title: Dose Optimization in TOF-PET/MR Compared to TOF-PET/CT.
abstract: OBJECTIVE
  To evaluate the possible activity reduction in FDG-imaging in a Time-of-Flight (TOF) PET/MR, based on cross-evaluation of patient-based NECR (noise equivalent count rate) measurements in PET/CT, cross referencing with phantom-based NECR curves as well as initial evaluation of TOF-PET/MR with reduced activity.
  
  
  METHODS
  A total of 75 consecutive patients were evaluated in this study. PET/CT imaging was performed on a PET/CT (time-of-flight (TOF) Discovery D 690 PET/CT). Initial PET/MR imaging was performed on a newly available simultaneous TOF-PET/MR (Signa PET/MR). An optimal NECR for diagnostic purposes was defined in clinical patients (NECRP) in PET/CT. Subsequent optimal activity concentration at the acquisition time ([A]0) and target NECR (NECRT) were obtained. These data were used to predict the theoretical FDG activity requirement of the new TOF-PET/MR system. Twenty-five initial patients were acquired with (retrospectively reconstructed) different imaging times equivalent for different activities on the simultaneous PET/MR for the evaluation of clinically realistic FDG-activities.
  
  
  RESULTS
  The obtained values for NECRP, [A]0 and NECRT were 114.6 (± 14.2) kcps (Kilocounts per second), 4.0 (± 0.7) kBq/mL and 45 kcps, respectively. Evaluating the NECRT together with the phantom curve of the TOF-PET/MR device, the theoretical optimal activity concentration was found to be approximately 1.3 kBq/mL, which represents 35% of the activity concentration required by the TOF-PET/CT. Initial evaluation on patients in the simultaneous TOF-PET/MR shows clinically realistic activities of 1.8 kBq/mL, which represent 44% of the required activity.
  
  
  CONCLUSIONS
  The new TOF-PET/MR device requires significantly less activity to generate PET-images with good-to-excellent image quality, due to improvements in detector geometry and detector technologies. The theoretically achievable dose reduction accounts for up to 65% but cannot be fully translated into clinical routine based on the coils within the FOV and MR-sequences applied at the same time. The clinically realistic reduction in activity is slightly more than 50%. Further studies in a larger number of patients are needed to confirm our findings.
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Marcelo A
authors.initials: MA
authors.lastname: Queiroz
authors.affiliation: GE Healthcare, Waukesha, Wisconsin, United States of America.
authors.forename: Gaspar
authors.initials: G
authors.lastname: Delso
authors.affiliation: GE Healthcare, Waukesha, Wisconsin, United States of America.
authors.forename: Scott
authors.initials: S
authors.lastname: Wollenweber
authors.affiliation: GE Healthcare, Waukesha, Wisconsin, United States of America.
authors.forename: Timothy
authors.initials: T
authors.lastname: Deller
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Konstantinos
authors.initials: K
authors.lastname: Zeimpekis
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland; Department of Medical Imaging, Diagnostic and Interventional Radiology, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Martin
authors.initials: M
authors.lastname: Huellner
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Felipe
authors.initials: F
authors.lastname: de Galiza Barbosa
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Gustav
authors.initials: G
authors.lastname: von Schulthess
authors.affiliation: Department of Medical Imaging, Nuclear Medicine, University Hospital Zurich, Zurich, Switzerland; Department of Medical Imaging, Diagnostic and Interventional Radiology, University Hospital Zurich, Zurich, Switzerland.
authors.forename: Patrick
authors.initials: P
authors.lastname: Veit-Haibach
mesh_terms: D006801:Humans; D008279:Magnetic Resonance Imaging; D064847:Multimodal Imaging; D049268:Positron-Emission Tomography; D011829:Radiation Dosage; D014057:Tomography, X-Ray Computed