Impact of head CT organ dose modulation on lens dose and image quality

Computed tomography (CT) of the head is one of the most common CT examinations, particularly in the case of hospitals with dedicated emergency departments. Given the potential necessity for repeat exposures an associated concern is increased radiation dose to the lens of the eye. Organ dose modulation (ODM) is a scan technique that permits exposure reduction at the anterior-view. Herein, we evaluate the impact of ODM on the lens dose and the resultant image quality. For a cohort of 60 adult patients (30 from use of a standard protocol and 30 from use of an ODM protocol) head CT data have been retrieved and analyzed retrospectively. With the aid of CTEXPO v2.5 and VirtualDoseTM CT software the lens equivalent dose was estimated for each member of the cohort. Image quality was evaluated within a clinical regime, qualitatively and quantitatively. The clinical qualitative merit was based on four categories: grey-white matter differentiation, posterior fossa quality, diagnostic acceptability, and image noise. Quantitative assessment was based on measuring the signal-to-noise ratio (SNR) in white and grey matter as well as the contrast-to-noise ratio (CNR). Using an unpaired parametric T-Test a significantly lower lens dose was observed for the ODM protocol versus the standard protocol (8.6 vs 9.3, p = 0.0003). Increase in the noise level was found in head CT images acquired with ODM, by up to 28% and 38% in grey and white matter respectively. A significant difference in lens dose was found using CT-EXPO v2.5 and VirtualDoseTM CT software. With no trade-off in acceptability of images, using ODM significant lens dose reduction of 14 % has been found achievable for head CT exams.