A comparison of reduction in CT dose through the use of gantry angulations or bismuth shields

This study investigated the effect of gantry angulation and eye shielding on radiation dose to the eye lens during brain Computed Tomography (CT), and secondly the effectiveness of thyroid and breast bismuth shielding during routine neck and chest CT. An anthropomorphic 'Rando' phantom was scanned at three commonly used gantry angles using this centre's normal adult brain protocol, and for normal adult neck and chest protocols. Bismuth shields were placed over the appropriate organs and dose measured using thermoluminescent dosimeters (TLD). Angling the gantry along the supraorbital meatal plane could reduce the dose by approximately 88% relative to the hard palate and Reid's base line protocols. Bismuth eye shields reduce dose by up to 48% when using either of the first two angles but gave no significant reduction in the supra-orbital plane. Reduction in thyroid dose for neck and chest scans were 55% and 47% respectively, and dose reduction in breast was 23%. We thus conclude that angling the gantry to avoid the orbits is the more effective method of reducing radiation dose to the eyes, with single use bismuth shields to be used where this is not feasible. Thyroid and breast shields should be used where the resultant artefact is not detrimental to image quality.