The rate of clinical progression of PD is highly variable and unpredictable (77). In clinical studies, several clinical endpoints for progressive functional decline in PD have been used, including the Unified Parkinson's Disease Rating Scale (UPDRS) in the "defined off" state or after drug washout up to two weeks, time to need for dopaminergic therapy, or time to development of motor fluctuations (85-89). Clinical rating scales are extremely useful, but ratings may be investigator-dependent and are frequently confounded by changes in symptomatic treatment. Pathological studies investigating rate of progression have been limited and rely entirely on cross-sectional data (63,64). These studies have in general considered patients with severe illness of long duration. In vivo imaging studies provide the opportunity to evaluate patients longitudinally from early to late disease using an objective biomarker for dopaminergic degeneration.
In several studies, neuroreceptor imaging of the nigrostriatal dopaminergic system has been used as a research tool to monitor progressive dopaminergic neuron loss in PD. In longitudinal studies of PD progression both 18F-DOPA and DAT imaging [P CIT(2P-carboxymethoxy-3P(4-iodophenyl)tropane) and CFT] using both
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