Myelin water fraction (MWF) (1,2) is a promising quantitative biomarker of myelin loss and repair in multiple sclerosis (3), which has been validated by histopathology (4,5). However, the translation of MWF mapping into clinical MRI has been hindered by long acquisition time (>10 min) and noisy maps (6-10). To address these challenges, Fast Acquisition with Spiral Trajectory and T2prep (FAST-T2) has been developed recently to reduce whole brain scan time to 4 min while producing reliable MWF maps with excellent intra-scanner reproducibility (11). The purpose of this study was to evaluate the inter-scanner reproducibility of FAST-T2 myelin mapping on three scanners from two vendors at the clinically relevant field strengths of 1.5T and 3T.

Imaging experiment. Seven healthy volunteers (29 years ± 7) were imaged within 5 days on three commercial whole-body scanners: 1.5T GE HD23 (33 mT/m gradient strength, 120 T/m/s slew rate), 3T GE LX Echospeed (40 mT/m, 150 T/m/s), and 3T Siemens Biograph mMR (45 mT/m, 200 T/m/s). The scanners are located in three separate buildings on the same medical campus and will be referred to as 1.5T Vendor 1, 3T Vendor 1, and 3T Vendor 2. Typical FAST-T2 parameters: 192x192x32 matrix, 24 cm FOV, 5 mm slice, spiral TR/TE = 7.8-9.2/0.5-0.8 ms with 32 spiral leaves, 6 geometric TEs as in (11), 10 ms adiabatic T2prep, 4 min whole brain scan time. An 8-channel brain coil and a 20-channel head coil was used for signal reception on scanners from Vendor 1 and 2, respectively.

Data analysis. MWF maps were extracted using a multi-voxel spatially constrained non-linear least squares algorithm with a L-BFGS iterative solver (11). The regularization parameter and the stopping criteria (<1e-3 relative change in objective function, max 150 iterations) were fixed for all data. Source images and MWF maps were co-registered using FLIRT algorithm (12). ROI analysis was performed in five major WM (genu and splenium of corpus callosum, minor forceps, major forceps, internal capsules) and three major GM (putamen, caudate head, thalamus) regions. The reproducibility of regional and voxel-wise MWF measurements was assessed using regression and correlation analysis, Bland-Altman plots, as well as coefficient of variance (COV).

Figure 1 shows an example of high quality MWF maps with good visual agreement obtained with FAST-T2 on the three different MRI scanners, particularly at 3T. Regional MWF measurements were found to be highly reproducible with excellent inter-scanner correlation (R>0.95) and small MWF bias of under 1% (Fig.2). There was a stronger MWF agreement between 3T scanners (Bland-Altman 95% limits of agreement [-2.3%,1.9%] for 3T Vendor 1 vs. 3T Vendor 2) compared to that between a 1.5T and a 3T scanner ([-1.8%,3.1%] for 1.5T Vendor 1 vs. 3T Vendor 1 and [-1.7%,3.5%] for 1.5T Vendor 1 vs. 3T Vendor 2). Reflecting this trend, the average COV for regional MWF measured in WM was 3.7 ± 0.9% for 3T Vendor 1 vs. 3T Vendor 2, which was better than COV values of 5.1 ± 2.0% for 1.5T Vendor 1 vs. 3T Vendor 1 and 5.2 ± 2.9% for 1.5T Vendor 1 vs. 3T Vendor 2. On a per voxel basis over the whole brain, the inter-scanner agreement was moderate (mean correlation R~0.7, mean MWF bias <1%, and mean 95% limits of agreement within ±6%, n=7).Our preliminary data obtained in healthy volunteers demonstrated that fast whole brain myelin mapping using the recently developed 4 min FAST-T2 sequence has high inter-scanner reproducibility on a regional basis among the three MRI platforms considered in this study. The better agreement observed between the two 3T scanners can be attributed to the approximately double SNR advantage of 3T vs. 1.5T acquisition. FAST-T2 shows similar reproducibility to the conventional slower acquisition methods (13,14), although it must be noted that previous studies mainly focused on scanners from a single vendor. A limitation of this study is a lack of MS patients, which will be addressed in our future work. In conclusion, FAST-T2 has the potential to provide a clinically reliable imaging tool for multi-center study and drug trials in MS.