Chapter 14: Imaging

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. ...