Chapter 20 Musculoskeletal ultrasound imaging

INTRODUCTION

Ultrasound imaging enables the clinician to look directly at the soft tissues of the body. The dynamic, real-time, moving image that is produced is ideal for evaluating the structure and behaviour of muscles, tendons and other soft tissues. Not only can ultrasound be used to evaluate pathological changes, it is also possible to analyse muscle contractions, the movement of tendons and joints, and their effect on surrounding structures.

The relatively low cost and safety (owing to the absence of ionizing radiation) of ultrasound scanners make them a practical tool in the clinical setting and, unsurprisingly, they are no longer the preserve of radiology and maternity departments. For many clinicians, such as cardiologists, anaesthetists and surgeons, using ultrasound is now seen as a natural extension of their clinical examination in much the same way as the stethoscope.

The use of ultrasound imaging in physiotherapy is still in its infancy but has already developed applications beyond those traditionally performed in radiology. Monitoring response to treatment, along with evaluating function and the dynamic interaction between structures, is often of more relevance to the therapist. This chapter introduces the reader to ultrasound imaging and highlights those applications that might be relevant to physiotherapy practice.

HISTORY

The use of ultrasound to image the human body began shortly after the Second World War, using technology borrowed from military and metallurgy applications. Early studies required the subject to be immersed in a water bath, and produced a single, static, cross-sectional image.

By the early 1980s, a moving image could be produced. This was acquired by applying a small probe to the skin surface, which allowed quick and straightforward examinations of many of the organs of the body. At around this time CT and MRI also became available as alternative methods of producing cross-sectional images. Each modality was found to have its own strengths and weaknesses and, despite rapid developments over the past 20 years, they remain complementary, rather than rival approaches to imaging.

Musculoskeletal ultrasound applications began to emerge in the mid-1980s (Crass et al 1984) as the resolution of scanners improved. Physiotherapy researchers found ultrasound to be ideal for the direct visualization and evaluation of both muscle size and activity (Stokes&Young 1986). By the late 1990s, the development of high-frequency probes, with resolution in some respects superior to MRI, had led to renewed interest in musculoskeletal ultrasound. This continues today as falling costs and increased portability have made high-resolution ultrasound imaging feasible in the clinic and on the ward.

Ultrasound continues to evolve, with numer-ous recent innovations including the use of contrast agents, software that can measure the elasticity of tissue and three-dimensional (3D) and four-dimensional (4D) imaging. These will lead to new insights and applications, which will enable clinicians to further refine the accuracy and effectiveness of diagnoses and therapeutic interventions.

PRINCIPLES

GREY SCALE

When a sound wave passes through the body, a small percentage of the wave’s energy (typically less than 1%) is reflected at each boundary that is encountered. The ultrasound image is acquired using a probe that generates a series of short, tightly focused pulses. These pass through the body and the sound waves reflected back to the probe by successive boundaries are recorded. Each pulse produces a single line of data, and a single image may be made up over a hundred such lines.

With each image taking as little as 1/100th of a second to construct, it can be updated continuously, giving a real time cinematic view of the body that allows even fast-moving structures such as the heart, active muscles, or even small children to be imaged.

SCANNERS

Scanners are typically made up of one or more hand-held probes attached via flexible cables to a central unit, which processes and displays the image (Fig. 20.1). The probes operate at one or more fundamental frequencies, which determine the maximum depth and resolution of the images they generate. At higher frequencies, the shorter wavelengths mean that shorter pulses can be produced, giving better resolution. At lower frequencies, the resolution is reduced but the ultrasound is absorbed less readily as it passes through the body, enabling deeper structures to be examined.

Figure 20.1 Portable ultrasound scanner (A) and probes (B).

(courtesy of Sonosite)

The most commonly used probes are the linear and curvilinear probes (Fig. 20.1B). Linear probes are normally designed to operate at frequencies of 7.5 MHz and above; these produce high-resolution, rectangular images to a depth of around 5 cm. Curvilinear probes generate a fan-shaped image with a wider field of view than a linear probe. They operate at lower frequencies, typically between 2 and 5 MHz, and can produce images to a depth of up to 15 cm but at the cost of lower resolution.

A detailed discussion of resolution and image generation lies beyond the scope of this chapter and the reader will find more comprehensive descriptions in dedicated ultrasound texts, in particular Hedrick et al (2005).