Quantitative magnetic resonance mapping of the myelin bilayer reflects pathology in multiple sclerosis brain tissue

UTE MRI for assessing demyelination in an mTBI mouse model: An open-field low-intensity blast study

Mild traumatic brain injury (mTBI) is a leading cause of long-term disability. Following mTBI, secondary chemical cascades and neuroinflammation can result in myelin damage, significantly impairing cognitive function. This study aims to assess demyelination in mice with mTBI induced by open-field low-intensity blast (LIB) using a novel three-dimensional short repetition time adiabatic inversion recovery UTE (3D STAIR-UTE) magnetic resonance imaging (MRI) sequence. Thirty male C57BL/6 mice, with 15 experiencing mTBI and 15 serving as sham controls, were included in this study. Behavioral tests were performed starting at 5 days post-injury to assess motor activity and anxiety-like responses followed by STAIR-UTE imaging using a pre-clinical 3T MRI scanner. Additionally, a proton density-weighted UTE sequence was scanned alongside the STAIR-UTE for quantification of myelin proton fraction (MPF). Luxol fast blue (LFB) staining was performed to evaluate myelin changes between the mTBI group and the control group. The behavioral tests indicated decreased motor activity in the center zone and increased anxiety-like response in the mTBI mice compared to sham controls. The STAIR-UTE sequence revealed significantly lower MPFs in the corpus callosum of mTBI mice (8.4 ± 0.4 % vs. 8.7 ± 0.4 %; P = 0.003), consistent with the myelin reduction observed in the LFB staining (0.77 ± 0.22 vs. 1.09 ± 0.15; P = 0.004). Our findings demonstrate that the STAIR-UTE sequence facilitates quantitative myelin imaging at 3T MRI, enabling the detection of demyelination in the white matter of the mouse brain associated with alterations in motor and anxiety domains post-LIB exposure.

3D balanced SSFP UTE MRI for multiple contrasts whole brain imaging

PURPOSE

This study aimed to develop a new high-resolution MRI sequence for the imaging of the ultra-short transverse relaxation time (uT2) components in the brain, while simultaneously providing proton density (PD) contrast for reference and quantification.

THEORY

The sequence combines low flip angle balanced SSFP (bSSFP) and UTE techniques, together with a 3D dual-echo rosette k-space trajectory for readout.

METHODS

The expected image contrast was evaluated by simulations. A study cohort of six healthy volunteers and eight multiple sclerosis (MS) patients was recruited to test the proposed sequence. Subtraction between two TEs was performed to extract uT2 signals. In addition, conventional longitudinal relaxation time (T1) weighted, T2-weighted, and PD-weighted MRI sequences were also acquired for comparison.

RESULTS

Typical PD-contrast was found in the second TE images, while uT2 signals were selectively captured in the first TE images. The subtraction images presented signals primarily originating from uT2 components, but only if the first TE is short enough. Lesions in the MS subjects showed hyperintense signals in the second TE images but were hypointense signals in the subtraction images. The lesions had significantly lower signal intensity in subtraction images than normal white matter (WM), which indicated a reduction of uT2 components likely associated with myelin.

CONCLUSION

3D isotropic sub-millimeter (0.94 mm) spatial resolution images were acquired with the novel bSSFP UTE sequence within 3 min. It provided easy extraction of uT2 signals and PD-contrast for reference within a single acquisition.

Yet more evidence that non-aqueous myelin lipids can be directly imaged with ultrashort echo time (UTE) MRI on a clinical 3T scanner: a lyophilized red blood cell membrane lipid study

Direct imaging of semi-solid lipids, such as myelin, is of great interest as a noninvasive biomarker of neurodegenerative diseases. Yet, the short T2 relaxation times of semi-solid lipid protons hamper direct detection through conventional magnetic resonance imaging (MRI) pulse sequences. In this study, we examined whether a three-dimensional ultrashort echo time (3D UTE) sequence can directly acquire signals from membrane lipids. Membrane lipids from red blood cells (RBC) were collected from commercially available blood as a general model of the myelin lipid bilayer and subjected to D2O exchange and freeze-drying for complete water removal. Sufficiently high MR signals were detected with the 3D UTE sequence, which showed an ultrashort T2* of ∼77-271 µs and a short T1 of ∼189 ms for semi-solid RBC membrane lipids. These measurements can guide designing UTE-based sequences for direct in vivo imaging of membrane lipids.