Reducing the radiation dose to the eye lens region during CT brain examination: the potential beneficial effect of the combined use of bolus and a bismuth shield

Abstract

-Objective:Computed Tomography (CT) is the leading contributor to medical exposure to ionizing radiation. Although the use of CT brain scans for patients with head injuries and convulsions has shown a tremendous growth, it has raised substantial concerns in the general public because of the risk of radiation-induced cataracts: the current available strategies to reduce the radiation dose to the eye lens region are limited. Therefore, the present research project was initiated with the aim of evaluating the potential benefit of the combined use of bolus and a bismuth shield on reducing the radiation dose to the eye lens region during CT brain examination. Materials and methods: We conducted a series of phantom studies to measure the entrance surface dose (ESD) that is delivered to the eye lens region during CT brain examination under the effect of different scanning and shielding setups. Results: Our results indicated, during CT brain examination: (1) a drastic reduction of 92.5% in the ESD to the eye lens region was found when the CT gantry was tilted from 0• (overall ESD = 30.7 mGy) to 30• cranially (overall ESD = 2.4 mGy), and (2) when the CT gantry was positioned at 0• (the common practice in the clinical setting), the setups with the application of a) a bismuth shield, b) a bismuth shield with a face shield (air gap), c) a bismuth shield with bolus, and d) a bismuth shield with bolus and an air gap can result in an acceptable level of image quality with a smaller overall ESD delivered to the eye lens region (overall ESD = 23.2 mGy, 24 mGy, 21 mGy and 19.9 mGy, respectively) than the setup without the bismuth shield applied (overall ESD = 30.7 mGy). Conclusion: When the primary beam scanning through the eye lens region is unavoidable during CT brain examination, the combined use of a bismuth shield with bolus and a face shield is an easyto-use and inexpensive shielding setup to reduce the radiation dose delivered to the eye lens region while maintaining the correct CT number and a low degree of image noise in the resultant image.

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

“…Of the trials which utilised a degree of gantry tilt as a component of their reference scanning technique, doses were significantly lower overall, mainly where alignment could be achieved along the supraorbital margin such that the lenses of the eyes were no longer within the primary irradiation beam. However, of these studies, the majority of those who were able to achieve adequate alignment were phantom trials . While the use of anthropomorphic phantoms in such studies allows precise documentation of absorbed dose, variations in patient size and position are not able to be taken into account.…”

“…The number and placement of dosimeters differ from each trial, which could affect the consistency and reproducibility of the results with only one or two devices. As well as this, the inclusion of the dosimeter in the scout images, as opposed to being attached after the scout images have been taken, could have implications on the end dose; however, admittedly its contribution would likely be relatively low.…”

“…While diagnostically valuable and relatively quick to perform, CT scans of the head do not come without risks. In conjunction with the consequences associated with direct irradiation of the brain, the formation of visually impairing opacities and radiation cataracts in the lens of the eye may be of concern . When paired with a lack of standardised scanning techniques and the growing tendency for brain scans to be over‐referred, the potential for radiation cataracts to become a more widespread phenomenon becomes tangible.…”

The Current Use and Effectiveness of Bismuth Shielding in Computed Tomography: A Systematic Review

“…Several studies have shown a drift in CT numbers between 50 and 65%. 25,26,28 There are also many studies that support the use of bismuth, suggesting no significant impact on image quality. 19,20,28 Where as these studies agree that streak artifacts disrupt image quality, the majority suggest that by creating a small gap between the eye lens and the shield, artifacts can be reduced to an acceptable diagnostic level.…”

“…19,20,28 Where as these studies agree that streak artifacts disrupt image quality, the majority suggest that by creating a small gap between the eye lens and the shield, artifacts can be reduced to an acceptable diagnostic level. 9,17,19,24,26 Our study by Raissaki et al 24 reduced artifacts to a negligible level by elevating the shield from the eyes using folded gauzes. Distances of 5, 10 and 20 millimeters were evaluated and found to reduce artifacts resulting in only slight decreases to dose reductions.…”