CT foot (protocol)

The CT foot protocol serves as an examination for the bony assessment of the fore and mid-foot and is almost always performed as a non-contrast study. It can also be combined with a CT ankle protocol or can be acquired as dual-energy CT depending on the clinical question.

Note: This article aims to frame a general concept of a CT protocol for the assessment of the foot. 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 foot might look like as follows:

Indications

Typical indications include trauma of the forefoot and or the depiction of bony lesions as e.g. in gout ^1-3:

• fractures or dislocations of the forefoot or midfoot
• Lisfranc injury
• foreign body
• inflammatory arthritis (e.g. gout)
• infectious processes
• postoperative evaluation
• bone and soft tissue tumors
• image guidance

Purpose

The purpose of a foot CT is the depiction of the bony structures, the classification of fractures and the depiction of bony erosions, osteolytic lesions, calcifications and soft tissue gas in the setting of neoplastic, inflammatory and infectious disorders ^2,3.

In the setting of suspected gout, the purpose is visualization and characterization of monosodium urate crystal depositions.

Technique

• patient position
  • supine position
• tube voltage
  • ≤120 kVp
• tube current:
  • ≤100 mAs
• scout
  • whole foot
• scan extent
  • variable depends on the proximal extent of the pathology
  • tip of the toes to Chopart joint
• scan direction
  • craniocaudal
• scan geometry
  • field of view (FOV): 100-160 mm (should be adjusted to increase in-plane resolution)
  • slice thickness: ≤0.625 mm, interval: ≤0.3 mm
  • reconstruction algorithm: bone, soft tissue
• multiplanar reconstructions
  • axial images: parallel to the long axis of the metatarsal bones
  • coronal images: perpendicular to the long axis 1st or 2nd and 3rd metatarsal bones
  • sagittal images: parallel to the axis of 1st or the 2nd and 3rd metatarsal bones
  • slice thickness: ≤1.5 mm, overlap 50%

Practical points

• the extent of the examination can and should be tailored to the specific indication or clinical question
• acquisition options:
  • foot-holder with toes pointing straight up
  • knee angulated 90-120° and the foot resting flat on the examination table (without ankle)