Diagnostic criteria and dose limiting approaches for image modalities in odontology

Objectives: Technical innovations in radiography enable the development of new approaches to reduce the dose to patients. The aim of this thesis was to explore dose optimization approaches for the most frequently used radiographic modalities in dentistry.

Material and Methods: Intraoral radiography: The performance of the Automatic exposure control function (AEC) was tested on dry mandibles with soft tissue equivalent of different thicknesses. Furthermore the image quality was compared between images exposed manually and with AEC function. Two different generations of direct digital intraoral sensors, based on charged couple device (CCD) and complementary metal oxide semi-conductor (CMOS) were compared in terms of dose response function, minimal perceptible contrast details and minimal perceptible exposure difference. Panoramic radiography: Effective doses obtained from panoramic examinations with ten different collimation features were assessed using the metal-oxide semiconductor field-effect transistor (MOSFET) method. In addition, the applicability of the collimation function under clinical situations was evaluated. Cone Beam Computed Tomography: Optimized exposure protocols for temporomandibular joint (TMJ) examinations on a phantom were obtained for CBCT and MSCT through subjective image quality analysis. Effective doses, before and after, optimization were compared for CBCT and MSCT using thermoluminescent dosimeter (TLD) technique.

Results: The exposure times using AEC were adjusted automatically according to the thickness of the objects and the resulting image quality was considered adequate by observers. The CMOS sensor was more sensitive to radiation and presented better image quality on low contrast details perception compared to the CCD sensor. The calculated effective dose of a full size panoramic radiograph was 17.6 μSv at 8mA and 66kV. In 61% of the studied referrals, a collimation including the dental alveolar region was applicable, providing a dose reduction by 40.3%. The effective doses for bilateral TMJ examination was 92 μSv for CBCT and 124 μSv for MSCT. The image quality of CBCT was considered better than that of MSCT.

Conclusions: AEC might be a feasible approach for acquiring intraoral digital radiographs with good image quality. ProSensor with CMOS technique was preferred in comparison to Dixi sensor with CCD technique due to lower exposure and better detectability of low contrast details. Collimating panoramic radiographs was an effective approach to reduce radiation dose to patients when clinical indication allowed. For TMJ examination CBCT was preferred to MSCT due to better image quality at comparable effective doses.

List of scientific papers

I. Benchimol D, Näsström K, Shi XQ. Evaluation of automatic exposure control in a direct digital intraoral system. Dentomaxillofacial Radiology. 2009;38:407-412.
https://doi.org/10.1259/dmfr/22255081

II. Shi XQ, Benchimol D, Näsström K. Comparison of psychophysical properties of two intraoral digital sensors on low-contrast perceptibility. Dentomaxillofacial Radiology. 2013;42(10):20130249.
https://doi.org/10.1259/dmfr.20130249

III. Kadesjö N, Benchimol D, Falahat B, Näsström K, Shi XQ. Evaluation of the effective dose of cone beam CT and multislice CT for temporomandibular joint examinations at optimized exposure levels. Dentomaxillofacial Radiology. 2015;44:20150041.
https://doi.org/10.1259/dmfr.20150041

IV. Benchimol D, Kadesjö N, Koivisto J, Shi XQ. Effective dose reduction using segmenting function in digital panoramic radiography and possible clinical implications in dentistry. [Submitted]