Ultrasound microangiography of the metacarophalangeal joint using ultrafast Doppler

Rheumatoid arthritis (RA) is a common autoimmune disease associated with chronic inflammation, referred to as synovitis, and ultimately joint destruction. It is acknowledged that ultrasound Power Doppler imaging can reveal subclinical synovitis but the quantification of inflammation stages is currently limited by the coarse resolution and sensitivity of conventional Doppler imaging. Here we show that ultrafast Doppler imaging characterizes metacarophalangeal joint microvasculature with an unprecedented accuracy, making it a promising microangiography method for the early diagnosis of RA. We made use of a 15 MHz probe (256 elements linear array, 0.125 mm pitch) connected to a programmable ultrafast ultrasound scanner. We insonified the second metacarpophalangeal joint of 13 healthy volunteers with a dedicated ultrafast Doppler imaging sequence consisting of 41 plane wave transmissions at a pulse repetition frequency of 20 kHz during one second. The received ultrasound data were beamformed and digitally filtered to get rid of tissue clutter. Power Doppler maps were computed and overlaid on co-registered Bmode images of the joint anatomy. Ultrafast Doppler imaging allowed for the detection of healthy metacarpophalangeal joint microvasculature, which is invisible in conventional Power Doppler imaging. We imaged microvascular blood flow in 12 out of 13 healthy joints, with Doppler signal to noise ratios of the order of 5 dB. In addition, we computed for each individual a functional capillary density (defined as the length of perfused capillaries in mm per tissue area in mm2) and obtained values of the order of 0.6 ± 0.1 mm microvessel/mm2 tissue. The method, which can be readily implemented on ultrafast ultrasound scanners, shows strong potential for the early diagnosis of RA and has the advantage of being fully noninvasive. A group of RA patients with different stages of inflammation will be investigated next.