OPTIMIZATION OF PET ACTIVATION STUDIES BASED ON THE SNR MEASURED IN THE 3-D HOFFMAN BRAIN PHANTOM

This work investigates the noise properties of O-15 water PET images i n an attempt to increase the sensitivity of activation studies, A meth od fur computing the amount of noise within a region of interest (ROI) from the uncertainty in the raw data was implemented for three-dimens ional (3-D) positron emission tomography (PET), The method was used to study the signal-to-noise ratio (SNR) of regions-of-interest (ROI's) inside a 3-D Hoffman brain phantom, Saturation occurs at an activity c oncentration of 2.2 mCi/l, which corresponds to a 75-mCi O-15 water in jection into a normal person of average weight. This establishes the u pper limit for injections for human brain studies using 3-D PET on the Siemens ECAT 921 EXACT scanner. Data from human brain activation stud ies on four normal volunteers using two-dimensional (2-D) PET were ana lyzed, The biological variation was found to be 5% ill l-ml ROI's, The variance for a complete activation study was calculated, for a variet y of protocols, by combining the Poisson noise propagated from the raw data in the phantom experiments with the biological variation. A prot ocol that is predicted to maximize the SNR in dual-condition activatio n experiments while remaining below the radiation safety limit is: tel l scans with 45 mCi per injection. The data should not be corrected fo r random or scatter events since they do not help in the identificatio n of activation sites while they do add noise to the image. Due to the lower noise level of 3-D PET, the threshold for detecting a true chan ge in activity concentration is 10%-20% lower than 2-D FIST, Because o f this, a 3-D activation experiment using the Siemens 921 scanner requ ires fewer subjects for equal statistical power.