Tradeoffs between Radiation Exposure to the Lens of the Eyes and Diagnostic Image Quality in Pediatric Brain Computed Tomography

Abstract

Background: Computed tomography (CT) of the brain is associated with radiation exposure to the lens of the eyes. Therefore, it is necessary to optimize scan settings to keep radiation exposure as low as reasonably achievable without compromising diagnostic image information. The aim of this study was to compare the effectiveness of the five practical techniques for lowering eye radiation exposure and their effects on diagnostic image quality in pediatric brain CT.

Methods: The following scan protocols were performed: reference scan, 0.06-mm Pbeq bismuth shield, 30% globally lowering tube current (GLTC), reducing tube voltage (RTV) from 120 to 90 kVp, gantry tilting, and combination of gantry tilting with bismuth shielding. Radiation measurements were performed using thermoluminescence dosimeters. Objective and subjective image quality was evaluated.

Results: All strategies significantly reduced eye dose, and increased the posterior fossa artifact index and the temporal lobe artifact index, relative to the reference scan. GLTC and RTV increased image noise, leading to a decrease signal-to-noise ratio and contrast-to-noise ratio. Except for bismuth shielding, subjective image quality was relatively the same as the reference scan.

Conclusions: Gantry tilting may be the most effective method for reducing eye radiation exposure in pediatric brain CT. When the scanner does not support gantry tilting, GLTC might be an alternative.

Keywords: Brain computed tomography; eye lens; image quality; radiation exposure.

Figure 1

The geometric profiles used in different scan protocols: The reference scan, bismuth shielding, GLTC and RTV (a), gantry tilting and gantry tilting with bismuth shielding (b). GLTC – Globally lowering the tube current, RTV – Reducing the tube voltage

Figure 2

Axial, unenhanced brain scans of an 8-year-old pediatric patient with reference scan settings at the level of basal ganglia (a) and cerebellum (b) with circular ROI used for objective analysis of image quality. The ROIs in the gray matter (black circles) and subcortical white matter (red circles) of the frontal and occipital lobes, in gray matter of the thalamus and white matter of the posterior limb of the internal capsule (a) and the temporal lobes and interpetrous region of the posterior fossa (white circles, b). ROI – Regions-of-interest

Figure 3

The mean and SD of the subjective image quality scores in different scan protocols for pediatric patients under 16 years old (SDs are shown as error bars). SD – Standard deviation; GLTC – Globally lowering tube current; GM-WM – Gray matter-white matter

Figure 4

The mean and SD of the subjective image quality scores in the reference scan and the reduced tube potential for pediatric patients under 5 years old (SDs are shown as error bars). SD – Standard deviation; GM-WM – Gray matter-white matter; RTV – reducing tube voltage

Figure 5

An example of axial image in each scan setting: (a) an axial scan of a 15-year-old pediatric patient with an orbital bismuth shield and a 1-cm shield-to-eyelid spacer. There are severe artifacts around the orbit (arrows), (b) an axial scan of a 8-year-old pediatric patient with 30% GLTC, (c) an axial scan of a 10-year-old pediatric patient with tube voltage of 90-kVp, (d) an axial scan of a 15-year-old pediatric patient with gantry tilted along the SOML at the levels of temporal lobes. GLTC – Globally lowering the tube current, SOML – Supraorbital meatal line