The lens dose for a single scan series is considerably less than the threshold dose for cataract formation. Consideration must be given, however, to the cumulative effect on patients who have multiple examinations, potentially at a younger age, and to growing life expectancy.⁵ There is no evidence that diagnostic imaging procedures lead to the formation of lens cataracts, but this could potentially become an issue if CT usage continues to increase and dose issues are not addressed.^10,11

Various authors have suggested ways to reduce the dose to the lens of the eye. Most proposals involve the adoption of different baselines/gantry angulations, and the use of in-plane shielding.

FIGURE 1. Infra orbital baseline.		FIGURE 2. Supra orbital baseline.

Either the infraorbital meatal baseline (Figure 1) or the supraorbital baseline (Figure 2) may be used when scanning a patient's head. The choice of baseline typically depends on hospital tradition, radiologists' preference, patient population, and the referring neurology center's protocol.

Changes in the scanning plane by using different gantry angulations can affect the radiation dose to the lens. Large dose reductions (maximum 88%) have been observed when the supraorbital baseline has been adopted.^11,12 The lens of the eye receives only scattered radiation with this technique.

An audit of local clinical practice suggests that even when the supraorbital baseline is chosen, two to seven slices will pass through the eye in sequential scanning. The eye can also receive up to 0.5 Gy, although it is not the area of interest. This is exacerbated by the overscan principle during spiral acquisition.

One argument against the use of the supraorbital baseline is that quality will be compromised. This is mainly due to beam hardening and artifacts caused by the petrous ridge in the posterior fossa. Evidence suggests that there is no significant disadvantage to the diagnostic quality of the images produced.¹¹

Between 29% and 32% of centers involved in two of the studies mentioned above^5,12 routinely used the supraorbital baseline to avoid the orbits. This practice was confirmed in our local survey of 50 CT units in the U.K. Some of these centers that routinely used the infraorbital meatal baseline changed their protocol for pediatric patients and used the supra-orbital baseline instead.