Quantitative myelin water imaging using short TR adiabatic inversion recovery prepared echo-planar imaging (STAIR-EPI) sequence

Abstract

Introduction: Numerous techniques for myelin water imaging (MWI) have been devised to specifically assess alterations in myelin. The biomarker employed to measure changes in myelin content is known as the myelin water fraction (MWF). The short TR adiabatic inversion recovery (STAIR) sequence has recently been identified as a highly effective method for calculating MWF. The purpose of this study is to develop a new clinical transitional myelin water imaging (MWI) technique that combines STAIR preparation and echo-planar imaging (EPI) (STAIR-EPI) sequence for data acquisition.

Methods: Myelin water (MW) in the brain has shorter T₁ and T₂ relaxation times than intracellular and extracellular water. In the proposed STAIR-EPI sequence, a short TR (e.g., ≤300 ms) together with an optimized inversion time enable robust long T₁ water suppression with a wide range of T₁ values [i.e., (600, 2,000) ms]. The EPI allows fast data acquisition of the remaining MW signals. Seven healthy volunteers and seven patients with multiple sclerosis (MS) were recruited and scanned in this study. The apparent myelin water fraction (aMWF), defined as the signal ratio of MW to total water, was measured in the lesions and normal-appearing white matter (NAWM) in MS patients and compared with those measured in the normal white matter (NWM) in healthy volunteers.

Results: As seen in the STAIR-EPI images acquired from MS patients, the MS lesions show lower signal intensities than NAWM do. The aMWF measurements for both MS lesions (3.6 ± 1.3%) and NAWM (8.6 ± 1.2%) in MS patients are significantly lower than NWM (10 ± 1.3%) in healthy volunteers (P < 0.001).

Discussion: The proposed STAIR-EPI technique, which can be implemented in MRI scanners from all vendors, is able to detect myelin loss in both MS lesions and NAWM in MS patients.

Keywords: EPI; STAIR; aMWF; multiple sclerosis; myelin water imaging.

Figure 1

Sequence diagrams for the STAIR-EPI sequence. An AFP pulse and a short TR (e.g., 250 ms) together with an optimized TI in STAIR-EPI enables robust long T₁ water suppression. The EPI allows fast data acquisition of the remaining MW signals.

Figure 2

Representative non-lesion ROIs (orange ovals) of the eight WM regions including the left and right centrum semioval, subcortical white matter, periventricular regions, splenium, and genu of the corpus callosum for healthy volunteers and MS patients.

Figure 3

Representative STAIR-EPI (columns 1 and 2) and PD-EPI (columns 3 and 4) images, as well as the corresponding aMWF maps (columns 5 and 6) from a 31-year-old healthy male volunteer. White matter regions have a much higher myelin water content than gray matter regions.

Figure 4

Representative STAIR-EPI (first column), PD-EPI (second column), and aMWF (third column), as well as T₂-FLAIR (fourth column) from three patients with MS (patient #1 is a 57-year-old female, patient #2 is a 52-year-old male, and patient #3 is a 67-year-old female). Hyperintense lesions detected on T₂-FLAIR images (arrows) show a signal loss on the corresponding myelin water images and aMWF maps.

Figure 5

Paired comparisons of aMWF measurements between NWM, NAWM, and MS lesions. The individual data points for each group are also shown in the bar plots. Significantly lower aMWF values are found in both MS lesions and NAWM regions in MS patients in comparison with NWM in healthy volunteers. (“***” indicates p < 0.001).