Ultrasound Findings in Gout: A Question about a Periarticular Structure

Thank you, Stephen Bird. You always help me answer many challenging questions. I have another issue today that requires your assistance. In gout patients, aside from observing synovitis on the dorsal side of the first metatarsophalangeal joint, we often notice hypoechoic areas on the medial side of the first metatarsophalangeal joint. These areas have heterogeneous internal echoes and appear to be relatively distant from the joint capsule. Additionally, in this case, it can be observed that part of this region has abundant blood flow signals, while another part does not. What structure could this be? How can we explain the distribution characteristics of the blood flow signals? Thank you very much!