Assessment of x-ray scatter for the micro-CT subsystem of the FLEX Triumph™ preclinical PET-CT scanner

This work aims at assessing x-ray scatter and more specifically the scatter to primary ratio (SPR) for the micro-CT subsystem of the FLEX Triumph™ preclinical PET-CT scanner. Two approaches were used: the experimental single blocker method and Monte Carlo (MC) simulations using the MCNP4C code. For the experimental setup, five cylindrical blockers with diameters ranging between 3.0 and 11.65 mm were used to assess the SPR using a polyethylene phantom (d=50mm). The beams energy influence was studied by scanning the phantom at 30, 50 and 80 kV (mag=1.3). Likewise, additional acquisitions at 50 kV were performed with a magnification of 2.0 to evaluate the impact of geometrical magnification and using a Plexiglas phantom (d=25mm) filled with water at 50 kV (mag=1.3) to assess the influence of the size and composition of the phantom. For each condition, the five blocker SPR results were linearly interpolated to obtain the SPR without the blocker. Central and peripheral SPR values were obtained by rotating the source and detector and were fitted with a Gaussian function. MC simulations were carried out using the MCNP4C code where the estimates were also fitted with a Gaussian function. The comparison showed that MC simulations can reproduce well the experimental results, at least in the region inside the phantom. The highest difference was obtained with the small phantom in the peripheral regions. The maximum SPR (0.562) was obtained at 30kV and magnification of 1.3 using the large phantom. The full SPR profile was calculated using MC simulations and used to express its dependency on beam energy and phantom diameter (quadratic), and air gap (asymptotic). The obtained results are in good agreement with theoretical predictions. MC is a very good alternative to experimental measurements of x-ray scatter in micro-CT imaging and can be used to validate novel scatter correction techniques. This will also improve the accuracy of CTAC on preclinical PET-CT systems.