We don’t routinely use ultrasound to assess ankle injuries.
A plain XR excludes ankle fractures & 5th metatarsal base fractures
If the bruising is typical of a sprain, and XR is normal, physiotherapy is appropriate.

Red flags are:
excessive swelling extending above the ankle suggests a syndesmosis sprain.
excessive swelling below the ankle on both sides sometimes represents a subtalar joint injury.
an inability to weight bear is unusual for an ordinary ankle sprain by a day.

For normal sprains, early physiotherapy is the most appropriate treatment.  If not behaving as expected, orthopaedic assessment is reasonable.
ATFL injuries alone do not require orthopaedic care, 95% will heal with conservative management, and few require reconstruction for instability.

Indications for surgery after normal ankle sprain are ongoing pain or ongoing symptomatic instability.

David Mitchell & Shaun English