In vivo T-1 and T-2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

Purpose Low-field (B-0 < 0.1 T) MRI has generated much interest as a means of increased accessibility via reduced cost and improved portability compared to conventional clinical systems (B-0 >= 1.5 Tesla). Here we measure MR relaxation times at 50 mT and compare results with commonly used models based on both in vivo and ex vivo measurements. Methods Using 3D turbo spin echo readouts, T-1 and T-2 maps of the human brain and lower leg were acquired on a custom-built 50 mT MRI scanner using inversion-recovery and multi-echo-based sequences, respectively. Image segmentation was performed based on a histogram analysis of the relaxation times. Results The average T-1 times of gray matter, white matter, and cerebrospinal fluid (CSF) were 327 +/- 10 ms, 275 +/- 5 ms, and 3695 +/- 287 ms, respectively. Corresponding values of T-2 were 102 +/- 6 ms, 102 +/- 6 ms, and 1584 +/- 124 ms. T-1 times in the calf muscle were measured to be 171 +/- 11 ms and were 130 +/- 5 ms in subcutaneous and bone marrow lipid. Corresponding T-2 times were 39 +/- 2 ms in muscle and 90 +/- 13 ms in lipid. Conclusions For tissues except for CSF, the measured T-1 times are much shorter than reported at higher fields and generally lie within the range of different models in the literature. As expected, T-2 times are similar to those seen at typical clinical field strengths. Analysis of the relaxation maps indicates that segmentation of white and gray matter based purely on T-1 or T-2 will be quite challenging at low field given the relatively small difference in relaxation times.