Radiation protection of patients with cataract

Frequently asked questions by the health professionals

» Which X-ray procedures and clinical conditions are associated with elevated eye lens doses to the patient? 
» What are typical eye lens doses to patients associated with diagnostic and interventional procedures? 
» How can I manage eye lens dose and prevent injuries in patients? 

» Which X-ray procedures and clinical conditions are associated with elevated eye lens doses to the patient?

In procedures such as head CT, paranasal sinus CT, temporal bone and orbital CT and neuro-interventional procedures, the eye is most likely in the field of the primary X-ray. It will have the potential to receive a higher dose if appropriate techniques to optimize protection of the eye are not used. Table below lists radiation dose to the eye in different examinations and procedures. When the eye is not in the primary beam (for examinations of body areas excluding the head), the scattered radiation dose is very small. Patients with recurrent and chronic conditions are among those who require frequent examination. As an example, 26% of patients with hydrocephalus receive radiation dose to lenses higher than 150 mSv within 3 years.  In paediatric patients, repeated head CT examinations result in average cumulative dose to lens of the eye of 26 mGy over a few years, but it can be as high as 1.3 Gy BE.

» What are typical eye lens doses to patients associated with diagnostic and interventional procedures?

Typical values in terms of absorbed dose to the patient’s lens of the eye per procedure are presented in Table below:

Table 1: Typical eye lens doses to patients’ eyes for various X-ray procedures in children and adults*

* These are typical doses that may be increased or (using new dose-sparing additions and iterative reconstruction) lowered by a factor of at least 2.

» How can I manage eye lens dose and prevent injuries in patients?

Whenever feasible, critical organs (eye lenses) should be excluded from the scan range. Depending on the type of procedure, there are many actions that may result in eye lens dose reduction without compromising diagnostic outcome:

• In head CT, the scan length should be selected individually and should be kept as short as necessary. The use of a different scan plane (different beam angulation by gantry angulation) to avoid the orbits or use helical instead of sequential acquisition mode has been shown to reduce eye lens dose. In helical scanning, the scanning range is usually longer than in sequential to facilitate multiplanar reformation. In sequential scanning, gantry angulation results in a decrease in CTDI to some degree, which is unavailable in helical scanning. The effective dose (0.80 mSv) of temporal bone CT in the helical scanning mode is higher than the doses from direct axial (0.64 mSv) and coronal scanning (0.57 mSv) alone, but lower than their combined effective doses. The two direct scanning modes are usually performed consecutively in a single examination leading to a gross effective dose of 1.2 mSv. The lens dose of 53 mGy in sequential scanning mode is higher than in helical scanning (40 mGy). In helical temporal bone CT, changing the scanning baseline to avoid the lens provides further eye lens dose reduction.  In this way, the lens dose could be reduced to 10 mGy . In scanners without gantry tilt possibility, head CT should be performed with careful head positioning. As in most scanning procedures, the exposure of patients can be varied over a wide range by changing the mAs;

• In interventional procedures, all actions that lead to dose reduction in fluoroscopy apply for eye lens dose reduction, as avoidance of unnecessary fluoroscopy and image acquisition, tight collimation, limitation of fluoroscopy time and number of frames, low-rate pulsed fluoroscopy, and use of means to monitor radiation exposure. The use of certain projections, like an oblique-lateral projection, allows simultaneous improvement of procedure efficiency and eye lens dose reduction. There are also procedure specific dose management strategies. For example, in balloon dacryocystoplasty, radiation dose to the lens of the treated eye is significantly lower than dose to the untreated eye (4.6 mGy compared to 38 mGy). In this procedure, minimal use of lateral projection and tight collimation for treated eye reduces dose to a healthy untreated eye; 

• In some cases, shielding against external scatter radiation, when the eye is outside the scanned area of the body, can be used. Use of shielding for organs located superficially within the area under examination, although providing significant organ dose reduction, must be considered carefully as it might cause artifacts and reduce the image quality. As an example, in CT, organ-based tube current modulation provides dose reduction of 30.4%, while with bismuth shielding, dose reduction is 26.4%. Comparable dose reduction is also achievable with simple mAs reduction by 30%. However, organ-based tube current modulation provides better image quality than the use of bismuth shielding, while similarly reducing dose to the eye.

Read more:

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