SPECT/CT/MRI coregistration and SPECT regional analysis

SPECT/CT/MRI scans underwent inter-modality registration and the brain was parcellated using a MR-based 3D digital mouse atlas for regional analyses as described previously^17,19. A 6-parameter rigid body registration was carried out using an established method¹⁹ between the mouse MR and a standard brain from which the digital atlas was derived using Automated Image Registration 5.3.0²¹. Higher spatial transformations (warping) were applied to the standard brain and corresponding warped MR atlas¹⁹. All MRI, warped MR atlas, and SPECT/CT-fused images were imported into AMIDE to affine registration between modalities^19,20.

To evaluate brain perfusion, SPECT volume of interest (VOI) statistics were generated for six brain ROIs defined by the MR atlas: (i) whole brain (excluding cerebellum, brain stem, and olfactory bulb), (ii) neocortex, (iii) hippocampal formation, (iv) amygdala, (v) thalamus, and (vi) basal ganglia. Semiquantitative estimates of perfusion were determined for each of the ROIs via SUV, expressed as SUV = (activity/unit dose)/g. SUVR were also evaluated, using a whole brain (wb) reference region, determined as follows: SUVR_roi = SUV_roi/SUVR_wb. This normalized metric was employed to reduce any possible inter-subject or inter-scan variability in the study¹⁹.