Ultrasound in emergency medicine

Published on 14/03/2015 by admin

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Chapter 5 Ultrasound in emergency medicine

Andrew Finckh, Julie Leung

Emergency department ultrasound continues to be an area of rapidly expanding importance in the practice of emergency medicine. It is a safe and reliable technology that is relatively inexpensive and easily learned.

The Australasian College for Emergency Medicine (ACEM) promotes the use of ultrasound in the emergency department and advocates the availability of timely ultrasound examinations 24 hours per day. The emphasis has been on the ability of the emergency physician and trainee to perform focused emergency department ultrasound examinations on trauma patients (focused assessment with sonography in trauma, or FAST) and on patients with suspected abdominal aortic aneurysm. The advantages of ultrasound examinations in these clinical entities have been clearly established. Other applications of ultrasound in the emergency department include providing guidance for difficult procedures and identifying pathological conditions such as ectopic pregnancy and deep vein thrombosis.

This chapter focuses on: the basic physical principles of ultrasound; ultrasound equipment; common applications of ultrasound in the emergency department; training, credentialling and quality review.

BASIC PHYSICAL PRINCIPLES

‘Ultrasound’ is sound whose frequency is above the range of human hearing. The audible human range is 20–20,000 hertz (Hz), while diagnostic ultrasound employs frequencies of 1–20 megahertz (MHz), with the most common range being 3–12 MHz.

Propagation of sound is the transfer of energy, not matter, from one place to another within a medium.

Frequency and wavelength

Frequency and wavelength are inversely related: the higher the frequency, the shorter the wavelength. The shorter the wavelength is, the less the tissue penetration but the greater the resolution and, therefore, the clearer the image.

Piezoelectric effect

Artificially grown crystals are commonly used for modern transducers. These are treated with high temperatures and strong electric fields to produce the piezoelectric properties necessary to generate sound waves. These properties mean that, when a crystal in the ultrasound transducer has an applied voltage, the crystal is deformed and produces a pressure, i.e. the transducer sends an ultrasound. Conversely, an applied pressure received by the transducer deforms the crystal to produce a voltage. This voltage is then analysed by the system. In essence, the piezoelectric crystal acts as both speaker and microphone.

Most transducers use many small crystal elements for the formation of each pulse.

Image resolution

Resolution is the ability to distinguish between echoes. It is clearly an important characteristic of an ultrasound machine. There are three kinds of image resolution.

1. Contrast resolution is the ability of an ultrasound system to differentiate between tissues with varying characteristics, e.g. between liver and spleen.
2. Temporal resolution is the ability of an ultrasound system to show changes in the underlying anatomy over time. This is particularly important in echocardiography.
3. Spatial resolution is the ability of an ultrasound system to detect and display structures that are close together.

a. Axial resolution is the ability to display small targets along the path of the beam as separate entities. The most important determinant of this is the length of the pulse used to form the beam. The shorter the pulse length, the better the axial resolution. Simply put, the higher frequency probes have better axial resolution but lower penetration.
b. Lateral resolution is the ability to distinguish between two separate targets perpendicular to the beam: the wider the beam, the poorer the lateral resolution. Correct positioning of the focal zones is critical to gaining the best lateral resolution for a given transducer.

Interaction of sound with tissue

Attenuation is the term used to describe the factors affecting the echoes returning to the transducer.

The four main processes in attenuation are:

1. Reflection. This occurs at interfaces between soft tissues of differing acoustic impedance. Impedance is the resistance to propagation of sound. The percentage of the sound reflected is dependent on the magnitude of the impedance mismatch and the angle of approach to the interface, for example:

Soft tissue/air interface 99% reflected
Soft tissue/bone interface 40% reflected
Liver/kidney interface 2% reflected

2. Refraction. This is the deviation in the path of the beam that occurs when the beam passes through interfaces between tissues of differing speeds of sound when the angle of incidence is not 90°.
3. Absorption. This is the transfer of some of the energy of the beam to the material through which sound is travelling. It explains why high-frequency transducers cannot be used for examining deep structures within the body.
4. Scattering. This is the reflection of sound off objects that are irregular or smaller than the ultrasound beam. It occurs at interfaces within the sound beam path. The scatter pattern relies on the size of the interface relative to the wavelength of the sound.

Artefacts

Ultrasound systems operate on the basis of certain assumptions relating to the interaction of the sound beam with soft tissue interfaces. Some artefacts help ultrasound diagnosis. The common types of artefacts include:

Acoustic shadowing. Occurs when beam energy attenuation at a given interface is high, such as when an object of high density blocks signal transmission.

Acoustic enhancement. Occurs when there is an area of increased brightness relative to echoes from adjacent tissues.

Edge shadowing. Results from a combination of reflection and refraction occurring at the edge of rounded structures. It can be falsely interpreted as acoustic shadowing.

Reverberation artefact. Results from repeated reflections of sound between two interfaces. It is usually generated by high-level mismatch interfaces when the echo amplitude is very high.

Beam width effect. Occurs because the ultrasound beam is not a single line, although the system assumes that it is.

Velocity artefact. Occurs because various tissues have velocities that are different from the constant velocity assumed by the system. This results in incorrect placement of an object.

Mirror image. Where a single image is displayed twice due to reflection off an interface.

ULTRASOUND EQUIPMENT

Types of transducers

Linear and curved array transducers are most commonly used in general ultrasound.

Linear array transducer is constructed with multiple small crystal elements arranged in a straight line across the face of the probe. The beam generated by this transducer travels at 90° to the transducer face.

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