Rheumatoid arthritis (RA) is an autoimmune disease with synovial lesions as the main manifestation. With advances in ultrasound engineering and the clinical use of high-frequency and ultra-high-frequency ultrasound, ultrasound can sensitively detect structural damage to joints and the mobility of lesions. Methodologically, ultrasound has the advantage of dynamic detection and can detect multiple joints simultaneously, thus playing an increasingly important role in the diagnosis, efficacy follow-up and prognosis assessment of RA. I. Ultrasound assessment of structural damage to RA joints 1. cartilage damage Cartilage thickening may appear on the sonogram in early RA patients, suggesting edema. Chronic inflammation of the cartilage may lead to permanent damage to the joint surface, which is shown on ultrasound as an unsmooth cartilage surface with uneven thickness. MCP and PIP articular cartilage thicknesses of the 2nd through 5th fingers can be measured more accurately. In advanced stages of the lesion, subchondral cysts are seen at the epiphysis, and hypoechoic vascular opacities may be present within the cysts. Ultrasound can be used to semi-quantitatively classify cartilage lesions into 5 levels. Ultrasound observation of articular cartilage can distinguish early RA and early symptomatic osteoarthritis (OA) from normal joints. The occurrence of bone erosion suggests aggressive arthritis. Bone erosion is mostly located at the junction between the synovial membrane and the articular surface, and ultrasound suggests that the sign of bone erosion is a bone cortical defect in two vertical sections. Due to the good spatial resolution of high-frequency ultrasound, ultrasound can suggest early bone cortical damage, which is manifested by hairy, unsmooth, and hairy cortical line changes. In recent years, these early signs of cortical damage suggested by ultrasound have attracted increasing clinical attention in recent years, so this early cortical damage is often reversible, and further damage may lead to irreversible changes in joint structure. Ultrasound assessment of lesion activity Compared with clinical symptoms, ultrasound can more accurately assess the activity of RA arthritis. Ultrasound evaluation of RA activity generally includes joint cavity effusion, active synovitis, synovial vascular opacity formation, bursitis, and tendon synovitis. The obvious advantage of ultrasound to assess lesion activity is that it is generally done without the use of contrast techniques and can be done with conventional color Doppler or energy Doppler. However, the obvious drawback of ultrasound is that it does not show bone marrow edema. Joint effusions present as an echogenic area within the joint that is compressible and has no blood flow signal. Energy Doppler ultrasound has the advantages of not being affected by the angle between sound velocity and blood flow velocity and being able to detect lower velocity blood flow, so it has improved the sensitivity of detecting intra-synovial blood flow signals and is the most commonly used technique for color flow detection in RA patients, but energy Doppler still has limitations in detecting very low blood flow. In recent years, new ultrasound display techniques such as ultrasonography or advanced flow imaging have been used in clinical practice, which can more sensitively display low-velocity blood flow in the synovium. The various ultrasound techniques for assessing the activity of RA are now graded using a semi-quantitative method, which is useful for longitudinal assessment of patient outcomes. The outstanding advantage of ultrasound in RA patients, especially in patients with treatment follow-up, is the imaging assessment of subclinical inflammation. The value of ultrasound for the diagnosis of RA has been increasingly appreciated for the early diagnosis of RA. When RA is clinically suspected, imaging examinations including ultrasound can significantly improve the accuracy of early diagnosis, allowing the diagnosis of some undifferentiated arthritis to be clarified. Although the imaging manifestations suggested by ultrasound are often not specific, more signs have systemic specificity with rheumatologic-immunologic significance, such as tendon attachment-end inflammation, early damage to the bone cortex and secondary entrapment of the median nerve in the carpal tunnel, so ultrasound is also of great value in the differential diagnosis of RA. The main goal of RA treatment is to achieve remission, which means minimizing the activity of the arthritis to improve the prognosis. However, up to 90% and 60% of RA patients in clinical remission still have synovial thickening and active synovitis, and 30% still exhibit further structural damage to the joint on imaging; therefore, in recent years, there has been an increasing clinical emphasis on imaging remission in RA patients. In recent years, biologic agents have been increasingly used in RA clinical practice, which requires more accurate assessment of patient activity and treatment effects in order to adjust treatment regimens and doses in a timely manner. In a follow-up of 112 patients with RA treated with biologics, the authors found a significant improvement in ultrasound sonographic indices of lesion activity in the early biologics group. Accordingly, timely adjustment of the treatment regimen based on imaging performance during the window period that may lead to structural damage may reduce the possibility of structural damage in patients in the distant future. V. OUTLOOK With the higher clinical demand for RA attainment treatment and the increasing understanding of RA, the application of imaging examinations, including ultrasound, in RA has been a hot spot for clinical research. Three-dimensional ultrasound, image fusion technology, and higher-sensitivity color blood flow display technology are also gradually being applied. However, ultrasound also has its limitations, high operator dependence, and there is no widely accepted consistent standard for ultrasound evaluation of RA, and related work still needs further in-depth study.