Emergency Point-of-Care Ultrasound

James A. Connolly, Anthony J. Dean, Beatrice Hoffmann, Robert D. Jarman, James A. Connolly, Anthony J. Dean, Beatrice Hoffmann, Robert D. Jarman

• Publisher: Wiley-Blackwell
• Year: 2017
• Print ISBN: 9780470657577
• eBook ISBN: 9781119072898
• Edition: 2
• Topic: Medicine
• Subtopic: Emergency Medicine & Critical Care

Featuring contributions from internationally recognized experts in point-of-care sonography, Emergency Point-of-Care Ultrasound, Second Edition combines a wealth of images with clear, succinct text to help beginners, as well as experienced sonographers, develop and refine their sonography skills.

The book contains chapters devoted to scanning the chest, abdomen, head and neck, and extremities, as well as paediatric evaluations, ultrasound-guided vascular access, and more. An entire section is devoted to the syndromic approach for an array of symptoms and patient populations, including chest and abdominal pain, respiratory distress, HIV and TB coinfected patients, and pregnant patients. Also included is expert guidance on administering ultrasound in a variety of challenging environments, such as communities and regions with underdeveloped healthcare systems, hostile environments, and cyberspace.

Each chapter begins with an introduction to the focused scan under discussion and a detailed description of methods for obtaining useful images. This is followed by examples of normal and abnormal scans, along with discussions of potential pitfalls of the technique, valuable insights from experienced users, and summaries of the most up-to-date evidence.

Emergency Point-of-Care Ultrasound, Second Edition is a valuable working resource for emergency medicine residents and trainees, practitioners who are just bringing ultrasound scanning into their practices, and clinicians with many years of sonographic experience.

Part 1
Physics

1
How Does Ultrasound Work?

Heather Venables

Introduction

The aim of this chapter is to outline the basics of how ultrasound works. The construction of an image and some of the physical principles that govern the behaviour of sound in tissue will be introduced.

What is Ultrasound?

Sound is simply the transfer of mechanical energy from a vibrating source through a medium. Ultrasound is defined as sound of a frequency above the human audible range, that is, above 20 kHz.

Piezoelectric crystals within the face of the transducer have the property of contracting or expanding when a voltage is applied across them. A thin layer of a synthetic piezoelectric material can be constructed to vibrate at a resonant frequency within the required range. This acts as a source of ultrasound. A very short (approximately 1 µs) pulse is generated by the transducer and transmitted into the soft tissues. After generation of the ‘pulse’, the transducer receives no further electricity for a period of time (typically about 100–300 µs) and acts as a ‘listening device’ to detect returning echoes generated within the medium of the soft tissues.

As the ultrasound wave of a returning ‘echo’ hits the transducer surface, the piezoelectric crystals vibrate, causing them to generate an alternating electric current. This is transmitted back to the ultrasound machine through the wires attached to the transducer. The magnitude of the voltage of this current is related directly to the amount of energy carried by the returning echo, and will determine the brightness level displayed for this location on the monitor. The machine measures the time that elapses between the pulse and the echo, and by using the known velocity of sound in soft tissues (1540 m s^–1) the distance to the echoing object can be calculated. Many animals (e.g., bats and marine mammals) use the same principle for echo‐location of objects in their environment. (It is worth noting that the construction of the transducer with its sensitive crystal elements does not respond favourably if it is dropped or if the wheels of the machine run over its wires.) Diagnostic ultrasound utilises the pulse‐echo principle to construct a two‐dimensional sectional image of anatomical structures (Figure 1.1).

Figure 1.1 The time taken (t) for the echo to return to the transducer, and the speed of sound in soft tissue (v), can be used to calculate the depth (d) of the reflecting interface, where d = vt/2.

Constructing the Image

Each pulse of sound transmitted into the patient generates a stream of echoes from multiple reflectors at various depths. As noted, the energy carried by each echo is converted into electrical energy by the piezoelectric crystals. In simple terms, these values are then stored within a computer memory as a single ‘scan line’ of information, and used to determine the brightness levels allocated to points in a vertical line on the image to represent corresponding depths in the patient. By firing pulses of sound in sequence from multiple adjacent crystals across the face of the transducer, numerous contiguous scan lines can be generated and a single ‘frame’ of information is produced to represent a two‐dimensional anatomical cross‐section (Figure 1.2). This type of ultrasound imaging is referred to as ‘brightness mode’ (‘B‐mode’ or ‘gray‐scale’) because the strength of the echoes are represented by the brightness of the ultrasound image at that location.

Figure 1.2 Pulses of sound are fired in sequence from multiple adjacent crystals across the face of the transducer. These are used to produce contiguous scan lines from which a single brightness mode (B‐mode) ‘frame’ of information can be produced that represents a two‐dimensional anatomical cross‐section.

If performed fast enough, the rapid update of frames can create a ‘real‐time’ dynamic image of the scanning plane. Frame rate is limited by several factors. The ultrasound machine ‘waits’ for the echoes to return from the maximum depth of interest along each scan line before the next pulse is sent out. Thus, the frame rate depends on the depth of interest and the total number of scan lines of the image (field of view). Adjusting the depth and field of view allows the operator of the ultrasound machine to optimise the frame rate and the resolution of the image. In general, the image should be adjusted to the minimum depth that will include the entire object of interest.

Making Sense of Ultrasound Images

During an ultrasound examination, most of the diagnostic conclusions about normal and abnormal appearances are based on pattern recognition. This includes a number of key observations:

• the spatial definition of tissue boundaries;
• relative tissue reflectivity;
• echo‐texture; and
• the effect of tissue on the transmission of sound.

These appearances are determined by the physical properties of the ultrasound waves and their interactions with tissues. Some of these key interactions are outlined below.

What Happens to a Pulse of Sound as it Travels Through a Patient?

Reflection, scattering and refraction are common to both sound and light waves. An appreciation of this helps us make sense of why structures appear as they do in an ultrasound image.

Reflection

Reflection of the ultrasound pulse occurs at interfaces between two media that have differences in acoustic impedance, which is a medium’s physical properties as a transmitter of sound. Impedance is determined primarily by the medium’s density and elasticity). At such boundaries, a proportion of the sound energy will be reflected, while the remaining sound energy is transmitted beyond the boundary. If the impedance difference at a boundary is high enough, for example at a soft tissue/air interface or at a soft tissue/solid interface, total reflection occurs and no sound energy is transmitted to deeper structures. Gas‐filled structures and bone are therefore a significant challenge in ultrasound imaging.

Specular Reflection

If a reflective boundary is smooth and large, specular reflection occurs. This is similar to when light is reflected from a smooth surface. Typical specular reflectors include the diaph...

Table of contents

• Cover
• Title Page
• Table of Contents
• List of Contributors
• About the Companion Website
• Introduction
• Part 1: Physics
• Part 2: Ultrasound by Region
• Part 3: Paediatrics
• Part 4: Adjunct to Practical Procedures
• Part 5: Syndromic Approach
• Part 6: Different Environments
• Appendix A1: Selected Protocols for Cardiac and Critical Care Ultrasound
• Index
• End User License Agreement