Purpose: This study was performed to establish a procedure for simulated low-dose cone-beam computed tomography (CBCT) scans and to investigate whether the resulting images are comparable in diagnostic accuracy to those obtained using a clinical low-dose protocol. Materials and Methods: ImageJ was used to manipulate the sinogram data from CBCT scans acquired at 5 mA to mimic a technical setting of 2 mA by adding noise to the Radon-transformed projection data before image reconstruction. Four observers compared the simulated 2 mA CBCT scans with original clinical 2 mA CBCT scans acquired previously. The CBCT images were analysed using a protocol with a ranking scale, and the observers were required to select only 1 category for each variable. The Wilcoxon signed-rank test was used to assess differences between the 2 CBCT scan types, with a significance level of P<0.05. Intra-observer agreement was evaluated using the Cohen kappa. Results: Pairwise observer comparisons of the simulated and clinical low-dose CBCT scans showed no significantdifferences in image quality. Intra-observer agreement was acceptable, and in 5 comparisons, the results indicated a high degree of agreement. Conclusion Simulated low-dose CBCT scans can be generated using ImageJ. No significant differences in imagequality were observed between simulated and clinical low-dose CBCT scans when evaluating mandibular third molars. These findings suggest that manipulation of sinogram data is a promising radiation-free approach for simulating low-dose images in optimisation efforts.

Purpose: The aim of this study was to assess the artefacts of 12 fixed orthodontic appliances in magnetic resonance images obtained using 1.5-T and 3-T scanners, and to evaluate different imaging sequences designed to suppress metal artefacts. Materials and Methods: In vitro, study casts of 1 adult with normal occlusion were used. Twelve orthodontic appliances were attached to the study casts and scanned. Turbo spin echo (TSE), TSE with high readout bandwidth, and TSE with view angle tilting and slice encoding for metal artefact correction were used to suppress metal artefacts. Artefacts were measured. In vivo, 6 appliances were scanned: 1) conventional stainless-steel brackets; 2) nickel-free brackets; 3) titanium brackets; 4) a Herbst appliance; 5) a fixed retainer; and 6) a rapid maxillary expander. The maxilla, mandible, nasopharynx, tongue, temporomandibular joints, and cranial base/eye globes were assessed. Scores of 0, 1, 2, and 3 indicated no artefacts and minor, moderate, and major artefacts, respectively. Results: In vitro, titanium brackets and the fixed retainer created minor artefacts. In vivo, titanium brackets caused minor artefacts. Conventional stainless-steel and nickel free brackets, the fixed retainer, and the rapid maxillary expander caused major artefacts in the maxilla and mandible. Conventional stainless-steel and nickel-free brackets caused major artefacts in the eye globe (3-T). TSE with high readout bandwidth reduced image artefacts in both scanners. Conclusion Titanium brackets, the Herbst appliance, and the fixed retainer caused minor artefacts in images of neurocranial structures (1.5-T and 3-T) when using TSE with high readout bandwidth.