Guidelines for musculoskeletal ultrasound in rheumatology.

1. Introduction Within the past decade, musculoskeletal ultra-sound (US) has become an established imaging technique for the diagnosis and follow up of patients with rheumatic diseases. 1–5 This has been made possible through technological improvements, resulting in faster computers and higher frequency transducers. US is most commonly used in the assessment of soft tissue disease or detection of fluid collection and can also be used to visualise other structures, such as cartilage and bone surfaces. 6 7 Owing to the better axial and lateral resolution of US, even minute bone surface abnormalities may be depicted. Thus destructive and/or reparative/ hypertrophic changes on the bone surface may be seen before they are apparent on plain x rays or even magnetic resonance imaging. 8 However , US wave frequencies cannot penetrate into bone, therefore imaging of intra-articular disease is usually not possible. The " real time " capability of US allows dynamic assessment of joint and tendon movements, which can often aid the detection of structural abnormalities. Advantages of US include its non-invasiveness, portability, relative inexpensiveness, lack of ionising radiation, and its ability to be repeated as often as necessary, making it particularly useful for the monitoring of treatment. US can also be used for guidance of aspiration, biopsy, and injection treatment. 9 Most musculoskeletal work is performed using " grey scale " , which means images are produced in a black and white format; each white dot in the image represents a reflected sound wave. Sound waves travel in a similar way to light waves and therefore the denser a material is—for example, bone cortex, the more reflective it is and the whiter it appears on the screen. Water is the least reflective body material and therefore appears as black as the sound waves travel straight through it. Newer US techniques, which are currently being evaluated, include colour and power Doppler imaging, which provide colour maps of tissues. Here the amount of colour is related to the degree of blood flow, which may be of use in assessment of vascular tissues as may occur in soft tissue inflammation. 10 To increase further the sensitivity of power Doppler intravenous bubble contrast agents are under development.