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

¹ Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hung Hom, HKSAR, P.R. China.
² Medical Physics & Research Department, Hong Kong Sanatorium & Hospital, Happy Valley, HKSAR, P.R. China.
³ Department of Diagnostic & Interventional Radiology, Hong Kong Sanatorium & Hospital, Happy Valley, HKSAR, P.R. China.

^* chris.lai@polyu.edu.hk

Received: 27 August 2014
Accepted: 16 January 2015

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 easy-to-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.