Development of a fillable, tapered PET/CT phantom

PET system performance and image quality are degraded as body size increases; as size increases so do scatter fractions, random fractions, and attenuation. Based on simulated and acquired data, a fillable, tapering phantom is being developed to measure PET scanner performance and image quality as a function of body size over a wide range of sizes. Design constraints included shape, cross-sectional dimensions, material, wall thickness, and taper angle. For fixed end sizes (large at one end, small at the other) a narrow taper leads to a long, heavy phantom. A steep taper (where the diameter changes greatly within the scanner field of view) may not represent any particular cross section very well. Monte Carlo simulations were performed to determine an acceptable taper angle that balances the pros and cons of steep and narrow taper rates. The simulations were validated with measurements on existing physical phantoms. Then, tapering phantoms with different taper angles and phantoms with constant cross-sectional dimensions were simulated at different positions in the axial field of view to investigate properties of different taper rates. Scatter fractions of simulated taper phantoms and phantoms with constant cross-sectional dimensions were compared. Of the different taper angles simulated, it was determined that a 25° taper will yield a phantom that can represent a continuum of cross-sectional dimensions while still being a practical size. The final design parameters for this phantom include the 25° taper, an oval cross section ranging from 38.5 × 49.5 cm to 6.8 × 17.8 cm, a length of 51.1 cm, a mass of 6 kg (empty), a mass of 42 kg (water filled), and acrylic walls 1.25 cm thick.