Optimizing rod window width in positron emission tomography.

A technique determines the optimal window width for orbiting rod transmission studies in positron emission tomography (PET). Windowing reduces noise in orbiting rod transmission studies. Lines-of-response (LOR) which intersect the rods generate primarily true coincidence events. LOR which pass far from the rods generate random and scatter events. Since the angular position of the orbiting rods is known in real-time, LOR which produce mostly noise are gated off. When optimally determined, the rod window width maximizes the noise equivalent counts (NEC) collected in the transmission study. Transaxial rod projection profiles of trues, randoms, and scatter produce NEC versus window width plots. For the ECAT EXACT line of PET systems and a 20-cm water cylinder, optimal is five LOR wide.

Design features and performance of a PET system for animal research.

The design features of a PET system designed for animal studies are described and its performance evaluated. The system employs a two-dimensional modular detector array consisting of bismuth germanate detector elements that are 3.5 mm (transaxially) by 6.25 mm (axially) by 30 mm (deep). These arrays are optically coupled to a pair of dual-photo-multiplier tubes (PMT). The detector ring is 64 cm in diameter with a field of view (FOV) of 40 cm by 5.4 cm axially, acquiring 15 slices at 3.4 mm spacing. These features include: (1) digitization of PMT signals from each block for improved position and energy discrimination of coincident events and (2) dual-window energy discrimination for simultaneous but separate acquisition of photopeak and scatter data. Intrinsic resolution averages 3.5 mm at the center of the FOV, while reconstructed resolution (ramp filter) ranges from 3.8 mm at the center of the FOV to 4.6 mm at an 8 cm radius. Axial resolution averages 4.4 and 4.9 mm and sensitivity averages 4.2 and 6.1 kcps/microCi/cc for cross planes and enhanced direct planes, respectively. Randoms fraction is high due to reduced interplane shielding, giving a peak true count rate of 103 kcps for a 10 cm cylinder. Scatter as a fraction of trues is 16% for a 10 cm cylinder at a lower energy threshold of 350 keV. All parameters are sensitive to energy threshold. Spatial resolution improves by 11% transaxially and 9% axially, scatter fraction drops to 10%, and overall sensitivity drops by 48% when the threshold value is increased from 350 keV to 450 keV.

Assessment of accuracy of PET utilizing a 3-D phantom to simulate the activity distribution of [18F]fluorodeoxyglucose uptake in the human brain.

A three-dimensional brain phantom has been developed to simulate the activity distributions found in human brain studies currently employed in positron emission tomography (PET). The phantom has a single contiguous chamber and utilizes thin layers of lucite to provide apparent relative concentrations of 5, 1, and 0 for gray matter, white matter, and CSF structures, respectively. The phantom and an ideal image set were created from the same set of data. Thus, the user has a basis for comparing measured images with an ideal set that allows a quantitative evaluation of errors in PET studies with an activity distribution similar to that found in patients. The phantom was employed in a study of the effect of deadtime and scatter on accuracy in quantitation on a current PET system. Deadtime correction factors were found to be significant (1.1-2.5) at count rates found in clinical studies. Deadtime correction techniques were found to be accurate to within 5%. Scatter in emission and attenuation correction data consistently caused 5-15% errors in quantitation, whereas correction for scatter in both types of data reduced errors in accuracy to less than 5%.