"Ultrasound," "MRI" and "x-ray" can appear together on foot and ankle paperwork around Des Plaines. This page keeps ultrasound apart from the other two and uses what RadiologyInfo.org says about it.
What the ultrasound label covers
RadiologyInfo.org states the scope this way: "Ultrasound images of the musculoskeletal system provide pictures of muscles, tendons, ligaments, joints, nerves and soft tissues throughout the body." The list is long and soft-tissue heavy. Tendons and joints, the two named in this page's title, are both on it.
The feature that sets it apart
RadiologyInfo.org adds a point about timing: "Because ultrasound images are captured in real time, they can show the movement of a soft tissue structure such as a tendon, joint or an extremity." Real-time capture is the feature to hold on to. A description that only says "ultrasound" has not told you that the picture can follow movement.
The neighboring scans
Two MedlinePlus entries cover the scans that get confused with it. The Leg MRI scan entry and the Joint x-ray entry each stay under their own name, so a reader can check each label against its own page instead of a brochure's summary.
Ankle pain across the imaging names
For ankle pain in particular, RadiologyInfo.org carries a page titled Appropriateness criteria | Chronic Ankle Pain, whose title ties imaging criteria to a single complaint. A research paper in the same area, MRI Evaluation of Traumatic and Non-traumatic Pathologies of the Ankle Joint and Hindfoot: A Single-Center Observational Study, is framed around MRI of the ankle joint and hindfoot.
The question that decides it
Ask whether the problem is one where seeing a tendon or joint move matters. If it is, ultrasound's real-time capture is the documented feature. If the request instead names a leg MRI or a joint x-ray, read that entry before assuming it does the same job.