title: Accurate PET/MR quantification using time of flight MLAA image reconstruction.
abstract: OBJECTIVE
  Quantification of positron emission tomography/magnetic resonance imaging (PET/MRI) studies is hampered by inaccurate MR-based attenuation correction (MR-AC). To date, most studies on MR-AC have been performed using PET/MR systems without time of flight (TOF). Maximum likelihood reconstruction of attenuation and activity (MLAA), however, has the potential to improve MR-AC by exploiting TOF. The purpose of this study is to assess the impact of MR-AC on PET image quantification for TOF-PET/MR systems and to evaluate PET accuracy when using TOF in combination with MLAA (TOF-MLAA).
  
  
  METHODS
  Simulations were designed to evaluate (1) the impact of MR-AC on PET quantification for different TOF windows (667, 500, 333 and 167 ps) and (2) use of TOF-MLAA for improving PET quantification. TOF-ordered subset expectation maximisation (OSEM) and TOF-MLAA reconstructions using MR-AC were compared with those obtained using TOF-OSEM with computed tomography-based AC (CT-AC).
  
  
  RESULTS
  OSEM reconstructions without TOF showed a negative MR-AC-induced bias of -50 % in the bone. TOF-OSEM was able to reduce this bias down to -15 %, with more accurate results for better TOF. TOF-MLAA was able to reduce the bias to within 5 % but at the cost of a ∼40 % increase in image variance.
  
  
  CONCLUSIONS
  TOF-MLAA can improve quantitative PET accuracy of PET/MR studies. Further improvements are anticipated with improving TOF performance.
authors.affiliation: Department of Radiology and Nuclear Medicine, VU University Medical Centre, Amsterdam, The Netherlands, r.boellaard@vumc.nl.
authors.forename: R
authors.initials: R
authors.lastname: Boellaard
authors.forename: M B M
authors.initials: MB
authors.lastname: Hofman
authors.forename: O S
authors.initials: OS
authors.lastname: Hoekstra
authors.forename: A A
authors.initials: AA
authors.lastname: Lammertsma
mesh_terms: D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted; D016013:Likelihood Functions; D008279:Magnetic Resonance Imaging; D019047:Phantoms, Imaging; D049268:Positron-Emission Tomography; D013997:Time Factors