The susceptibility effects arising from metal implants may challenge the accuracy of MR-AC. However, in the absence of CT or traditional transmission sources in PET/MR systems, the MR images are also used for MR-based AC (MR-AC). Combined PET/MR helps present PET information in the context of sequentially or simultaneously acquired MR data. Recently, combined PET/MR systems have become available to clinical users and found to be of value in cancer imaging. The accuracy of the CT-AC is limited by beam-hardening artifacts caused by the significantly higher photon absorption from high-Z materials compared with low-Z materials (e.g., soft tissues) at CT energies. Combined PET/CT imaging offers the additional advantage of using the CT images for noise-limited attenuation correction (CT-based attenuation correction, CT-AC). Combined positron emission tomography/computed tomography (PET/CT) has been shown to provide intrinsically aligned functional and anatomical image information from a single patient's examination. PET is a powerful and accurate diagnostic imaging method for the assessment of oncology patients.

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. The resulting PET/MR artifacts may exceed the actual volume of the dental fillings. Metallic dental work may cause severe MR signal voids. 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. The mean and maximum SUVs averaged across all patients increased after inpainting by 52% (± 11%) and 28% (± 11%), respectively, in the corrected region. The corresponding/resulting bias of the reconstructed tracer distribution was localized mainly in the area of the signal void. 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 volume of the artifacts and the computed relative differences in mean and max standardized uptake value (SUV) between the two PET images are reported. The reconstructed PET images were evaluated visually and quantitatively using regions of interests in reference regions. 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. MR-AC was based on either standard MR-AC DIXON or MR-AC INPAINTED where the susceptibility-induced signal voids were substituted with soft tissue information. The PET/MR data were acquired over a single-bed position of 25.8 cm covering the head and neck.

Patients were injected with -FDG, -PiB, -FET, or -DOTATATE. MethodsĪ total of 148 PET/MR patients with clear visual signal voids on the attenuation map in the dental region were included in this study.

The purpose of this study was to assess the frequency and magnitude of subsequent PET image distortions following MR-AC. The susceptibility effects due to metal implants challenge MR-AC in the neck region of patients with dental implants. 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).

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