Background In many patients with rheumatoid arthritis (RA) subclinical disease activity can be detected with ultrasound (US), especially using power Doppler US (PDUS).[1-3] However, in our experience, PDUS may be highly dependent on machine type. To clarify how the PDUS signal differs between machines a flow phantom could be used.

Objectives Determine the lowest flow that the power Doppler (PD) modality of 5 US machines could detect by creating a small microvessel (150micron) flow phantom.

Methods The flow phantom, consisted of an acrylic (PMMA) container filled with tissue mimicking material(TMM)[4], with three microvessels (2000 micron, 1000 micron and 150 micron). The blood mimicking fluid was based on the recipe by Ramnarine.[5] A syringe pump (Harvard Apparatus Pump 11 Elite) was used to generate different flows. Five US machines were used, the Esaote MyLab60 (probe LA 435), Aloka α7 (UST-5411), Philips iU22 (L9-3), Ultrasonix SonixTouch (L14-5/38) and VisualSonics Vevo2100 (MS200). Settings were optimised to detect lowest flows by adjusting pulse repetition frequency, velocity range, filters, frequency and gain. We recorded the lowest possible flow in each of the vessels that still resulted in a continuous PDUS signal for each machine. Flow velocities were calculated from pump flow setting and microvessel diameter.

Conclusions PD was highly variable between machines. The Esaote detected the lowest flow. The very low flow detected by the Esaote in the 2mm vessel was checked multiple times by multiple observers. The machines predominantly used in research (Ultrasonix, VisualSonics) were not able to detect flows in the smallest vessel (150 micron). The differences found between the machines are apparently caused by fundamental differences in processing of the PD signal or internal settings inaccessible to users. Probe frequency does not explain the large differences found. A spurious finding was that the minimal detectable flow velocity was lower in the 1mm vessel than in the 2mm vessel. Possibly, the 1mm vessel was compressed by the TMM so that the calculated flow velocity was underestimated. The relative differences between machines are unchanged by this. For a reliable and reproducible detection of microvessel flow in inflamed joints, the choice of the US machine and its settings seems very important. By investigating the flow velocity of blood in an inflamed joint, the minimal flows that rheumatological US machines need to detect will be known. Then, our flow phantom would be a very useful instrument in the process of optimising US machines and for rheumatologists when buying a new US machine.

References

1. Peluso (2011) Ann Rheum Dis

2. Saleem (2011) Ann Rheum Dis

3. Brown (2008) Arthritis Rheum

4. Teirlinck (1998) Ultrasonics

5. Ramnarine (1998) Ultrasound Med Biol

References

Disclosure of Interest None Declared