SIGNAL-ENHANCING SWITCHED PROTOCOLS TO STUDY HIGHER-ORDER COGNITIVE TASKS WITH PET

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 tile signal-to-noise ratio (S/N) and statistical significance of the activation signals detected in PET im ages 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 whil e they were performing synonym-generation and word-repetition tasks (a ctivation and baseline: tasks, respectively), Each volunteer had three activation/baseline and three baseline/ activation scans. Data for ea ch scan were collected in two frames (60 and 120 sec long). During the first 60 sec, data were collected using a standard activation protoco l, Subjects then switched tasks, and acquisition continued for 120 sec . Two images were obtained from each scan: an rCBF image using-the fir st 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 s ignals yielded by the two acquisition methods were compared. Results: All the activation foci detected using measurements oi rCBF were detec ted when the switched protocol was used; this protocol, in addition, y ielded better S/N values, The cognitive component introduced by task-s witching in switched protocols did not yield extra statistically signi ficant foci. In single subjects, the average improvement in the signal significance from regions of activation, at a 95% confidence level, w as between 6% and 25%, When scans were averaged across subjects, the s witched protocol yielded improvements in signal statistical significan ce of up to 38%. Conclusion: We present evidence suggesting that switc hed protocols can be used to study higher-order cognitive tasks and th at they yield activation fool with S/N values that are greater than th ose oi equivalent loci detected using an rCBF protocol. Switched proto cols appear lo be easy to apply to the testing of higher-order cogniti ve functions, However, the extra cognitive requirement of switching ta sks during data acquisition may be a limiting factor when switched pro tocols are used to study memory processes.