Radiation dose and risk to the lens of the eye during CT examinations of the brain.

Cataracts are the leading cause of blindness and visual disability worldwide. Of the known contributing factors to this condition, ionising radiation is considered the primary concern in a radiological context given the particular radiosensitivity of the lens of the eye. In light of the substantially increased application of computed tomography in brain imaging, an investigation of the relevent literature is warranted to assess thresholds, lens radiation doses and dose reduction techniques in respect to the cataractogenic risk of such examinations. The value and very existence of a lens dose threshold is debatable given different considerations of radiation dose, latency, opacity classifications and historical sample populations, though ICRP guidelines suggest a threshold of 0.5 Gy. Documented CT-specific radiation doses to the eye following scans of the brain are highly variable between studies (2-130 mGy), primarily owing to discrepancies in scanning technique. These findings, when coupled with the relative ambiguity of known threshold values, present difficulties in assessing the overall risk of cataracts following serial CT examinations to the head. In the absence of definitive risk evaluations, a cautionary approach is advised. The implementation of gantry tilt along the supraorbital margin is recommended as standard practice on account of its highly effective radiation dose reduction outcomes. Organ-based tube modulation and reductions in tube current may also be considered beneficial. Bismuth eye shielding is only advised where gantry tilting is unachievable, and in such cases, ensure careful adherence to appropriate shield placement and infection control measures.