Indirect detection of myelin water by T2-relaxation during the RF pulse

Abstract: Introduction: The axonal myelin sheath is the main cause of magnetic resonance imaging (MRI) contrast between gray matter (GM) and white matter (WM) in the brain. It encloses a small pool of myelin water (MW) with a short T2 of about 15 ms. Common signal equations of MRI sequence assume an instantaneous RF excitation pulse followed by free relaxation. As known from ultra-short echo time (UTE) MRI, deviations from this signal behavior may occur when the relaxation times are shorter than or of the same order of magnitude as the duration of radio frequency (RF) excitation pulse. In this MSc thesis project, it was studied whether such transverse in-pulse relaxation effects can be used to detect MW by increasing the RF pulse duration to the range of the MW T2. By such an approach, the in-pulse relaxation effects, which alter the MRI signal, would arise mainly from MW. Material and methods: By numerical integration of the Bloch equations, in-pulse relaxation effects on the magnetization were studied for rectangular (RECT), Gaussian and sinc-shaped (SINC) pulses of 10 ms pulse duration in order to find the optimal RF pulse for imposing different degrees of saturation onto the longitudinal magnetizations of MW and intra–/extra- axonal water with optimal excitation profile, i.e., minimal degradation in the signal intensities. The effect of longitudinal relaxation during the RF pulse was neglected because T1 of MW and intra– and extra-axonal water (IE-water) is much longer than the duration of the RF pulse. The fast low angle shot (FLASH) pulse sequence of a 3 T MR scanner (Siemens Magnetom Skyra) was modified to provide two pulse durations of 0.5 ms and 10 ms using a Gaussian RF pulse. FLASH MRI at variable flip angles was carried out on three cream phantoms (12 %, 27 % and 40 % fat content), a formalin-fixated pig brain and a healthy volunteer. The measurement with short pulse duration served as a reference to the difference in apparent T1. Results: The simulations indicated that in-pulse relaxation would result in a reduced partial saturation of MW magnetization which is largely independent on flip angle and amounting to between 6 % (SINC) and 21 % (RECT). A Gaussian shape (11 % reduced partial saturation) was implemented experimentally, as this shape was less sensitive to frequency offsets, due to the shape of the excitation profile, than the RECT. With the long pulse, the apparent T1 was about 25 % shorter in WM and 10 % shorter in GM, for both the fixated brain and in vivo, and the effects were thus much larger than expected from the simulations. The spatial distribution of the T1 reduction showed more pronounced reduction in the WM, where MW is localized. The simulations for the cream phantoms indicated a halving of the in-pulse relaxation effects compared with the MW and IE-water in vivo, and thus no mapping of the apparent T1 was performed. Conclusion: The reference measurement was most likely affected by magnetization transfer (MT) effects from macromolecules, for which saturation is also influenced by RF pulse duration. Thus, the shorter the pulse duration, the more the MT effects are pronounced. Compared with in-pulse relaxation effects, the MT effects seemed to be dominating, and further studies are needed to separate these effects.