Short TE ⁷Li-MRS confirms Bi-exponential lithium T2 relaxation in humans and clearly delineates two patient subtypes

Purpose: (i) To develop an MRS technique to measure ⁷Li levels in human brain in a reasonable scan time, (ii) to develop a technique to quantify ⁷Li T2 relaxation times as measured from human brain in patients taking lithium for the treatment of their bipolar disorder, and (iii) to confirm or refute the presence of bi-exponential ⁷Li T2 relaxation in human brain. Materials and Methods: We modified a spin-echo MRS pulse sequence to decrease its minimum echo time. With IRB approval, we performed lithium MRS with the modified pulse sequence on 13 euthymic bipolar patients stable on long-term lithium to treat their disease. Results: We were able to achieve a total scan time per sample of 8:20; total scan time including imaging, calibration and MRS was approximately 1 h 15 min. We observed bi-exponential T2 relaxation in the majority of patients, with an average short decay time of 5.3 ± 1.4 ms and an average long decay time of 68.2 ± 10.2 ms. However, in two patients we observed strongly mono-exponential T2 relaxation with an average decay time of 47.4 ± 1.3 ms. Conclusion: ⁷Li relaxation patterns may prove useful to distinguish between lithium-responsive and lithium nonresponsive bipolar patients.