Background Currently available radiological methods do not completely capture the diversity of multiple sclerosis (MS) lesion subtypes. This lack of information hampers the understanding of disease progression and potential treatment stratification. For example, inflammation persists in some lesions after gadolinium (Gd) enhancement resolves. Novel metabolic and molecular imaging methods may improve the current assessments of MS pathophysiology. Purpose To compare the in vivo proton exchange rate (k(ex)) MRI with Gd-enhanced MRI for characterizing MS lesions. Study Type Retrospective. Subjects Sixteen consecutively diagnosed relapsing-remitting multiple sclerosis (RRMS) patients. Field Strength/Sequence 3.0T MRI with T-2-weighted imaging, postcontrast T-1-weighted imaging, and single-slice chemical exchange saturation transfer imaging. Assessment MS lesions in white matter were assessed for Gd enhancement andk(ex)elevation compared to normal-appearing white matter (NAWM). Statistical Tests Student'st-test was used for analyzing the difference ofk(ex)values between lesions and NAWM, with statistical significance set at 0.05. Results Of all 153 MS lesions, 78 (51%) lesions were Gd-enhancing and 75 (49%) were Gd-negative. Without exception, all 78 Gd-enhancing lesions showed significantly elevatedk(ex)values compared to NAWM (924 +/- 130 s(-1)vs. 735 +/- 61 s(-1),P < 0.05). Of 75 Gd-negative lesions, 18 lesions (24%) showed nok(ex)elevation (762 +/- 29 s(-1)vs. 755 +/- 28 s(-1),P= 0.47) and 57 (76%) showed significantk(ex)elevation (950 +/- 124 s(-1)vs. 759 +/- 48 s(-1),P < 0.05) compared to NAWM. MS lesions withk(ex)elevation appeared nodular (118, 87.4%), ring-like (15, 11.1%), or irregular-shaped (2, 1.5%). Data Conclusion For Gd-enhancing lesions,k(ex)MRI is highly consistent with Gd-enhanced images by showing 100% of elevatedk(ex). For all Gd-negative lesions, the discrepancy onk(ex)MRI may further differentiate active slowly expanding lesions or chronic inactive lesions, supportingk(ex)as an imaging biomarker for tissue oxidative stress and inflammation. Level of Evidence 2 Technical Efficacy Stage 3