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Most current PET detector designs suffer from event mispositioning at high count rates, as scintillation light from nearby and nearly simultaneous gamma ray conversions becomes mixed. We have used the NEMA NU 2-2001 70 cm test phantom and a Na-22 point source to quantify this effect as a function of activity on two block-detector tomographs (the Siemens/CTI HR+ and the General Electric Discovery LS), and two Anger-type PET tomographs (the Siemens ECAM DUET and the Philips CPET+). After accounting for event losses due to dead time, we find that the number of counts on LORs passing through a cylinder of diameter and height equal to the point-source full width at tenth-maximum measured at low rate surrounding the point source decreases by between 9% (HR+) and 35% (CPET+) at the activity giving rise to peak noise equivalent count (NEC) rate. Mispositioned events act to reduce signal-to-noise ratio, both by reducing apparent activity at the originating location and by increasing the signal background. We have reformulated the conventional expression for NEC rate to account for this phenomenon. The new formulation of NEC, which we call NEC*, results in a lower peak value which in turn occurs at a lower activity concentration than for the conventional formulation. Index Terms--Count-rate, data quality, NEC, PET, pileup.

We demonstrate that dead-time mismatch between the normalization and emission acquisitions in three-dimensional positron emission tomography (3D PET) can introduce high-frequency circular image artifacts. We describe a simple correction scheme for this problem where appropriate correction factors are calculated from the emission data itself, thus avoiding the necessity of acquiring normalization data at a range of different count-rates. In this scheme 'pseudo detector efficiencies' are calculated from the emission data by application of a conventional normalization algorithm. The mean systematic variation in these pseudo-efficiencies with position within the block-detector is then calculated, and appropriate correction factors applied to the original emission data. We show that this 'self-normalization' scheme can remove normalization artifacts in both uniform and nonuniform phantom studies, and that it is also effective in vivo.

Preliminary results of an assessment of the effects of changing the axial field of view (AFOV) and detector ring diameter (DRD) of a cylindrical PET tomograph on count-rate performance are presented. The assessment was made using Monte Carlo simulations of an anthropomorphic phantom based on the Zubal phantom. This phantom was modified to include cylinders approximating arms and legs, and was sequentially stepped through the AFOV to simulate a wholebody scan covering an axial region of interest of 1 m. DRD was varied from ~60 cm to ~108 cm, and AFOV was varied from 10 cm to 60 cm. A simple activity distribution and dead-time model was assumed to allow the calculation of noise-equivalent count (NEC) rates for a situation similar to that of a typical 18F-FDG study. Both the scatter fraction and singles flux were found to be strongly dependent on DRD, but only weakly dependent on AFOV when the latter was greater than ~25 cm. Trues and randoms sensitivity were strongly dependent on AFOV, and randoms sensitivity was also strongly dependent on DRD. Scatter and singles flux do not appear to be limiting factors for extended AFOV configurations, and randoms rates, while high, appear to be manageable with existing detector technology. This initial assessment suggests that for whole-body applications, substantial gains in NEC may be possible by extending the AFOV.