OPTIMIZATION OF NONINVASIVE ACTIVATION STUDIES WITH O-15-WATER AND 3-DIMENSIONAL POSITRON EMISSION TOMOGRAPHY

We investigated the effects of varying the injected dose, speed of inj ection, and scan duration to maximize the sensitivity of noninvasive a ctivation studies with O-15-water and three-dimensional positron emiss ion tomography. A covert word generation task was used in four subject s with bolus injections of 2.5 to 30 mCi of O-15-water. The noise equi valent counts (NEC) for the whole brain peaked at an injected dose of 12 to 15 mCi. This was lower than expected from phantom studies, presu mably because of the effect of radioactivity outside of the brain. A 1 0 mCi injection gave an NEC of 92.4 +/- 2.2% of the peak value. As the scan duration increased from 60 to 90 to 120 seconds, the areas of ac tivation decreased in size or were no longer detected. Therefore, we s elected a 1 minute scan using 10 mCi for bolus injections. We then per formed simulation studies to evaluate, for a given CBF change, the eff ect on signal-to-noise ratio (S/N) of longer scan duration with slow t racer infusions. Using a measured arterial input function from a bolus injection, new input functions for longer duration injections and the corresponding tissue data were simulated, Combining information about image noise derived from Hoffman brain phantom studies with the simul ated tissue data allowed calculation of the SM for a given CBF change. The simulation shows that a slow infusion permits longer scan acquisi tions with only a small loss in S/N. This allows the investigator to c hoose the injection duration, and thus the time period during which sc an values are sensitive to regional CBF.