Accurate Reference Region Calculation in Mouse Brain [¹⁸F]fallypride Studies

Quantification of the dopamine D_2/3 receptors tracer [¹⁸F]fallypride is usually performed by defining the cerebellum (CB) as the reference region for its use in kinetic modeling. In mouse studies, [¹⁸F]fallypride is defluorinated causing gradual uptake in the skull which, in addition to extra-striatal binding, can contaminate the CB activity, therefore introducing errors in reference region kinetic modeling. Here we propose a method using non-negative matrix factorization (NMF) to accurately extract the reference region time activity curve (TAC) unaffected by spill-over from surrounding regions. We compared the NMF method with template-based CB reference region (erodedCB), where the region was eroded to avoid spill-over. The different methods were applied in a drug challenge study using RX821002, and compared with results obtained using [¹¹C]raclopride. Striatal and brain parametric maps of nondisplaceable binding potential (BP_ND) were calculated with the different methods, and differences between baseline and challenge were investigated. The NMF reference region TACs showed higher peak and lower tail activity compared with erodedCB. Striatal BP_ND values calculated with NMF were about 20% higher compared to those calculated with erodedCB, and difference between baseline and challenge increased using NMF (NMF: 11.5%, erodedCB: 7.0%). Parametric t-statistic maps show clusters with significant differences using NMF but not with erodedCB. [¹¹C]raclopride BP_ND differences between baseline and challenge were lower (6.1%) than with [¹⁸F]fallypride, but BP_ND variability was lower. In summary, NMF allowed us to extract reference region TACs without contamination from skull or extra-striatal uptake, which improved voxel-wise detection of differences in a drug challenge study.