Settings and artefacts relevant in colour/power Doppler ultrasound in rheumatology

Abstract

The paper explains the most important parameters for the use of colour and power Doppler in rheumatology. Recommendations for machine settings are given. The commonly encountered artefacts and their importance for image interpretation are explained.Most musculoskeletal ultrasound (US) is performed using grey-scale US, but newer US techniques include the use of Doppler US in the assessment of changes in tissue vascularisation that may occur in inflammatory conditions. 1-3 The Doppler evaluation provides useful clinical information regarding the presence or absence of flow. Guidelines have been suggested by the European League Against Rheumatism (EULAR) work group for the use of grey-scale US in musculoskeletal disease. 4 The guidelines address technical issues, training and standardisation of image acquisitions. However, no such guidelines exist for Doppler US. Standardisations of the methods for evaluating inflammation and the effect of the quality of the machine and image processing still need to be established.Correct interpretation of flow images requires knowledge of physical and technical factors that influence the Doppler signal. Artefacts caused by physical limitations of the modality or inappropriate equipment settings may result in displayed flow conditions that may differ considerably from the actual physiological situation. As a consequence, artefacts in Doppler imaging may be confusing and lead to misinterpretation of flow information.In this paper, we review colour Doppler and power Doppler (PD) US, as well as the artefacts and settings relevant for musculoskeletal US in rheumatology with focus only on soft tissue and joint inflammation. DOPPLER SIGNALThe Doppler effect is a change in wavelength (frequency) of sound resulting from motion of a source, receiver or reflector. As the US transducer is a stationary source and receiver, the Doppler effect arises from reflectors in motion-for all practical purposes these are the erythrocytes. When a pulse is reflected from erythrocytes, the frequency of the wave received differs from that, which is transmitted. This difference is known as the Doppler shift, named after the Austrian physicist and mathematician Chr. Andreas Doppler, who first described the phenomenon for light in 1843. 5There are two successive Doppler shifts involved. First, the sound from the stationary transmitting transducer is received by the moving erythrocytes. Second, the erythrocytes act as moving sources as they re-eradiate the US back toward the transducer, which is now the stationary receiver. These two Doppler shifts account for factor 2 in the Doppler equation:where: f D is the Doppler shift, f t is the transmitted frequency, f r is the received frequency, v is the blood velocity, h is the insonation angle (the angle between the US beam and the blood flow), and c is the speed of sound. The Doppler shift is thus directly proportional to the velocity of the flow, v, cosine to the insonation angle, h, and the transmitted frequency of the US, f t . 6 Pulsed DopplerThe Dopple...

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