Ultrasound Imaging of Joint Disease
Ultrasound machines with high-resolution probes are readily available in most radiology departments and are routinely used to assess both articular and periarticular disorders. They are also becoming commonplace in rheumatology departments, reflecting the important role they now play in rheumatological disease. Ultrasound has many advantages over other imaging techniques with the ability to carry out rapid assessment of multiple joints at different locations, undertake dynamic imaging and guide diagnostic and therapeutic injections. This chapter focuses on the role of ultrasound in joint disease and particularly rheumatological conditions, the technical aspects of joint ultrasound examination and the imaging findings.
The development of higher-frequency transducers has allowed for improved resolution and, with the majority of joints lying relatively superficially, linear array probes of frequencies of 10 MHz or higher can be effectively utilized. Curvilinear probes, although rarely required for musculoskeletal imaging, may be useful for examining deeper joints such as the hip joint. Although transducer selection primarily depends upon the frequency, the probe footprint (the surface area of the transducer in contact with the skin) should be considered. Small footprint probes can be easily manoeuvred to image small superficial structures, small joints and at bony prominences such as the malleoli, where the skin surface does not allow adequate contact with larger probes. Stand-off gel pads can prove useful to reduce the amount of near-field reverberation when examining superficial structures; however, with modern probes these are rarely necessary and the use of liberal amounts of ultrasound jelly is usually all that may be required in practice.
Many of the pitfalls and limitations of ultrasound are dealt with elsewhere in this book. They include anisotropy and beam edge artifact. However, when undertaking ultrasound of joint disease certain specific pitfalls should be considered.
If it proves difficult to maintain probe contact with the skin surface using minimal probe pressure, a thick application of sonographic jelly may help.
Disadvantages compared to MRI include a small field of view and difficulty in demonstrating cartilage and deep joints in their entirety. Contrast-enhanced MRI provides a better measure of capillary permeability and enhancement characteristics, although the advent of 3-D ultrasound may narrow this gap. At present, both ultrasound and MRI are increasingly important in the diagnosis and management of early rheumatoid arthritis with no current clear winner as the imaging modality of choice.
Expertise is important in the interpretation of both ultrasound and MRI; however, unlike with MRI, reevaluation of ultrasound requires the patient to be recalled. Thus standardization of ultrasound criteria and validation of training both of the radiologist and the rheumatologist who perform these studies are paramount.
While an all-inclusive examination of the small joints may be desirable, this is daunting and time consuming and can be modified by omitting joints that are frequently uninvolved, such as the distal interphalangeal joints (IPJs) and thumbs in rheumatoid arthritis.
It is the authors’ routine to examine the index, middle, ring and little fingers, although this may be adapted for specific clinical indications. Superficial structures such as the tendon and tendon sheath are assessed prior to the joint itself, where standard sagittal images form the basis of the examination, with axial (metacarpophalangeal joint, MCPJ) and coronal and axial (proximal interphalangeal joint, PIPJ) images used as adjuncts.
Routine examination of the extensor aspect of the MCPJs, followed by the extensor, ulnar and radial aspects of the IPJs, is performed.
Coronal images of the IPJs are obtained by asking the patient to hyperextend the metacarpophalangeal of the finger being examined.
When examining the extensor surfaces of the finger joints, it is commonplace to assess the dorsal aspects of the wrist and associated tendons prior to turning to the palmar side. Whether both the extensor and flexor sides need to be examined remains debatable; however, published literature suggests that a significant proportion of synovitis would be overlooked if limited to one or the other, and it is the authors’ practice to examine the flexor aspects of the MCP and proximal IPJs at this time. Dynamic sonographic assessment by moving the joint can be useful to facilitate the detection of low-volume synovial thickening, which bunches up in the proximal extensor recess on flexion. Articular cartilage over the metacarpal and phalangeal heads can be more comprehensively demonstrated when the joint is examined in flexion as well as extension.
It is essential to appreciate the normal sonographic anatomy of the small joints to be able to identify pathology (Fig. 32.1). Superficial and deep flexor tendons can be identified as they pass over the MCPJs into the flexor tendon sheath of the fingers on the volar aspect of the joints. Dynamic assessment with finger movement can help identify them individually. The tendons are maintained in place by pulleys, seen as thin hypoechoic linear structures; the pulleys and other aspects of tendon pathology are discussed in Chapter 15.
Figure 32.1 Small joint anatomy. (A, B) Sagittal view of the flexor aspect of the MCPJ. The flexor tendons lie anterior to the joint capsule. The volar plate, articular cartilage (*) and A1 pulley (arrow) are visible. (C, D) Sagittal view of the extensor surface of the middle MCPJ. The normal articular cartilage (*) is seen clearly. There is a small amount of joint fluid present that can be seen in the dorsal joint recess (arrows).
Several connective tissue structures such as the collateral ligament, accessory collateral ligament and the volar plate strengthen the flexor side of the MCPJs and IPJs and can be identified on ultrasound. The proximal recess of the joint is the area between the volar aspect of the metacarpal neck and the joint capsule and contains intracapsular, but extrasynovial, fat, allowing close approximation of the two layers of synovium.
It is important not to misdiagnose intracapsular fat as synovial thickening, particularly as the proximal recess is where early and prominent synovial thickening may occur.
On the extensor surface of the joint the extensor tendon complex is identified. The joint line is evident and articular cartilage may be seen, especially over the metacarpal heads. An important recess to the MCP and IP joints is found over the dorsal aspect of the metacarpal or phalanx on the proximal side of the joint that may contain synovitis or fluid and should not be mistaken for a bursa or tenosynovitis.
Absolute measurements of normal joints remain undefined and most authors use the point of maximal joint distension for assessment of the joint; an increase in joint dimension of more than 1 mm above normal is sufficient to suggest abnormality.
Ultrasound can be used to assess involvement in areas that are clinically occult as well as determine the precise structures involved. Serial examinations can assess current activity and disease distribution, and monitor progression or therapeutic response.