Several rheumatic diseases are affecting blood vessels. Vascular ultrasound is relevant in rheumatology particularly for large-vessel vasculitis, Raynaud's phenomenon and venous thrombosis.
Flow velocities are higher in large vessels when compared to intra-synovial vessels. Colour Doppler, not power Doppler ultrasound should be performed for evaluating large vessel. Power Doppler is used for low blood flow velocities in order to increase the sensitivity for detecting signals. Colour signal sensitivity is usually not a problem in large vessels. Furthermore, colour Doppler ultrasound allows detecting stenoses by demonstrating a phenomenon called “aliasing”: Due to increased flow velocity in stenoses a mixture of colours appears.
B-mode and colour Doppler frequency needs to be adjusted according how deeply the vessel is localized below the skin surface. Pulse repetition frequency (PRF) needs to be adjusted according to the blood flow velocities in the vessel (Table). It needs to be increased in case of fast flow for preventing aliasing and impaired image quality. It needs to be decreased in case of low flow velocities in order to provide enough sensitivity for detecting colour signals.
In relatively small vessels like the temporal arteries the sonographer needs to make sure that the colour signal does not cover the wall when the colour gain is too high or that it only covers the centre of the lumen when the colour gain is too low. Gain settings need to be tested in normals before ultrasound is done for diagnosis of temporal arteritis.
The optimal angle between sound waves and the vessel is 45°. If these are perpendicular the colour quality becomes inadaequate. Ultrasound machines provide a steer function for the colour box that needs to be used in this case.
Conclusion Settings for large-vessel colour Doppler ultrasound include PRF, B mode frequency, colour frequency and colour gain. The colour gain needs to be adjusted so that the colour does not cover the vessel wall or, if too low, only covers the centre of the vessel lumen. The colour box needs to be steered in order to arrive at a good colour quality.
Disclosure of Interest W. Schmidt Grant/Research support from: GE, Esaote, Siemens
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