In vivo T1 and T2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

MRM Papers Volume 87(2) - February 2022
1

link to paper

In vivo T1 and T2 relaxation time maps of brain tissue, skeletal muscle, and lipid measured in healthy volunteers at 50 mT

Thomas O’Reilly, Andrew G. Webb

Abstract

Purpose

Low-field (B0 < 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 (B0 ≥ 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, T1 and T2 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 T1 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 T2 were 102 ± 6 ms, 102 ± 6 ms, and 1584 ± 124 ms. T1 times in the calf muscle were measured to be 171 ± 11 ms and were 130 ± 5 ms in subcutaneous and bone marrow lipid. Corresponding T2 times were 39 ± 2 ms in muscle and 90 ± 13 ms in lipid.

Conclusions

For tissues except for CSF, the measured T1 times are much shorter than reported at higher fields and generally lie within the range of different models in the literature. As expected, T2 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 T1 or T2 will be quite challenging at low field given the relatively small difference in relaxation times.

2

The conclusion states “line widths over the entire 3D imaging volume were less than 1 kHz and therefore unlikely to introduce significant [diffusion-effect-induced] errors”. The body of the text mentions that the line widths are less than 1kHz, but I did not readily find the reasoning that this leads to a low likelihood of errors. After all, 1kHz is not that small, and if it reflects localized inhomogeneity, it could (in theory) stil lead to diffusion effects. But maybe the reasoning could be as follows: “local inhomogeneities scale linearly with field strength and diffusion effects scale quadratically with local field variation, so the effect on T2 measurements will be negligible”