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 discusses the various considerations for optimal visualization and patient safety when using diagnostic radiography and computed tomography (CT). Magnetic resonance imaging (MRI) is discussed in a separate chapter. Point-of-care ultrasound (POCUS) is discussed in a separate chapter, but this differs from formal ultrasound studies performed by technicians and radiologists. Ultrasonography is the best means to confirm pyloric stenosis, testicular torsion, ectopic pregnancy, ovarian torsion, intussusception, appendicitis, and many other clinical entities; however, it does not identify all cases. Thus, a high clinical suspicion for a condition in which surgical intervention is highly time-dependent should not be ignored when not confirmed by ultrasonography, which can be very operator and interpreter dependent.

Plain radiography accounts for approximately 75% of all imaging studies.¹ The image obtained from plain radiography is acquired with the aid of x-rays, 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 30,000 eV of energy.² The large amount of energy that x-rays contain allows them to ionize atoms that they encounter, hence labeling x-ray as a form of ionizing radiation.

Approximately 1% of x-rays navigate all the way through the patient to the film (Fig. 15-1).² The remainder of the radiographs are either absorbed or scattered. Absorption of a radiograph results in a white appearance on the film because the x-rays do 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 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 x-rays 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-ray 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.

SAFETY CONSIDERATIONS

X-ray can cause damage to biologic tissue. This occurs when the x-ray radiation 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.

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 radiography devices make it available at the bedside, limiting the need for transport to the radiology department.

One limitation of a plain film radiograph 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 (e.g., plastic) cannot be seen. Thus, when dealing with foreign bodies, the utility of plain film radiographs may be limited depending on the suspected object (Fig. 15-2).

DIAGNOSTIC AND PROCEDURAL CONSIDERATIONS

Diagnostic radiographs are the most commonly ordered imaging study in the emergency department (ED) setting. They are the standard for diagnosing fractures and are generally used to evaluate for chest and abdominal pathology. Diagnostic radiographs also have the benefit of allowing the practitioner to assess for multiple causes for the patient’s symptoms at the same time (Fig. 15-3).