Realistic Monte Carlo PET simulation with pixellated block detectors, light sharing and randoms modeling

We have developed a realistic Monte Carlo simulation based on the SimSET software to model photon transport and detection in PET scanners using the block design, modeling explicitly random coincidences, pixellated block detectors and light sharing among crystals elements. Our modeling was validated by comparison to a general purpose Monte Carlo transport code that has been extensively validated previously (GATE) as well as comparison to measurements made on two PET scanners. We have modified the SimSET software to allow tracking singles in the object/collimator. For each single photon in the detector, the energy weighted average of interaction points is computed and a blurring model is applied to account for light-sharing. The detection crystal is then determined. Primary, scattered and random coincidences are binned in 3D sinograms while modeling the curvature effects. Energy spectra and single distributions predicted by this model in 2D and 3D modes were compared to GATE as well as two GE DST PET scanners for NEMA NU-2 2001 recommended tests. Very good agreement was found between GATE and our improved modeling of photon transport. Simulated spatial resolutions @ 1 cm and 10 cm agreed with measurements within the variations in measurements made on the two DST scanner (1 mm FWHM, 1 mm FWTM in every direction). Simulated scatter fractions were 18.7/spl plusmn/1.2% in 2D (measured: 20.6%) and 45.5/spl plusmn/5.5% (measured: 44.1%) in 3D. Mean error between simulated and measured sensitivity were 2.5% in 2D mode and 2.4% in 3D. The new model allows accurate and fast modeling of PET acquisition under realistic activity and attenuation distributions.