Doppler ultrasound allows the visualization of perfusion in the inflamed synovium and therefore allows the diagnosis and quantification of inflammatory activity in a joint, tendon or enthesis. The issue when choosing Doppler modality in a rheumatological setting is the sensitivity for slow flow. This may be colour of power Doppler depending on the machine. Always choose the Doppler modality that provides the most flow information.
The preset on the ultrasound machines are not per se set optimally for slow flow. The most important parameters to adjust are frequency, gain, PRF and thereby the filters, focus and colour priority.
Doppler frequency behaves as the grey-scale frequency. High Doppler frequency gives better resolution on the expense of penetration. Gain determines the sensitivity of the system to slow flow and is adjusted by increasing the gain until noise pixels are present in the image and then gradually lowering the gain until only very few noise pixels are seen in the image. By lowering the gain further noise and motion artefacts may be prevented but also loss of slow flow information. The PRF is the pulse repetition frequency and determines the sensitivity to slow flow. The PRF must be kept as low as possible (around 500 Hz) because linked with the PRF adjustments are the filters. The higher the PRF the higher the filters are and high filters will eliminate slow flow. Focus in Doppler is like focus in grey-scale. It must be palce in the region of interest to get optimal resolution. Colour priority may not be adjusted easily in all machine but is a function that determines what is the most important in the image: the colours or the greys. Also adjust to a 100% to ensure maximal flow information.
Adjusting the many parameters of the Doppler should not be done at every examination. Fortunately, there is little difference from patient to patient and from joint to joint with respect to Doppler settings. An exception is the hip joint because of its deep location. Once the sensitivity of the Doppler has been maximised, the settings may be saved as a set-up, which the machine reverts to at every new exam. It is possible to have many different set-ups, which can be named to our liking such as knee, obese knee, wrist etc.
Using the same settings from patient to patient and from visit to visit is important if we wish to gauge the treatment response longitudinally or compare patients. Therefore it is also important that the same make and model of ultrasound machine are used from visit to visit.
Disclosure of Interest None declared
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