Radiotracer Dose Reduction in Integrated PET/MR: Implications from National Electrical Manufacturers Association Phantom Studies

Abstract

With the replacement of ionizing CT by MR imaging, integrated PET/MR in selected clinical applications may reduce the overall patient radiation dose when compared with PET/CT. Further potential for radiotracer dose reduction, while maintaining PET image quality (IQ) in integrated PET/MR, may be achieved by increasing the PET acquisition duration to match the longer time needed for MR data acquisition. To systematically verify this hypothesis under controlled conditions, this dose-reduction study was performed using a standardized phantom following the National Electrical Manufacturers Association (NEMA) IQ protocol. Methods: All measurements were performed on an integrated PET/MR whole-body hybrid system. The NEMA IQ phantom was filled with water and a total activity of 50.35 MBq of 18 F-FDG. The sphere-to-background activity ratio was 8:1. Multiple PET data blocks of 20-min acquisition time were acquired in list-mode format and were started periodically at multiples of the 18 F-FDG half-lives. Different sinograms (2, 4, 8, and 16 min in duration) were reconstructed. Attenuation correction of the filled NEMA phantom was performed using a CT-based attenuation map template. The attenuation-corrected PET images were then quantitatively evaluated following the NEMA IQ protocol, investigating contrast recovery, background variability, and signalto-noise ratio. Image groups with half the activity and twice the acquisition time were evaluated. For better statistics, the experiment was repeated 3 times. Results: Contrast recovery, background variability, and signal-to-noise ratio remained almost constant over 3 half-life periods when the decreasing radiotracer activity (100%, 50%, 25%, and 12.5%) was compensated by increasing acquisition time (2, 4, 8, and 16 min). The variation of contrast recovery over 3 half-life periods was small (−6% to 17%), with a mean variation of 2%, compared with the reference setting (100%, 2 min). The signal-to-noise ratio of the hot spheres showed only minor variations over 3 half-life periods (5%). Image readers could not distinguish subjective IQ between the different PET acquisition setups. Conclusion: An approach to reduce the injected radiotracer activity in integrated PET/MR imaging, while maintaining PET IQ, was presented and verified under idealized experimental conditions. This experiment may serve as a basis for further clinical PET/MR studies using reduced radiotracer dose as compared with conventional PET/CT studies.

A method to assess image quality for Low-dose PET: analysis of SNR, CNR, bias and image noise

“…A number of studies to investigate the effect of different count levels on PET image quality with phantom have been reported [13, 14]. Our previous study demonstrated count statistics as low as 5 × 10 6 counts could achieve a fairly high detectability level using a data set of 18 F-FDG PET images of tuberculosis (TB) patients acquired on a PET/MR scanner [15].…”

Oncological whole-body staging in integrated 18F-FDG PET/MR: Value of different MR sequences for simultaneous PET and MR reading

“…However, this disadvantage could be turned into an advantage: The extra time required for high quality MRI could be used for longer PETacquisition times, either increasing the image quality of PET [20] or allowing for a significant reduction of the injected tracer dose [21]. Consequently, especially in patients requiring repeated PETexaminations (e.g.…”

Combined PET/MRI: Multi-modality Multi-parametric Imaging Is Here

“…Further dose reduction, if desired, could be achieved by trading off lower injected radiotracer activities at the expense of extended emission scan times, assuming constant image quality and no further advances in performance of the PET system in use [31]. Extended imaging times in these patients may require longer anaesthesia times that, in turn, may pose a higher risk than the perceived risk from exposure to ionising radiation.…”