Quantification of brain proton longitudinal relaxation (T1 ) in lithium-treated and lithium-naïve patients with bipolar disorder in comparison to healthy controls.

Abstract

BackgroundProton longitudinal relaxation (T1) is a quantitative MRI‐derived tissue parameter sensitive to myelin, macromolecular, iron and water content. There is some evidence to suggest that cortical T1 is elevated in bipolar disorder and that lithium administration reduces cortical T1. However, T1 has not yet been quantified in separate groups containing lithium‐treated patients, lithium‐naïve patients, and matched healthy controls.MethodsEuthymic patients with bipolar disorder receiving lithium (n = 18, BDL) and those on other medications but naïve to lithium (n = 20, BDC) underwent quantitative T1 mapping alongside healthy controls (n = 18, HC). T1 was compared between groups within the cortex, white matter and subcortical structures using regions of interest (ROI) derived from the Desikan‐Killiany atlas. Effect sizes for each ROI were computed for BDC vs BDL groups and Bipolar Disorder vs HC groups.ResultsNo significant differences in T1 were identified between BDL and BDC groups when corrected for multiple comparisons. Patients with bipolar disorder had significantly higher mean T1 in a range of ROIs compared to healthy controls, including bilateral motor, somatosensory and superior temporal regions, subcortical structures and white matter.ConclusionsThe higher T1 values observed in the patients with bipolar disorder may reflect abnormal tissue microstructure. Whilst the precise mechanism remains unknown, these findings may have a basis in differences in myelination, macromolecular content, iron and water content between patients and controls.