Inversion recovery ultrashort echo time imaging of ultrashort T 2 tissue components in ovine brain at 3 T: a sequential D 2 O exchange study.

Inversion recovery ultrashort echo time (IR-UTE) imaging holds the potential to directly characterize MR signals from ultrashort T ₂ 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 T ₂ * signals are contaminated by water protons, including those associated with myelin sheaths. This study investigated such a possibility in an ovine brain D ₂ O exchange model and explored the potential of IR-UTE imaging for the quantification of ultrashort T ₂ * signals in both white and gray matter at 3 T. Six specimens were examined before and after sequential immersion in 99.9% D ₂ O. Long T ₂ 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 T ₂ signals (TI _null ). Then, at this TI _null , images with echo times (TEs) of 0.01-4 ms were acquired to measure the T ₂ * values of STCs. The PD-FSE signal dropped to near zero after 24 h of immersion in D ₂ O. A wide range of TI _null values were used at different time points (240-330 ms for white matter and 320-350 ms for gray matter at TR = 1000 ms) because the T ₁ values of the long T ₂ tissue components changed significantly. The T ₂ * values of STCs were 200-300 μs in both white and gray matter (comparable with the values obtained from myelin powder and its mixture with D ₂ O or H ₂ O), and showed minimal changes after sequential immersion. The ultrashort T ₂ * signals seen on IR-UTE images are unlikely to be from water protons as they are exchangeable with deuterons in D ₂ 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.
(Copyright © 2017 John Wiley & Sons, Ltd.)