Richard Carson

CHAPTER 20 – Optimization of Noninvasive Activation Studies with O-15-Water and 3D PET

To maximize the sensitivity of noninvasive activation studies with O-15-water and three-dimensional positron emission tomography (PET), the effects of varying the injected dose, speed of injection, and scan duration are investigated. A covert word generation task is used in four subjects with bolus injections of 92.5–1110 MBq of O-15-water. The noise equivalent count (NEC) for the whole brain peaked at an injected dose of 444–555 MBq. A 370 MBq injection gave an NEC of 92.4 ± 2.2% of the peak value. The significance of activation and the size of the activated area in Brocas area decreased as scan time increased from 60 to 120 seconds. Therefore, a 1-minute scan using 370 MBq for bolus injections is selected. Then simulation studies are performed to evaluate the effect on signal-to-noise ratio (S/N) of longer scan duration with slow tracer infusions. Using a measured arterial input function obtained with a bolus injection, new input functions for longer duration injections are simulated. Combining noise information derived from Hoffman brain phantom studies with simulated tissue count data allowed calculation of the S/N for a given cerebral blood flow change. Simulations showed an equivalent S/N for a 60-second scan with a bolus injection and a single longer scan with a slow infusion, given the same injected dose. This permits investigators to customize scanning parameters for different activation paradigms while maintaining the S/N.