IMPROVED SIGNAL-TO-NOISE IN PET ACTIVATION STUDIES USING SWITCHED PARADIGMS

PET activation studies employing the autoradiographic technique and O- 15-water or O-15-butanol use the difference between images acquired un der baseline conditions and during activation to detect focal changes in cerebral blood flow which occur upon stimulus presentation. Typical ly, the activating task or baseline conditions are maintained througho ut the entire imaging period. Simulations of the kinetics of these fre ely diffusible tracers suggest there may be an advantage to switching between activation and baseline conditions during the course of the st udy which results in images which maximize the difference signal rathe r than seeking to quantitate blood flow. We examine the potential of t hese switched protocols to increase signal-to-noise (S/N) in PET activ ation studies. Methods: We examined S/N in activation studies using bo th standard and switched paradigms with a simple switched protocol and dynamic three-dimensional PET data from human subjects. With tracer k inetic simulations, we investigated the sensitivity of the S/N gain to factors such as the shape of the input function, the time at which th e conditions are switched and the magnitude of the activation. Results : In human studies of activation sites in the visual cortex, primary m otor and premotor areas, S/N improvements of 20%-30% were detected usi ng the switched paradigms. Simulations show that this gain is virtuall y independent of activation magnitude and that there is a broad time w indow of 20 sec for making the switch between conditions. To obtain th e highest S/N gain, a rapid bolus injection is required. Conclusion: S witched paradigms have the potential to significantly increase S/N in PET activation studies. In human studies, the S/N increase averaged 25 % which is equivalent to increasing the number of counts collected by 50%. Switched paradigms can be used to maximize the difference signal in many activation studies, and do not preclude the absolute quantitat ion of blood flow using the standard autoradiographic technique.