The contribution made by CT to the cumulative population radiation dose, as a proportion of all medical examinations, continues to rise. The number of CT scans performed in the U.S. increased from 2.8 million 1981 to 50 million in 2003.1 CT contributed 20% of the collective radiation dose in the U.K. in 1989.2 By 2003 this had risen to 47%.3
Today, CT accounts for 50% of the U.K. population's medical radiation burden, but just 6% of diagnostic examinations. This is most likely to be due to the wide availability of multislice CT systems and the increasing number of clinical applications where CT is called for.
All individuals involved with a patient's care have a responsibility to ensure the implementation of dose reduction strategies. Clinicians must ensure that the indications warrant CT, radiologists should be available to accept or reject requests, and radiographers should make certain that the optimum diagnostic imaging acquired at a dose which is as low as is reasonably practicable (ALARP).
A compromise must be met between diagnostic quality and radiation dose in most investigations that use ionizing radiation. In the modern protocol-driven healthcare setting, however, the issue of optimum quality has yet to be resolved.
Each individual center will use their own, preferred technique to produce the highest quality diagnostic images by balancing a variety of factors. This practice is often based on tradition and/or manufacturers' recommendations, and may lead to differing dose levels. Results from clinical surveys confirm that there is a wide variation in the practice of head CT and the dose levels involved in these scans. One survey of 945 head CT examinations, performed at many different centers, found that 94% of patients were scanned sequentially and 6% helically - but there were 118 different protocols.4
Substantial dose reduction can be achieved through the careful selection of exposure factors and appropriate positioning techniques. Such variables should ideally be standardized if variations in patient radiation dose are to be reduced.5
Specific patient dose limits are not enforced by legislation, though the ALARP principle dictates we should be able to justify our techniques, protocols, and procedures in terms of the radiation dose delivered. Dose reference levels (DRLs)6 have now become firmly embedded in CT culture, and are a first step towards the introduction of standardized doses for individual CT examinations.4
SENSITIVE SITES
The lens of the eye is the most radiosensitive organ within the head. A standard CT head scan delivers approximately 50 mGy of radiation to the eyes. A dose of 0.5 to 2 Gy can cause detectable opacities, whilst anything above 4 Gy can cause visual impairment.7,8 Children's eyes are even more radiosensitive. It has been suggested that a dose of 0.5 to 1 Gy is cataractogenic.9
