Maxillofacial cone beam computed tomography (CBCT) : aspetcs on optimisation

Sammanfattning: Maxillofacial Cone Beam Computed Tomography (CBCT) has become a common modality for imaging of the facial skeleton. The increased, and sometimes inappropriate, use of this modality and the fact that radi-ation doses from CBCT examinations are generally higher than those from conventional radiography will result in an increase in the radiation dose and radiation-associated risk to which patients are exposed. There-fore, the radiation protection principles, justification and optimisation of protection, recommended by the International Commission on Radiological Protection (ICRP) should be applied. Justification of clinical indication is the most important aspect of reducing radiation dose with CBCT scanning. In terms of optimisation, the examination should be performed exposing the patient to the lowest possible radiation dose whilst simultaneously obtaining the image quality required for the diagnostic task. The overall objective of this thesis was to clarify some aspects of optimisation for CBCT examinations. STUDY I comprised a systematic review with the aim to estimate effec-tive dose of CBCT of the facial skeleton with focus on measurement methods and scanning protocols that were used when measuring and es-timating the radiation dosage and effective doses range. The review ad-hered to the preferred reporting items for systematic reviews (PRISMA) statement. Three electronic databases were searched and 38 studies ultimately met the inclusion criteria. Heterogeneity in measurement methods and scanning protocols between studies made comparisons of effective doses of different CBCT units and scanning protocols difficult. A model with minimum data set on important parameters based on this observation of heterogeneity was proposed. Few studies related ef-fective dose to image quality and consequently the review revealed a need for studies on radiation dosages related to image quality. STUDY II demonstrated the feasibility of GafChromic® film XR-QA2 (ISP Corp., Wayne, NJ) as a dosimeter when performing measurements of the effective dose from three different CBCT units and comparing doses from 3 common dental clinical situations. The CBCT units used were Veraviewepocs 3De® (J Morita MFG Corp., Kyoto, Japan), Pro-Max® 3D (Planmeca, Helsinki, Finland) and NewTom VGi® (Quantita-tive Radiology, Verona, Italy). Depending on availability, medium and smaller field of view (FOV) scanning modes were used. Additionally, radiation doses from the three CBCT units were compared with radia-tion doses from three digital panoramic units. GafChromic XR-QA2 films were placed between selected layers of the head and neck of a tis-sue-equivalent human skull (RANDO® phantom; The Phantom Labora-tory, Salem, NY). The effective dose was estimated using the 2007 ICRP formalism. The lowest effective dose of a CBCT unit was observed for ProMax 3D, FOV 4 X 5 cm (10 µSv), the highest for NewTom VGi, FOV 8 X 8 cm—high resolution (129 µSv). The range of effective doses for panoramic units measured was 8-14 µSv. STUDY III investigated the relationship between dose and image quali-ty for 3D Accuitomo® 170 CBCT scanner (J. Morita, Kyoto, Japan) us-ing 12 different scanning protocols for assessment of periodontal struc-tures. The SedentexCT IQ phantom (Leeds Test Objects Ltd, Borough-bridge, UK) was used to investigate the relationship between contrast-to-noise ratio (CNR) and dose–area product (DAP). Subjective image quality assessment was achieved using a small adult skull phantom (RANDO®; The Phantom Laboratory, Salem, NY) for the same range of exposure settings. Five independent raters assessed the images for three anatomical landmarks using a three-point visual grade analysis. Objective and subjective image quality was evaluated and correlated to radiation dose. By altering tube potential and current for the 360° rotation protocol, the conclusion was reached that assessment of periodontal structures can be performed with a smaller dose without substantially affecting visualisation. STUDY IV comprised a clinical study of ten adolescents with the aim of evaluating the reliability of measurements of root lengths and marginal bone levels in CBCT images, and periapical (PA) and bitewing radiographs (BW). Six raters performed all available measurements. CBCT was the most reliable imaging method for root length meas-urements while reliability for marginal bone level measurements was about the same for all methods.