Computed tomography in children
Authors: Paul D Griffiths, Giles D Morrison
Journal: BMJ (2004)

Apaper published recently in the BMJ reported results from a historical study of intellectual capacity in people receiving head and neck radiotherapy before the age of 18 months.1 Hall et al concluded that the severity of intellectual impairment was a dose dependent result of radiotherapy. They further commented that radiation doses associated with contemporary diagnostic computed tomography of the head were comparable with radiotherapy doses quoted from their study and concluded that this was a cause for concern. The “What this paper adds” box stated: “Diagnostic evaluation of children with minor head injuries by computed tomography needs to be re-evaluated.” We believe that the data presented in the paper do not support that conclusion. For “This week in the BMJ,” the chosen the headline was: “Computed tomography scanning in infancy may affect later learning.” This was unfortunate, as the paper did not address that topic. Equally unfortunately, the press releases to the media carried this message, which was subsequently published in many newspapers. Those of us working in paediatric imaging expected a deluge of inquiries from worried parents. We were not disappointed.

Radiology professionals are required to know the radiation burdens of x ray techniques used, which can be assessed by several different methods. Figures are usually quoted for effective whole body dose expressed in milliSievert (mSv), whereas in other situations individual organ doses are more appropriate, measured in milliGray (mGy). The important distinction between the two is that whole body dose measurements are required to estimate the level of risk of any future adverse complications, which include a range of malignancies. Hall et al were not explicit in defining the doses referred to but quote from a document from the Swedish Radiation Protection Society for doses associated with computed tomography of the head (68 mGy for adults and approximately 90 mGy for children).2 We have interpreted their figures as being for the organ (brain) dose. If so, their figures are considerably higher than estimated doses at Sheffield Children's Hospital. Our standard computed tomography protocol is slices of 3 mm thickness through the posterior fossa and of 5 mm thickness to the vertex on a GE Light speed plus (4 multislice) scanner using 200 mA and 120 kV. For a child of 2 years the effective whole body dose is 0.98 mSv, and the brain dose is 23 mGy. For a child of 12 years the figures are 1.95 mSv and 46 mGy (ImPACT computed tomography patient dosimetry calculator version 0.99n, as used in 2003). These doses are categorised as very low risk by the National Radiological Protection Board. This level of data is cumbersome and probably without meaning to many readers of the BMJ, but this depth of information is needed to open an informed debate about radiation doses and risks.

A further confounding factor, which may have led to bias in the data provided by Hall et al, was the caseload. Although many of the cutaneous lesions that were treated were likely to be isolated abnormalities, some patients with Sturge-Weber syndrome could have been included in the study. These children have severe brain abnormalities that lead to poor academic performance.3 A few cases of Sturge-Weber syndrome could seriously bias the results. Moreover, many other reasons exist why children with disfiguring head and neck haemangiomas may not perform well at school, and the lack appropriate controls in this study was a problem.

Our major criticism, however, is that the authors quote 68-90 mGy for the dose associated with computed tomography of the head, and yet they say that no consistent difference was seen between the two lowest dose categories (1-20 mGy and 20-100 mGy). The odds ratios quoted for those groups are not significantly different from doses of zero, therefore no measurable effect seems to occur below 100 mGy, which includes their estimated computed tomography doses—for example, the multivariate odds ratios for the frontal regions were 1.14 (95% confidence interval 0.86 to 1.51) mGy and 1.06 (0.88 to 1.36) mGy, respectively.

The radiologist acts as an adviser to clinical teams about the most appropriate way to investigate a particular problem by formulating an analysis of risk and benefits, considering such variables as the expected pathology, radiation dose, condition of the patient, and the effect of missing the diagnosis. In most centres, however, it falls to the paediatrician to give the information to patients and parents and to obtain informed consent. One of the most difficult areas is when ionising radiation is used for reassuring worried parents that, for example, their child with headaches does not have a brain tumour. Although this sounds like a misuse of radiation, 8-10% of children with brain tumours may have headache as their sole presenting complaint.4,5 We need to ensure that adequate facilities exist to perform this task, without recourse to radiation based techniques using magnetic resonance imaging or ultrasonography. As well as eliminating the risk associated with ionising radiation, magnetic resonance imaging has been shown to have superior diagnostic accuracy for many brain pathologies in children compared with computed tomography. This is not true for all clinical situations. Magnetic resonance imaging produces poor bone detail, and acute haemorrhage can be imaged only with difficulty.

Computed tomography remains the imaging method of choice for investigating trauma (including non-accidental injury) and when haemorrhage is suspected—these account for a considerable portion of neuroimaging in children and young adults. The speed of modern computed tomography scanners is such that sedation or general anaesthesia are rarely required, whereas magnetic resonance imaging in children may require anaesthesia because it takes much longer to carry out. The small risks associated with anaesthesia are comparable to, or sometimes greater than, the risks of exposure to radiation from computed tomography.

The more politically charged issues, such as availability of scanners, long waiting times, and local expertise to interpret the scans also need to be considered. A good example of magnetic resonance imaging replacing computed tomography is in the follow up of children with treated brain tumours, which is particularly relevant for children with tumours such as low grade astrocytomas, who have a good long term prognosis. Other areas present further challenges. An early version of a document to be released by the National Institute for Clinical Excellence recommends neuroimaging for most children with focal epilepsy.6 These children should have magnetic resonance imaging, but we do not have sufficient numbers of scanners and experts to do this. As a result many of these children will be scanned by computed tomography. This not only carries a radiation burden, but also has an exceptionally low chance of detecting the pathologies sought for in focal epilepsy.

Better access to magnetic resonance imaging would reduce the radiation dose in children and we must strive to obtain the required level of equipment and expertise. In some situations, however, computed tomography remains the imaging method of choice irrespective of availability of magnetic resonance imaging or ultrasound. We need to stress that no imaging method is completely safe and that computed tomography of the head in children should be used only after careful clinical consideration. We do not believe, however, that an examination of the head by computed tomography in a child presents a serious threat to intellectual development.