Direct magnitude and phase imaging of myelin using ultrashort echo time (UTE) pulse sequences: A feasibility study

Abstract

In this paper, we aimed to investigate the feasibility of direct visualization of myelin, including myelin lipid and myelin basic protein (MBP), using two-dimensional ultrashort echo time (2D UTE) sequences and utilize phase information as a contrast mechanism in phantoms and in volunteers. The standard UTE sequence was used to detect both myelin and long T2 signal. An adiabatic inversion recovery UTE (IR-UTE) sequence was used to selectively detect myelin by suppressing signal from long T2 water protons. Magnitude and phase imagingand T2* were investigated on myelin lipid and MBP in the forms of lyophilized powders as well as paste-like phantoms with the powder mixed with D2O, and rubber phantoms as well as healthy volunteers. Contrast to noise ratio (CNR) between white and gray matter was measured. Both magnitude and phase images were generated for myelin and rubber phantoms as well white matter in vivo using the IR-UTE sequence. T2* values of ∼300 μs were comparable for myelin paste phantoms and the short T2* component in white matter of the brain in vivo. Mean CNR between white and gray matter in IR-UTE imaging was increased from -7.3 for the magnitude images to 57.4 for the phase images. The preliminary results suggest that the IR-UTEsequence allows simultaneous magnitude and phase imaging of myelin in vitro and in vivo.

Volumetric imaging of myelin in vivo using 3D inversion recovery‐prepared ultrashort echo time cones magnetic resonance imaging

“…The T 2 * value of 0.241 ms measured with 3D IR-UTE-Cones for a healthy volunteer was close to the T 2 * value of 0.20 ms measured with the same sequence for an ex vivo MS brain and was close to the 0.33 ms measured with 3D UTE-Cones of a myelin lipid/D 2 O phantom at a similar concentration. These values are also similar to the T 2 * values of myelin phantoms and D 2 O-exchanged white matter observed in prior studies using 2D UTE sequences and nuclear magnetic resonance at 3 T. [23][24][25][27][28][29] The longer T 2 * of the myelin phantom is expected from a small amount of residual semi-heavy water (HDO). 26 Myelin is defined based on its ultrastructural configuration of tightly compacted multilamellar lipid membranes, which requires active maintenance by oligodendrocytes or Schwann cells to maintain.…”

“…[24][25][26] The 2D IR-UTE sequence has been shown to detect MS lesions not previously detected with clinical sequences, 27,28 and has demonstrated its ability to obtain high myelin contrast on a clinical 3 T scanner. 9,27,29 However, 2D UTE sequences are susceptible to eddy current distortion artifacts during slice selection and have inefficient excitation of short T 2 components from the relatively long half-sinc excitation pulse. Furthermore, it is difficult to cover the whole brain using 2D UTE sequences, which are subject to stronger eddy currents and gradient distortion for off-center slices.…”

Ultrashort echo time (UTE) magnetic resonance imaging of myelin: technical developments and challenges

“…Myelin specific information, such as its T 1 and T 2 * as well as its proton density are not widely known. Ultrashort echo time (UTE) sequences with minimum nominal TEs of 8-100 µs, which are ~100 times shorter than the TEs of conventional clinical sequences, make it possible to directly detect signal from myelin using whole body clinical MRI scanners (18)(19)(20)(21)(22)(23)(24)(25)(26)(27)(28). Herein we review recent technical developments in UTE imaging of myelin ex vivo and in vivo.…”

Decoding the microstructural properties of white matter using realistic models

“…Direct MR imaging of the myelin is challenging due to the ultra-short transverse relaxation time of the phospholipid proton (T * 2 = 0.3 ms). Nevertheless, several attempts have been performed using zero or ultra short echo time techniques [4], [5]. Alternatively, myelin can be probed indirectly using magnetization transfer techniques [6], [7], multi-echo spin-or gradient-echo sequences [8].…”