title: On the quantification accuracy, homogeneity, and stability of simultaneous positron emission tomography/magnetic resonance imaging systems.
abstract: OBJECTIVE
  A potential major application of simultaneous avalanche photodiode-based positron emission tomography (PET)/magnetic resonance imaging (MRI) systems are quantitative brain studies for cerebral blood flow measurements in combination with blood-oxygen-level-dependent or perfusion MRI, requiring a high performance for both modalities. Thus, we evaluated PET quantification accuracy and homogeneity for 2 different simultaneous PET/MRI systems (whole-body and brain scanner) compared with those of a state-of-the-art PET detector (PET/computed tomography) using phantom studies. In addition, we investigated the long-term stability of PET and quality of functional MRI measurements of a clinical whole-body PET/MRI scanner.
  
  
  METHODS
  Phantom measurements were conducted using spheres filled with [F]-fluoride distributed in a homogeneous cylinder phantom at different positions inside the PET field of view. Recovery values and standard deviations were extracted from resulting PET images. The influence of magnetic resonance-based attenuation correction and that of activity outside the PET field of view on the recovery values of these spheres was evaluated. Furthermore, long-term PET stability of the whole-body PET/MRI system was assessed by evaluating position profiles, energy spectra, count rates, and recovery values from [Ge]-phantom scans. Functional MRI applicability was tested in accordance with the functional Biomedical Information Research Network procedure.
  
  
  RESULTS
  The BrainPET system showed high recovery values (up to 99%) but also increased variability (up to 7.4%). Significant underestimations in PET quantification near activity outside the PET field of view were found (up to 80%). Using magnetic resonance-based attenuation correction led to an underestimation in PET activity of approximately 7%. In distinction, the whole-body PET/MRI system revealed performance similar to the PET/computed tomographic scanner (recovery values up to approximately 60% with a variability of approximately 4%). Long-term stability and fMRI performance of the whole-body PET/MRI scanner showed no degradation compared with stand-alone systems.
  
  
  CONCLUSIONS
  Homogeneity and accuracy of avalanche photodiode-based PET detectors is comparable with those of the state-of-the-art detectors based on photomultiplier tubes. However, attenuation correction on PET/MRI systems has to be adapted carefully for quantitative PET measurements. The BrainPET system needs improved scatter correction to perform quantitative brain studies. The whole-body PET/MRI scanner, however, is applicable for quantitative brain studies.
authors.affiliation: From the *Diagnostic and Interventional Radiology, Department of Radiology, †Laboratory for Preclinical Imaging and Imaging Technology of the Werner Siemens-Foundation, Department of Preclinical Imaging and Radiopharmacy, University of Tübingen; ‡Department of Empirical Inference, Max Planck Institute for Intelligent Systems; and §Nuclear Medicine, Department of Radiology, University of Tübingen, Tübingen, Germany.
authors.forename: Holger
authors.initials: H
authors.lastname: Schmidt
authors.forename: Nina F
authors.initials: NF
authors.lastname: Schwenzer
authors.forename: Ilja
authors.initials: I
authors.lastname: Bezrukov
authors.forename: Frederic
authors.initials: F
authors.lastname: Mantlik
authors.forename: Armin
authors.initials: A
authors.lastname: Kolb
authors.forename: Jürgen
authors.initials: J
authors.lastname: Kupferschläger
authors.forename: Bernd J
authors.initials: BJ
authors.lastname: Pichler
mesh_terms: D001921:Brain; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000295:instrumentation*; D064847:Multimodal Imaging; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography / Q000295:instrumentation*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation*; D051598:Whole Body Imaging / Q000295:instrumentation*