Parametric imaging of the rate constant Ki using [18Fluoro]-L-dopa positron emission tomography in progressive supranuclear palsy.

Positron emission tomography (PET) studies using [18F]-L-dopa were carried out in 9 patients with supranuclear palsy and 13 controls. For quantification of PET data a rate constant Ki was calculated for the radiotracer using a graphical method. Corrections for nonspecific activity were performed in both arterial plasma and brain tissue. The purpose of this study was to test the hypothesis that parametric images of the rate constant K mapping can be obtained on a pixel-by-pixel basis using an appropriate mathematical algorithm. Ki values from these parametric images and the graphical approach were compared. Both correlated closely, with y = 0.013 + 0.947x, r = 0.992 and y = -0.052 + 1.018x, r = 0.965 in patients and controls, respectively. Contrast measurements were also performed and showed a striking increase in contrast on parametric images. K mapping offers several advantages over the graphical approach, since parametric images are time-independent, i.e. one image represents the quantitative result of the study. In addition, parametric images of the rate constant are normalized to arterial plasma radioactivity and corrected for tissue metabolites. Thus, parametric images of Ki in different individuals can be compared directly without further processing in order to assess the nigrostriatal integrity.