SU-E-I-05: Dose Reduction in Head CBCT Examinations After Protocol Optimization

CBCT systems for head examinations especially in otorhinolaryngology offer better than required image quality at relatively low purchasing costs. Due to this fact the number of dedicated head scanners increases steadily. Regarding image quality in relation to patient dose these systems are frequently not optimized. Examination parameter optimization is an iterative process, which should be done carefully by physicians in cooperation with physicists. After a successful optimization, Monte Carlo Simulations can be used to quantify the dose reduction to the patient.

Images of an anatomic head phantom were obtained using different settings of kVp, mAs and rotation angle (360°, 210°). The resulting images were anonymized and the examination parameters were removed. Radiologists and otorhinolaryngologists evaluated these images and rated them into excellent image quality, good image quality, bad image quality and not readable. Based on the ratings new parameters for the adequate image quality were set. The dose reduction was calculated using Monte Carlo simulations.

The organ dose of radiation sensitive organs was reduced significantly. The dose reductions compared to the standard settings are: eyes: 85%, eye lenses: 88%, salivary glands: 40%, thyroid: 60%, blood vessels: 40%, brain: 60%, teeth: 80% and tonsils: 65%. Dose reduction was best, when a short scan (210° rotation angle) was used, where the eyes are not in the direct exposure field.

The results show the potential for dose optimizations in otorhinolaryngological CBCT examinations compared to standard vendor settings without loss in diagnostic information. Furthermore this study points out the great opportunities that Monte Carlo-based dose-calculations methods offer to quantify the increase of dose efficiency after examination protocol optimizations. It could also be shown that CBCT has advantages when doing short scan mode around the rear head. This protects radiation sensitive organs such as the eye lenses from being directly radiated.