G.I. Ogbole
Abstract
Computed tomography (CT) is a powerful tool for the accurate and effective diagnosis and treatment of a variety of conditions because it allows high resolution three-dimensional images to be acquired very quickly. However as the number of CT procedures performed globally have continued to increase; with growing concerns about patient protection. Currently, no system is in place to track patient doses and the lifetime cumulative dose from medical sources. The widespread use of CT even in developing countries has raised questions regarding the possible threat to public health especially in children. The best available risk estimates suggest that paediatric CT will result in significantly increased lifetime radiation risk over adult CT. Studies have shown that lower milliampere-second (mAs) settings can be used for children without significant loss of information. Although the risk–benefit balance is still strongly tilted toward benefit, there is still need for caution. Furthermore since the frequency of paediatric CT examinations is rapidly increasing, and estimates suggest that quantitative lifetime radiation risks for children are not negligible, efforts should be made toward more active reduction of CT exposure settings in paediatric patients. This article hopes to address this concerns and draw attention to the fact that children are not ‘small adults ’ and should therefore be treated differently.
Keywords: Computed Tomography, Radiation Risk, Radiation Dose, Patient Dose Reduction, Children.
Correspondence:
Dr. G.I. Ogbole
Department of Radiology
University College Hospital,
Ibadan.
Introduction
‘One of the first duties of the physician is to educate the masses not to take medicine’. William Osler (1849 – 1919)
In 1972, Computed tomography (CT), a technique that produces non-superimposed, cross-sectional images of the body, was introduced into clinical practice. Its introduction revolutionized diagnostic radiology, as it experienced rapid technological developments (fast acquisition and reconstruction times, spiral acquisition mode, multislice capability). Its use has also in the last decade grown considerably. As a result, the numbers of examinations have increased to the extent that CT has made a substantial impact on not only patient care, but also patient and population exposure from medical x-rays. This relatively high dose modality, represents about 5–10% of all x-ray examinations, but contributes between 41% and 75% of the collective dose from diagnostic radiology in some countries.1-3
It is estimated that more than 62 million CT scans per year are currently obtained in the United States,including at least 4 million for children.2 By its nature, CT involves larger radiation doses than the more common conventional X-ray imaging procedures.
This review will examine the main clinical applications of CT scanning particularly in children. It will focus on the associated radiation doses they receive and the consequent cancer risks. It will also describe the various dose descriptors of CT and measures that can be employed to reduce radiation doses in children and the general patient population.
The Use of CT
The use of CT has increased rapidly, both globally as well as in Nigeria with more centers acquiring CT scanners in the last 3-5 years. In the United States CT scans have risen from an estimated 3 million per year in 1980 to more than 62 million currently.2 This sharp increase has been driven largely by advances in CT technology that make it extremely user-friendly,for both the patient and the physician. Most importantly the largest increases in CT use have been in the category of pediatric diagnosis4, 5 and this trend can only be expected to continue as investigative medicine remains standard practice for the next few years.
The growth of CT use in children has been driven primarily bythe decrease in the time needed to perform a scan – now less than 1 second – largely eliminating the need for anesthesia or sedation to prevent the child from moving during image acquisition4. In developed countries like the United States of America, the major growth area in CT use for children has been presurgical diagnosis of appendicitis, for which CT appears to be both accurate and cost-effective; though arguably no more so than ultrasonography in most cases.6
Other more common areas of CT use in children include the diagnosis, monitoring, treatment for infectious or inflammatory disorders, abdominal masses, seizures and injury-related conditions. It is also performed to evaluate blood vessels serving the brain, face or neck, the spinal cord and the spinal column. It is especially useful in cases of head injury, where the examination can display or rule out serious complications such as intracranial haemorrhage and other forms of brain injury. Except for the chest xray, CT is the most commonly used imaging procedure for evaluating the chest. Using multidetector CT, it is possible to obtain very detailed images of the heart and mediastinum in children, even newborn infants. CT is well-suited for visualizing diseases or injury of important organs in the abdomen including the liver, kidney and spleen and to detect abdominal tumors, birth defects or stones in the urinary tract. In the pelvis, CT scans can help demonstrate cysts or tumors of the ovary or abnormalities of the bladder and pelvic bones. An international IAEA study has shown that some countries are over-exposing children to radiation when performing computed tomography (CT) scans. These children are receiving adult-sized radiation doses. In addition, the study showed that pediatric CT scans occur more frequently in Africa than in Asia and Eastern Europe. The frequency has been attributed to the limited availability of alternative medical imaging techniques, such as MRI and ultrasound, which do not involve ionizing radiation, or because some CT scans are performed unnecessarily7.4
The records at the reputedly largest teaching hospital in Nigeria show that the proportion of CT studies that are currently performed in children range between 14% and 18%. This CT facility at the time of writing also did not use a documented guideline and protocol for imaging children.
RADIATION DOSE
The amount of radiation energy deposited in a medium is called the radiation dose. Different x-ray modalities address radiation dose in different ways. For example, in chest radiography it is the entrance exposure (not the dose) that is the commonly quoted comparison entity. In mammography, the average glandular dose is the standard measure of dose. The distribution of radiation dose in CT is markedly different than in radiography, because of the unique way in which radiation dose is deposited. There are three aspects of radiation dose in CT that are unique in comparison to x-ray projection imaging8.
First, because a single CT image is acquired in a highly collimated manner, the volume of tissue that is irradiated by the primary x-ray beam is substantially small compared with, for example the average chest radiograph.
Second, the volume of tissue irradiated, is exposed to the x-ray beam from almost all angles during the rotational acquisition, and this more evenly distributes the radiation dose to the tissues in the beam. In radiography, the tissue irradiated by the entrance beam experiences exponentially more dose than the tissue near the exit surface of the patient.
Finally, CT acquisition requires a high Signal to Noise Ratio (SNR) to achieve high contrast resolution, and therefore the radiation dose to the slice volume is much higher because the techniques used (kV and mAs) are higher. As a rough comparison, a typical PA (posterioranterior) chest radiograph may be acquired with the use of 120 kV and 5 mAs whereas a thoracic CT image is typically acquired at 120 kV and 200 mAs. 8