THE PHYSICS AND TECHNOLOGY OF DIAGNOSTIC ULTRASOUND: A PRACTITIONER'S GUIDE
National Library of Australia Cataloguing-in-Publication entry:
Author: Gill, Robert.
Title: The physics and technology of diagnostic ultrasound : a practitioner's guide / by Robert Gill.
ISBN: 9780987292131 (e-book)
Subjects: Diagnostic ultrasonic imaging.
Dewey Number: 616.07543
All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording, or any information and retrieval system, without permission in writing from the publisher.
Every effort has been made in preparing this book to provide accurate and up-to-date information which is in accord with accepted standards and practice at the time of publication. Nevertheless the author, editors and publishers can make no warranties that the information contained herein is totally free from error, not least because clinical standards are constantly changing through research and regulation. The author, editors and publisher therefore disclaim all liability for direct or consequential damages resulting from the use of the material contained in this book. Readers are strongly advised to pay careful attention to information provided by the manufacturer of any drugs or equipment that they plan to use.
Copyright © 2012 High Frequency Publishing, Sydney, Australia
Foreword
This book has been written to help medical professionals (and others) to develop a sound understanding of the physical and technical principles of diagnostic ultrasound. It is intended for use either in self-guided study or in the context of a formal course or training program. It is assumed that the reader has access to ultrasound equipment and opportunities for scanning patients or volunteer subjects while they are studying from this book.
Inevitably the choice of topics and the depth to which they are covered has been selective. The coverage of the book has been designed to suit the typical university or professional course of study. Practitioners in highly specialised areas such as echocardiography may therefore need to supplement the material here by studying other resources.
The author is well aware that many students of ultrasound find the physics and technology very abstract and challenging. For this reason concepts are introduced throughout the book as they are needed, with each chapter building on the preceding chapters. It is therefore strongly recommended that the chapters be read in the order they are presented.
Chapter 1: Introduction
Introductory comments
Ultrasound (i.e. high frequency sound) is a very widely used medical imaging modality. It is based on the concept that low levels of ultrasound energy (generally in the form of short pulses) can be transmitted into the body. As it travels through the body the ultrasound interacts with the tissues, creating a series of "echoes".
These echoes can be detected by the machine's probe and processed to produce an image of part of the patient's anatomy. Other forms of processing can produce information about movement (e.g. the flow of blood) using the Doppler effect.
There are a number of reasons why it is essential to have an understanding of both the underlying physics and the technology (i.e. the principles of operation) of ultrasound instruments to be a competent practitioner.
First, ultrasound machines are complex. They contain a mixture of electronic circuits and digital processing, with the trend being towards increasing the level of digitisation. They provide many functions and so there are a large number of user controls. A feature of the industry has been constant technological innovation, with new modes of operation being added on a regular basis.
Secondly, an ultrasound examination is highly interactive. One reason for this is that every patient is different, and that the precise reason for the scan varies from patient to patient. Even something as well defined as a scan of the kidneys will vary depending on the question being addressed. In addition, the details of the examination will be dictated by what is found as the examination progresses.
Thirdly, ultrasound has significant limitations. An important example is the limits to its ability to penetrate deep into the body due to attenuation of the ultrasound energy as it passes through tissues. Equally important is the severe attenuation of ultrasound when it encounters air or bone. This means that the "acoustic window" through which many body regions can be scanned is quite limited.
An extreme example is echocardiology, where windows must be found that allow the ultrasound to pass through to the heart while avoiding the ribs and lungs. The sonographer or sonologist must be aware of these limitations and must be able to optimise both the scanning technique and the equipment settings to minimise their impact.
Fourthly, ultrasound images contain "artifacts", i.e. features in the images that do not accurately reflect the tissues being scanned. These artifacts need to be recognised as such during the examination, and their negative impact minimised. Measurements of structures must be made using correct scanning and measurement technique to minimise the impact of equipment limitations and artifacts on their accuracy.
Taken together, these considerations mean that the sonographer or sonologist must have a deep understanding of ultrasound physics and technology. They must also have a sound understanding of the controls of the machine and how they can be used to optimise the information displayed.
Finally, it is important to recognise that the main priority for the sonographer or sonologist must be to focus on the patient and on the information being displayed. Interaction with the machine's controls and the assessment of technical factors such as artifacts must therefore become intuitive so they do not distract attention from these priorities.
Suggested activities
1 Observe an experienced sonographer or sonologist as they scan.
2 Note how often they interact with the equipment controls and how much (or little) this interaction distracts their focus from the patient and the display.
Physics and mathematics
Physics deals with concepts that help us to understand the physical world – concepts such as mass, force, velocity, temperature and pressure. Most importantly, it also deals with the relationships between these quantities. These relationships are normally expressed using mathematical equations, since any other way of expressing them would be very cumbersome and difficult to use.
You will therefore need to be able to understand equations and what they say about the relationships between quantities. For example, the simple equation λ = c/f says that the wavelength of the ultrasound (λ) can be calculated by dividing the ultrasound propagation speed (c) by the ultrasound
