Comparison of PET [O-15]water studies with 6-minute and 10-minute interscan intervals: Single-subject and group analyses

The authors recently showed that [O-15]water PET data obtained with a short interscan interval (6 minutes) produced similar results whether or not the residual background from the previous scan is subtracted. The purpose of t he present study was to compare scans obtained during motor activation usin g a short (6-minute) interscan interval protocol with those obtained with a standard (10-minute) protocol in the same scanning session. Single-subject and group analyses were performed using Worsley's method, which uses a poo led variance estimate and statistical parametric mapping with a local varia nce estimate. High consistency in both the activation maps, i.e., the numbe r of activated motor brain structures and the Talairach coordinates of peak intensities of the activated regions, was obtained in the 6- and 10-minute studies in both single-subject and group analyses. However, in comparison to the 6-minute studies, a larger cluster size of activated brain regions a nd an approximately 20% higher peak activation in these regions were observ ed in the 10-minute studies with the same number of replicates. Analysis of these results suggests that using a 6-minute interval with an increased nu mber of replications, i.e., without changing the subject's total study dura tion, should produce comparable statistical power to that of the 10-minute interval for group analysis and increased statistical power for single-subj ect analyses that use a local variance estimate because of increased degree s of freedom. Alternatively, with a small increase in the number of scans a nd the use of a 6-minute interscan interval, a comparable level of statisti cal significance may be achieved for single-subject experiments that use a local variance estimate, with an overall shortening of the study duration.