Simultaneous T2 and T2∗ mapping of multiple sclerosis lesions with radial RARE-EPI

Abstract

Purpose: The characteristic MRI features of multiple sclerosis (MS) lesions make it conceptually appealing to pursue parametric mapping techniques that support simultaneous generation of quantitative maps of 2 or more MR contrast mechanisms. We present a modular rapid acquisition with relaxation enhancement (RARE)-EPI hybrid that facilitates simultaneous T₂ and $T_2^{\ast }$ mapping (2in1-RARE-EPI).

Methods: In 2in1-RARE-EPI the first echoes in the echo train are acquired with a RARE module, later echoes are acquired with an EPI module. To define the fraction of echoes covered by the RARE and EPI module, an error analysis of T₂ and $T_2^{\ast }$ was conducted with Monte Carlo simulations. Radial k-space (under)sampling was implemented for acceleration (R = 2). The feasibility of 2in1-RARE-EPI for simultaneous T₂ and $T_2^{\ast }$ mapping was examined in a phantom study mimicking T₂ and $T_2^{\ast }$ relaxation times of the brain. For validation, 2in1-RARE-EPI was benchmarked versus multi spin-echo (MSE) and multi gradient-echo (MGRE) techniques. The clinical applicability of 2in1-RARE-EPI was demonstrated in healthy subjects and MS patients.

Results: There was a good agreement between T₂ / $T_2^{\ast }$ values derived from 2in1-RARE-EPI and T₂ / $T_2^{\ast }$ reference values obtained from MSE and MGRE in both phantoms and healthy subjects. In patients, MS lesions in T₂ and $T_2^{\ast }$ maps deduced from 2in1-RARE-EPI could be just as clearly delineated as in reference maps calculated from MSE/MGRE.

Conclusion: This work demonstrates the feasibility of radially (under)sampled 2in1-RARE-EPI for simultaneous T₂ and $T_2^{\ast }$ mapping in MS patients.

Keywords: EPI; MRI; RARE; brain; multiple sclerosis; parametric mapping; radial k-space sampling; transversal relaxation time.