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Radiation dose in CT: are we meeting the challenge?

¹Department of Radiology, University of Oxford and ²National Radiological Protection Board, Chilton, Didcot, Oxon OX11 0RQ, UK

What challenge?

CT has developed dramatically: scanning is faster, images are better, applications have grown and, of course, radiation doses have come down, have they not? Well, no, actually; quite the contrary in fact.

When CT was new it was appreciated that it was a relatively high dose technique but there was overriding clinical justification for using it; in the brain no technique could approach it and when body CT began it concentrated on patients with malignant disease, where radiation dose was of less concern. Now circumstances are different. The technique is widely used, extensively in benign disease as well as in young patients for whom radiation protection considerations are paramount. Awareness dawned of the magnitude of the challenge following a national dose survey in the UK. In 1989 the National Radiological Protection Board showed that despite comprising only 2% of all examinations, CT contributed around 20% of the collective dose to the population from diagnostic imaging [1]. Subsequent analyses for the UK suggest that this latter figure may have risen to 40% [2]. One department in the USA has claimed that CT now represents 67% of the collective dose that it delivers [3]. Patient doses from CT are among the highest in diagnostic radiology; an abdominal examination in an adult with an effective dose of 10 mSv has been estimated to increase the lifetime risk of fatal cancer by 1 in 2000 [4]. Radiation exposure from CT is rising, not falling.

This increase is not simply owing to CT replacing other techniques. We know that large variations in CT practice exist; experience from review clinics suggests volumes of exposure, number of slices and repeat exposures can be widely different within the same clinical application, with little apparent clinical justification. Surveys of practice and dose suggest that (effective) dose for a given examination may vary by a factor of 40 between departments for a UK patient [1], or a factor of 20 in Norway [5]. Overall, the evidence indicates a strong trend of increasing population dose owing to rising use of CT and to increased dose per examination. It now seems clear that variations in practice have become more important than scanner technology in determining the dose to the patient [6].

Ease of use may be contributing to this syndrome. Early CT scanners were rigid tools and extending an examination implied a time penalty. The introduction of spiral CT reduced this dramatically and probably contributed significantly to new variations in practice, as few technique guidelines were available when the new technology was introduced. For example, contrast enhanced studies have become more widespread and multiphase enhancement has become common [7]. Although this latter technique has extended the application of CT, its use when a smaller number of phases would suffice cannot be justified. There is also anecdotal evidence that workload pressure may be adding to the problem. In the face of rising demand, radiologists may rely on CT examinations being performed with standard "catch all" protocols while they concentrate on other work; retrospective reporting of a comprehensive examination is efficient for the radiologist. Inexperienced radiologists in particular are likely to feel more confident the more sections they have available to read. However, this removes one of the key elements of radiation protection in CT, namely supervision by the radiologist, who should terminate the examination at the point at which it has delivered the information required for clinical management. The approach of "imaging overkill" may be tempting in current circumstances but cannot withstand serious enquiry.

A new urgency exists in the form of multislice CT [8]. This technique offers almost no resistance to extending the examination, introduces new applications and will be widely used despite the fact that, examination for examination, absorbed dose may be up to 40% higher [9, 10]. Interventional CT and CT fluoroscopy pose a particular problem. The latter may use an exposure rate 10 times that of conventional fluoroscopy and it is not difficult for the hands of the operator to reach the annual occupational dose limits [11]. If procedures under CT fluoroscopy becomeestablished, a particularly intense approach to protecting both patient and operator is indicated [12].

What should be done?

Our situation is given new impetus by new European and national regulations [13] that require departments to introduce robust procedures for the protection of the patient, including the twin elements of ensuring clinical justification of the examination and optimization of the technique. In CT this translates into strong consideration of other techniques, notably ultrasound and MRI. Where these are a practical alternative, rigorous adoption of clinical guidelines for vetting and accepting requests, as well as ensuring that examination parameters are appropriate to the indication is required. UK departments are currently adapting their practice to the new regulations, but a major challenge faces the task in hand. Common experience suggests that clinicians have come to rely progressively on imaging where hitherto clinical examination would have been regarded as sufficient. This is especially true for doctors in training, who find an authoritative result reassuring in the face of inexperience inclinical skills. Some requests are not easy to decline; for example it is not easy to exclude pulmonary embolism clinically in patients presenting with chest pain, and CT pulmonary arteriography threatens to become a ubiquitous screening tool unless precise clinical criteria for examination are defined [14, 15]. Similar constraints may need to be applied to "exciting" new applications such as CT colonography or CT bronchography [8]. In that sense, attempts to conform to the new regulations are swimming against a tide. The need for effective justification in children is even more important, as a higher radiosensitivity owing to the longer opportunity for delayed effects to appear is compounded by the fact that effective doses may also be higher [16].

It is a golden rule of investigative medicine that the benefit of the procedure should outweigh the risk. In CT, justification depends on the probability that clinical management will be positively influenced by the results. Practitioners are in fact quite fortunate in the amount of research that has been carried out into the clinical applications of CT, and the evidence base is now strong. Clinical guidelines such as those produced in the UK by the Royal College of Radiologists [17] provide strong guidance for radiologists' consideration of whether clinical requests are sufficiently justified to merit acceptance, although continued update is clearly required in view of the propensity of CT to develop new uses.

Optimization of CT is more problematic. In conventional radiography it is usually clear when over exposure has taken place. This is not true inCT, as the technology compensates for wide variations in exposure parameters and it is possible for the difference between an adequate image and a high quality one obtained at much higher exposure not to be immediately evident. The "as low as reasonably achievable" (ALARA)principle applies just as much to CT as it does to conventional radiography. We know that by manipulating exposure factors the radiologist can readily alter the dose to the patient by a significant amount [11]. However, these factors also determine image quality and therefore clinical efficacy. Contrast resolution, which may be critical in some common examinations such as detecting soft tissue lesions in the liver, is detracted by image noise and therefore improved by increasing exposure. Ideally we would know what represents acceptable image quality (and therefore exposure) for each application, but unfortunately we suffer from a serious lack of evidence in this area [6]. In areas of high natural contrast, such as the chest or skeleton, image noise is less critical and clinically acceptable images can be obtained with limited exposure factors, especially reduced tube current [18]. A dose reduction of 50% is possible in the chest without diagnostic loss [19], and in orbital trauma a low dose approach may produce a reduction by an order of magnitude [20]. In fact, where the clinical objective is very limited and the image quality not critical, a 25-fold reduction is possible [21]. In most other major areas of use we have a serious need for studies indicating the threshold exposure factors that will deliver clinically effective image quality, including pitch factors and section thickness with the new technology [6, 22, 23]. National regulations [13] to promote optimization of patient protection now require the formal use in CT departments of diagnostic reference levels as investigation levels to help identify unusually high doses.

Mounting concern about radiation dose in CT is reflected in international efforts to raise the profile of the issue and to advise on practice. In 1994 a European Commission (EC) Working Party began work on European guidelines for good practice in CT [24], including the development of reference dosimetry based on the practical dose quantities of weighted CT dose index (CTDI_w) and dose–length product (DLP) [25]. The European Co-ordination Committee of the Radiological and Electromedical Industries has recently published a handbook on radiation exposure in CT [18], and the International Commission for Radiological Protection (ICRP) set up a task group in 1999 to report on the management of patient dose in CT. This latter document has been through its consultation phase and publication is imminent [11]. The report concentrates on the magnitude of doses, the practical steps to reduce them and recommendations to manufacturers on technological developments.

Although containment of dose has not been as prominent in CT development as other features, manufacturers have an important role in ensuring that patient dose is kept to a minimum. One recommendation of the EC Working Group was that scanner consoles should give an indication ofpatient dose, and the International Electrotechnical Commission now recommends the display of CTDI_w [26], albeit modified by the pitch setting, although some manufacturers also helpfully include values of DLP. Another development that is strongly recommended by the ICRP is on-line tube current control; this adapts the exposure to the shape of the body and may achieve a dose saving of 15–50% [27, 28]. The use of solid-state detectors may also reduce dose by 30% [29].

The European guidelines on quality criteria for CT

The quality criteria concept has been developed by the EC as an effective method for optimization in medical imaging. The definition of quality criteria for CT was carried out by a multinational group composed of radiologists and physicists. Prescribing quality criteria is necessarily more complex in CT than in conventional radiography in view of the specific requirements of multiple applications. The Working Party therefore concentrated on common applications of CT and the resulting guidelines cover six major areas of use [30]. For each of these areas, recommendations are made on preparatory steps before investigation, on the criteria for acceptable images and for good imaging practice, and on clinical conditions that impact on good imaging performance. The proposed guidelines underwent extensive consultation, including an open workshop in Aarhus, and were published early in 2000. They are also available on a website (http://www.drs.dk/guidelines/ct/quality/). Integral to the Working Party's approach was the concept of reference doselevels.These are relatively easy to define in conventionalradiography but less readily provided in CT owing to the complexity of dosimetry and the variability ofexaminations [18]. Initial values of CTDI_w and DLP have been suggested for some common examinations on adult [24] and paediatric patients [31].

In view of the magnitude of its task, the EC Working Party has continued under Euratom Framework Programmes 4 and 5. One of the tasks completed by the group is a pilot study regarding application of the criteria, covering five types of examination over four countries [32]. This study was instrumental in providing reference doses to complement those available from previous surveys and also supported the contention that diagnostic criteria could be used to optimize CT to achieve dose reduction. The Working Party is currently planning a European field survey focusing on evaluating examination protocols and assessing patient dose, with a further survey intended to target image quality and dose in a selected patient group. Also planned are refinements to methods for calculating effective doses from CT, as well as an Internet database for dosimetric data for CT, together with revision and extension of the European guidelines. It is intended that this work, together with that of other centres, should provide a scientific foundation for effective examination guidance in the future, particularly in relation to reference dose levels and image quality.

For the present it behoves all involved in CT to observe the following:

1. There must be clear justification for CT. This means active consideration of whether the examination is required, whether it could be replaced by ultrasound or MRI and, if accepted, whether it conforms to current clinical guidelines.
   
2. The examination technique must be targeted to the clinical application and the exposure parameters must be adjusted to those settings delivering the minimum dose necessary, where these are known.
   
3. A single spiral exposure or sequence of serial scans should be used where this alone will satisfy clinical need.
   
4. Additional scans with contrast enhancement should only be used when there is clear clinical evidence to support their application.
   
5. Tube current should be reduced to a minimum where possible, especially in high resolution studies.
   
6. The literature should be observed constantly and practice should be adapted as more evidence becomes available.
   
7. Centres should participate in further surveys of dose and national initiatives to refine diagnostic reference levels for CT.
   

Radiation protection in CT must not become subject to paranoia or a witch-hunt, but equally there is no place for complacency. Both patients and purchasers have the right to expect that staff conform to best practice principles. As far as staff and departments are concerned, this means an awareness of emerging evidence and the implied changes in practice, with revision of protocols to take account of advances. It also means establishing local dose audit to ensure that examinations conform to available reference dose levels except where there is clinical justification for exceeding these. This continuing process must be embraced by all who practice CT.

Received for publication March 27, 2001. Revision received May 31, 2001. Accepted for publication June 18, 2001.

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