Improved volumetric myelin imaging in human brain using 3D dual echo inversion recovery-prepared UTE with complex echo subtraction

Purpose: Inversion recovery-based UTE (IR-UTE) sequences have been proposed to directly image myelin with extremely short $T_2^{\ast }$ (~0.3 ms). In this study, we demonstrate the feasibility of complex echo subtraction to improve 3D IR-UTE imaging of myelin in white matter of the brain in vivo.

Methods: In IR-UTE imaging, long T₂ components in white matter (i.e., water) are suppressed using an adiabatic inversion recovery preparation pulse. Dual echo UTE data acquisition and magnitude echo subtraction are used to suppress the residual white matter and gray matter signals, providing high myelin contrast. Complex echo subtraction may further improve the myelin contrast by reducing the residual long T₂ water signal contamination caused by regional T₁ variations. To verify the efficacy of the complex subtraction technique, in vivo experiments were performed with 5 non-symptomatic healthy volunteers and 5 multiple sclerosis patients on a 3T clinical MR system. Signal enhancement between the complex subtraction and the magnitude subtraction was introduced to evaluate the improvement.

Results: The complex subtraction improved myelin contrast over the magnitude subtraction in both healthy and patient groups, with more fine myelin structures being revealed. The foci of the demyelinated lesion were more clearly detected by complex subtraction. An average signal enhancement of up to 135.9% was achieved with the complex subtraction over the magnitude subtraction.

Conclusion: The complex echo subtraction improves 3D IR-UTE morphologic imaging of myelin in white matter of the brain.