title: Clinical Assessment of Emission- and Segmentation-Based MR-Guided Attenuation Correction in Whole-Body Time-of-Flight PET/MR Imaging.
abstract: The joint maximum-likelihood reconstruction of activity and attenuation (MLAA) for emission-based attenuation correction has regained attention since the advent of time-of-flight PET/MR imaging. Recently, we improved the performance of the MLAA algorithm using an MR imaging-constrained gaussian mixture model (GMM). In this study, we compare the performance of our proposed algorithm with standard 4-class MR-based attenuation correction (MRAC) implemented on commercial systems.
  
  
  METHODS
  Five head and neck (18)F-FDG patients were scanned on PET/MR imaging and PET/CT scanners. Dixon fat and water MR images were registered to CT images. MRAC maps were derived by segmenting the MR images into 4 tissue classes and assigning predefined attenuation coefficients. For MLAA-GMM, MR images were segmented into known tissue classes, including fat, soft tissue, lung, background air, and an unknown MR low-intensity class encompassing cortical bones, air cavities, and metal artifacts. A coregistered bone probability map was also included in the unknown tissue class. Finally, the GMM prior was constrained over known tissue classes of attenuation maps using unimodal gaussians parameterized over a patient population.
  
  
  RESULTS
  The results showed that the MLAA-GMM algorithm outperformed the MRAC method by differentiating bones from air gaps and providing more accurate patient-specific attenuation coefficients of soft tissue and lungs. It was found that the MRAC and MLAA-GMM methods resulted in average standardized uptake value errors of -5.4% and -3.5% in the lungs, -7.4% and -5.0% in soft tissues/lesions, and -18.4% and -10.2% in bones, respectively.
  
  
  CONCLUSIONS
  The proposed MLAA algorithm is promising for accurate derivation of attenuation maps on time-of-flight PET/MR systems.
authors: Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, Geneva, Switzerland.
authors: Abolfazl
authors: A
authors: Mehranian
authors: Division of Nuclear Medicine and Molecular Imaging, Geneva University Hospital, Geneva, Switzerland Geneva Neuroscience Centre, University of Geneva, Geneva, Switzerland; and Department of Nuclear Medicine and Molecular Imaging, University of Groningen, Groningen, The Netherlands habib.zaidi@hcuge.ch.
authors: Habib
authors: H
authors: Zaidi
mesh_terms: D000465:Algorithms; D015992:Body Mass Index; D019788:Fluorodeoxyglucose F18; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000532:radiotherapy; D006801:Humans; D007091:Image Processing, Computer-Assisted; D016013:Likelihood Functions; D008279:Magnetic Resonance Imaging; D008875:Middle Aged; D015233:Models, Statistical; D064847:Multimodal Imaging; D016011:Normal Distribution; D049268:Positron-Emission Tomography; D012189:Retrospective Studies; D014057:Tomography, X-Ray Computed; D051598:Whole Body Imaging
chemical_list: D019788:Fluorodeoxyglucose F18
keywords: PET/MRI; attenuation correction; quantification; time-of-flight; whole-body imaging