title: Systematic evaluation of phantom fluids for simultaneous PET/MR hybrid imaging.
abstract: With the recent advent of integrated PET/MR hybrid systems, the need for simultaneous PET and MR phantom measurements arises. Phantom fluids that are used in stand-alone MR systems, especially in larger phantoms and at a high magnetic field strength, are not necessarily applicable in PET imaging and vice versa. In this study, different approaches to fluid selection were considered and systematically evaluated with respect to their usability for simultaneous PET/MR phantom imaging.
  
  
  METHODS
  Demineralized water, water with increased electrical conductivity, a water-oil emulsion, and monoethylene and triethylene glycol were investigated in MR and PET measurements using the most common PET tracer (18)F-FDG. As an alternative to (18)F-FDG, a modified PET tracer ((18)F-fluoride Kryptofix 222 complex) was investigated toward its ability to dissolve in pure oil, which provides good signal homogeneity in MR imaging. Measurements were performed on a 3.0 T integrated PET/MR whole-body system using a National Electrical Manufacturers Association quality-standard phantom.
  
  
  RESULTS
  All tested fluids dissolved the radiotracer (18)F-FDG homogeneously. Regarding their suitability for MR at 3.0 T, all fluids significantly improved the homogeneity compared to pure water (increase of excitation flip angle within the tested phantom by a factor of 2.0). When the use of (18)F-FDG was preferred, triethylene glycol provided the best compromise (flip angle increase by a factor of 1.13). The potential alternative tracer, (18)F-fluoride Kryptofix 222 complex, dissolved in pure oil; however, it is not optimal in its tested composition because it accumulates at the bottom of the phantom during the time of measurement.
  
  
  CONCLUSIONS
  This study provides a systematic approach toward phantom fluid selection for imaging a given quality-standard body phantom--and phantoms of comparable size--at 3.0 T. For simultaneous PET/MR scans using the standard tracer (18)F-FDG, an alternative fluid to water and oil is proposed that serves as a viable option for both imaging modalities. Nevertheless, when water is preferred, ways to improve MR image homogeneity are presented. The tested alternative PET tracer enables the use of pure oil in combined scans, but the tracer composition needs to be optimized for phantom measurement applications.
authors.affiliation: Institute of Medical Physics, University of Erlangen-Nuremberg, Erlangen, Germany. susanne.ziegler@imp.uni-erlangen.de
authors.forename: Susanne
authors.initials: S
authors.lastname: Ziegler
authors.forename: Harald
authors.initials: H
authors.lastname: Braun
authors.forename: Philipp
authors.initials: P
authors.lastname: Ritt
authors.forename: Carsten
authors.initials: C
authors.lastname: Hocke
authors.forename: Torsten
authors.initials: T
authors.lastname: Kuwert
authors.forename: Harald H
authors.initials: HH
authors.lastname: Quick
mesh_terms: D005240:Feasibility Studies; D005459:Fluorides; D019788:Fluorodeoxyglucose F18; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000295:instrumentation*; D013997:Time Factors
chemical_list: D019788:Fluorodeoxyglucose F18; D005459:Fluorides
keywords: NEMA standard phantom; PET/MR phantom measurements; inhomogeneous radiofrequency excitation; integrated PET/MR hybrid imaging; radiofrequency artifact