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Signal-Enhancing Switched Protocols to Study Higher-Order Cognitive Tasks with PET

Department of Neurology and Neurosurgery, Neuropsychology/Cognitive Neuroscience Unit and McConneli Brain Imaging Centre, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada

Correspondence: For correspondence or reprints contact: Christopher J. Thompson DSc, Montreal Neurological Institute, Room 798, 3801 University Street, Montreal, Quebec, Canada H3A 2B4.

ABSTRACT

We tested the effectiveness of a switched protocol when it is used to detect signals that result from the study of a higher-order cognitive task with PET. Using language tasks that have been studied extensively in our laboratories, we compared the signal-to-noise ratio (S/N) and statistical significance of the activation signals detected in PET images of regional cerebral blood flow (rCBF), obtained using a standard activation protocol, and of activity concentration, obtained using a switched protocol. Methods: Four volunteers were studied with PET while they were performing synonym-generation and word-repetition tasks (activation and baseline tasks, respectively). Each volunteer had three activation/baseline and three baseline/activation scans. Data for each scan were collected in two frames (60 and 120 sec long). During the first 60 sec, data were collected using a standard activation protocol. Subjects then switched tasks, and acquisition continued for 120 sec. Two images were obtained from each scan: an rCBF image using the first frame and an activity-concentration image using both frames. Images were transformed into Talairach space, subtracted and averaged within and across subjects. Parametric t-statistic images were generated for each protocol, and the magnitude and significance of the activation signals yielded by the two acquisition methods were compared. Results: All the activation foci detected using measurements of rCBF were detected when the switched protocol was used; this protocol, in addition, yielded better S/N values. The cognitive component introduced by task-switching in switched protocols did not yield extra statistically significant foci. In single subjects, the average improvement in the signal significance from regions of activation, at a 95% confidence level, was between 6% and 25%. When scans were averaged across subjects, the switched protocol yielded improvements in signal statistical significance of up to 38%. Conclusion: We present evidence suggesting that switched protocols can be used to study higher-order cognitive tasks and that they yield activation foci with S/N values that are greater than those of equivalent foci detected using an rCBF protocol. Switched protocols appear to be easy to apply to the testing of higher-order cognitive functions. However, the extra cognitive requirement of switching tasks during data acquisition may be a limiting factor when switched protocols are used to study memory processes.

Key Words: PET • switched protocols • activation studies • oxygen-15-water • cerebral blood flow