A Recommendation for Revised Dose Calibrator Measurement Procedures for 89Zr and 124I

Because of their chemical properties and multiday half lives, iodine-124 and zirconium-89 are being used in a growing number of PET imaging studies. Some aspects of their quantitation, however, still need attention. For 89 Zr the PET images should, in principle, be as quantitatively accurate as similarly reconstructed 18 F measurements. We found, however, that images of a 20 cm well calibration phantom containing 89 Zr underestimated the activity by approximately 10% relative to a dose calibrator measurement (Capintec CRC-15R) using a published calibration setting number of 465. PET images of 124 I, in contrast, are complicated by the contribution of decays in cascade that add spurious coincident events to the PET data. When these cascade coincidences are properly accounted for, quantitatively accurate images should be possible. We found, however, that even with this correction we still encountered what appeared to be a large variability in the accuracy of the PET images when compared to dose calibrator measurements made using the calibration setting number, 570, recommended by Capintec. We derive new calibration setting numbers for 89 Zr and 124 I based on their 511 keV photon peaks as measured on an HPGe detector. The peaks were calibrated relative to an 18 F standard, the activity level of which was precisely measured in a dose calibrator under well-defined measurement conditions. When measuring 89 Zr on a Capintec CRC-15R we propose the use of calibration setting number 517. And for 124 I, we recommend the use of a copper filter surrounding the sample and the use of calibration setting number 494. The new dose calibrator measurement procedures we propose will result in more consistent and accurate radioactivity measurements of 89 Zr and 124 I. These and other positron emitting radionuclides can be accurately calibrated relative to 18 F based on measurements of their 511 keV peaks and knowledge of their relative positron abundances.