Assessment of scatter for the micro-CT subsystem of the trimodality FLEX Triumph preclinical scanner.

PURPOSE This work aims at assessing, through experimental measurements and Monte Carlo calculations, the scatter to primary ratio (SPR) for the micro-CT subsystem of the FLEX Triumph preclinical PET-CT scanner to improve its quantitative capabilities. METHODS Experimental measurements were carried out using the single blocker method, where five cylindrical blockers with diameters ranging between 3 and 11.65 mm were used to assess the SPR without the blocker through interpolation. Because of the vertical layout of the imaging device, the blocker was placed over rat-sized and mouse-sized phantoms and central and peripheral SPR values were obtained by rotating the source and detector. The influence of beam energy (30, 50, and 80 kVp), geometrical magnification (1.3 and 2.0) and phantom diameter (25 and 50 mm) and density (polyethylene and water) were investigated. Monte Carlo (MC) simulations using the MCNP4C code were also performed and compared to experimental results to validate their accuracy. RESULTS The highest difference was found in the extreme peripheral region of the small phantom, while the maximum difference at the center of the phantom is about 6%, indicating that MC simulations can reproduce well the experimental results, at least in the region inside the phantom. The maximal SPR (0.562) was obtained for the large phantom at 30 kVp and a magnification of 1.3. The full SPR profile was calculated using MC simulations and used to express its dependency on beam energy (quadratic), air gap (asymptotic), and phantom diameter (quadratic). CONCLUSIONS The obtained results are in good agreement with theoretical predictions. MC simulations were valuable for the evaluation of the influence of various acquisition parameters on the SPR estimates.