BS AV A M an ua l o f C anin e a nd F elin e M usc ul osk ele tal Ima gin g, se co nd e di tio n
BSAVA Manual of
Canine and Feline
Musculoskeletal Imaging
second edition
Edited by
Robert M. Kirberger and
Fintan J. McEvoy With CD
Musculoskeletal Imaging
Second edition
Edited by Robert M. Kirberger and Fintan J. McEvoy
This, the second edition of the BSAVA Manual of Canine and Feline Musculoskeletal Imaging, has been extensively updated reflecting the dramatic changes that have taken place in imaging over the past 10 years.
With the increasing availability of digital radiography in general veterinary practice and a greater accessibility to cross-sectional imaging techniques, five new chapters have been included covering the basics of the different imaging modalities with particular reference to their use in musculoskeletal imaging. Comparisons are drawn between the different techniques, and a generous use of illustrations and images brings an example driven clarity and understanding to the written word.
As in the previous edition, the manual is structured along anatomical lines and the updated chapters have all been expanded to include information on ultrasonography, as well as CT, MRI and scintigraphy. Additionally, there is an accompanying CD featuring all the images used throughout the manual plus three bonus video clips.
The practical nature of the manual makes it ideal for use in general practice as well as being a rich source of information for students and newly qualified veterinary surgeons.
However, the depth of information supplied by an international panel of authors also makes the BSAVA Manual of Canine and Feline Musculoskeletal Imaging, second edition a useful reference for orthopaedic surgeons and specialist radiologists.
CONTENTS: Basics of musculoskeletal radiography and radiology; Basics of musculoskeletal ultrasonography; Basics of musculoskeletal computed tomography; Basics of musculoskeletal magnetic resonance imaging; Basics of musculoskeletal nuclear medicine; Soft tissues;
Bones – general; Long bones – juvenile; Long bones – mature; Long bones – fractures; Joints – general; The shoulder joint and scapula; The elbow joint; The hip joint and pelvis; The stifle joint; Distal limbs – carpus/tarsus and distally; Skull – general; Skull – nasal chambers and frontal sinuses; Skull – teeth; Spine – general; Spine – conditions not related to intervertebral disc disease; Spine – intervertebral disc disease and ‘wobbler syndrome’; Spine – lumbosacral region and cauda equina syndrome; Index
Fintan J. McEvoy
MVB PhD DVSc DVR DipECVDI After qualifying as a veterinarian from University College Dublin in 1983, Fintan McEvoy went on to work in private practice for 4 years and then in academia at the University of London, the Swedish University of Agricultural Sciences and for the last 16 years at the University of Copenhagen, where he is currently professor of veterinary radiology.
Robert M. Kirberger
BVSc DVSc MMedVet(Rad) DipECVDI Robert Kirberger qualified as a veterinarian from the University of Pretoria in 1971.
He spent 17 years in private practice before going on to qualify as a specialist veterinary radiologist from the University of Pretoria in 1990. He is currently a professor of diagnostic imaging at the University of Pretoria where he has spent his whole academic career.
with CD included
BSAVA Manual of Canine and Feline
Musculoskeletal Imaging
second edition
Editors:
Robert M. Kirberger
BVSc DVSc MMedVet(Rad) DipECVDI
Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort 0110, South Africa
Fintan J. McEvoy
MVB PhD DVSc DVR DipECVDI
Department of Veterinary Clinical and Animal Sciences, University of Copenhagen, Dyrlægevej 16,
1870 Frederiksberg C, Denmark
Published by:
British Small Animal Veterinary Association Woodrow House, 1 Telford Way,
Waterwells Business Park, Quedgeley, Gloucester GL2 2AB
A Company Limited by Guarantee in England Registered Company No. 2837793
Registered as a Charity Copyright © 2016 BSAVA
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in form or by any means, electronic, mechanical, photocopying, recording or otherwise without prior written permission of the copyright holder.
Figures 7.1, 7.8, 7.18, 7.22ab, 11.1, 12.3a, 12.4a, 12.5, 12.10ab, 13.2b, 13.6b, 14.2, 14.3a, 14.10, 14.11, 14.13ab, 14.16, 14.24, 14.30, 15.6, 15.9a, 15.10, 16.12, 16.13, 16.15ab, 16.18a, 19.10a, 19.15a, 19.16a, 19.17a, 19.18a, 19.19a, 19.22a, 20.11, 20.13 and 20.20 were drawn by S.J. Elmhurst BA Hons (www.livingart.org.uk) and are printed with her permission.
Figures 10.2, 10.15, 10.16 and 10.17 were drawn by Vicki Martin and are printed with her permission.
A catalogue record for this book is available from the British Library.
ISBN 978 1 905319 78 7 e-ISBN 978 1 910443 29 3
Disclaimer: This eBook does not include ancillary media that was packaged with the printed version of the book.
Contact publications@bsava.com for further information
The publishers, editors and contributors cannot take responsibility for information provided on dosages and methods of application of drugs mentioned or referred to in this publication. Details of this kind must be verified in each case by individual users from up to date literature published by the manufacturers or suppliers of those drugs. Veterinary surgeons are reminded that in each case they must follow all appropriate national legislation and regulations (for example, in the United Kingdom, the prescribing cascade) from time to time in force.
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Titles in the BSAVA Manuals series
Manual of Canine & Feline Abdominal Imaging Manual of Canine & Feline Abdominal Surgery
Manual of Canine & Feline Advanced Veterinary Nursing Manual of Canine & Feline Anaesthesia and Analgesia Manual of Canine & Feline Behavioural Medicine Manual of Canine & Feline Cardiorespiratory Medicine Manual of Canine & Feline Clinical Pathology
Manual of Canine & Feline Dentistry Manual of Canine & Feline Dermatology
Manual of Canine & Feline Emergency and Critical Care Manual of Canine & Feline Endocrinology
Manual of Canine & Feline Endoscopy and Endosurgery Manual of Canine & Feline Fracture Repair and Management Manual of Canine & Feline Gastroenterology
Manual of Canine & Feline Haematology and Transfusion Medicine Manual of Canine & Feline Head, Neck and Thoracic Surgery Manual of Canine & Feline Musculoskeletal Disorders
Manual of Canine & Feline Musculoskeletal Imaging Manual of Canine & Feline Nephrology and Urology Manual of Canine & Feline Neurology
Manual of Canine & Feline Oncology Manual of Canine & Feline Ophthalmology
Manual of Canine & Feline Radiography and Radiology: A Foundation Manual Manual of Canine & Feline Rehabilitation, Supportive and Palliative Care:
Case Studies in Patient Management
Manual of Canine & Feline Reproduction and Neonatology
Manual of Canine & Feline Surgical Principles: A Foundation Manual Manual of Canine & Feline Thoracic Imaging
Manual of Canine & Feline Ultrasonography
Manual of Canine & Feline Wound Management and Reconstruction Manual of Canine Practice: A Foundation Manual
Manual of Exotic Pet and Wildlife Nursing Manual of Exotic Pets: A Foundation Manual Manual of Feline Practice: A Foundation Manual Manual of Ornamental Fish
Manual of Practical Animal Care Manual of Practical Veterinary Nursing Manual of Psittacine Birds
Manual of Rabbit Medicine
Manual of Rabbit Surgery, Dentistry and Imaging Manual of Raptors, Pigeons and Passerine Birds Manual of Reptiles
Manual of Rodents and Ferrets
Manual of Small Animal Practice Management and Development Manual of Wildlife Casualties
For further information on these and all BSAVA publications, please visit our website:
www.bsava.com
Contents
List of contributors v
Foreword vii
Preface viii
1
Basics of musculoskeletal radiography and radiology 1
Eberhard Ludewig and Fintan J. McEvoy
2
Basics of musculoskeletal ultrasonography 15
Nele Ondreka and Martin Kramer
3
Basics of musculoskeletal computed tomography 28
Ingrid Gielen
4
Basics of musculoskeletal magnetic resonance imaging 33
J. Fraser McConnell
5
Basics of musculoskeletal nuclear medicine 58
Federica Morandi
6
Soft tissues 65
Frances Barr and Sally Birch
7
Bones – general 75
Robert M. Kirberger
8
Long bones – juvenile 87
Nerissa Stander and Nicky Cassel
9
Long bones – mature 108
Hester McAllister and Emma Tobin
10
Long bones – fractures 133
Steven J. Butterworth
11
Joints – general 156
Sarah Davies, Graeme Allan and Robert Nicoll
12
The shoulder joint and scapula 171
Ingrid Gielen, Annemie Van Caelenberg and Henri van Bree
13
The elbow joint 189
Robert M. Kirberger
Kerstin H. Von Pückler, Bernd Tellhelm and Robert M. Kirberger
15
The stifle joint 233
Eithne J. Comerford
16
Distal limbs – carpus/tarsus and distally 251
Robert Nicoll, Graeme Allan and Sarah Davies
17
Skull – general 275
Ruth Dennis
18
Skull – nasal chambers and frontal sinuses 301
Christopher R. Lamb
19
Skull – teeth 316
Gerhard Steenkamp
20
Spine – general 333
Robert M. Kirberger
21
Spine – conditions not related to intervertebral disc disease 347
Fintan J. McEvoy and Hugo Schmökel
22
Spine – intervertebral disc disease and ‘wobbler syndrome’ 365
Jeremy V. Davies and Francois-Xavier Liebel
23
Spine – lumbosacral region and cauda equina syndrome 380
Johann Lang and Karine Gendron
Index 395
CD Contents Video clips
Avulsion fracture of the supraglenoid tubercle in a young dog Subcutaneous cellulitis with a grass awn in longitudinal section Subcutaneous cellulitis with a grass awn in transverse section
The CD that accompanies the BSAVA Manual of Canine and Feline Musculoskeletal Imaging, second edition also contains all the images featured in the manual.
Contributors
Graeme Allan
BVSc MVSc DVSc FACVSc DipACVR MRCVS
Veterinary Imaging Associates and Online VeTS Pty Ltd, PO Box 300, St Leonards NSW 2065, Australia
Frances Barr
MA VetMB PhD DVR DipECVDI MRCVS
British Small Animal Veterinary Association, Woodrow House, 1 Telford Way,
Waterwells Business Park, Quedgeley, Gloucester GL2 2AB, UK
Sally Birch
BVSc CertAVP DipECVDI MRCVS
Willows Veterinary Centre and Referral Service, Highlands Road, Shirley,
Solihull, West Midlands B90 4NH, UK
Steven J. Butterworth
MA VetMB CertVR DSAO MRCVS
Weighbridge Referrals,
Kemys Way, Swansea Enterprise Park, Swansea SA6 8QF, UK
Nicky Cassel
BSc BVSc MMedVet(Diagnostic Imaging) DipECVDI
Vet Imaging Specialists,
Postnet Suite 653, Private Bag X1, The Willows, 0041, Pretoria, South Africa
Eithne J. Comerford
MVB PhD CertVR CertSAS PGCertHE DipECVS FHEA MRCVS
Institute of Ageing and Chronic Disease and School of Veterinary Science, University of Liverpool,
Leahurst Campus, Chester High Road, Neston, Cheshire CH64 7TE, UK
Jeremy V. Davies
BVetMed PhD DVR DipECVS DipECVDI MRCVS
Davies Veterinary Specialists, Manor Farm Business Park,
Higham Gobion, Hertfordshire SG5 3HR, UK
Sarah Davies
BVSc MS DipACVR
Veterinary Imaging Associates and Online VeTS Pty Ltd, PO Box 300, St Leonards NSW 2065, Australia
Ruth Dennis
MA VetMB DVR DipECVDI MRCVS
Centre for Small Animal Studies,
Animal Health Trust, Lanwades Park, Kentford, ewmarket, Suffolk CB8 7UU, UK
Karine Gendron
DMV DipECVDI
College of Veterinary Medicine, Cornell University, VMC Box 36, Room C1-120,
Ithaca, NY 14853, USA
Ingrid Gielen
DVM PhD MSc
Department of Veterinary Medical Imaging and Small Animal Orthopaedics, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133,
9820 Merelbeke, Belgium
Robert M. Kirberger
BVSc DVSc MMedVet(Rad) DipECVDI
Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, University of Pretoria, Private Bag X04, Onderstepoort 0110, South Africa
Martin Kramer
Prof Dr med vet Dr hc DipECVDI
Clinic for Small Animals, Surgery and Diagnostic Imaging, Department of Veterinary Clinical Science, Justus-Liebig-University Giessen,
Frankfurter Strasse 108, 35392 Giessen, Germany
Christopher R. Lamb
MA VetMB DipACVR DipECVDI FHEA MRCVS
Department of Clinical Science and Services, The Royal Veterinary College, University of London, Hawkshead Lane, North Mymms,
Hatfield, Hertfordshire AL9 7TA, UK
Johann Lang
Prof emeritus DipECVDI
Division of Clinical Radiology, Department of Clinical Veterinary Medicine, Vetsuisse-Faculty,
University of Bern,
CH-3001 Bern, Switzerland
Francois-Xavier Liebel
DVM DipECVN MRCVS
Davies Veterinary Specialists,
Manor Farm Business Park, Higham Gobion, Hertfordshire SG5 3HR, UK
Eberhard Ludewig
Prof Dr med vet DipECVDI
University of Veterinary Medicine Vienna (Vetmeduni Vienna), Department for Companion Animals and Horses, Clinical Unit of Diagnostic Imaging, Veterinärplatz 1, A - 1210 Vienna, Austria
Hester McAllister
MVB DVR DipECVDI MRCVS
UCD Veterinary Hospital, School of Veterinary Medicine, Veterinary Sciences Centre, University College Dublin, Belfield, Dublin 4, Ireland
J. Fraser McConnell
BVM&S DVR DipECVDI CertSAM MRCVS
Small Animal Teaching Hospital,
School of Veterinary Science, University of Liverpool, Leahurst Campus, Chester High Road,
Neston, Cheshire CH64 7TE, UK
Department of Veterinary Clinical and Animal Sciences,
University of Copenhagen, Dyrlægevej 16, 1870 Frederiksberg C, Denmark
Federica Morandi
DMV MS DipACVR DipECVDI
Department of Small Animal Clinical Sciences, College of Veterinary Medicine,
University of Tennessee, 2407 River Drive, Knoxville, TN 37996, USA
Robert Nicoll
BSc(Vet) BVSc DipACVR
Veterinary Imaging Associates and Online VeTS Pty Ltd, PO Box 300, St Leonards NSW 2065, Australia
Nele Ondreka
DVM Dr med vet DipECVDI
Department of Small Animal Clinical Sciences, Justus-Liebig-University Giessen,
Frankfurter Strasse 108, 35392 Giessen, Germany
Hugo Schmökel
Dr med vet PhD DipECVS
Evidensia Spine Center, Djursjukhusvägen 11, 73494 Strömsholm, Sweden
Nerissa Stander
BVSc MMedVet (Diagnostic Imaging) DipECVDI
Western Australian Veterinary Emergency and Specialty, Unit 1, 640 Beeliar Drive, Success, WA 6164,
Australia
Dental and Maxillofacial Surgery Clinic, Onderstepoort Veterinary Academic Hospital, Faculty of Veterinary Science, University of Pretoria, Pretoria, South Africa
Bernd Tellhelm
Dr med vet DipECVDI
Clinic for Small Animals, Surgery and Diagnostic Imaging, Department of Veterinary Clinical Science, Justus-Liebig-University Giessen,
Frankfurter Strasse 108, 35392 Giessen, Germany
Emma Tobin
MVB MVM CertVR DipECVDI
School of Veterinary Medicine,
Veterinary Sciences Centre, University College Dublin, Belfield, Dublin 4, Ireland
Henri van Bree
Prof emeritus DVM PhD DipECVDI DipECVS
Department of Veterinary Medical Imaging and Small Animal Orthopaedics, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133,
9820 Merelbeke, Belgium
Annemie Van Caelenberg
DVM CertLAS PhD
Department of Veterinary Medical Imaging and Small Animal Orthopaedics, Faculty of Veterinary Medicine, Ghent University, Salisburylaan 133,
9820 Merelbeke, Belgium
Kerstin von Pückler
Dr med vet DipECVDI
Department of Small Animal Clinical Sciences, Justus-Liebig-University Giessen,
Frankfurter Strasse 108, 35392 Giessen, Germany
Foreword
It has been 10 years since the first edition of the BSAVA Manual of Canine and Feline Musculoskeletal Imaging was published and, indeed, that edition followed the concept of splitting the highly successful BSAVA Manual of Small Animal Diagnostic Imaging, edited by Professor Robin Lee, into its constituent parts. This second edition, edited by Robert Kirberger and Fintan McEvoy, has taken the BSAVA Manual of Canine and Feline Musculoskeletal Imaging to new heights including a wealth of relevant, up-to-date information particularly in respect of new technology and advanced imaging techniques.
It was Albert Einstein who said,
“Any fool can know. The point is to understand”
For this new edition the editors have assembled a group of experts in the field to deliver an authoritative, easy-to-follow and well-explained text which complements the excellent quality of the illustrations and line drawings, thus enabling clinicians to understand the principles and limitations underlying their interpretation of the images they make or receive.
New for this edition is a practical and informative section on imaging techniques with chapters on radiography and radiology, musculoskeletal ultrasonography, computed tomography, magnetic resonance imaging and nuclear medicine. This enables the clinician to provide informed and objective recommendations on the most appropriate imaging technique(s) to use for a particular case. The chapter on radiography and radiology includes an excellent section on computed radiography and direct radiography, which also clearly explains and illustrates the artefacts that can occur with these types of digital imaging.
The remaining chapters, covering the clinical evaluation of soft tissues, bones (general, juvenile, mature and fractures), joints (general and individual), as well as the chapters on the skull (general, nasal chambers and teeth) and the spine (general, intervertebral disc disease and lumbosacral disease), have all been substantively revised and updated with new and clear illustrations of radiographs, ultrasound, CT and MRI scans, which help improve our understanding and interpretation of what we see.
In tune with other BSAVA manuals, both the clarity of the text and the logical layout of the BSAVA Manual of Canine and Feline Musculoskeletal Imaging enable busy clinicians to find appropriate and relevant information quickly and easily. The succinct explanations facilitate an understanding of the rationale behind their decision-making, which will be to the benefit of the many patients that pass through their care.
Michael E. Herrtage MA BVSc DVSc DVD DSAM DipECVDI DipECVIM MRCVS
University of Cambridge
It is now more than 10 years since the publication of the first edition of the BSAVA Manual of Canine and Feline Musculoskeletal Imaging. The first edition replaced the original manual edited by Professor Robin Lee and was the initial manual of a subsequent series of sister BSAVA imaging manuals. These were the Manuals of Canine and Feline Thoracic Imaging (2008), Canine and Feline Abdominal Imaging (2009), Canine and Feline Ultrasonography (2011) and Canine and Feline Radiography and Radiology: A Foundation Manual (2013). This is the first of the second editions of these manuals, written to keep pace with the rapid changes taking place in diagnostic imaging, increasing availability of diagnostic ultrasound machines in veterinary practice and the greater accessibility to cross-sectional imaging techniques.
This manual has thus been considerably expanded with five new chapters to cover the basics of the different imaging modalities with particular reference to their use in musculoskeletal imaging. The remaining chapters cover the same areas as in the first edition but have all been expanded to include more ultrasound as well as cross-sectional imaging techniques.
The dramatic changes over the past 10 years in radiography, from hard copy analogue films, which stood us in good stead for more than a hundred years, to digital imaging, have been extraordinary. In first world countries digital radiography is now probably available in more than 80% of practices and in 5 years’ time hard copy films are likely to be part of history.
Although ubiquitous, digital radiography is still poorly understood by many practitioners.
Teleradiology companies see a vast array of inappropriate use of digital systems and it is hoped that the first chapter of this second edition will enable practitioners to understand this powerful diagnostic tool and enable them to gain the maximum benefit from this modality in order to make good quality diagnostic digital radiographs in their practices. The second chapter covers the basics of diagnostic ultrasound and its applications in musculoskeletal imaging. Although many practitioners are familiar with the technique in some of its applications, not many are utilizing the modality for enhancing their musculoskeletal diagnostic capabilities. Chapters 3 to 5 will hopefully assist practitioners in understanding the basic concepts of the imaging techniques, CT, MRI and scintigraphy, that are usually accessed through referral for investigation of the musculoskeletal system.
We would like to express our thanks to the members of the BSAVA editorial team for all their hard work in making this second edition a reality and for ensuring that the BSAVA manuals remain up-to-date by commissioning new editions. Additionally, to our first edition authors who have updated their chapters, as well as the new authors contributing to the second edition, our sincere thanks go to you for your contributions and forbearance in dealing with editorial queries. We are grateful that you have found time in your busy schedules to share your extensive specialized imaging knowledge with all veterinarians who have an interest in improving their diagnostic imaging capabilities.
Robert M. Kirberger Fintan J. McEvoy July 2016
Preface
Basics of musculoskeletal radiography and radiology
Terminology
CR Computed radiography CT Computed tomography
DICOM Digital imaging and communications in medicine
DQE Detective quantum efficiency DR Direct radiography
LCD Liquid crystal display lp/mm line pairs per millimetre LUT Look-up table
MTF Modulation transfer function
PACS Picture archiving and communication system
TFT Thin film transistor
Eberhard Ludewig and Fintan J. McEvoy
Radiography is an excellent tool to generate diagnostic information in cases where either a skeletal lesion or a sys- temic disease with skeletal manifestation is suspected.
The fact that radiographic equipment is available in almost all veterinary practices makes it the imaging method most often used initially to detect and characterize lesions of bones and joints.
Radiography techniques
Techniques of image recording
Screen film radiograp y
In screen–film systems one or two intensifying screens transform incoming X-rays into light photons. Subsequently, this light exposes the film. The ‘intensifying’ (i.e. dose- saving) effect results from the fact that film is much more sensitive to light than to X-rays. Screen–film systems can be categorized according to the thickness of the phosphor layer of the screen. There is a negative correlation between the sensitivity of a screen–film system and its resolving power (Figure 1.1). In musculoskeletal radiography it is important to visualize subtle structures; therefore, fine-detail screen–film systems speed class 100 should be used. In cases where a grid is required to remove scatter radiation thickness 10 cm , ‘faster’ systems offering greater inten- sification but a lower spatial resolution (speed class about 400 are a useful compromise to achieve exposure times short enough to avoid a lack of sharpness due to patient motion.
Speed class Equivalent dose
( Sv) Spatial resolution (approximate lp/mm)
50 20 7.4
100 10 5.7
200 5 4.5
400 2.5 3.6
800 1.25 3.2
Speed classes of screen–film systems. There is a reciprocal relationship between the equivalent dose (in microSieverts, Sv) required to generate a certain level of film blackening (optical density = 1) and the spatial resolution measured in line-pairs per millimetre (lp mm).
1.1
igital radiograp y
In recent years, digital radiography systems have become more common in veterinary diagnostic radiology. Today, manufacturers provide a variety of digital imaging solu- tions based on various detector and readout technologies.
Common technical features or characteristics of all current digital systems include:
• An imaging chain comprising four separate technical steps: signal acquisition, signal processing, image distribution and archiving, and image presentation
• For each of the individual elements of this chain, several technical solutions exist. These steps can be optimized separately. Therefore, they can be adjusted to a specific user’s predefined requirements
• The capabilities of a digital radiography system are dependent on the interplay of these parts. The weakest part determines the overall performance.
Two types of digital detector exist: CR and DR. There are important differences between different detector technologies that affect system performance and image quality. To be useful for veterinary radiography the de - t ector has to fulfil a number of prerequisites related to detector and pixel size, sensitivity (DQE), dynamic range and readout speed. These concepts are explained later in this chapter.
Principles of computed radiography: CR uses a storage- phosphor image plate covered by a cassette. The X-ray sensitive part of the plate consists of a detector layer of photostimulable crystals. It absorbs the X-ray energy and temporarily stores it during an exposure. In this way a
latent image is formed. To release the stored energy the cassette is put into an image reading device (Figure 1.2), where it is opened automatically. The image plate is removed to be scanned by a laser beam. This process sets the stored energy free as visible light. Photodiodes capture the light emitted and convert it into a digital signal.
Because a part of the latent image remains in the image plate, the residual latent information must be removed so the image plate can be reused. This task is accomplished inside the reader by exposing the image plate to intense white light.
Depending on the size of the image plate and the scan-matrix the entire readout process takes about 0 to 40 seconds. To shorten the readout process some of the devices can operate multiple cassettes in parallel (Figure 1.2).
CR image reading devices. Numerous types of systems are on the market. They differ in size, the number of stackers and other technical features such as matrix and scanning speed.
(a) Single-cassette reader. (b) Four-cassette reader.
1.2 (a)
(b)
Since its first release, CR technology has undergone significant improvements. Recent developments (e.g. ‘dual- reading’ image plates, needle-structure plates) have resulted in new systems with improved sensitivity, spatial resolution and readout speed (Figure 1.3).
Principles of direct radiography: Flat-panel detectors convert X-rays into electrical charge by means of a direct readout process (‘direct radiography’) (Figure 1.4). De - pending on the type of conversion, flat-panel detectors can be categorized into direct and indirect converting systems (Figure 1.5). Both technologies have advantages and disad- vantages with respect to sensitivity and spatial resolution:
• Direct conversion detectors have a layer of photoconductors (e.g. amorphous selenium) that directly transforms incoming X-ray energy into electric charge. Underneath this layer there is a layer of electrodes for transmission of the released electrons to an array of TFTs, which form the third layer. The TFTs sample and store the energy of the electrons for the readout process
• Indirect conversion systems also consist of layers.
First, in the scintillator layer (caesium iodide, gadolinium- or lanthanum-oxide sulfide), X-rays are converted into visible light. In the second step, a photodiode array of amorphous silicon produces electric charge from light.
The next layer is formed by an array of TFTs.
Flat-panel detectors are available in different sizes.
Signal transfer from the detector to the host computer is carried out via a wired or wireless connection.
Basic principles of processing, display and storage of digital images: Other important features of digital systems that significantly affect both performance and image quality are signal processing, image distribution and archiving, and image presentation.
Processing means that region-related specific mathe- matical algorithms are used to optimize contrast, to reduce noise and to emphasize structures of interest. An impor- tant integral part of processing is an instruction telling the computer how to display the processed picture on the monitor. Such a translation instruction is called a ‘look-up table’. The LUT defines window settings (contrast, bright- ness), image orientation, magnification and annotations.
Adequate processing results in improved visualization of structures, whereas insufficient or improper processing can obscure structures or even create artefacts (Figure 1.6).
Processing errors and incorrect LUTs are frequent errors.
Very often they have a significant impact on the quality of the displayed image.
It is, therefore, vital to choose appropriate settings. For example, a radiograph from the elbow will be processed and translated very differently from a thoracic radiograph.
Processing and LUT should be adjusted continuously to improve image quality.
An adequate workstation setup (hardware, software, network capabilities) and a well designed reading environ- ment can substantially improve diagnostic accuracy and/or increase productivity. Viewing monitors are the key hard- ware components of the workstation. A workstation for primary image viewing should be equipped with a minimum of two side-by-side monitors (Figure 1.7). It has been shown that large-screen standard computer liquid crystal displays (LCDs) with high brightness and resolution
screen diameter 1 inches . cm , maximum lumi- nance 00 Candela cd /m2; matrix megapixels have
Types of CR systems. (a) Single-side readout system. In ‘conventional’ systems, during the readout process, a laser beam scans the image on a single side of the plate pixel by pixel. The photostimulated luminescence is proportional to the absorbed radiograph intensity. The output of the photomultiplier is logarithmically amplified and subsequently digitized by an analogue–digital converter. The phosphor layer has a grainy structure.
To avoid excessive light spread that results in increasing intrinsic ‘unsharpness’, the thickness of the layer is limited. (b) Dual-side reading system. In comparison with ‘conventional’ single-side readings, in dual-side reading systems a transparent support is used that allows light to pass through this layer of the image plate. The phosphor layer is thicker. This system results in improved quantum e ciency without a loss of resolution in comparison with single-side reading systems. (c) Line scanning system. In contrast to flying-spot scanners (a–b), in this system a line scanner is used. In this approach, an entire line is illuminated with a set of stimulation sources (e.g. a row of solid-state laser diodes). The light from this line is read by an array of
photodetectors. The stimulation sources, light-collecting optics, photodetectors and other technical components are contained in a scan head that is as wide as the screen. Therefore, the screen surface can be scanned while moving the scan head along the image plate. Because of the needle structure of the image plate, a thicker phosphor layer can be used, resulting in a higher D E and improved resolution. CCD = charge-coupled device.
(Reproduced and modified from Ludewig et al. (2012) Clinical technique digital radiography in exotic pets – important practical differences compared with traditional radiography. Journal of Exotic Pet Medicine 21, 71–79, with permission from Elsevier)
1.3
Laser beam
Light Light
O ptical g uid e
I m ag e pl ate
M irror Photosensor
Phosphor
Support
Laser beam
Light Light
Light O ptical g uid e
I m ag e pl ate
M irror Photosensor
Phosphor
O ptical g uid e
Photosensor
Support ( transparent)
Lenses Laser Line scanner
CCD
Phosphor Photosensor
Support (a)
(b)
(c)
DR system. The wireless detector is stored in a drawer under the tabletop. The grid is positioned in a slot between the tabletop and the detector. If the grid is not required for a radiograph it can be removed. Alternatively, the detector can be positioned directly on the tabletop.
1.4
Types of flat-panel detector. (a) A direct flat-panel detector uses a semiconductor material layered between two electrodes, and electron hole pairs are directly produced as a result of local radiograph energy absorption. A high-voltage bias placed between the electrodes separates the charge pairs with little or no lateral spread, allowing for high intrinsic spatial resolution. (b) An indirect flat-panel detector has a scintillator to convert absorbed energy into visible light. Thus, sensitivity (D E) is high. The photodiode layer electrode on the surface of the array produces photo-induced charge within each detector element and the resultant charge is stored in the local TFT. aSe = amorphous selenium; aSi = amorphous silicon;
CsI = caesium iodide; Gd2O2S = gadolinium-oxide sulfide.
(Reproduced from Ludewig et al. (2012) Clinical technique digital radiography in exotic pets – important practical differences compared with traditional radiography. Journal of Exotic Pet Medicine 21, 71–79, with permission from Elsevier)
1.5
Photoconductor (e.g. aSe)
TFT array Charge readout
Electric charge X-rays
Photodiode (e.g. aSi) Scintillator layer (e.g. Csl, Gd2O2S)
TFT array Charge readout
Light
Electric charge X-rays
(b) (a)
Image processing. Multiscale processing was used to create three different versions of the same image data. One out of several parameters of this filter, namely frequency enhancement, has been changed stepwise. The modification resulted in a changed appearance of image details such as bone contour and implant surface.
1.6
similar performance to more expensive medical-grade greyscale monitors and, therefore, domestic monitors offer a useful alternative. Monitor reading is characterized by a dynamic and changeable presentation of the image. This is achieved using software designed for viewing images.
There are many different versions of DICOM viewing soft- ware that range from freely downloadable to expensive commercial programs. Whatever software is chosen, it should at least include tools to: control contrast and brightness; flip and rotate images; magnify images; zoom image details; and measure distances and angles.
Basically, the software must be user friendly so that little or no special training is required to work with the system.
Because the capabilities and functionality of viewing software differ among programs, it is very helpful to explore
• Digital radiography can be easily integrated into complex information systems where individual components interact. There are numerous options for individualized design of such a system, in which images are stored digitally and are available at any time and everywhere:
• Exchange of information with a radiology
information system (RIS) and hospital information system (HIS) enables a paperless workflow
• Several modalities can be brought together on a uniform platform, which makes the viewing process more productive
• Large amounts of image data can be sent to specialists worldwide for consultancy. In teleradiology services the analysis of images is locally separated from image recording
• Image quality depends on the performance and the interplay of the individual steps of the imaging chain.
This provides opportunities to monitor performance and make changes that will result in an overall improvement in system performance
• The number of recorded attenuation differences (the dynamic range) is much larger. This property can be exploited to generate more diagnostic information from the image
• The much wider exposure range can be used to reduce dose. Exposure faults are well tolerated so repeat exposure can be reduced
• Major advantages of a film-less environment include:
• Faster image distribution
• Reduced workforce required for film handling
• Abandonment of chemicals for film processing
• Saving of room for film storage and darkroom space.
The major limitations of digital radiography are:
• The costs are high for purchase and maintenance
• Overexposures can be easily overlooked (Figure 1.8)
• In most countries no authoritative regulations or guidelines exist to define the minimum technical prerequisites for components sold for veterinary use or for acceptable exposure limits. Consequently, there is the real danger that systems are used that are not able to achieve diagnostic image quality or that achieve acceptable image quality through unjustified high exposures.
There are numerous digital systems on the market.
Factors that may influence the decision on what system to buy are given in Figure 1.9.
posure
Exposure tables specific for particular imaging systems are highly recommended if optimal results are to be achieved. They are required for screen–film radiography to achieve adequate film blackening and contrast, and for digital radiography to avoid under- and overexposure (Figure 1.8).
General principles for choosing exposure settings in musculoskeletal imaging are:
• Moderate–low kilovoltage (kVp) and moderate–high milliampereseconds (mAs) for adequate image contrast. For orientation:
• For cm thickness without use of a grid 40 to 0 k p and about 10 mAs
• For 1 cm thickness with use of a grid 0 to 70 k p and about 0 mAs
Ideally, a workstation should be equipped with a minimum of two side-by-side display monitors and a typing monitor. Two monitors are required because it is necessary to display multiple radiographs simultaneously at an adequate size. If only one monitor is used the images can be displayed either consecutively or side-by-side in a smaller format.
1.7
them by hands-on testing to identify the optimum function- ality required to meet the user’s specific needs, and to check user comfort.
The computer system that manages the acquisition, transmission, storage, distribution, display and interpre- tation of medical images is called a picture archiving and communication system. The PACS infrastructure includes imaging modalities, network and archive components, workstations, software and interfaces with the hospital or radiology information system.
The antiscatter grid
The most important source of scatter radiation is the body of the patient. Image contrast decreases as scatter radia- tion increases. If the thickness of the musculoskeletal region to be radiographed exceeds 10 cm, a grid should be used. This requirement applies in general to both screen–film and digital radiography. One exception to this rule of thumb seems to be possible when a digital detec- tor with a very high sensitivity D E 0 at 0 lp/mm is used. Such a detector transforms a higher proportion of radiation into signal for image information. In conse- quence, a lower dose is sufficient to achieve adequate image quality. Owing to the lower patient dose the amount of scatter radiation produced is significantly lower, and a grid is not needed.
Given that the antiscatter grid absorbs both multidirec- tional (‘scatter’) radiation and a certain amount of the primary radiation, the total exposure must be increased to achieve the required level of detector dose. The magnitude of the increase in dose depends on the type of grid used, and in particular on the grid-ratio, which is the ratio of the height of the lead strips to the width of the interspacing of the antiscatter grid. At least a doubling of the dose is necessary. Owing to these extra dose requirements a grid should not be used in regions where it is not required, i.e.
when the region to be examined has a thickness 10 cm.
Advantages and disadvantages of digital radiograp y versus screen film radiograp y
Digital radiography offers a number of advantages over screen–film radiography:
Signal response
Dynamic range
Digital detector
cree fil s ste
Dose ( Sv)
10,000 1000
100 10 1
0.01 0.1 1 10 100
Characteristic curves.
The characteristic curve of a digital detector is linear. The high dynamic range means that a wide range of attenuation differences are registered and subsequently can be displayed. The system can compensate for vast exposure differences. In contrast, screen–film systems have a sigmoid-shaped curve with ‘toe’
(too bright), linear and ‘shoulder’
(too dark) regions. The dynamic and dose ranges are narrow.
(Reproduced from Ludewig et al. (2012) Clinical technique digital radiography in exotic pets – important practical differences compared with traditional radiography. Journal of Exotic Pet Medicine 21 , 71–79, with permission from Elsevier)
1.8
1. at are t e fields o o eratio (exclusively small animal practice versus small AND large animal practice)
The combined usage requires higher detector mobility. Computed radiography (CR) and wireless direct radiography (DR) systems allow more flexibility than DR systems with a wired connection. owever, in small animal radiology horizontal beam views are occasionally required, which is only possible with a CR or wireless DR system and not with a permanently installed wired DR system in the Bucky tray.
2. o ou a t to i tegrate t e detector i a e isti g ra ta le This is generally easier with CR cassettes.
3. at are t e re uired i i u s ecificatio s i de e de t ro t e detector tec olog
Detector format 40 (35) x 40 cm2 Pixel pitch 200 m Detection quantum e ciency (D E) The higher the better 4. d fi all or
i direct
conversion direct conversion Image resolution
(lp mm) 2.5–5 ~3.5 ~3.5
Dynamic range 1 10,000 1 10,000 1 10,000
Image depth (bit) 10–12 12–16 12–16
D E ( ) 20–45 40–70 ~35
Image acquisition
(seconds) 20–40 5 5
Start-up costs Moderate igh igh
Resistant to damage Yes –
moderately No No
Factors influencing the choice of a digital detector.
1.9
• For areas where patient motion could cause
unsharpness (e.g. spine, pelvis), exposure time should not exceed 0.0 seconds
• For soft tissue presentation with screen–film systems, a reduction of the exposure setting may be necessary
• Digital systems with high DQE allow lower exposure settings than those with lower D E Figure 1.10 .
Image quality: fundamental technical considerations
Poor image quality is a common reason for misdiagnosis.
Therefore, the initial goal of a radiographic examination is
the production of images of adequate quality. The required level of image quality is achieved when the images contain the information necessary to distinguish and describe normal versus abnormal structures.
The design of an imaging protocol is influenced by the physical and anatomical characteristics of the area of interest. The musculoskeletal system exhibits a mixture of challenging and less challenging conditions:
• Owing to marked differences in X-ray absorption between bone and the adjacent soft tissues, favourable conditions exist to transform these absorption
differences into image contrast
• If absorption differences are low, such as between soft tissue and fat, the resulting low image contrast makes the identification of abnormalities challenging
• Gas accumulations and mineralized structures exhibit high absorption differences. Consequently, even small amounts can be seen
• Because no attenuation differences exist between soft tissues and fluid, changes cannot be identified unless there is a mass effect
• Some patients and the structures we aim to image within them are very small. High spatial resolution is mandatory to visualize subtle changes within these small structures.
The fundamental physical parameters characterizing image quality are spatial resolution, contrast, noise and artefacts.
Spatial resolution
This describes the ability of the system to distinguish or discriminate between neighbouring structures. Spatial resolution is correlated with the intrinsic sharpness of the detector employed. In musculoskeletal radiography, intrin- sic sharpness substantially determines the overall sharp- ness. Other factors are patient movement and subject contrast. The effects of these patient- or object-related factors on the final image are determined by beam quality, scattered radiation and the size of the focal spot.
The intrinsic sharpness of screen–film systems is dependent on the thickness of the screen layer and the size of the grains in the film emulsion. Lead line grids are used to measure the resolving power of radiography
60
40
20
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Spatial frequency (lp/mm) Computed radiography
cree fil s ste
Flat-panel detector
(direct conversion)
Flat-panel detector (indirect conversion)
DQE (%) 80
D E of various detectors. The D E describes how well a detector is able to transform radiation into image information. A detector that outputs the same signal-to- noise ratio as it receives as input will have a D E of 100 . If the output has a higher signal-to- noise ratio than the input, the D E is less than 100 . An ideal system has a D E of 100 for structures of any size. The D E of any real imaging system is always below 100 and D E generally decreases with increasing spatial frequency.
ence it becomes more di cult to maintain the incident signal-to-noise ratio at higher spatial frequencies (i.e. in regions of fine image detail).
(Data adapted from Neitzel, 2005;
reproduced from Ludewig et al. (2012) Clinical technique digital radiography in exotic pets – important practical differences compared with traditional radiography. Journal of Exotic Pet Medicine 21, 71–79, with permission from Elsevier)
1.10
systems. This parameter is expressed in units that describe the number of line pairs per millimetre (lp/mm) that can be resolved in a test or calibration image. This parameter is often quoted when describing imaging systems; the higher the number, the better the spatial resolution. In screen–film systems there is a negative correlation between spatial resolution and sensitivity (speed) (Figure 1.11).
Image sharpness for digital detectors is affected by several factors. The theoretically achievable resolution (cut-off frequency, Nyquist frequency, fmax) is limited by the distance between the centres of neighbouring pixels (pixel pitch, p) which is a measure of pixel size. It can be calculated using the equation:
fmax (lp/mm) = 1 2 x p
The pixel pitch of the majority of large-area detectors is in the range of 100 to 00 m. The resulting cut-off frequencies of 5–2.5 lp/mm are comparable with speed class 400 screen–film systems. Because the real resolving power of a detector system is determined not only by the capture element of the detector but also by coupling and collector elements, lead line grid measurements rather than calculated resolution are preferable as descriptors of detector resolution. The American College of Veterinary Radiology recommends that digital detectors should exceed 2.5 lp/mm. An alternative and superior parameter, the MTF, better characterizes how well a digital imaging system reproduces high-contrast structures of varying size (Figure 1.11).
Contrast
This is defined as a measure of the relative difference in brightness between two locations in an image. The char- acteristic curve can be used to characterize the contrast response of a radiographic system (see Figure 1.8).
Screen–film systems have a sigmoid-shaped curve with
‘toe’ (too bright), linear and ‘shoulder’ (too dark) regions.
Dynamic and dose ranges are narrow if exposures are kept (as they should be) within the linear response seg- ment of the screen–film system’s characteristic curve. In this region, the number of greyscale steps is low and film density is a direct indicator of patient dose.
Digital imaging detectors have a different characteristic curve. It has no ‘toe’ or ‘shoulder’ region, but rather is linear in response to a very large range of exposures. The large number of recorded X-ray absorption differences means that even small attenuation differences can be transformed into image contrast. The number of recorded attenuation differences is termed ‘image depth’. It varies among digital systems. Recent systems range from 10 bit
10 4 shades of grey to 1 bit , 3 shades of grey . The large response range allows wide dose ranges and, for this reason, digital systems ‘forgive’ many exposure faults.
Image retakes for reasons of incorrect exposure are very seldom required. These systems are, however, not immune to exposure error. Extreme underexposure leads to ‘noisy’
(grainy) images. Unnecessarily high exposure cannot be identified by viewing the image, because a digital detector does not set the limit, as film does, with respect to film blackening, and the risk of chronic overexposure is real.
Extreme overexposure results in detector saturation, in which the ability of the digital detectors to record at low dose differences becomes lost.
Noise
Noise produces random variations in signal that obscure useful information in an image. Noisy images appear mottled or pixellated. Noise arises from several sources.
Quantum noise results from the number of photons applied to produce an image. The interaction of X-rays with tissues is essentially a statistical effect. If too few photons are used, a random fluctuation of image intensity will be seen in areas that attenuate X-rays uniformly.
Electronic noise originates from the detector and is a con- stant parameter.
Image quality in digital radiography. The pelvic radiographs of a young Pug were acquired by the use of two different CR systems. Identical exposure settings were applied. There are significant differences in image quality. (a) The image is of poor, non-diagnostic quality. The grainy appearance, caused by insu cient detector performance, inadequate signal processing or a combination of these factors, hampers the evaluation of bone. (b) This CR system has adequate performance, allowing the evaluation of subtle changes such as the bony structure of the femoral head and neck.
1.12 (a)
(b)
Flat-panel detector (direct conversion)
Flat-panel detector (indirect conversion)
Computed radiography
Spatial frequency (lp/mm)
1.0
0.8
0.6
0.4
0.2
0
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
cree fil s ste
cree fil s ste
MTF of various detectors for unprocessed digital images. Modulation is a measure of contrast. MTF communicates how well a system
‘transfers’ contrast at ever-increasing levels of detail. The usual trend will be for loss of contrast between object and background with increasing levels of image detail (decreasing modulation with increasing spatial frequency). Ideally, the curve is a horizontal line at a modulation of 1 (equivalent to 100 retention of contrast). In reality, because of technical factors, there is a loss of contrast between objects and background, which is greater for smaller structures (higher spatial frequencies, higher lp mm) in comparison with larger structures (lower spatial frequencies, lower lp mm).
As a result, the MTF progressively decreases with increasing spatial frequency. The curves of digital systems end at the Nyquist frequency value determined by the detector element size. For comparison, MTF curves of screen–film systems of two speed classes (100, 400) are shown.
(Data adapted from Neitzel, 2005; reproduced from Ludewig et al. (2012) Clinical technique digital radiography in exotic pets – important practical differences compared with traditional radiography.
Journal of Exotic Pet Medicine 21, 71–79, with permission from Elsevier)
1.11
The level of electronic noise depends on the type of detector used. Therefore, noise can only be decreased by addressing the first of these sources, i.e. by using more photons (a higher dose) to create the image. The DQE describes how effectively a detector is able to transform radiation into image information. With identical exposure settings, a system with higher DQE produces a higher signal-to-noise ratio. The image, in comparison to a low DQE system, is less pixellated: detail visibility is superior.
The role of these various parameters differs between screen–film and digital radiography. In screen–film radio- graphy, image quality is contrast limited. In digital radiography, noise is the major limiting factor in object detection.
Artefacts
Both technologies are prone to artefacts. Artefacts in radiography can cause errors while reading radiographs by decreasing visualization or altering the appearance of structures of interest. Digital radiographs may have similar artefacts to screen–film artefacts, but they additionally have specific, technology-related artefacts (Figure 1.12).
Artefacts in digital radiography can be categorized according to the step of their creation, such as pre- exposure, exposure, post-exposure, reading and work- station artefacts. Figure 1.13 describes and illustrates common artefacts. More comprehensive overviews on this specific topic are given in the literature (e.g. Drost et al., 008; Jimene and Armbrust, 009 .
rte act a e a eara ce a d eli i atio
alo e ect ersc i ger arte act
Black halo or a dark band along metal implants or bone (mimics osteolysis of bone next to metal implants)
rigi processing error (edge-enhancing algorithms were applied)
li i atio reprocessing of image data with less or no edge-sharpening algorithms
oire atter aliasi g Series of repeating lines which change in appearance with changing image size
rigi error of signal recording (signal registration interacts with low-frequency antiscatter grid lines)
li i atio retake image using a higher-frequency antiscatter grid
ua tu ottle lo sig al to oise ratio N Pixellated, mottled radiograph
rigi underexposure – too few X-rays reached the detector. In relation to the given detector noise the added signal is too low (low SNR)li i atio retake image with higher exposure settings (monitor exposure settings by use of the dose indicator)
Detector saturation
Black areas on the radiograph primarily in thinner body part with lower attenuation (‘tissue disappeared’);
increasing brightness does not restore lost information
rigi overexposure – too many X-rays reached the detector. The detector became insensitive to additional input
li i atio retake image with lower exposure settings (monitor exposure settings by use of the dose indicator)
(a–h) Common artefacts in digital radiography. CR = computed radiography; DR = direct radiography; L T = look-up table.
1.13
rte act a e a eara ce a d eli i atio li i g
Black areas on the radiographs primarily in thinner body part with lower attenuation (‘tissue disappeared’); increasing brightness does not restore lost information
LUT errors
Image is displayed too bright or too dark
rigi processing error (inadequate L T). li i atio reprocessing of image data with an adjusted L T. Do not change exposure settings
ite s ots o i ages
White ‘creatures’ on the radiograph. rigi dirt, hair and other debris on the image plate, or scratches on the image plate. li i atio contaminants clean the image plate; scratches replace the image plate
ess oard atter o i ages
Rectangular fields of varying brightness fill the complete image format rigi poor detector calibration. li i atio recalibrate according to user manual
rigi processing error (wrong algorithm)
li i atio reprocessing of image data with an adapted algorithms (mostly increased latitude)
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
Basics of musculoskeletal radiography
Indications
There are numerous indications for musculoskeletal radio- graphy. Lameness is the major reason to take limb radiographs. Neurological disorders and spinal pain domi- nate as indications for spinal radiography. In comparison, the list of indications for skull radiographs is much longer.
The results of the clinical examination are funda- mental to determining the diagnostic information required and this should be specified as precisely as possible.
The questions that have to be answered by use of the radiographic examination have immediate impact on the radiographic technique that should be applied.
Positioning, centring and collimation
General considerations include:
• The clinical status of the patient must be considered before beginning the radiographic examination. If the animal is in distress it must first be stabilized
• Position and centring errors hamper the discrimination between normal and abnormal findings. Adequate training and daily practice, with attention to detail, are necessary to produce high quality radiographs consistently
• Positioning aids such as foam wedges, sandbags, ties and troughs should be used to their full advantage.
There is much space for creativity. Because they are needed for almost every patient they must be readily available next to the X-ray table (Figure 1.14)
• The primary X-ray beam should be centred on the area of interest. Radiographs of long bones should include the complete adjacent joints. Radiographs of joints must display the entire joint and part of the adjacent bones
• Collimation of the beam to the area of interest improves contrast and reduces dose
• The detection of subtle lesions is improved when radiographs from the contralateral limb or follow-up images are available.
estraint and patient preparation
Manual restraint
Radiation safety law in the UK specifies that manual restraint of small animals is only permitted in exceptional circumstances where restraint by other means is impos- sible. In other countries, the law allows small animals to be held by hand. However, laws across all jurisdictions demand that the dose for assisting personnel must be kept as low as reasonably achievable/practicable (the
‘ALARA/ALARP’ principle). The interpretation of what is
‘reasonable’ differs among countries and results in differ- ent practices. In countries where manual restraint is considered reasonable some general considerations for musculoskeletal radiography must be applied:
• Appropriate personal protection, such as lead aprons, lead gloves and thyroid shielding, should be used
• The distance from the radiation worker to the patient and the primary beam must be maximized to make use of the inverse square law (doubling the distance quarters the radiation exposure)
• Exposure settings – especially in digital radiography – must be as low as possible
• Personal dosimetric monitoring is necessary
• Entrance to designated areas where there is the potential to receive doses higher than background and above certain specified levels is restricted for certain groups of people (pregnant women, persons under the age of 18).
Physical restraint
Physical restraint, using positioning aids only, often leads to limited image quality. However, in some instances certain diagnostic questions can be answered sufficiently to make a diagnosis. Additional sedation (see below) improves posi- tioning significantly and makes the examin ation stress-free (Figure 1.15).
Chemical restraint
Sedation and anaesthesia are essential prerequisites to:
• Ensure consistent proper positioning
• Minimize unsharpness due to patient motion
• Reduce stress for the animal (and personnel)
• Avoid radiation exposure of personnel.
In comparison with radiographs taken of manually restrained animals, image retakes are less frequent.
If the animal is in pain, sedation will not occur unless analgesic drugs are also administered. Anaesthesia should also be considered in these cases, as well as for animals where the radiographic examination can be combined with other diagnostic procedures that require anaesthesia, or if surgery is to follow immediately after radiography.
There are numerous options for chemical restraint. The reader is referred to the BSAVA Manual of Canine and Feline Anaesthesia and Analgesia for appropriate protocols.
adiograp ic vie s
At least two orthogonal radiographs (taken at an angle of 90 degrees to each other are required for a basic evalu- ation. The standard views are:
• For radiography of the limbs, mediolateral and craniocaudal/caudocranial or dorsopalmar/
dorsoplantar views Positioning aids such as foam wedges, sandbags, tie-downs
and troughs are essential prerequisites to achieve good image quality. For practical reasons it is advisable to keep these aids next to the X-ray machine. A mobile table can be used to store the objects.
1.14
