Comparative analysis of striatal FDOPA uptake in Parkinson's disease: ratio method versus graphical approach

Striatal-to-occipital ratio (SOR) and influx constant K(i)(occ) are commonly used as analytic parameters in L-3,4-dihydroxy-6-(18)F-fluorophenylalanine (FDOPA) PET studies. Both have been shown to be useful in discriminating Parkinson's disease (PD) patients from healthy subjects. We evaluated the relative performance of SOR and influx constant (K(i)(occ)) in the clinical assessment of nigrostriatal dopaminergic function in PD.

Twenty-one parkinsonian patients (Hoehn and Yahr scale I-IV; mean age +/- SD, 56 +/- 9.2 y) and 11 healthy subjects (mean age, 60 +/- 16 y) underwent 3-dimensional dynamic FDOPA scanning from 0 to 100 min. After spatial realignment, PET images at each frame were integrated by summing 4 central striatal slices, and time-activity curves (TACs) were generated after placing a standard set of elliptic regions of interest over striatal and occipital structures. SOR and K(i)(occ) values for each subject were then computed from TACs at different times using an input function from the occipital cortex.

Both SOR and K(i)(occ) showed significant bilateral decreases in striatal dopamine uptake in the PD group compared with the control group. SOR values estimated for 10-min frames between 65 and 95 min are statistically equivalent in group discrimination. In addition, SOR values in the caudate and putamen correlated strongly with K(i)(occ), especially toward the end of the scanning epoch. Both parameters correlated significantly and comparably with Unified Parkinson's Disease Rating Scale motor scores.

These results suggest that SOR determined from a single 10-min scan at 95 min is as accurate as K(i)(occ) in separating PD patients from healthy subjects and in predicting clinical measures of disease severity.