A MSUS image of the left foot plantar metatarsal area of a study participant with RA. The MSUS image demonstrates bursitis as a demarcated complex mass protruding beyond the 3rd and 4th metatarsal heads with hypertrophied synovium and anechoic spaces containing synovial fluid (arrow). The image is seen from the plantar aspect and in the transverse plane. M4: 4th metatarsal head; M3: 3rd metatarsal head; P: plantar fat pad.