Fig. 1. Variability of volumes of interest across the subjects (N = 6). The color scale overlaid on the nonlinear-averaged T1-weighed MRI represents probabilistic distribution of manually traced VOIs. Mid-sagittal sections highlight the anterior cingulate cortex, the midbrain raphe and hypothalamus in (a), while the hippocampus is depicted in (b). The axial sections highlight the thalamus, anterior insula and the anterior cingulate cortex in (c), while the hippocampus and the midbrain raphe are shown in (d). Note the low anatomical variability across subjects.

Fig. 2. Plate depicts representative maps of α-[¹¹C]MT K [µl/g/min] (a), the corresponding slice on the anatomical MRI (b), and the fusion between both images (c) in three anatomical levels. The first row shows the mid-sagittal plane (y = − 7), the second row shows an axial slice at the level of basal ganglia (z = − 4) and the third slice is at the level of the head of the hippocampus (z = 13).

Fig. 3. Plate depicts representative maps of [¹⁸F]MPPF BP_ND (a), the corresponding slice on the anatomical MRI (b), and the fusion between both images (c) in three anatomical levels. The first row shows the mid-sagittal plane (y = − 7), the second row shows an axial slice at the level of basal ganglia (z = − 4) and the third slice is at the level of the head of the hippocampus (z = 13).

Fig. 4. The color scale overlaid on the nonlinear-averaged T1-weighed MRI (N = 6) illustrates voxel-by-voxel correlation maps between α-[¹¹C]MT K and [¹⁸F]MPPF BP_ND. Significant correlations (r ≤ − 0.73; p < 0.05) were observed bilaterally in hippocampus (a) and anterior insula (b) and in the left anterior cingulate cortex (c).

Fig. 5. Regression plots between α-[¹¹C]MT K and [¹⁸F]MPPF BP_ND using the values from the most significant clusters obtained in the voxel-wise analysis. Significant correlations were found in the clusters located in five cluster-based VOIs selected a priori: left hippocampus (r = − 0.97; p = 0.0007; df = 5), right hippocampus (r = − 0.83; p = 0.01; df = 5), left anterior insula (r = − 0.92; p = 0.005; df = 5), right anterior insula (r = − 0.99; p = 0.0001; df = 5), left anterior cingulate cortex (r = − 0.93; p = 0.004; df = 5). The bold lines represent the fitting of the linear model and the dotted lines represent 95% confidence interval.