CT has become an established examination in the evaluation of the paranasal sinuses. Until recently this was achieved by the direct coronal technique on conventional and single slice helical scanners. With the advent of multislice technology, thin slice axial CT with excellent coronal and sagittal reconstructions is now the norm. We describe a study designed to evaluate the radiation dose to the lens of the eye and thyroid gland in the axial and coronal planes on a Siemens Volume Zoom quad slice scanner at 140 kV and effective mAs of 100 using 1 mm collimation. Thermoluminescent dosimeters were placed on the eyelid and thyroid gland of 29 patients scanned axially in the supine position and a further 28 patients scanned coronally in the prone position with gantry tilt. The results show mean doses of 35.1 mGy (lens) and 2.9 mGy (thyroid gland) in the coronal plane compared with 24.5 mGy (lens) and 1.4 mGy (thyroid gland) in the axial plane. Results obtained from a head phantom and from using the ImPACT CT dose calculator were comparable. The kV and mAs were then reduced to 120 and 40, respectively, and the axial study repeated using the head phantom and predicted doses using the ImPACT CT dose calculator. The low dose scanning technique revealed a lens dose of 9.2 mGy and thyroid dose of 0.4 mGy. The eye dose on a multislice scanner is still substantially less than the threshold dose of 0.5-2 Gy for detectable lens opacities. These results indicate that, in addition to the established perceived advantages of multislice axial sinus CT, i.e. patient comfort, no artefact from dental amalgam and reproducible true coronal images, should be included a decreased radiation dose to both the eye lens and thyroid gland compared with direct coronal scanning.

There is limited data currently available for making dose and risk assessments for paediatric patients undergoing computed tomographic examination. A method has been developed to correlate the risk-related quantity, effective dose, to the more simply derived quantity dose-length product. This involved scanning a series of paediatric anthropomorphic phantoms containing thermoluminescent dosimeters to measure effective dose for scans of various anatomic regions. The quantity effective dose per dose-length product was calculated and plotted as a function of patient size. This showed a simple exponential relationship, and equations of fit were derived to enable the calculation of effective dose for a patient of any size. Measurements carried out on a second scanner and for alternative scan volumes indicated that the method could be generally utilized.

A method for the systematic selection of paediatric CT technique factors is described. The approach is based on the assumption that the level of image noise acceptable for a given adult CT image is also acceptable for the equivalent paediatric examination. A simple exponential attenuation model is proposed. Effective linear attenuation coefficients were initially established from a series of phantom measurements simulating head, chest and abdomen examinations at 120 kVp, then extended for a range of tube potentials and beam qualities using a beam spectral model. Application of the method is demonstrated using phantoms representing head, chest and abdomen sections for neonate and ages 1 year, 5 years, 10 years, 15 years and adult.