T1/T2 Ratio Imaging Improves Cortical Lesion Contrast in Multiple Sclerosis on 3T MRI

Background: Cortical demyelinated lesions are prevalent in multiple sclerosis (MS) and associated with disability; their presence on MRI has recently been incorporated into MS diagnostic criteria. Presently, advanced and ultra-high-field MRI - not routinely available in clinical practice - are the most sensitive methods for detection of cortical lesions, and approaches utilizing MRI sequences obtainable in routine clinical practice remain an unmet need. Objective: To assess the sensitivity of the ratio of T1-weighted and T2-weighted (T1/T2) signal intensity for focal cortical lesions in comparison to other established, sensitive, advanced and high-field imaging methods. Methods: 3-tesla (3T) and 7-tesla (7T) MRI collected from 10 adults with MS participating in a natural history study at the National Institutes of Health were included in the study. T1/T2 images were calculated by dividing 3T T1w images by 3T T2w fluid-attenuated inversion recovery (FLAIR) images for each participant. Cortical lesions were identified using 7T T2*w and T1w images and corresponding voxels were assessed on registered 3T images. For each participant, ratios derived from the median signal intensity of nonlesional tissue in the cortical region of the lesion and the median lesional voxel intensity were computed. These values were compared across 3T imaging sequences, including the calculated T1/T2 image, as well as T1w, T2w, and Inversion Recovery Susceptibility Weighted Imaging with Enhanced T2 weighting (IR-SWIET) images. Results: 614 cortical lesions were identified on 7T images. 3T T1/T2 images demonstrated a larger contrast between median nonlesional cortical signal intensity and median cortical lesion signal intensity (median ratio = 1.29, range 1.19 - 1.38) when compared to T1w (1.01, 0.97 - 1.10, p<0.002), T2w (1.17, 1.07 - 1.26, p<0.002), and IR-SWIET (1.21, 1.01 - 1.29, p<0.03). Conclusion: T1/T2 images are sensitive to cortical lesions. Approaches incorporating T1/T2 could improve the accessibility of cortical lesion detection in research settings and clinical practice.