CT hip (protocol)

The CT hip protocol serves as an examination for the evaluation of the hip joint. It is often performed as a non-contrast study. However, it can be combined with a CT arthrogram for the evaluation of chondral and/or labral tears or a femoral neck version scan.

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

Indications

Typical indications include the following ^1-6:

• occult proximal femoral or acetabular fractures
• preoperative planning
  • osteoarthritis of the hip
  • femoroacetabular impingement
  • acetabular dysplasia
• complications of hip protheses
• inflammatory or septic arthritis
• bone and soft tissue tumours
• image guidance
• cartilage injury or labral tears (CT arthrogram) in cases where MRI is contraindicated

Purpose

The main purpose of a dedicated hip CT is the depiction of the three-dimensional bony morphology of the femoral head-neck area and the acetabulum. This is especially useful for preoperative planning or the assessment of the postoperative hip and includes a depiction of the following ^1-6:

• proximal femoral and acetabular fracture morphology
• degenerative changes
  • joint space narrowing and osteophytes
  • femoral neck cystic change
  • acetabular rim abnormalities (fragmentation, subchondral cysts, os acetabuli)
• hip joint effusion
• cam morphology
• acetabular overcoverage (anterior coverage, coxa profunda, protrusio acetabuli)
• postoperative changes and complications
  • size of osteoplasty
  • residual cam lesion
  • infected hip prothesis

A CT arthrogram of the hip can be done for the assessment of the articular cartilage and the detection of labral tears if MRI is contraindicated ⁴.

Technique

• patient position
  • supine position
• tube voltage
  • ≤120 kVp
• tube current
  • as suggested by the automated current adjustment mode
• scout
  • iliac crest to the proximal half of the femur
• scan extent
  • might vary depending on the indication e.g. preoperative planning or implants
  • should include the anterior inferior iliac spine and 3-5cm below the lesser trochanter
• 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 kernel: bone kernel (e.g. B60-U70), soft tissue kernel (e.g. B20-30)
• multiplanar reconstructions
  • axial images:strictly axial to the body axis
  • coronal images: strictly coronal to the body axis
  • sagittal images: strictly sagittal to the body axis
  • axial oblique: parallel to the femoral neck axis
  • slice thickness: ≤2 mm, overlap 50%
  • additional coronal and sagittal centred on the femoral stem might be obtained

Practical points

• mindful patient positioning with slight internal rotation of the lower limb prior to scanning might reduce reconstructions
• the extent of the examination should be tailored to the specific indication or clinical question
• reconstructions should be centred on the hip in question but should also include a wide field of view to depict acetabular and captured pelvic morphology ⁶
• imaging of implants ⁶
  • the whole implant should be captured
  • use metal artifact reduction algorithms 
  • use monochromatic reconstructions in dual-energy CT scans
  • use additional wide window setting
• 3D reconstruction techniques might be beneficial in the setting of preoperative planning and/or visualisation of fractures