MRI. Recently studies improved the detection of cortical lesions using 7T T2(⁎), 7T MPRAGE and 3T DIR but it seems that we are still able to detect only "the tip of the iceberg". In this study we report for the first time the systematic use of high resolution MTR in MS and compare MTR lesion detection with 7T MPRAGE, 7T T2(⁎) and 3T 3D DIR.

Objectives: We report the use of high resolution, fast, magnetisation transfer imaging (MTI) at 7T in MS focusing on the detection of cortical lesions.

Subjects And Methods: Eighteen patients with MS were scanned (Expanded Disability Status Scale score: 3.0, mean age: 48 years, mean disease duration: 7.25 years). The scans were compared to nine healthy control subjects (mean age 36.5 years).

Data Acquisition: We acquired 7T MPRAGE images, 7T MTR maps, 7T T2(⁎)and 3T 3D DIR. The WM was segmented from the MPRAGE and removed to obtain only the cortical grey matter ribbon (cGMR) mask. The mask was then applied to the different modalities (MPRAGE, MTR, DIR, T2(⁎)w) previously registered onto the MPRAGE volume. The analysis of the cGMR was performed by two observers blinded to the disease state.

Results: In patients with MS 365 lesions in total were detected with 7T MTR (mean 20.28 lesions per patient), 289 lesions were detected with 7T MPRAGE (mean 16.06 lesions) and 231 lesions were detected with 7T T2(⁎) (mean 12.83 lesions). In the 8 MS subjects who had 3T 3D DIR acquired on the same day, a total of 136 lesions (mean 17 lesions per patient) were detected as opposed to 171 lesions with 7T MTR, 147 lesions were detected with 7T MPRAGE and 126 lesions with 7T T2(⁎) in the same patients.

Conclusion: We found that 7T MTR, in less than 10min scanning time, was able to detect cortical lesions. In this study we found that 7T MTR was better in detecting intracortical lesions in comparison with 7T T2(⁎), 7T MPRAGE, and 3T 3D DIR. since only a very few intracortical lesions were detected in healthy controls in our blind assessment, it is likely that the lesions detected represent focal grey matter demyelination. High resolution MT imaging has especially revealed cortical changes that have not been recognised by other MR sequences. MTR maps were noisier than MPRAGE, T2(⁎) and DIR, but also better in localising cortical lesions. As MTR is more pathologically specific than other sequences in detecting tissue myelination, it raises the possibility that high resolution MTR will be able to demonstrate cortical remyelination in vivo.