Abstract

Inversion recovery ultrashort echo time (IR-UTE) imaging holds the potential to directly characterize MR signals from ultrashort T2 tissue components (STCs), such as collagen in cartilage and myelin in brain. The application of IR-UTE for myelin imaging has been challenging because of the high water content in brain and the possibility that the ultrashort T2 * signals are contaminated by water protons, including those associated with myelin sheaths. This study investigated such a possibility in an ovine brain D2 O exchange model and explored the potential of IR-UTE imaging for the quantification of ultrashort T2 * signals in both white and gray matter at 3T. Six specimens were examined before and after sequential immersion in 99.9% D2 O. Long T2 MR signals were measured using a clinical proton density-weighted fast spin echo (PD-FSE) sequence. IR-UTE images were first acquired with different inversion times to determine the optimal inversion time to null the long T2 signals (TInull ). Then, at this TInull , images with echo times (TEs) of 0.01-4ms were acquired to measure the T2 * values of STCs. The PD-FSE signal dropped to near zero after 24h of immersion in D2 O. A wide range of TInull values were used at different time points (240-330ms for white matter and 320-350ms for gray matter at TR=1000ms) because the T1 values of the long T2 tissue components changed significantly. The T2 * values of STCs were 200-300μs in both white and gray matter (comparable with the values obtained from myelin powder and its mixture with D2 O or H2 O), and showed minimal changes after sequential immersion. The ultrashort T2 * signals seen on IR-UTE images are unlikely to be from water protons as they are exchangeable with deuterons in D2 O. The source is more likely to be myelin itself in white matter, and might also be associated with other membranous structures in gray matter.

Highlights

  • The myelin sheath is a lipid‐protein lamellar membranous structure

  • The present study aimed to explore myelin as a source of the ultrashort T2 signals seen in white matter (WM) on Inversion recovery ultrashort echo time (IR‐ultrashort echo time (UTE)) images, and to assess the feasibility of using IR‐UTE to image the ultrashort T2 tissue components (STCs) in gray matter (GM) on a 3‐T clinical scanner in an ovine brain D2O exchange model

  • After sequential exchange with D2O, the specimens in this study showed a gradual decrease in proton density (PD)‐fast spin echo (FSE) signals

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Summary

Introduction

The myelin sheath is a lipid‐protein lamellar membranous structure. It envelops axons and is essential for the rapid propagation of electrical signals in the nervous system. Several relaxometry studies have characterized T2 distributions in brain tissue, reporting values of 10–50 ms in water associated with myelin sheaths.[5,6,7,8] Signals from these distributions are detectable with conventional clinical pulse sequences. These include T1‐ and T2‐weighted fast spin echo (FSE) imaging, gadolinium‐enhanced MRI, diffusion tensor imaging and magnetization transfer imaging. These techniques have all shown high sensitivity for MS lesions,[9,10,11,12,13,14] but measures obtained with these techniques have not been strongly correlated with clinical manifestations.[10,11,12,13,14,15,16,17] It is possible that this lack of correlation is because these measures cannot differentiate demyelination and remyelination from other pathological substrates, such as axonal loss and gliosis, which are associated with heterogeneous clinical manifestations of MS.[18,19] Methods for direct myelin imaging in vivo provide a more specific and sensitive evaluation of myelin, and might be of considerable value

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