Reduction of radiation exposure in PET examinations by data acquisition inthe 3D mode

Aim: Modern volume PET systems offer the possibility to measure without the shadowing effect of interplane septa (2D mode) and thus to detect coincide nt events between detectors on distant rings (3D mode). It was the aim of t he present paper to characterize the count rate behaviour of a latest-gener ation whole-body PET system in the 2D and 3D mode as well as to discuss the consequences for the radiation hygiene of PET examinations with 2-[F-18]-f luoro-2-deoxyglucose (18-F-FDG). Methods: Ail experiments were performed wi th the PET system ECAT EXACT HR+. For 2D data acquisition, a collimator of thin tungsten septa was positioned in the field-of-view. The count rate beh aviour of the scanner was evaluated in the 2D and 3D mode over a wide range of F-18 activity concentrations following the NEMA protocol. Moreover, PET images of the EEC whole-body phantom with different inserts were acquired in the 2D and 3D mode over a period of 15 min each. For the 3D measurement, the activity concentrations of the F-18 solution were only half of those u sed for the 2D measurement. Results: For the circular NEMA phantom (empty s et = 19.4 cm, length = 19,0 cm), we observed an increase of the system sens itivity in the 3D mode by a factor of about 5 with respect to the 2D mode ( 27.7 vs. 5.7 cps/Bq/ml). The evaluation of the activity distributions of th e EEC phantom reconstructed from the 3D data set revealed a superior image quality compared to the corresponding 2D images despite the fact that the a ctivity concentrations were only half as high. Conclusion: By using the 3D data acquisition mode, it is possible to markedly reduce the amount of acti vity to be applied to the patient and nevertheless to improve image quality . In our experience, it is sufficient to administer an activity of 150-200 MBq for whole-body examinations with F-18-FDG, which results in an effectiv e equivalent dose of 3 or 4 mSv, respectively.