CT ankle (protocol)

The CT ankle protocol serves as an examination for the bony assessment of the ankle and rearfoot and is almost always performed as a non-contrast study. It can be also combined with a CT arthrogram for the evaluation of chondral and osteochondral injuries or can encompass the whole foot in certain indications.

Note: This article aims to frame a general concept of a CT protocol for the assessment of the ankle and rearfoot. Protocol specifics will vary depending on CT  scanner type, specific hardware and software, radiologist and perhaps referrer preference, patient factors e.g. implants, specific indications.

A typical CT of the ankle might look like as follows:

Indications

Typical indications include foot and ankle trauma and or the depiction of bony lesions ^1-7:

• distal tibial fractures
• malleolar fractures
• talar and calcaneal fractures
• midtarsal injury
  • midtarsal sprain
  • Chopart fracture-dislocation
• distal tibiofibular syndesmosis injury
• subtalar dislocation
• subtalar instability
• osteoarthritis of the ankle
• inflammatory processes
• infectious processes
• postoperative evaluation
• bone and soft tissue tumors
• image guidance (e.g. subtalar joint injection)
• cartilage injury (CT arthrogram)

Purpose

The most common purpose of an ankle CT is the depiction of the bony structures and precise classification of ankle fractures, talar and calcaneal fractures including avulsion injuries ^1-3.

The purpose in the setting of inflammatory and neoplastic processes is to depict bony erosions and osteolysis ^4,5.

In the setting of distal tibiofibular syndesmosis injury, CT demonstrates the position of the lateral malleolus in relation to the tibial incisure and can be used to assess tibiofibular groove morphology ^7,8.

The main purpose of a CT arthrogram of the ankle is the detection and assessment of cartilage or osteochondral injury ⁹.

Technique

• patient position
  • supine position
• scout
  • mid/distal third of lower leg to the skin of the heel
• tube voltage and tube current
  • ≤120 kV and ≤100 mAs
• scan extent
  • variable depends on the proximal extent of the pathology
  • minimum: ~2 cm above the tibiotalar joint to the bottom of the calcaneus
• scan direction
  • caudocranial
• scan geometry
  • field of view (FOV): 100-160 mm (should be adjusted to increase in-plane resolution)
  • slice thickness: ≤1.5 mm, overlap ~50%
  • reconstruction algorithm: bone, soft tissue
• multiplanar reconstructions
  • axial images: perpendicular to the distal tibia and parallel to the tibiotalar joint
  • coronal images: parallel to the malleolar axis 
  • sagittal images: perpendicular to the malleolar axis 
  • axial oblique images*: posteroinferior tilt of ~45° or perpendicular to the posterior facet of the calcaneus

Practical points

• footrests are advisable if tolerated by the patient especially in a setting where both ankles and/or the distal tibiofibular syndesmosis is evaluated
• the extent of the examination can and should be tailored to the specific indication or clinical question