Introduction

The use of ultrasound in emergency medicine, also known as bedside ultrasound or emergency ultrasound (EUS), has grown rapidly over the past 10 to 15 years. What began primarily as an adjunct for the evaluation of trauma patients has expanded to provide a use for the evaluation of almost every organ system. Ultrasound, previously thought of as the domain of radiologists, is now used by faculty physicians, residents, medical students, and even nursing staff in the emergency department (ED) for the day-to-day evaluation and management of patients. EUS also has a place outside the ED, and its role in international and disaster medicine continues to be defined.

Distinction must be made between EUS and ultrasound performed by other specialties. The American College of Emergency Physicians (ACEP) has defined EUS as “the medical use of ultrasound for the bedside diagnostic evaluation of emergency medical conditions and diagnoses, resuscitation of the acutely ill, critically ill or injured, guidance of high risk or difficult procedures, monitoring of certain pathologic states and as an adjunct to therapy.”¹ EUS is performed or directly supervised by emergency physicians in a variety of settings. Importantly, EUS is primarily distinguished from consultant-performed ultrasound in that it is a goal-directed, focused ultrasound examination.² In short, EUS seeks to answer a brief or “yes or no” question. For example, in a patient with a complaint of abdominal pain and a positive urine pregnancy test, EUS seeks to answer the question “Is an intrauterine pregnancy present?” to guide diagnosis and treatment of the patient. Ultrasound performed by a consultant answers the question in a more expansive manner. For example, in the aforementioned patient, ultrasound performed by radiology or obstetrics and gynecology seeks to evaluate all the organs of the pelvis and to describe any abnormalities identified.

What We Are Looking For

The most common uses for EUS are listed in Box 1. Additionally, the ACEP released the Emergency Ultrasound Imaging Criteria Compendium in 2006,³ which further elucidates and defines the indications, technique, and interpretation of each of the eight primary types of bedside ultrasound. Although this list is extensive, applications for EUS continue to be refined and grow as emergency physicians respond to the challenges of patient care.

Physics of Ultrasound

In its most basic terms, ultrasound uses sound waves with a frequency of between 2 and 20 megahertz (MHz). Crystals in the ultrasound transducer are affected by electrical energy, in a process known as the piezoelectric effect, and emit sound waves at varying frequencies. These sound waves then travel into the body and are reflected (or echoed) back to the transducer with varying speed and strength. Based on these characteristics of return, the ultrasound transducer is able to determine the characteristics of the tissues being scanned.

Tissues that are similar to water transmit sound waves very well and reflect less sound back to the transducer. Examples of these types of tissue in the body are the bladder, blood within the arteries and veins, and the vitreous humor of the eye. These tissues are seen as black or anechoic (without echoes) on the screen (Fig. 1). Conversely, tissue that has less water and is more dense reflects most or all of the sound waves that strike it. Examples of these types of tissue in the body are bones, tendons, and gallstones. These tissues are seen as white or hyperechoic (a lot of echoes) on the screen (Fig. 2). Continuing this logic, all other tissues that are somewhere in the middle of these textures fall into the range of shades of gray (Fig. 3). Objects that are mostly solid but contain some water are a lighter shade of gray, whereas objects that contain more water but are somewhat solid are a darker shade of gray.

Modifiers of echogenicity are used frequently, especially to compare one object with another. For example, one object may be described as more echogenic (creating more echoes) than surrounding objects. It may also be described as hyperechoic with respect to surrounding tissues. Two tissues that are similar in nature are described as being isoechoic (the same amount of echoes) with respect to each other.

Fig. 1 Ultrasound image of the bladder.

Note that urine in the bladder, which is similar to fluid or water, is black or anechoic.

Fig. 2 Ultrasound image of a gallbladder containing gallstones.

Note that the gallstones (arrow), which are hard with little water content, are white or hyperechoic.

Fig. 3 Ultrasound image of the right upper quadrant demonstrating the spectrum between hyperechoic (white) and anechoic (black).

The liver is seen on the left of the image and is dark gray because it contains a large amount of water but is more dense than a purely anechoic object (for example, the bladder). The kidney is seen on the right of the image. Its cortex appears similar to the parenchyma of the liver (isoechoic), but its central collecting system, which is more dense, is hyperechoic (white).

Selecting a Transducer

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