New type of imaging method is in demand clinically for early detection and treatment guiding of RA. Photoacoustic imaging (PAI), a hot spot in medical imaging society, which combines the merits of optical imaging and US and has a capacity in morphological, functional and molecular imaging, has shown potential in visualizing superficial organs, including small joints. The investigators developed a multi-modal photoacoustic/ultrasound (PA/US) imaging system, equipped with a handheld 3D PA/US probe, which wass able to provide 3D volume data of dual-wavelength PA imaging of small joints, in addition to real-time 2D PA/US imaging. The investigators plan to recruit RA patients in different disease conditions and healthy volunteers to receive the 2D and 3D PA/US examination on the 2nd and 3rd MCP, PIP joints and wrist joints using this imaging system, and to evaluate the potential clinical role of the imaging system in RA, and its added value to current imaging methods.

The multi-modal PA/US imaging system was built on a high-end commercial US platform, and allowed real-time imaging of grey-scale US imaging, Color-Doppler US (CDUS) imaging and dual-wavelength PA imaging. The PA/US images were acquired using a hand-held probe integrated optical devices and US transducers. 3D volume data of the selected joint of each participant was collected using an automatic scanning device after the dual-modal imaging. The investigators plan to recruit RA patients from the outpatient department of Rheumatology of our hospital, who have been diagnosed as RA by experienced rheumatologists. The patients and healthy volunteers will receive multi-modal imaging examinations using the novel imaging system. The MCP 2 and 3, PIP 2 and 3, and wrist of bilateral hand were chosen for the multi-modal imaging. Conventional US scanning of the joints, including grey-scale US, CDUS, and PDUS, will be carried out by an experienced US operator. Afterwards, 2D and 3D PA/US imaging will be implemented for each joint by the same operator. All of the imaging data will be analysed by the Matlab software.

The inclusion criteria

1. The patients were above 18 years-old;
2. The patients were diagnosed with RA according to the American College of Rheumatology (ACR) criteria, and the current clinical state was in-active or relapse;
3. The patients were consent to receive examinations of photoacoustic imaging.

The exclusion criteria

1. The patients had mental disease or other severe diseases, and were not capable of receiving imaging examinations;
2. The patients had strong reaction to laser light.

The RA patients are diagnosed as RA by two experienced rheumatologists.