Computed tomography (CT) was developed in the late 1960s by the British engineer Geoffry Hounsfield.2 Hounsfield recognized that by compiling image data from numerous different angles, the attenuation properties of each object could be determined. A computer was used to accumulate the data and to compose a cross-sectional image.
There are approximately 62 million CT scans performed in the United States annually, with 2 to 4 million CT scans performed on children.3,4 This reflects an increase in the utilization of CT scans over the last 10 years, where the use of CT in adults and children has risen 7- to 10-fold.3 It is hypothesized that the increased usage is attributed to increased availability and improved technology.
Physics and Pathophysiology
The basic principle of CT scans is similar to plain radiography as x-rays are used to visualize different densities within the body of interest. The patient lies on a table that slides through the CT machine (Fig. 14-4). Within the CT machine, there is a narrow opening where the x-ray source emits a thin, fan-shaped beam. Instead of film, the x-rays are received by x-ray detectors, which are located on the opposite side of the patient. The x-ray source and detectors rotate simultaneously around the patient during the examination. The information from the detectors is processed by a computer to create a three-dimensional image.
Picture of a typical CT scanner.
Recent advances in CT scanners have allowed CT to be done more rapidly. Axial CT scans are done where the table moves intermittently between rotations of the x-ray source and detectors. Helical (also called spiral) CT scanners, which were developed in the 1990s, allow the table to move the patient continually through the examination, and not have to wait for each rotation.5 Multidetectors, also known as multislice CT scanners, now have multiple rows of x-ray detectors that rotate next to each other allowing multiple slices to be processed concurrently. This further decreases the time needed to perform a CT scan.
X-rays, as noted above, are a form of ionizing radiation that can release electrons from atoms and molecules. In the case of water molecules, hydroxy radicals are released that can damage deoxyribonucleic acid (DNA) causing strands to break and bases to be harmed. DNA can also be directly ionized by x-rays. The damage to DNA can be repaired by the cell in most cases, but the double-strand breaks are less easily mended. During the repair of double-strand breaks, there can be incorrect repairs that can lead to point mutations, translocations, gene fusions, and ultimately cancers. Because of the serious side effects brought about by ionizing radiation, the safety of radiologic studies has been brought into question.
Ionizing radiation is present in the environment from natural sources in addition to man-made sources. Natural sources of radiation include cosmic rays, terrestrial rocks, and mountains. For people living in the United States, the average background radiation is estimated to be 3 mSv per year.5,6 Man-made sources of radiation include those from diagnostic x-rays and CT scans. Ionizing radiation can alter biologic tissues and thus cause a safety hazard.
In considering the safety of CT scans, there are some terms that should be defined. The absorbed dose is the radiation dose delivered to an organ. It is measured in grays (Gy), where 1 Gy is equal to 1 J of absorbed radiation energy per kilogram.4 The organ dose represents the deposition of the radiation within the organ. The effective dose is used when calculating the risk to nonhomogeneous tissues (e.g., different organs in an abdominal CT). The effective dose takes into account the amount of radiation each organ receives as well as the radiosensitivity of the specific organs. The effective dose is measured in mSv. For x-ray radiation, a whole-body radiation dose of 1 mGy is the same as an effective dose of 1 mSv.
CT scans provide a significantly higher organ dose than a plain radiograph. For example, with anterior–posterior abdominal x-ray, the stomach receives an organ dose of approximately 0.25 mGy, which is 50 times less than what the stomach would get from an abdominal CT.4 The Food and Drug Administration (FDA) estimates that a CT scan with an effective dose of 10 mSv (e.g., abdominal CT) increases the chance of a person dying from a fatal cancer to 1 in 2000.7 The lifetime cancer mortality risk to a child from an abdominal CT scan is estimated to be 1 in 550.7 The increase in radiation-induced cancer mortality risk is alarming, but it is also important to keep this in perspective with the overall cancer mortality risk from all causes, which is approximately one in five individuals.6
The increase in cancer mortality risk is relatively small compared with the overall cancer risk, but with the increased utilization of CT scans, especially in children, there is greater concern for the consequences in the future. Children are more vulnerable to ionizing radiation from CT scans than adults for three reasons. The first reason is that the tissues and organs of children are more radiosensitive, because they are still developing. Thus, given the same radiation dose, the child's organs would be more susceptible to the ionizing effects. The second reason is that since there is a latent period between the time of exposure and the development of cancer, a child who presumably has a longer life expectancy would have more time for the cancer to become evident. The latent period for leukemia is the shortest at 2 to 5 years postradiation exposure, and solid malignancies have the longest latent period of 10 to 20 years.7 The third reason children are more susceptible is that when children undergo a CT scan, they are often exposed to adult parameters. This is commonly seen in adult institutions where parameters may not be adjusted to children.
In addition to cancer risk, CT scans of the brain in children have some risk of cognitive harm. Although controversial, a Swedish study demonstrated progressive lowering of cognitive performance as the dose of infant cranial radiation increased.8 It makes sense that developing brain tissue in infants is more susceptible to radiation-induced harm compared with adult brain tissue.
Radiologists and CT technicians should adjust parameters to use the radiation dose that is as low as reasonably achievable (ALARA) to get an adequate scan.6 Decreasing ionizing radiation to children should follow the ALARA principle. The CT parameters should be adjusted to the individual patient. By decreasing the CT radiation dose, there may be a more speckled appearance to the scan, but this usually does not sacrifice the diagnostic accuracy. A 50% to 90% decrease in the radiation dose from adult to child parameters has been used without any compromise to the interpretation of the scan.6 For the practitioner, it is essential that the reason for the scan be indicated so that the CT parameters can be tailored to the indication. Second, CT as well as other diagnostic x-ray studies should only be done when necessary. In some cases, alternative studies such as ultrasound and MRI can be used as an alternative means of imaging to avoid unnecessary ionizing radiation. Informed consent principles require that physicians advise patients and parents of the radiation risk of x-ray/CT.
CT scans are readily available to most emergency medicine practitioners. The main benefit of CT is its ability to provide useful information quickly. Since most CT scans are performed rapidly, many can be done without the need for sedation.
The main limitation of CT is that it provides a significant radiation exposure, especially in the pediatric age group. Second, the interpretation of CT in many cases must be by a trained radiologist. Third, to obtain a CT scan the patient needs to be transported to the radiology department, which may limit its use in unstable patients. Finally, contrast is sometimes needed to better visualize structures, but has some notable side effects. Approximately 5% of patients will have mild reaction such as nausea, vomiting, rash, or a metallic taste.1 Approximately 1 in 1000 patients will have a severe reaction including hypotension, laryngeal edema, and possibly cardiac arrest.1 Contrast can cause impairment of renal function, thus cannot be used in patients with renal dysfunction or multiple myeloma.
Diagnostic and Procedural Considerations
Overall, the benefits, limitations, and safety considerations must be weighed carefully with the clinical picture to determine the best diagnostic option. Common conditions that utilize CT scans are discussed below.
The diagnosis of appendicitis can be made based on clinical examination. As discussed previously, ultrasound can be used to diagnose appendicitis and should be used as the first diagnostic option when available. When ultrasound cannot determine the diagnosis, abdominal CT scan can be obtained with a sensitivity and specificity of 87% to 100% and 83% to 97%, respectively.9,10
In pediatric trauma patients, CT scans can provide a great deal of information quickly. The information can be used to prompt or obviate the need for surgical intervention. Also, the need for airway intervention may remove the ability of the practitioner to clinically assess and monitor the patient, thus CT scans are the practical approach to diagnose suspected intracranial and intra-abdominal injury.
In the unstable trauma patient, CT often has the disadvantage of requiring transport to the radiology department. It also may require moving the patient from the gurney onto the scanner bed, which can be labor-intensive. Furthermore, in cases where a patient is unstable and needs operative intervention, an abdominal CT scan has little to add to the management.