A Subtracted-Added-Divided Inversion Recovery (dSIR) Approach to Visualise the Effects of Microstructure on T1 Contrast in Human White Matter

Recent evidence has demonstrated that several white matter (WM) microstructural features, such as axon diameter, fibre configurations and fibre orientation in respect to the magnetic field influence T1 relaxation. The effects from microstructural features on T1 are small in size, thus, visualising the effects of WM microstructure remains challenging in standard T1 weighted MRI in vivo. Here, we have studied an algebraic approach involving subtraction, addition and division of closely spaced inversion time images in WM imaging, the so-called dSIR approach. Images collected with short TI (300 ms at 3T and 600 ms at 7T) and long TI (600 ms at 3T and 1000 ms at 7T) with MP2RAGE MRI were combined using the dSIR processing. dSIR signal intensities were compared with absolute T1 images. We found that dSIR was linearly related with T1 relaxation time over approximately 200 ms both at 3T and 7T. The slope of the dSIR versus T1 plot was 1.6 times greater at 7T than at 3T indicative of higher dSIR contrast at 7T. dSIR contrast revealed WM tracts that are oriented with high angle (fibre-to-field angle > 75°), in addition, dSIR signal showed angular patterns that closely resembled those of T1 at both fields. The dSIR contrast due to intratissue T1 difference of order of ~50 ms generated by microstructural features, including axon fibre orientation as well as by the presence of large and giant axons in somato-motor subsection of corpus callosum were visualised. It is concluded that dSIR signal mimics T1 and that the dSIR contrast is higher at 7T than at 3T; thus, the approach will help to visualise the effects of microstructure on T1 to evaluate WM integrity.