Biexponential longitudinal relaxation in white matter: Characterization and impact on T₁ mapping with IR‐FSE and MP2RAGE

Abstract

Purpose Magnetization transfer in white matter (WM) causes biexponential relaxation, but most quantitative T1 measurements fit data assuming monoexponential relaxation. The resulting monoexponential T1 estimate varies based on scan parameters and represents a source of variation between studies, especially at high fields. In this study, we characterized WM T1 relaxation and performed simulations to determine how to minimize this deviation. Methods To characterize biexponential relaxation, four volunteers were scanned at 3T and 7T using inversion recovery fast spin echo (IR-FSE) with 13 inversion times (TIs). Three volunteers were scanned with IR-FSE using TIs chosen by simulations to reduce T1 deviation, and with MP2RAGE. Results At 3T, the biexponential relaxation has a short component of T1 = 48 ms (9%) and a long component of T1 = 939 ms. At 7T the short component is T1 = 57 ms (11%) and the long component is 1349 ms (89%). For IR-FSE, acquiring four TIs with a minimum of 150 ms (3T) or 200 ms (7T) yielded monoexponential T1 estimates that match the long component to within 10 ms. For MP2RAGE, significant differences (90 ms at 3T, 125 ms at 7T) remain at all parameter values. Conclusion Many T1 mapping sequences yield robust estimates of the long T1 component with suitable choice of TIs, allowing reproducible, sequence-independent T1 values to be measured. However, this is not true of MP2RAGE in its current implementation.

Effects of magnetization transfer on T 1 contrast in human brain white matter

“…8, indicating the minimum TI values at which the apparent (instantaneous) T 1 becomes independent of TI. This conclusion was also reached in a very recent paper investigation bi-exponential relaxation (Rioux, Levesque et al 2015). The effects of bi-exponential relaxation in IR experiments can be further minimized by the use of high power adiabatic inversions that fully invert WP and fully saturate MP.…”

In vivo observation and biophysical interpretation of time-dependent diffusion in human white matter

“…Biexponential T 1 relaxation has been reported, with a short T 1 component of about T 1 short ≈ 50 ms, originating, presumably, from water in-between myelin sheets (Does and Gore, 2002; Rioux et al, 2015). By itself, this compartment is NMR-invisible, since the relatively long TE of 100 ms in our experiment greatly exceeds myelin water T 2 value, measured around 10–20 ms (Does and Gore, 2002; Stanisz et al, 1999; Whittall et al, 1997).…”

Assessing White Matter Microstructure in Brain Regions with Different Myelin Architecture Using MRI

“…Therefore, we arguably chose to directly assess these MR parameters, rather than measure T1 in white matter tracts. Since a substantial gain in sensitivity towards microstructure can be obtained by separating the short from the long T1 components [131,132] future studies may prove that this parameter is actually a sensitive, if not specific, measure for assessing myelin.…”