Head CT - Lens Exclusion


Exclusion of the lens of the eye in routine head CT examinations.


The exclusion of the lens from the standard brain CT examination will:

• Reduce the radiation dose to the eye

• Reduce the likelihood of lens damage and cataract formation

The Cycle

The standard: 

All brain scans should be performed with the base line set so as to exclude the lens of the eye.



Assess local practice


Percentage of routine head CT scans performed with the lens excluded from the scan field.

Data items to be collected: 

Record the number of brain scans in which:

• The lens of one or both eyes is/are included in the (field of) examination

• The lenses of both eyes are not included

Suggested number: 

50 consecutive routine brain scans (where the examination of the orbits was not a requirement).

Suggestions for change if target not met: 

• Develop a protocol for brain CT in which the base line is set so as to exclude the eye lens

• Identify exclusions to this protocol (e.g. examination for diplopia or exophthalmos)

• Emphasise to radiographers the importance of excluding the eye

• Persuade supervising radiologists to avoid making exclusions to the protocol


- Image file review

- Radiographer or radiologist: 2 hours to examine 50 scans

- 1 hour for report

  1. Spencer, D. CT Scanning Angles Selected for Routine CT Brain Scanning. How does the Selected CT Scanning Angle Affect the Eye-Lens Dose? MSc dissertation. London: City University, 1992.

  2. NRPB ASP6 (1984). Radiography 1984;595:25–6.

  3. National Radiological Protection Board. Protection of the Patient in X-ray Computed Tomography. Didcot: NRPB, 1992.

Published Date: 
Monday 7 January 2008

Last Reviewed: 
Wednesday 10 April 2019