Inversion recovery ultrashort echo time imaging of ultrashort T2 tissue components in ovine brain at 3 T: a sequential D2O 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 D2O 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% D2O. 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 (TInull). Then, at this TInull, 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 D2O. A wide range of TInull 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 T1 values of the long T-2 tissue components changed significantly. The T-2* values of STCs were 200-300 mu s in both white and gray matter (comparable with the values obtained from myelin powder and its mixture with D2O or H2O), 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 D2O. The source is more likely to be myelin itself in white matter, and might also be associated with other membranous structures in gray matter.