Development of a Fillable, Tapered PET/CT Phantom

PET performance and image quality are degraded as body size increases; as size increases so do attenuation and background fractions. Based on simulated and acquired data, a fillable, tapered phantom was 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 (as large as possible at one end and small at the other) a lower taper (narrow) leads to a long, heavy phantom. A higher taper (steep so that the diameter changes greatly within the scanner field of view) may not represent any particular cross section well. Monte Carlo simulations were performed to determine an acceptable taper angle that balanced the benefits and drawbacks of lower and higher taper rates. The simulations were validated with measurements on existing physical phantoms. Then, tapered phantoms with different angles and non-tapered phantoms, which had dimensions similar to the tapered phantoms, were simulated at different positions in the PET axial field of view to investigate properties of different taper angles. Scatter fractions of simulated tapered and non-tapered phantoms were compared. Of the different angles simulated, the results indicated that a 25° taper would yield a phantom that represents a continuum of cross-sectional dimensions while still being a practical size. The 25 ° tapered phantom was machined with an oval cross-section ranging from 38.5 cm × 49.5 cm to 6.8 cm × 17.8 cm, a length of 51.1 cm, a mass of 6 kg (empty) or 42 kg (water filled), and acrylic walls 1.25-cm thick. Initial PET/CT images were acquired.