Yet more evidence that myelin protons can be directly imaged with UTE sequences on a clinical 3T scanner: Bicomponent <mml:msubsup>T2*</mml:msubsup> analysis of native and deuterated ovine brain specimens

PurposeUTE sequences with a minimal nominal TE of 8 mu s have shown promise for direct imaging of myelin protons (T-2,<1 ms). However, there is still debate about the efficiency of 2D slice-selective UTE sequences in exciting myelin protons because the half excitation pulses used in these sequences have a relatively long duration (e.g., 0.3-0.6 ms). Here, we compared UTE and inversion-recovery (IR) UTE sequences used with either hard or half excitation pulses (durations 32 mu s or 472 mu s, respectively) for imaging myelin in native and deuterated ovine brain at 3T. MethodsFreshly frozen ovine brains were dissected into approximate to 2mm-thick pure white matter and approximate to 3 to 8mm-thick cerebral hemisphere specimens, which were imaged before and/or after different immersion time in deuterium oxide. ResultsBicomponent analysis of UTE signals obtained with hard excitation pulses detected an ultrashort T-2 component (STC) fraction (f(S)) of 0% to 10% in native specimens, and up to approximate to 86% in heavily deuterated specimens. f(S) values were significantly affected by the TIs used in IR-UTE sequences with either hard or half excitation pulses in native specimens but not in heavily deuterated specimens. The STC was in the range of 150 to 400 mu s in all UTE and IR-UTE measurements obtained with either hard or half excitation pulses. ConclusionOur results further support myelin protons as the major source of the ultrashort signals seen on IR-UTE images and demonstrate the potential of IR-UTE sequences with half excitation pulses for directly imaging myelin using clinical scanners. Magn Reson Med 80:538-547, 2017. (c) 2017 International Society for Magnetic Resonance in Medicine.