Strange and Schafermeyer's Pediatric Emergency Medicine, Fourth Edition (Strange, Pediatric Emergency Medicine), 4th Ed.

CHAPTER 14. Imaging

Wendy C. Matsuno

HIGH-YIELD FACTS

• Ultrasonography is the imaging technique of choice for confirmation of pyloric stenosis, testicular torsion, ectopic pregnancy, ovarian torsion, and appendicitis.

• High clinical suspicion for testicular torsion or ovarian torsion should not be ignored when not confirmed by ultrasonography. Sensitivity and specificity are limited.

• Successful diagnosis with ultrasonography may be limited in obese children.

• Computed tomography (CT) is an extremely valuable imaging tool, but the risk of ionizing radiation exposure should be considered when ordering this test in young children.

• Magnetic resonance imaging (MRI) in the emergency department (ED) is usually reserved for emergent conditions such as cord compression and stroke.

INTRODUCTION

The practice of emergency medicine brings patients with a variety of complaints to our doorstep. In deciphering the many signs and symptoms, imaging is an important tool to reach a diagnosis. This chapter will discuss the various considerations for optimal visualization and patient safety when using diagnostic radiographs and computed tomography (CT). Ultrasound and magnetic resonance imaging (MRI) are discussed in separate chapters.

DIAGNOSTIC RADIOGRAPHS (X-RAYS)

Plain radiography accounts for approximately 80% of all imaging studies.1 The image obtained from plain radiography is acquired with the aid of an x-ray, which is a collection of electromagnetic energy called a photon. Electromagnetic energy travels at the speed of light at different frequencies, where the higher the frequency, the more energy it possesses. For example, the low frequency of a light photon has 1 eV of energy, compared with the high frequency of an x-ray photon that has 30 keV of energy.2 The large amount of energy that x-rays contain allows them to ionize atoms that they encounter, hence labeling x-rays as a form of ionizing radiation.

Approximately 1% of x-rays navigate all the way through the patient to the film (Fig. 14-1).2 The remainder of the x-rays are either absorbed or scattered. Absorption of an x-ray results in a white appearance on the film because the x-ray does not penetrate through the given object onto the film. Objects that have a high atomic number (e.g., bone) are more likely to absorb the x-ray and appear white on the image. Approximately one-third of the x-rays reaching the film are primary x-rays, which travel directly through the patient in a straight line.2 The other x-rays are the result of scattering, which occurs when an x-ray encounters an atom and bounces off in another direction. The scattered x-rays that reach the film appear as a gray color, which decreases the quality of the image. To remove the scattered x-rays, an antiscatter grid is often used, which consists of thin lead strips with intermingled radiolucent bands. The scattered x-rays are absorbed by the lead, whereas the primary x-rays are allowed to reach to the film. The grid usually slides over a little during the exposure process to prevent gridlines. The drawback of the antiscatter grid is that it may eliminate too much x-rays from reaching the film, resulting in an underpenetrated film. To fix this problem, a higher x-ray exposure is usually required. The techniques to decrease scatter and to increase absorption must be carefully balanced to provide the best image with the lowest radiation dose.

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FIGURE 14-1. Picture of a plain x-ray machine where the x-ray source is located on the left and the film is located on the right.

image SAFETY CONSIDERATIONS

X-rays can cause damage to biologic tissue. This occurs when the x-ray is either absorbed or scattered by its encounter with an atom causing electrons to shoot off and ionize surrounding atoms. Because of the damage x-rays can cause, lead shields are often used to protect body parts that are not being imaged. The high atomic number of lead prevents x-rays from penetrating through it, thus making it a good shield.

image BENEFITS AND LIMITATIONS

The benefit of using diagnostic radiography is that one is able to get a quick view of a large area with a relatively low amount of radiation. The images can be interpreted by emergency physicians for acute pathology. Portable x-ray devices make it available at the bedside limiting the need for transport to the radiology department.

One limitation of x-ray is that it is a two-dimensional image of a three-dimensional subject. This makes interpretation difficult and often necessitates the need to obtain multiple views. Another limitation is that radiolucent objects cannot be seen on film (e.g., plastic). Thus, when dealing with foreign bodies, the utility of x-rays may be limited depending on the suspected object (Fig. 14-2).

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FIGURE 14-2. Plain x-ray revealing a radio-opaque foreign body. Patient had reported accidentally swallowing a screwdriver.

image DIAGNOSTIC AND PROCEDURAL CONSIDERATIONS

Diagnostic x-rays are the most commonly ordered imaging study in the emergency department (ED) setting. It is the standard for diagnosing fractures and is generally used to evaluate for chest and abdominal pathology. Diagnostic x-rays also have the benefit of allowing the practitioner to assess for multiple causes for the patient’s symptoms at the same time (Fig. 14-3).

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FIGURE 14-3. X-ray of a trauma patient. Note rib fractures on the right with pneumothorax.

Procedures are usually not done by using diagnostic x-rays, but can be done with fluoroscopy. Fluoroscopy is similar to diagnostic rays except that the fluoroscopy screen is viewed directly with the help of an image intensifier.2Fluoroscopic procedures allow the practitioner to view real-time images, but the drawback is that the image quality is poorer and the patient is exposed to more radiation. Common fluoroscopic procedures include fracture reductions and contrast studies.

COMPUTED TOMOGRAPHY

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.

image 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.

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FIGURE 14-4. 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.

image SAFETY CONSIDERATIONS

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.

image BENEFITS AND LIMITATION

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.

image DIAGNOSTIC AND PROCEDURAL CONSIDERATION

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.

CONCLUSION

Diagnostic imaging is vital to the practice of pediatric emergency medicine. The familiarity and accessibility of x-rays and CT scans make them the usual first-choice study. With practitioners becoming more aware of radiation risk involved with x-rays and CT scan, and with nonionizing radiation modalities become more available, we will likely see a shift in diagnostic imaging selections.

REFERENCES

1. Mettler FA Jr. X-ray. In: Mettler FA Jr, ed. Essentials of Radiology. 2nd ed. Philadelphia, PA: Elsevier; 2005:2.

2. Dixon RL. The physical basis of diagnostic imaging. In: Chen MYM, Pope TL Jr, Ott DJ, eds. Basic Radiology. New York, NY: McGraw-Hill; 2004: Chapter 2http://www.accessmedicine.com. Accessed February 15, 2008.

3. National Cancer Institute. Radiation and Pediatric Computed Tomography: A Guide for Health Care Providers. Rockville, MD: National Cancer Institute; 2002. http://www.cancer.gov/cancertopics/causes/radiation-risks-pediatric-CT. Accessed February 15, 2008.

4. Brenner DJ, Hall EJ, Phil D. Computed tomography—an increasing source of radiation exposure. NEJM. 2007;357:2277.

5. Frush DP, Donnelly LF, Rosen NS. Computed tomography and radiation risks: what pediatric health care providers should know. Pediatrics. 2003;112:951.

6. Brody AS, Frush DP, Huda W, et al. Radiation risk to children from computed tomography. Pediatrics. 2007;120:677.

7. Semelka RC, Armao DM, Junior JE, et al. Imaging strategies to reduce the risk of radiation in CT studies, including selective substitution with MRI. J Magn Reson Imaging. 2007;25:900.

8. Hall P, Adami HO, Trichopoulos T, et al. Effect of low doses of ionizing radiation in infancy on cognitive function in adulthood: Swedish population based cohort study. British Med J. 2004;328:19.

9. Bachur RG. Abdominal emergencies. In: Fleisher GR, Ludwig S, Henretig FM et al., eds. Textbook of Pediatric Emergency Medicine. 5th ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2006:1605.

10. Dorias AS, Moineddin R, Kellenberger CJ, et al. US or CT for diagnosis of appendicitis in children and adults? A meta-analysis. Radiology. 2006;241:83.