2.6. Magnetic resonance and positron emission tomography imaging

Patients underwent integrated magnetic resonance-positron emission tomography (MR-PET) brain imaging at the Martinos Center for Biomedical Imaging at MGH (Boston, MA) on a 3 Tesla MAGNETOM Tim Trio scanner (Siemens Healthineers, Erlangen, Germany) with the Siemens Biograph mMR PET insert and using an 8-channel head coil. Patients completed MR-PET preoperatively and at one month postoperatively.

The radioligand [¹¹C]-PBR28 was synthesized on site [44] and injected as a slow intravenous bolus (up to 15 millicurie) through the catheter. PET images were acquired in a list mode format for 60 minutes scanning time beginning 30 minutes afterinjection to complete PET image acquisition in the 30–90 minutes after injection timeframe and, simultaneously, 60 minutes of MR imaging. The Biograph mMR Dixon sequence was used for attenuation correction during scanning, and an internal method was used after scanning for additional attenuation correction, as described previously [45]. PET data were reconstructed in 5-minute frames and a 30-minute frame over the 60-90 minute time point. We followed approaches detailed in previous publications [46,47].

For analysis of [¹¹C]-PBR28 PET data, we will follow established approaches [19,45,48]. Briefly, after acquisition and image reconstruction with corrections for normalization, dead time, isotope decay, photon attenuation, and expected random and scatter coincidences, attenuation correction maps will be created using MR-based methods. We will first create late-uptake [¹¹C]-PBR28 PET images for 60-90 min after injection and quantify the PET data as standardized uptake values (SUVs). Individual SUV 60–90 min images will then be registered to each individual's reconstructed T1-weighted MRI scan and to Montreal Neurological Institute space, spatially smoothed (6 mm full width at half maximum) and intensity-normalized. To account for the large interindividual variability in the global PET signal, SUV will be normalized by whole-brain PET uptake and will be expressed as SUVr (SUV ratios). We will analyze the PET using (1) whole brain voxel-wise analyses and (2) regions of interest (ROI) analyses. T1 images will be processed and analyzed using FreeSurfer (version 6.0) as we have done previously [[49], [50], [51]].