Introduction

Kimberly Palgrave1 and Jessica A. Kidd2

1Overland Animal Hospital, Denver, CO, USA; 2Oxfordshire, UK

Welcome to the first edition of the Atlas of Equine Ultrasonography.

The field of veterinary ultrasonography has blossomed in the last 30 years and the improvements in technology since its first use have been exponential. It is also now being used on structures and body systems that were not previously thought to be conducive to ultrasonography. Many vets in equine practice now have access to an ultrasound machine and, along with radiography, ultrasonography has become a mainstay of equine diagnostic imaging. It has the advantages of being non-invasive and complementary to radiography. The purpose of this book is to encourage further use of ultrasonography in clinical case management and expansion of the techniques utilized by vets in both general practice and at the referral level.

Ultrasonography is an excellent diagnostic tool which has many applications in veterinary practice. When considered in conjunction with relevant clinical information, such as patient history and physical examination findings, it can be an extremely useful aid in the clinical decision-making process. Developing the skills necessary to confidently acquire and interpret ultrasound images requires knowledge of normal equine anatomy and an understanding of the mechanisms displayed by individual body systems when reacting to various disease processes. We hope this book will help achieve this. A general appreciation of the physics of ultrasound is also necessary as this enables the ultrasonographer to optimize the diagnostic quality of ultrasound images obtained by appropriately altering their technique and machine settings. The aim of this introductory chapter is to provide an overview of ultrasound technology and the fundamental principles of image evaluation with a focus on the applications of ultrasound within equine practice.

How to Use This Book

The book is divided into three main sections: musculoskeletal, reproduction, and internal medicine. Each section is then further subdivided into chapters by anatomical region. Within each chapter is information on scanning technique for that area, a review of the normal anatomy and discussion of some of the more common ultrasonographic abnormalities. This is then followed by images that demonstrate normal and abnormal findings. The end of each chapter lists Recommended Reading for more extensive references relating to the chapter topics.

Physics of Ultrasound

Ultrasound physics is a vast and theoretically complex subject; however, a thorough understanding of this topic is not required for performing and utilizing diagnostic ultrasonography effectively in the clinical environment. Therefore, this text will cover the aspects of ultrasound physics that directly relate to the interaction of ultrasound waves with tissue and how these interactions translate to the displayed image. Additional sources covering this subject matter in greater detail are listed at the end of this chapter under Recommended Reading.

Features of Ultrasound Waves

Ultrasound waves have features in common with audible sound waves although they are of a higher frequency than audible sound and cannot be heard by the human ear; hence the term ultrasound. They are both created through the vibration of an object resulting in movement of surrounding molecules. Ultrasound waves are produced through the application of an electric current to piezoelectric crystals within the transducer (probe), causing the crystals to vibrate. This vibration is transmitted through the surrounding tissues in the form of sound waves. These waves interact with the tissues along their path of travel in various ways which may result in attenuation of the ultrasound beam.

Attenuation is defined as the progressive weakening in intensity of the ultrasound wave as it is transmitted through body tissues. Sound waves passing through tissues can either be reflected, refracted, scattered or absorbed. These are the primary causes of attenuation of the ultrasound wave and these phenomena are ultimately responsible for the formation of an ultrasound image.

Reflection and Acoustic Impedance

As ultrasound waves travel through the body, they come into contact with structures which reflect a proportion of the waves directly back towards the piezoelectric crystals, while the remainder of the waves continue to travel deeper into the tissues. The force of the returning waves or echoes results in vibration of the crystals and this vibration is then translated into an electrical signal, which is used to create the image displayed on the screen. Therefore, a unique feature of piezoelectric crystals is that they are capable of both emitting and receiving ultrasound waves.

It is important to realize that an ultrasound image is only produced when ultrasound waves are reflected back to the transducer. Reflection occurs when an ultrasound wave reaches an interface between two tissues as it is transmitted through the body and a portion of that wave is returned or “bounced back” to the probe while the remainder of the wave continues to travel deeper into the body. The strength of the returning wave and the length of time taken for that wave to travel through the tissues before returning to the probe is recognized and processed by the ultrasound machine in order to create the ultrasound image. These concepts will be later explored in the B-Mode and Echogenicity section.

The proportion of the emitted wave that is reflected back to the probe depends on the acoustic impedance of the interface between tissues and the angle at which the ultrasound wave strikes the interface. The acoustic impedance of a tissue is a product of the density of that tissue and the speed at which sounds waves travel through it. A dense tissue, such as bone, has a high acoustic impedance (7.8) compared to the relatively low acoustic impedance of air (0.0004), with soft tissues being in between (kidney – 1.62) (see Table I.1). However, it is the difference in acoustic impedance between tissue types that determines the reflective nature of a given tissue interface, not the acoustic impedance of a single tissue in isolation. For example, both a bone–soft tissue interface and an air–soft tissue interface have significant differences between the acoustic impedance values of the tissues at that interface, despite the fact that bone and air are at opposite ends of the acoustic impedance spectrum. Therefore, both bone–soft tissue and air–soft tissue interfaces are highly reflective, with the majority of the ultrasound waves being reflected back to the transducer in both scenarios. This also results in very little of the ultrasound wave remaining to penetrate into the deeper tissues beyond this highly reflective interface. By comparison, soft tissue–soft tissue interfaces (either between or within soft tissue structures) are less reflective due to the small differences between the acoustic impedances of these tissue types. Understanding this physical property of ultrasound wave propagation is essential to understanding how tissue variations translate into the ultrasound image appearance. This also justifies the need for appropriate patient preparation, including clipping of the haircoat where possible and application of ultrasound coupling gel to minimize the amount of air at the probe–skin interface. Differences in acoustic impedance also contribute to artifact formation, which will be discussed later in the chapter.

Table I.1    Approximate acoustic impedance in commonly encountered tissues. (Source: Adapted from Curry, TS III et al., 1990. Reproduced with permission of Lippincott Williams & Wilkins.)

The angle at which the ultrasound beam strikes the tissues is also integral to the degree of reflection of the ultrasound wave. Only ultrasound waves striking an interface which is perpendicular to the direction in which the wave is travelling will result in reflection of the wave directly back to the probe. If the wave reaches the tissue interface at an angle, the waves will be reflected into adjacent tissues instead of back to the probe, resulting in a lack of direct information from that area of the body. Therefore, the true reflective nature of that particular tissue will not be accurately represented in the displayed ultrasound image. In practical terms, imaging a structure when the ultrasound beam is not directed perpendicular to the region of interest may result in a smaller proportion of the wave being reflected to the probe and the resulting ultrasound appearance of that structure may appear “patchy” or irregular, although this effect can be used to the ultrasonographer's advantage, by allowing the margins of structures to be more easily recognized, for example.

Refraction, Scatter, and Absorption

In addition to reflection, other types of interaction between the ultrasound beam and...