Assessment of striatal dopamine D2/D3 receptor availability with PET and ¹⁸F-desmethoxyfallypride: Comparison of imaging protocols suited for clinical routine

Assessment of striatal dopamine receptor availability with ¹⁸F-desmethoxyfallypride PET is of high diagnostic utility in parkinsonism. The present study was undertaken to define the optimal clinical scan protocol with regard to quantification accuracy and scan time. Methods: Fourteen patients with parkinsonian syndromes underwent ¹⁸F- desmethoxyfallypride PET over 90 min. Volume-of-interest analyses were performed after spatial normalization, with the right and left caudate nuclei and putamina as target regions and the cerebellum as reference region. The estimate of target region binding potential (relative to nondisplaceable radioligand in tissue) (BP_ND) provided by the 2-step simplified reference tissue model (SRTM2) served as the reference standard. Additional analyses included the multilinear reference tissue model 2 (MRTM2), noninvasive graphical analyses, and single-scan analyses (peak-equilibrium analysis at 35- 65 min [PEA]; pseudoequilibrium analysis at 60-90 min [PsEA]). Results: SRTM2 and MRTM2 yielded virtually identical results (mean BP_ND difference = 0.1% ± 0.5%, r² = 1.0). Noninvasive graphical analyses with and without inclusion of the k₂′ term were affected by a small BP_ND bias (2.5% ± 3.6% and -5.0% ± 6.7%, respectively), although correlations with SRTM2 were still excellent (r² = 1.0 and 0.98, respectively). In turn, singlescan analyses suffered from limited precision (PEA, mean BP_ND bias = 0.7% ± 13.0%, r² = 0.90) or a considerable positive bias (PsEA, 19.2% ± 7.1%, r² = 0.98). Shortening scan time to 70 and 60 min resulted in an acceptable average BPND change (<5% decline) for SRTM2/MRTM2 and graphical analysis with inclusion of the k₂′ term, respectively. Conclusion: Kinetic reference tissue model analyses of ¹⁸F-desmethoxyfallypride PET data offer the least biased results at a well-tolerable scan duration and should thus be pursued whenever possible. Single-scan analyses may be pragmatic alternatives that, however, suffer from a relevant positive bias (PsEA) or limited precision (PEA).