Improving CT-Based PET Attenuation Correction in the Vicinity of Metal Implants by an Iterative Metal Artifact Reduction Algorithm of CT Data and Its Comparison to Dual-Energy–Based Strategies A Phantom Study Christoph Schabel, MD,* Sergios Gatidis, MD,* Malte Bongers, MD,* Fabian Hüttig, MD,† Georg Bier, MD,* Juergen Kupferschlaeger, PhD,‡ Fabian Bamberg, MD,* Christian la Fougère, MD,‡ Konstantin Nikolaou, MD,* and Christina Pfannenberg, MD* Objective: The aim of this study was to evaluate the potential of iterative metal artifact reduction (IMAR) for the improvement of computed tomography (CT)– based position emission tomography (PET) attenuation correction in the vicinity of metal implants and compare it with dual-energy–based metal artifact reduction strategies. Methods: A dedicated dental phantom was constructed consisting of a cylindri- cal tube filled with [18-F]FDG solution (5300 mL and 50.9 MBq) containing 2 artificial jaws with 1 nonprecious alloy fixed dental prosthesis and 3 single tooth crowns in the lower jaw. Computed tomography measurements of the phantom were acquired on a stand-alone dual-energy CT scanner equipped with IMAR capabilities. A series of 24 CT data sets were obtained using different scan parameters and monoener- getic extrapolation of dual-energy CT acquisitions with and without IMAR reconstruction. Position emission tomography measurements of the phantom were performed on a state-of-art PET/CT scanner. Position emission tomography data were recon- structed using all 24 previously acquired CT data sets. Relative errors in the quantification of activity concentrations using the different CT scanning and reconstruction parameters were quantified by placement of re- gions of interest within the phantom. Results: Metal artifacts of different extent were observed in all CT data sets. A marked reduction in CT metal artifacts was observed using IMAR. In general, activity concentrations were overestimated/underestimated in areas of high/ low-density metal artifacts, respectively. Relative errors in PET quantification ranged between -71% and +70% with- out IMAR. Using IMAR, these errors were reduced to a range between -40% and +12%. Averaged absolute values of relative PET quantification errors were 27% and 7% without and with the use of IMAR (P < 0.001), respectively. Iterative metal artifact reduction was superior compared with dual-energy–based metal ar- tifact reduction strategies, and the combination of both strategies did not result in further significant improvement of PET quantification. Conclusions: The use of IMAR in PET/CT is a promising approach for markedly improving image quality and PET quantification in the vicinity of metal implants. Further clinical studies are necessary to assess the clinical performance of this al- gorithm in patients. Key Words: PET/CT, metal artifact reduction, IMAR, CT, PET, phantom, denture, dual energy (Invest Radiol 2017;52: 61–65) P osition emission tomography (PET)/computed tomography (CT) has become the clinical standard for diagnostic imaging of a variety of oncologic, neurologic, and inflammatory disorders. The combination of high anatomic accuracy provided by CT together with the possibility of molecular imaging using PET allows for comprehensive and accurate characterization of diverse pathological processes. 1 For various reasons, the introduction of combined PET/CT marked an important step in the technical evolution of PET imaging— the combination of PET and CT within a single scanner provides high accuracy in spatial alignment of the 2 modalities, thus allowing for a precise spatial allocation of tracer uptake, resulting in an increase in diagnostic confidence. 2,3 The second major advantage of combining PET and CT lies in the possibility of using CT attenuation data for attenuation correction of PET images. Position emission tomography examination times can thus be reduced significantly, as the PET transmission scan, which is used for attenuation correction in PET-only studies, is replaced by the much faster CT scan. 4 However, 1 drawback of CT-based PET attenuation correction lies in its susceptibility for errors in regions where CT artifacts occur. This is, in particular, the case around high-density objects such as metal implants or fixed dental prostheses that typically cause radial streak artifacts in CT images. Tissue density and attenuation values (ie, Hounsfield Units) as measured by CT can be highly inaccurate within these regions. As a result, PET attenuation correction can be im- precise, causing falsely low or high local activity concentrations. 5–7 Several approaches have been introduced for the reduction of CT metal artifacts. On the acquisition side, using higher-energetic x-ray spectra have been suggested to reduce CT metal artifacts to a certain extent with the drawback of reduced image contrast. 8 As an alternative, the use of dual-energy CT acquisition provides strategies for metal artifact reduc- tion, by extrapolation of high photon energy scans and by computation of monoenergetic CT images. 9–12 On the postprocessing/reconstruction side, the introduction of advanced CT reconstruction techniques has shown to allow for a marked reduction in CT metal artifacts recently. Metal artifact reduction algorithms (MARs) have been proven beneficial for reduction of metal artifacts 13 and PET attenuation correction before. 14–18 A recently re- leased MAR algorithm that uses the combination of normalized and frequency-split metal artifact reduction, also referred as iterative metal Received for publication March 21, 2016; and accepted for publication, after revision, May 17, 2016. From the *Department of Radiology, Diagnostic and Interventional Radiology, †De- partment of Prosthodontics, University Clinic for Dentistry, Oral Medicine, and Maxillofacial Surgery, and ‡Department of Radiology, Nuclear Medicine and Clinical Molecular Imaging, University of Tübingen, Tübingen, Germany. Funding: Parts of the study were supported by a research grant from Siemens Healthcare, Germany. There was no additional external funding received for this study. The funder had no role in study design, data collection, and analysis or de- cision to publish. Conflicts of interest and sources of funding: Dr Christoph Schabel received lecture honoraria from Siemens Healthcare, Forchheim, Germany. The other authors de- clare that they have no conflicts of interest. Ethical approval: This article does not contain any studies with human participants or animals performed by any of the authors. Correspondence to: Christoph Schabel, MD, Department of Radiology, Diagnostic and Interventional Radiology, University of Tübingen, Hoppe-Seyler-Straße 3, 72076 Tübingen, Germany. E-mail: christoph.schabel@med.uni-tuebingen.de. Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved. ISSN: 0020-9996/17/5201–0061 DOI: 10.1097/RLI.0000000000000306 ORIGINAL ARTICLE Investigative Radiology • Volume 52, Number 1, January 2017 www.investigativeradiology.com 61 Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.