Cone-beam computed tomography (CBCT) is one of the most revolutionary innovations in dentistry, and was introduced into the dental field two decades ago as a three-dimensional (3D) imaging modality. Since then, it has gained general clinical acceptance, and is widespread among many dental specialties. The CBCT examination is a fast and user-friendly technique that provides multiplanar images with high spatial resolution, providing information that is unattainable with two-dimensional (2D) imaging in many diagnostic tasks. Thus, CBCT is an essential examination tool that can replace or complement other examinations. CBCT imaging plays a valuable role when hard tissue abnormalities are suspected in the temporomandibular joint (TMJ) by providing diagnostic information on cortical and subcortical boneintegrity or destruction/production changes. The growing availability and use of CBCT at dental clinics has led to concerns as to whether the information obtained by CBCT imaging justifies the additional exposure of the patient to radiation and the training required to examine and interpret the images. The radiation protection principles: justification and optimisation, should always be applied as the basis for protection. This thesis presents four studies on TMJ imaging using CBCT. In line with recent trends towards digital online education, two CBCT educational tools were developed for the interpretation of CBCT images of the TMJ and published on the Malmö University web site. The first was an educational tool including 35 CBCT examinations of TMJ, presented as 2D multiplane CBCT images. The second was a web-based programme containing 15 CBCT examinations of TMJ, presented as 3D multiplane CBCT images. Both tools included the image analysis criteria of the Diagnostic Criteria for Temporomandibular disorder (DC/TMD) as an assessment module, and were tested by dental students. It was found that they could be useful educational tools for TMJ assessment using CBCT images. The aim of the third study was to map and compare the distribution of absorbed doses using radiochromic film dosimeters in panoramic radiography and CBCT examinations of the TMJ using adult and child anthropomorphic head phantoms. Sheets of Gafchromic film (XR-QA2)were placed at five levels, corresponding to the radiographic examination, in the phantoms. The clinical protocols for panoramic and CBCT imaging of the TMJ of three dental X-ray units were used. The mean absorbed doses to a number of radiosensitive tissues within the oral and maxillofacial regions were estimated. The absorbed doses varied considerably among and within the radiosensitive tissues with examination type, X-ray unit, clinical setting, and patient age. The bonesurface and salivary glands received the highest absorbed doses in both radiographic examinations. The radiation burden was lower when using two small right and left fields of view (FOVs) than when using a single larger FOV. Accurate measurements of the absorbed dose in small dental radiation fields is challenging due to steep dose gradients. The use of Gafchromic film has shown promising results, allowing dose comparisons between different radiographic imaging. The last study was carried out to investigate the possibility of dose optimisation in CBCT examinations of the TMJ, in line with there commendations of the International Commission on Radiological Protection (ICRP) and the National Council on Radiation Protection and Measurements (NCRP). Thirty-four adult patients referred for CBCT imaging of the TMJ underwent two examinations with different scanning protocols, a manufacturer-recommended protocol (default), and a low-dose protocol in which the tube current was reduced to 20% of that in the default protocol. Three image stacks were reconstructed: the default protocol, the low-dose protocol, and the low-dose protocol processed using a noise reduction algorithm. Four radiologists evaluated the visibility of TMJ anatomic structures, image quality and radiographic findings. It was found that the visibility of the TMJ anatomical structures, the overall image quality and the radiographic findings using the low-dose protocol were comparable to those in the default protocol, indicating that diagnostically comparable results could be obtained with a five times lower radiation dose. Finally, this thesis highlights the diagnostics of TMJ imaging using CBCT in the context of image interpretation, imaging dosimetry and dose optimisation, all with the purpose of improving and optimising radiological TMJ diagnostics.
Mutual Information-Based Non-Local Total Variation Denoiser for Low-Dose Cone-Beam Computed Tomography
