1 Principles of ultrasonographic imaging of the bovine musculoskeletal system

Sébastien Buczinski, Isabelle Masseau

1.1 Introduction

Ultrasonography is an imaging technique based on the reflection and refraction of acoustic waves as they are transmitted through the tissues (Kirberger 1995). In veterinary medicine, it was initially applied to the diagnosis of pregnancy, to assess reproductive organs prior to insemination or in an attempt to determine causes of failure to induce pregnancy in cattle. Its affordable cost and ease of use have contributed to its popularity and explain that today many veterinary practitioners are equipped with an ultrasound machine dedicated to cattle reproduction management programs (King 2006, DesCôteaux et al. 2009, Fricke et al. 2016).

In parallel with the development and sophistication of ultrasonographic examinations in the field of reproduction, a number of clinical conditions have emerged for which ultrasonography has been evaluated for its potential aid as a complementary imaging diagnostic tool. Over time, numerous research studies and growing expertise have resulted in diversification of ultrasound use in cattle leading to the recognition of its diagnostic utility for various indications, including examinations of musculoskeletal structures in cases of lameness, joint instability or penetrating wounds, among others (Flückiger 1997, Buczinski 2009a, Kofler 2009, Braun and Attiger 2016, Re et al. 2016b).

Ultrasonographic evaluation of musculoskeletal structures is facilitated by the superficial location of a majority of them. Consequently, most rectal probes (transducers) employed today for ultrasonography of the reproductive system can also be utilized for the evaluation of musculoskeletal structures. Since most practitioners are already equipped with ultrasound units, they do not have to pay additional costs for acquisition of new probes. Another important advantage of ultrasonography is its portability, allowing for musculoskeletal examinations to be performed directly on the farm (Ollivett and Buczinski 2016).

Like any other diagnostic imaging tool, it is important to understand the physical principles responsible for generating ultrasound images and commonly encountered artifacts (Kirberger 1995, Blond and Buczinski 2009). Understanding how artifacts occur can help their avoidance whenever possible or to use them advantageously to document the nature of the tissues from which they originate (e. g. gas in an abscess, osteophytes, dystrophic mineralization within a tendon, etc.). A few parameter settings that optimize image quality will also be briefly discussed. Therefore, the aim of this introductory chapter is to provide the reader with a brief overview of these important topics.

1.2 Physics and acoustic principles

Ultrasound consists of high frequency vibrations generated by the crystals within a probe. When subjected to an electric field, the crystals inside the probe become excited, which triggers a movement or vibration, generating the emission of the ultrasound wave. This phenomenon is based on the inverse piezo-electric effect of certain materials. The speed at which transmitted ultrasound waves are propagated through a structure of interest varies according to the type of medium.

The speed of ultrasound waves through soft tissues is generally constant at approximately 1,540 m/s (Blond and Buczinski 2009).

A wave can be transmitted through a medium, as well as reflected, refracted and attenuated. Other types of effects such as diffraction, polarization, dispersion and interference can also occur.

The interference effect mentioned above is of particular interest for ultrasound examinations that are performed in the proximity of other wave-generating materials or electronic devices, such as ventilation fans in a barn (Kirberger 1995, Blond and Buczinski 2009, Hindi et al. 2013).

A transducer (probe) emits ultrasound waves for only a very small fraction of the time (< 0.1 %). The remaining time (99.9 %) is devoted to reception of ultrasound echoes reflected back to the probe from tissues. This returning signal will then be converted electronically to form an ultrasound image (sonogram). As a general concept, the time interval between the emission of ultrasound waves and their return as echoes is used to estimate the depth of a specific structure. Information derived from returning echoes and their depth estimation is converted into different shades of white/grey pixels over a black background, generating an image that can be displayed on an ultrasound monitor.

Tissues commonly encountered during ultrasonography of the musculoskeletal system include articular components (capsule, synovial cavities, articular cartilage, menisci), tendons, muscles, ligaments and bones. Although most of these tissues are considered to comprise "soft tissues", with the exception of bones, they have slightly different acoustic properties that will in turn influence the speed of propagation of ultrasound waves and the behavior of these waves as they travel through different types of media. Fig. 1-1 summarizes the basic principles of ultrasound propagation within a tissue consisting of two different media (ex: muscle/ tendon interface).

1.2.1 Specular reflection

Specular reflection is defined as the mechanism by which ultrasound waves, after encountering a smooth surface, return back to the probe in one direction (Hindi et al. 2013). Indeed, when the incidence of the ultrasound beam strikes a surface with an angle other than perpendicular, the waves can then be reflected with a similar angle (a), but in an opposite direction ( Fig. 1-2). The probe would, in turn, not receive any echoes and therefore, no image would be obtained. When a reflected wave actually reaches the probe, then the image of this point will be falsely represented due to the angles of reflection. Reflection only occurs when ultrasound waves reach an interface between two tissues with different acoustic characteristics (or impedance [Z]). Each tissue is characterized by a unique impedance measured in Rayl (for Dr. Rayleigh) equivalent to a unit in kg/(s × m2) (Bushberg et al. 2012). Tab. 1-1 summarizes the impedance of musculoskeletal tissues of interest examined with ultrasound (Sanches et al. 2012).

Fig. 1-1 Schematic image of ultrasound propagation characteristics: When a probe is applied over an interface between two tissues of different acoustic impedances, such as a muscle-tendon interface, the ultrasound waves emitted by the transducer strike the interface at an angle alpha (a). Since the impedance difference (z) between these two media is very small, a portion of the emitted ultrasound waves is reflected back to the probe at the same angle as the incident angle. A significant part of the waves is transmitted within the tendon at a refraction angle beta (ß). Scattering generally occurs when ultrasound waves strike a diffuse reflector such as blood cells or an irregular organ surface.

Fig. 1-2 Specular reflection associated with the bone/soft tissue interface: Transrectal sonogram of the lumbosacral joint of an adult Holstein cow with schematic interpretation. The ventral borders of both vertebrae are represented by the hyperechoic lines (white arrows). When ultrasound waves strike the soft tissue/bone interface, the high difference in impedance between the two tissues results in their reflection back to the probe. Consequently, there is no information from the deeper parts of the vertebrae and no image can be obtained distal to the vertebral hyperechoic borders. The intervertebral disc space and joint are illustrated (white stars).

Tab. 1-1 Impedance of tissues encountered in musculoskeletal Ultrasound

* The impedance values have been reproduced from human references (Sanches et al. 2012).

1.2.2 Diffuse reflection (scattering)

In contrast to specular reflection, diffuse reflection (scattering) occurs when ultrasound waves strike irregular or "rough" surfaces, allowing low amplitude reflection in multiple directions. This type of reflection also leads to attenuation of the ultrasound waves that are transmitted deeper into the tissues.

1.2.3 Attenuation

Attenuation of ultrasound waves, with reflection and refraction, constitutes an important component of image generation in ultrasonography. It is defined as a decrease in the amplitude of the ultrasound beam as it travels through a medium. Attenuation is influenced by absorption of wave energy by the tissue, and therefore varies according to the nature of the tissue. Since attenuation is positively correlated with frequency, high frequency probes will generate higher attenuation and hence permit a lesser maximal depth of examination than low frequency probes. Further, for the same frequency, ultrasound attenuation is lower for liquids (e. g. blood, synovial fluid) than for muscles or...