Chapter 1
General principles

INTRODUCTION

There are many books that describe the principles of radiography. This book does not attempt to provide detailed information in this area, and readers who do not have a working knowledge of radiography are advised to consult one of the standard texts in order to obtain the necessary understanding of radiographic physics. This book does aim to provide up-to-date information specific to the horse. As various forms of competitive and pleasure riding have become more popular, the demand on veterinary surgeons to provide the highest quality of treatment has increased. Similarly radiography of the horse in sickness as well as in health, for insurance and purchase examinations, has increased. The book is intended for all who radiograph horses and read radiographs, be they equine specialist, general practitioner or student. It gives information on equipment required, radiographic techniques, positioning, and the views required to examine the various areas of the horse adequately. It also provides information on the normal radiographic anatomy of the immature and skeletally mature horse, variations, and incidental findings. Finally it gives information on the types of lesion that may be detected, with examples of as many of the more common problems as practical, as well as brief clinical remarks where appropriate. The 'Further reading' lists at the end of each chapter are not intended to be complete lists of every paper written on the subject of the chapter. They list references that the authors consider of particular interest, and that are complementary to the text. Many of these references give more detailed information in specific areas than can be justified in a textbook of this type.

Interpreting the clinical significance of radiological changes is always difficult. We set out to indicate certain lesions that may always be regarded as clinically significant, and some that are known to have no clinical significance. The section in each chapter on 'Normal variation and incidental findings' attempts to differentiate between variations that have no clinical significance at any time (e.g. radiolucent lines in the fibula, that represent remnants of separate centres of ossification) and those that may be clinically significant for a specific but limited period of time, and therefore require further clinical investigation to determine their significance (e.g. entheseophyte formation). The radiograph is only a reflection of the state of the tissues at the fraction of a second when they were radiographed. There are many findings which indicate a past event that has 'left its mark', but which is no longer clinically significant. For example, entheseophyte formation at the insertion of a ligament may indicate a sprain to that ligament at some time in the past. As entheseophytes take time to form, once they are visible on radiographs they no longer represent an acute injury, but are the result of an incident that occurred at least several weeks previously; on the other hand, their radiographic appearance might be used to approximate their age.

Radiography is a continually developing science, and as more powerful and sophisticated equipment becomes generally available, the diagnostic possibilities for veterinary practitioners become ever greater. It is hoped that this book will enable veterinarians to get the best out of their equipment, to obtain diagnostic radiographs, and to give a correct and meaningful diagnosis from the radiographs. The information in the text has been collated from the literature where possible, and complemented by the authors' experience. In some areas, however, there is no published work, or published information is contradictory. In these circumstances the authors have relied on their own collective experience, but have only presented information if all the authors are in agreement. (For example, reported physeal closure times for some physes vary widely between texts. The times given are based on the authors' experience of radiographic closure, in some cases backed up by radiographic examinations of animals specifically to aid completion of this text.) The authors are experienced clinicians who routinely obtain and read equine radiographs, and it is hoped that the broad range of experience that they offer to the reader will prove to be of practical value. It is important to remember that, as radiography is a developing science, 'new' lesions and radiographic views are continually being found and described, and no text can hope to be complete when published, let alone as time progresses.

This text has made use of current terminology. Nomina Anatomica Veterinaria (5th edition, 2005) was consulted for anatomical terms and names. In some instances we refer first to the correct nomenclature, but make subsequent reference to the more commonly used colloquial name (e.g. distal sesamoid bone and navicular bone). It should be noted that long bones have cortices and a myeloid cavity (the medulla), sesamoid bones and short bones (e.g. the central and third tarsal bones) have compact bone and spongiosa. Radiographic views are described using the method advocated by the American College of Veterinary Radiologists, which first describes where the x-ray beam originates relative to the horse (e.g. dorsolateral), then where the beam is directed to (e.g. palmaromedial) (i.e. dorsolateral-palmaromedial oblique). Reference to Figure 1.1 may help to elucidate the current terminology used. While at first sight this may appear cumbersome, it does provide a specific description of the views, which allows them to be reproduced accurately. Terminology in common usage is included in parentheses and serves only to maintain continuity with other texts and references. A glossary (Appendix C) is also included and lists former and current scientific terminology as well as common lay terms.

Figure 1.1 Correct nomenclature to describe various aspects of the horse.

We have not set out to provide radiographs of every variation of all lesions. Rather we have given typical examples of lesions, and in the text have indicated how these may vary. We also hope that the reader will use this text as a basis to understand why certain types of radiographic lesions form, and the processes that are likely to cause them, so that an inexhaustible supply of radiographic variations would be superfluous. Although we have done our utmost to find radiographs that reproduce well, we ask the reader to remember that inevitably some detail is lost in the process of transferring radiographs to print, however all images can be viewed on the website, and this also provides additional images that are not present in the printed version.

PRINCIPLES OF RADIOGRAPHY

The following paragraphs serve only as a reintroduction to the subjects of image production and differentiation. For more detailed information the reader is referred to the standard radiography texts. It is important that any radiograph is of maximum quality and yields sufficient detail to allow subtle radiographic lesions to be detected.

Production of x-rays

An x-ray beam consists of high-energy electromagnetic radiation. It is produced by accelerating a beam of electrons into a tungsten target. This results in the production of a beam of x-rays, and the liberation of considerable energy as heat. A small target area produces a narrower beam of x-rays, and better definition on the resultant radiograph than a larger target area. The area of the target struck by electrons is called the 'focal spot'. Ninety-nine percent of the energy from the electron beam is given off as heat, not x-rays, and so there is a risk of the target being melted. Dissipating this heat and keeping the target as small as possible are major factors in design of x-ray tubes. For generators with a large output, the target in the tube is the edge of a disc. By rotating the disc at very high speeds during x-ray production, the area being heated is continually being changed, allowing a small focal spot in spite of high output. This is standard in large static x-ray generators. Smaller mobile or portable generators generally have fixed targets, which does limit the output possible. Any x-ray beam is made up of photons of mixed wavelengths. The older half- and full-wave rectification in small x-ray generators resulted in very marked variations in the energy of the individual photons of the x-ray beam. The high-frequency generators currently available have greatly improved the consistency of the x-ray beam produced, causing less scatter and a better resultant image.

Production of a radiographic image

An image is created by detecting the differential absorption of x-rays that pass through an object placed in the path of the primary x-ray beam. The x-rays that pass right through the object are either detected using conventional x-ray film, or digital images are created (see Chapter 2). The number of x-rays that are absorbed by a given thickness of a specific tissue varies between tissues, and thus affects the number of x-rays passing through to form the image. For example it is more difficult to penetrate bone than air, and therefore less x-rays reach the film if they have to penetrate bone rather than air. The areas of the image relating to relatively unobstructed x-rays are black, whereas the areas protected by bone, which absorbs or deflects a...