Dental artifacts in the head and neck region: implications for Dixon-based attenuation correction in PET/MR.
BACKGROUND
In the absence of CT or traditional transmission sources in combined clinical positron emission tomography/magnetic resonance (PET/MR) systems, MR images are used for MR-based attenuation correction (MR-AC). The susceptibility effects due to metal implants challenge MR-AC in the neck region of patients with dental implants. The purpose of this study was to assess the frequency and magnitude of subsequent PET image distortions following MR-AC.


METHODS
A total of 148 PET/MR patients with clear visual signal voids on the attenuation map in the dental region were included in this study. Patients were injected with [(18)F]-FDG, [(11)C]-PiB, [(18)F]-FET, or [(64)Cu]-DOTATATE. The PET/MR data were acquired over a single-bed position of 25.8 cm covering the head and neck. MR-AC was based on either standard MR-ACDIXON or MR-ACINPAINTED where the susceptibility-induced signal voids were substituted with soft tissue information. Our inpainting algorithm delineates the outer contour of signal voids breaching the anatomical volume using the non-attenuation-corrected PET image and classifies the inner air regions based on an aligned template of likely dental artifact areas. The reconstructed PET images were evaluated visually and quantitatively using regions of interests in reference regions. The volume of the artifacts and the computed relative differences in mean and max standardized uptake value (SUV) between the two PET images are reported.


RESULTS
The MR-based volume of the susceptibility-induced signal voids on the MR-AC attenuation maps was between 1.6 and 520.8 mL. The corresponding/resulting bias of the reconstructed tracer distribution was localized mainly in the area of the signal void. The mean and maximum SUVs averaged across all patients increased after inpainting by 52% (± 11%) and 28% (± 11%), respectively, in the corrected region. SUV underestimation decreased with the distance to the signal void and correlated with the volume of the susceptibility artifact on the MR-AC attenuation map.


CONCLUSIONS
Metallic dental work may cause severe MR signal voids. The resulting PET/MR artifacts may exceed the actual volume of the dental fillings. The subsequent bias in PET is severe in regions in and near the signal voids and may affect the conspicuity of lesions in the mandibular region.
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. claesnl@gmail.com.
Claes N
CN
Ladefoged
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. adam.espe.hansen@regionh.dk.
Adam E
AE
Hansen
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. sune.hoegild.keller@regionh.dk.
Sune H
SH
Keller
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. malene.fischer@regionh.dk.
Barbara M
BM
Fischer
Department of Oncology, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. jacobrasmu@gmail.com.
Jacob H
JH
Rasmussen
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. ian.law@regionh.dk.
Ian
I
Law
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. andreas.kjaer@regionh.dk.
Andreas
A
Kjær
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. liselotte.hoejgaard@rh.regionh.dk.
Liselotte
L
Højgaard
Department of Computer Science, University of Copenhagen, Universitetsparken 5, 2100, Copenhagen East, Denmark. francois@di.ku.dk.
Francois
F
Lauze
Centre for Medical Physics and Biomedical Engineering, Medical University of Vienna, Waehringer Guertel 18-20/4L, Vienna, A-1090, Austria. Thomas.Beyer@meduniwien.ac.at.
Thomas
T
Beyer
Department of Clinical Physiology, Nuclear Medicine and PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 9, 2100, Copenhagen East, Denmark. flemming.andersen@regionh.dk.
Flemming L
FL
Andersen
Attenuation correction; Inpainting; Metal artifacts; PET/MRI; Quantification