PET Imaging of [11C]MRB in MS and HC

PET imaging using [¹¹C]MRB was performed as described previously^{19 (link)}. Both the MRB standard and precursor were prepared following previously described protocols^{56 (link)}. In brief, [¹¹C]MRB was synthesized from [¹¹C]methyliodide ([¹¹C]CH₃I) using the TRACERLab FXC automated synthesis module (GE Healthcare, USA). The final formulated product was 98% radiochemically pure, the average injected mass was 0.021 ± 0.01 μg/kg. Dynamic PET was started after intravenous bolus injection (90 s) of 485.6 [477.2 – 489.9] MBq [¹¹C] MRB (MS patients) and 380.6 [357.2 – 485.1] MBq (HC; see Table 1) using the ECAT EXACT HR + scanner in 3D acquisition mode (Siemens, Erlangen, Germany; intrinsic resolution at the centre 4.3 mm [full-width at half maximum, FWHM], axial resolution: 5–6 mm, field of view: 15.5 cm). Emission scan duration was 90 min, acquiring 23 frames (4 × 0.25, 4 × 1, 5 × 2, 5 × 5, 5 × 10 min). We used a 10-min transmission scan (from three ⁶⁸Ga sources), which was performed prior to the emission scan, for attenuation correction. An iterative reconstruction (10 iterations, 16 subsets) was applied to a transverse image series (63 slices, 128 × 128 matrix, voxel size 2.6 × 2.6 × 2.4 mm³) with a Hann filter (cut-off 4.9 mm) for post-processing^{19 (link)}.