Radiation doses to the eye lenses in computed tomography of the orbit and the petrous bone.

Standard CT examinations of the orbit and the petrous bone were simulated with a tissue-equivalent human skull phantom on a Somatom DR 2 scanner. The orientation of the scan planes and the scan parameters including thickness, number of slices and mAs were varied according to the assumed diagnostic problems. The total radiation doses in the region of the eye lenses were measured with thermoluminescent dosimeters. If the globes are located within the X-ray field, the total radiation doses received by the lenses increased with the number and the thickness of the slices and the mAs. The highest radiation doses were found in contiguous thin-section scans of the orbit in axial (52 mGy) or direct coronal (47 mGy) orientation. In cases where the globes were located outside the X-ray field, the radiation doses received by the eye lenses could be reduced by a factor of 16, resulting in only 3.1-3.4 mGy for a complete axial study of the inner ear. Recommendations are given how to reduce the radiation exposure to the eye lenses without loss of diagnostical information.