2.4. Positron emission tomography

Radiosynthesis of [¹¹C]ABP688 was performed according to the method already reported (Elmenhorst et al., 2016). The average molar activity at the injection time was 101.70 ± 45.33 MBq/nmol. All subjects were measured using a 3 T hybrid MR‐BrainPET insert system (Herzog et al., 2011). Our bolus plus infusion protocol was optimized based on a previous publication that reported a K _bolus = 53 min and a steady‐state after 40 min (Burger et al., 2010). In our protocol, the bolus injection (50% of total activity), followed by 65 min of infusion with 92 ml/hr infusion rate, was applied after positioning the subject in the scanner. The average total injected activity per subject was 525 ± 55 MBq. A distribution equilibrium was observed after 30 min (also reported by other researchers; Akkus et al., 2014; Deschwanden et al., 2011). Starting simultaneously with the bolus injection, the PET data were acquired in list mode for 65 min. The image reconstruction was performed with 3D‐OP‐OSEM (van Velden et al., 2008; 2 subsets, 32 iterations), with an isotropic voxel of 1.25 mm into a volume consisting of 153 transverse slices of 256 × 256 pixels, and a frame scheme of 10 × 60 s; 11 × 300 s. The images were corrected for attenuation (Rota Kops, Herzog, & Shah, 2014), random and scattered coincidences, and dead time. Postprocessing was performed first by filtering with a 2.5 mm 3D Gaussian filter and then by performing motion correction with frame‐by‐frame realignment. Herby, the reference image was obtained, on average, at 5 min postinjection (Scheins et al., 2019 2019).

PMOD software version 3.9 was used to define the volumes of interest (VOIs) with T1 MPRAGE images serving as the anatomical reference. All images were normalized to the MNI space, and the Hammers atlas (Hammers et al., 2003) was used for activity concentration analysis. The maximum probability operation was applied to generate the VOIs related to the gray matter cortex (GM) in all brain temporofrontal regions except in basal ganglia regions. The cerebellum GM has been chosen as the most appropriate reference region for human brain studies on mGluR5 (Akkus et al., 2017; Akkus et al., 2018; Smart et al., 2018). In all cases, the nondisplaceable binding potential (BP_ND) of [¹¹C]ABP688 was estimated as:

where C _region is the specifically bound radioligand concentration and C _ref is the nondisplaceable ligand concentration in the cerebellar GM reference tissue. BP_ND values were calculated in an average of 5 min frame after 30 min p.i during the equilibrium plateau.