CT knee (protocol)

The CT knee protocol serves as an examination for the bony assessment of the knee the femoral condyles or the tibial plateau and the proximal tibiofibular joint. It is often performed as a non-contrast study. It can also be combined with a CT arthrogram in cases of suspected internal derangement where an MRI is contraindicated.

Note: This article aims to frame a general concept of a CT protocol for the assessment of the knee femoral condyles or tibial plateau. 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 knee might look like as follows:

Indications

Typical indications include imaging of the following ^1-6:

• distal femoral fractures
• proximal tibial fractures
• preoperative planning
  • osteoarthritis
  • trochlear dysplasia
  • patellar translation
• knee implants and complications
• inflammatory or septic arthritis
• bone and soft tissue tumors
• image guidance
• CT arthrography
  • contraindication to MRI
  • presence of metallic implants
  • internal derangement

Purpose

The purpose of a knee CT is the depiction of the bony structures and morphology furthermore the demonstration and classification of fractures as well as the visualization of bone tunnels and implants in a postoperative setting. 

In the case of neoplastic, inflammatory and infectious disorders a knee CT is done to demonstrate bony erosions, osteolytic lesions, calcifications, to evaluate bone matrix or soft tissue gas and their extent. If gout is suspected a dual-energy CT can visualize urate crystal deposits. 

If MRI is contraindicated or metallic implants are present a CT arthrogram of the knee can allow for evaluation of the articular cartilage and menisci in the setting of a previous repair and depict ligamentous injuries.

Technique

• patient position
  • supine position
• tube voltage
  • ≤120 kVp
• tube current
  • as suggested by the automated current adjustment mode
• scout
  • distal third of the femur and proximal half of the tibia
• scan extent
  • the whole patella and the fibular head should be included
• scan direction
  • craniocaudal
• scan geometry
  • field of view (FOV): 120-250 mm (should be adjusted to increase in-plane resolution)
  • slice thickness: ≤1.25 mm, interval: ≤0.625 mm
  • reconstruction algorithm: bone, soft tissue
• multiplanar reconstructions
  • axial images: parallel to the femorotibial joint line
  • coronal images: parallel to the transepicondylar axis 
  • sagittal images: perpendicular to the transepicondylar or parallel to the anteroposterior axis
  • slice thickness: ≤2 mm, overlap 50%

Practical points

• patient positioning with slight internal rotation of the lower limb before scanning
• the extent of the examination should be tailored to the specific indication or clinical question
• imaging of implants ⁷
  • the whole implant should be captured
  • use metal artifact reduction algorithms
  • use monochromatic reconstructions in dual-energy CT scans
  • use additional wide window setting
• in the setting of preoperative planning, 3D reconstruction techniques are highly recommended