A recovery coefficient study using pre-clinical hollow spheres in clinical PET/CT scanners

Abstract

Aim: To measure and study the recovery coefficients of pre-clinical hollow spheres in clinical PET/CT scanners. Materials and Methods: Seven small hollow spheres with inner diameters ranging from 3.95mm up to 15.43mm, were imaged in a Jaszczak ECT Phantom, in air and with sphere to background activity concentration ratios of 15 to 1 for 16 minutes. The images were reconstructed to 2×2×2 mm voxel size images using the default clinical reconstruction method supplied by the scanner. The imaging protocol was performed on five different PET/CT scanners; the Gemini TF and Vereos by Philips, the mCT and HRRT by Siemens, and the Discovery 710 by General Electric. Results: The NEMA NU2 recovery coefficient for the 15.43 mm and 12.43 mm diameter spheres were about the same within each scanner. The Philips Vereos and HRRT scanners had RC values of 108% and 104% respectively, while the GE Discovery 710 was 118% and the Siemens mCT was 126%. The Gemini TF was 76%. For the smallest hollow sphere of 3.95 mm, the RC were 6.6%, 3.6%, 13%, 8.9% and 6.1% for the Gemini TF, Vereos, Discovery 710, mCT and HRRT respectively. An anomalous increase in the RC for the Discovery 710 and mCT was found for sphere sizes 9.89 mm diameter spheres where the RC was 161% and 145% for each respectively. A related recovery coefficient parameter called RC I which measures the recovery coefficient of the integrated activity in the spheres did not show this anomaly of increased recovery above 100% at the 9.89 mm sphere diameter measurement. Conclusion: The recovery coefficient data resulting from scanning five different scanners is presented. An increase in the recovery coefficient is seen in two scanners which is possibly due to over correction by the iterative reconstruction used to generate the images. This effect is not present when measuring the total activity in the sphere.