1. ^*Human Motor Control Section, Medical Neurology Branch, National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, U.S.A.
2. ^‡Biometry and Field Studies Branch, National Institute of Neurological Disorders and Stroke, Bethesda, Maryland, U.S.A.
3. ^†PET Department, Warren Grant Magnuson Clinical Center, National Institutes of Health, Bethesda, Maryland, U.S.A.

Correspondence: Peter Herscovitch, PET Department, Building 10, Room 1C401, 10 Center DR, MSC 1180, Bethesda, MD 20892-1180, U.S.A.

¹Dr. Sadato is currently at the Biomedical Imaging Research Center, Fukui Medical School, Fukui, Japan.

Received 20 May 1996; Revised 9 December 1996; Accepted 20 February 1997.

Abstract

We investigated the effects of varying the injected dose, speed of injection, and scan duration to maximize the sensitivity of noninvasive activation studies with ¹⁵O-water and three-dimensional positron emission tomography. A covert word generation task was used in four subjects with bolus injections of 2.5 to 30 mCi of ¹⁵O-water. The noise equivalent counts (NEC) for the whole brain peaked at an injected dose of 12 to 15 mCi. This was lower than expected from phantom studies, presumably because of the effect of radioactivity outside of the brain. A 10 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 activation decreased in size or were no longer detected. Therefore, we selected a 1 minute scan using 10 mCi for bolus injections. We then performed simulation studies to evaluate, for a given CBF change, the effect on signal-to-noise ratio (S/N) of longer scan duration with slow tracer 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 simulated tissue data allowed calculation of the S/N for a given CBF change. The simulation shows that a slow infusion permits longer scan acquisitions with only a small loss in S/N. This allows the investigator to choose the injection duration, and thus the time period during which scan values are sensitive to regional CBF.