June 26, 2009
Diagnostic Imaging Europe.
In-plane shielding cuts
head CT dose overspill
A standard CT head scan delivers approximately 50 mGy of radiation to the eyes
BY LINDA M. CLARKSON, M.PHIL., AND PAUL M. ARNOLD, M.SC.
Pathology may also prompt a change in baseline. Good practice dictates that orbits should not be in the primary beam, though this is not always possible with MSCT. PREVENTIVE MEASURES 
Our research with single- and multislice CT (Table 1) supports earlier studies demonstrating an 87% reduction in dose to the eye when the supraorbital baseline is used, and that sequential scanning greatly reduces the dose to the lens.11,12 This previous work was based on single-slice CT examinations, though. A secondary aim of our research investigated the impact of MSCT on the dose delivered to the lens during routine head imaging. The routine protocol has traditionally been sequential scanning, which avoids the need to scan outside the volume of interest. This choice was largely due to equipment limitations: spiral scanning was not possible or limited on many single-slice systems. 
The wider availability of MSCT has now resulted in more centers switching to a helical protocol for routine head scans. An increase in dose is observed with spiral acquisition, using the supra-orbital baseline. Our data (Table 2) shows that the dose to lens of the eye increases by a mean of 94.5% (range -1% to 304%) when this technique is used, mainly due to the overscan principle. This suggests that MSCT sequential scanning should be used to reduce dose to the eye until the issue is addressed by manufacturers. Many centers have not adopted the supraorbital baseline, despite its proven success in lowering dose to the eye. Those centers that have may still need to adopt additional dose-reduction strategies, given the increasing use of spiral scanning in head CT, and the inclusion of part of the lens on initial scan slices with single-slice CT. Research has shown bismuth to be effective at reducing the primary radiation dose to superficial organs,13 for instance, the lens of the eye, the thyroid, and breast, without reducing the diagnostic quality of the examination.14 Evidence is mounting that in-plane shielding may protect the sensitive lens without leading to a significant increase in noise or causing artifacts on the brain images.7 Examination times do not need to be extended, and the shields are easy to use. 
Significant reductions in dose to the eye have been observed when bismuth shielding is applied during imaging with the infra-orbital meatal baseline. Manufacturers claim that the drop in dose is 40% to 50%.13 The authors of studies reporting an 87% dose reduction through the use of the supraorbital baseline argue that shielding is not required.11,12 Our data demonstrate that whilst this may be true for sequential scanning, it is not necessarily the case with spiral CT. We observed a mean dose decrease of 37% when bismuth shielding was used (Table 3). In-plane shielding can reduce dose to the lens of the eye when using a helical scanning protocol and the supraorbital baseline. In conclusion, using the supraorbital baseline in head CT, with sequential scanning, can reduce dose to the lens of the eye. Spiral MSCT protocols increase dose to the lens, though this can be reduced by using in-plane shielding. Shielding should also be applied when performing head CT with the infraorbital meatal baseline. Given the growing evidence that in-plane shielding can protect radiosensitive organs during CT examinations, practitioners should surely be encouraging and developing this practice in daily routine.
References
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12. Heaney DE, Norvill CA. A comparison of reduction in CT dose through the use of gantry angulations or bismuth shields. Australas Phys Eng Sci Med 2006;29(2):172-178.
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