T₁ Over Squared Proton Density Ratio to Characterize Multiple Sclerosis Lesions

Area of Science:

• Neuroimaging
• Quantitative MRI
• Multiple Sclerosis Pathophysiology

Background:

• Histological confirmation is the gold standard for differentiating remyelinated from demyelinated lesions in multiple sclerosis (MS).
• Current MRI techniques often struggle to reliably distinguish these lesion types non-invasively.
• Quantitative MRI (qMRI) metrics offer potential for improved lesion characterization.

Purpose of the Study:

• Introduce and evaluate the T1-to-PD ratio (TPR) imaging approach for MS lesion characterization.
• Assess the ability of TPR to differentiate lesion types compared to other qMRI metrics.
• Correlate TPR imaging findings with established qMRI parameters.

Main Methods:

• Thirty individuals with MS underwent 3T MRI using 3D gradient-echo sequences.
• Quantitative maps of T1, proton density (PD), T2*, quantitative susceptibility mapping (QSM), magnetization transfer ratio (MTR), and MT saturation index (MTsat) were generated.
• TPR images were derived from B1-corrected T1 and PD maps, and lesions were classified as hyper-, hypo-, or iso-intense on TPR.

Main Results:

• TPR imaging visually differentiated MS lesions into hyper-, hypo-, and iso-intense categories, unlike FLAIR where all lesions appeared hyperintense.
• Hyperintense TPR lesions showed significantly higher T1, PD, and T2* values, with lower MTR and MTsat, consistent with demyelination.
• Hypointense TPR lesions exhibited distinct quantitative MRI metrics, potentially indicating tissue repair processes like remyelination.

Conclusions:

• TPR imaging provides a novel visual method to categorize MS lesions based on their underlying tissue composition.
• Hyperintense TPR lesions correlate with increased water and reduced myelin, indicative of active demyelination.
• Hypointense TPR lesions may represent areas of remyelination or other repair mechanisms in MS.