Benefits and costs, an eternal balance
Article Outline
This ICRP Publication on managing the dose the patient receives in the era of multi-detector computed tomography (MDCT) is very timely. Not all workers involved in CT fully appreciate the specific parameters in MDCT which increase or decrease the dose imparted to patients compared with the original single slice CT machines. The various technological options, including automatic exposure control, are well covered in this report. Each institution must agree on various protocols to allow the production of diagnostically adequate images at reasonable radiation cost. Thus the report emphasises that image policy should be specified by the users and appropriately suited to the clinical task. For certain clinical conditions (e.g., the detection of lung nodules) there can be considerable savings in radiation with virtually no detriment to patient care. Similarly requests for imaging in paediatric practice and in pregnancy can be handled with marked dose reduction. Recently it has been shown that in CT colonography, there may be good grounds for not worrying too much about tiny lesions (Pickhardt et al., 2007); this may allow lower-dose protocols to be adopted with more confidence.
CT has become the pre-eminent method of examining patients with ionising radiation. Because of the versatility provided by newer multi-detector CT systems, it is fast replacing the clinical examination of the patient. For example, in the acute abdomen there is increasing evidence that the plain abdominal radiograph can be safely discarded and the whole triage of the patients now often depend on the CT findings (discharge, admit, observe, surgery – see Ng et al., 2002). So fast is the growth of CT that around 75 million CTs are performed annually in the USA for a total population of around 300 million. CT has virtually replaced the IVU, the barium enema, and much diagnostic angiography. In years to come it is possible that even the chest x-ray for a patient with dyspnoea may be replaced by low-dose CT.
Radiologists are de-skilling in the interpretation of CXRs; the lateral view is now performed much less commonly with nearly all patients with equivocal findings on a PA CXR being referred on for CT. CT is even needed when the CXR is definitely abnormal in order to clarify the nature of those abnormalities. Furthermore the image quality of the scout view/scannogram/digitally acquired radiograph is challenging the image quality of certain aspects of the CXR (lung bases, bones, etc.). Thus, for all these reasons, a review reminding everyone in the health care industry of the radiation issues is pertinent.
Many workers have shown the disproportionate radiation burden to the community from CT compared with the relatively low number of procedures performed. And this is particularly true in paediatric practice. Thus every time a child or young or pregnant patient is referred, one should think about whether the clinical question could be satisfactorily answered by a technique which avoids ionising radiation. We should be using US and MR whenever possible in children. In adult practice we should avoid lumbar spine CT; one exception is bone mineral quantification although such measurements can also be obtained at a much lower dose by techniques such as DEXA.
Even when CT is the optimal investigation for the clinical problem and recommended by guidelines (RCR, 2007), there are many tricks of the trade which can reduce dose. It has become widely appreciated that an un-enhanced series is not indicate for the majority of body CT examinations; perfusion studies and CT urography are obvious exceptions to this rule. Any technique that requires un-enhanced, early arterial, late arterial, early portal, late portal, and delayed imaging can lead to an enormous radiation dose (Dixon and Goldstone, 2002). Thus such repeat sequences should only be used in exceptional circumstances. Indeed nearly all clinical questions can usually be answered by two sequences at most.
Whilst it is possible to reduce the radiation dose to certain specific questions (such as the lung nodule identification alluded to above), it is controversial whether initial diagnostic CT examinations, where the diagnosis may be unknown, should be performed with reduced dose parameters. One of the inherent advantages of CT is that a full data set is obtained, providing full information about a range of tissues (bones, muscles, internal organs etc). This data set can be re-interrogated at a future date to answer specific questions which may emerge during a patient’s hospital admission.
For example, even if the initial clinical question was for ‘lung parenchyma for possible sarcoidosis’, it may turn out that the patient has a possible underlying malignancy, in which case close interrogation of the bones would also be needed. Bone texture would not be all that well demonstrated on a low-dose chest examination. This concept of obtaining the maximum possible information during any initial CT examination has been adopted by many departments. The CT data set is an information bank which can be interrogated at many different settings at a later date.
Indeed, if the all the raw data are saved, one can re-analyse at high special algorithm for high resolution images in order to demonstrate fine detail of the lung or sagittal images of the thoracic spine and vertebral canal. If the initial investigation is comprehensive, it may very well save a repeat CT and other investigation, some of which might involve ionising radiation. In the example of the patient with ‘query sarcoidosis’, who turns out to have a malignancy, an NM skeletal scintigram may be avoided if the bones have been comprehensively examined by CT. Such arguments are further extended by radiology centres that promote ‘whole body’ CT screening. Despite the high radiation dose such examinations appear to be on the increase, even though many counsel against widespread screening for asymptomatic patients (Dixon, 2004). It will be interesting to see how whole body MR screening competes with CT screening in the future.
This paper also highlights the need for additional training for all workers involved in CT. This should include the correct education of medical students about the dose implications of CT. It necessitates the continual education of clinicians requesting CT examinations. But, above all, there should be extensive and ongoing training for the CT technicians/radiographers who control the CT examination parameters. While those responsible for an institution will happily pay several hundreds of thousands of dollars/euros for a new CT machine, those same individuals are often very reluctant to invest much in radiographic training and continuing medical education, or in medical physics support to monitor the radiation aspects of CT.
In short it is an honour to write an accompanying editorial to this review. I very much hope that at least the summary points are read by the appropriate administrators.
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PII: S0146-6453(07)00022-X
doi:10.1016/j.icrp.2007.09.001
© 2007 Published by Elsevier Inc.