Introduction

Point-of-care ultrasound (PoCUS) is increasingly being used in the care of patients as it can lead to narrowed diagnostic possibilities in acutely unwell patients, increase safety of invasive procedures, improve efficiency of care and limit exposure to ionizing radiation. Improved access to bedside ultrasound has led to a demand for training as doctors are now reaching for an ultrasound probe to enhance their bedside assessments of patients.

The first step in learning to use PoCUS safely and effectively at the bedside involves a basic understanding of (1) how an ultrasound machine works, (2) how to operate it and (3) how to understand and interpret the images on the screen.

Ultrasound waves

In order to successfully operate the machine and interpret images one must have a basic understanding of the physics of sound. This is because an ultrasound machine uses sound waves to produce images. In essence, an ultrasound probe transmits sound waves into tissues. These waves are then reflected back to the ultrasound probe in varying amounts and intensity depending on tissue characteristics. In turn, the machine translates this signal into an image on the screen which can then be interpreted at bedside.

Understanding the properties of ultrasound waves and how they interact with tissues helps us acquire the best images as well as understand the images on the screen. The basic properties of sound waves are frequency, wavelength, and amplitude (Figure 1).

Figure 1: Anatomy of ultrasound waves

 

Wavelength is the distance between successive peaks or valleys of a waveform.

Frequency is the number of times a wave repeats or cycles in a one second period.

Amplitude refers to the height or intensity of a waveform.

Frequency and wavelength are important for image quality. High frequency ultrasound waves oscillate more quickly in tissues, resulting in a shorter wavelength and better resolution or clearer images. But as these short waves enter tissue their energy is quickly dissipated. As such, they do not penetrate well into deeper structures. On the contrary a low frequency ultrasound wave oscillates less vigorously with longer waves. This results in poorer resolution or clarity of structures, but the long waves have a better ability to penetrate into tissues and visualize deeper structures.

This is important because certain ultrasound (US) transducers emit high-frequency ultrasound waves and therefore are best at looking at superficial structures in fine detail whereas other low-frequency transducers can penetrate more deeply into tissue. This is at the cost of a coarser image.

Simply put frequency and penetration are inversely related while frequency and resolution are directly related:

Frequency α 1/Penetration

Frequency α Resolution