Porous phantoms for PET and SPECT performance evaluation and quality assurance

Characterization of PET and SPECT imaging performance often requires phantoms with complex radionuclide distributions. For example, lesion detection studies use multiple spherical regions of specific target-to-background ratios to simulate cancerous lesions. Such complex distributions are typically created using phantoms with multiple fillable chambers. However, such phantoms are typically difficult and time-consuming to prepare accurately and reproducibly. A new approach using a single-chamber phantom with a porous core can overcome these difficulties.

Prototypes of two designs of porous core phantoms were produced and evaluated. The "hot spheres" phantom contained a multitude of simulated spherical lesions with diameters ranging from 6.35 to 25.4 mm ("multi-resolution" slice) and with lesion-to-background ratios ranging from 1.6 to 4.4 ("multi-contrast" slice). The "multi-attenuation" phantom consisted of two halves. One half contained a porous core to produce regions of different attenuation but uniform activity. The other half mimicked the NEMA-94 design with cold inserts of different attenuation.

Both phantoms produced the expected radionuclide distributions while requiring the preparation of only a single radionuclide solution and with much reduced preparation time. In images taken on clinical PET and SPECT scanners, the porous core structures were found to contribute negligible background noise or artifact. The measured lesion-to-background ratios from the hot spheres phantom differed slightly from calculated values, with the differences attributed mainly to uncertainty in pore diameter. The measured attenuation coefficients from the multi-attenuation phantom agreed well with expected values. However, it was found that trapped air bubbles due to manufacturing defects in the porous core could potentially cause quantitative errors.

The hot spheres and multi-attenuation porous phantoms exhibited a wide range of imaging features providing thorough tests of lesion detection and of attenuation and scatter correction accuracy. Because the local activity concentration is set by the relative volume of radionuclide solution in the porous core, the quantitative accuracy is limited mainly by mechanical tolerance, and strict quality control during manufacturing is essential. Nonetheless, the single-chamber design of the porous core phantoms is inherently more reproducible and more practical for routine use compared to conventional multi-chamber phantoms.