Ankle protocol (MRI)

The MRI ankle protocol encompasses a set of MRI sequences for the routine assessment of the ankle joint.

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

Indications

Typical indications include pain in the ankle or rearfoot as in:

• ankle sprain 
• midtarsal sprain
• stress fractures
• suspected chondral injury
• ankle impingement 
• Achilles tendinopathy
• plantar fasciitis

1.5 vs 3 tesla

Musculoskeletal examinations are generally done on both 1.5 and 3 tesla. They profit from the improved spatial and contrast resolution of 3 tesla. Postoperative examinations in patients with metallic implants, however, should be done on 1.5 tesla with metal artifact reduction sequence. 

Patient positioning

An MRI of the ankle is conducted with the patient in the supine position.

Technical parameters

Coil

Multi-phased array coils are recommended.

• dedicated ankle coil
• alternative: flexible coil

Scan geometry

• in-plane spatial resolution: ≤0.3 x 0.3 mm
• field of view (FOV): 100-160 mm
• slice thickness: ≤3 mm

Planning

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

• axial images:
  • angulation: perpendicular to the distal tibia and parallel to the tibiotalar joint
  • volume: about 3-5 cm above the tibiotalar joint to the plantar fascia 
  • slice thickness: ≤3 mm
• coronal images:             
  • angulation: parallel to the malleolar axis 
  • volume: depends on the question but should include at least the whole tibiotalar joint, the talus and navicular bone but could sometimes include also the metatarsal bases
  • slice thickness: ≤3 mm
• sagittal images:
  • angulation: perpendicular to the malleolar axis 
  • volume: includes medial and lateral malleolus
  • slice thickness: ≤3 mm
• axial oblique images*:
  • angulation: with a posteroinferior tilt of about 45° perpendicular to the posterior facet of the calcaneus
  • volume: should include the tibiotalar joint and both malleoli
  • slice thickness: ≤3 mm
• sagittal oblique images*:
  • angulation: with an inferolateral tilt about 45° in course of the anterior tibiofibular ligament
  • volume: about 8 slices, should contain the anterior and posterior syndesmosis

Sequences

The mainstay in musculoskeletal imaging are water-sensitive sequences, this can be achieved with conventional STIR or fat-saturated images or with intermediate weighted images.

At least one T1-weighted sequence should be included to ease the assessment and interpretation of bone marrow and/or soft tissue lesions.

Frequently an MRI of the ankle does not require any contrast media.

Standard sequences

• intermediate-weighted (fat-saturated)
  • purpose: bone and/or soft-tissue characterization, detailed anatomy, including ligament and tendon anatomy
  • technique: IM fast spin echo / PD Dixon
  • planes: coronal, sagittal, axial
• T1-weighted
  • purpose: bone and/or soft-tissue characterization
  • technique:  T1 fast spin echo
  • planes: coronal, axial* (option in case of arthritis or synovitis)

Optional sequences

• T2-weighted
  • purpose: bone and soft tissue characterization, in particular, the depiction of the calcaneofibular ligament and the tibiofibular syndesmosis
  • technique: T2 FS fast spin echo
  • planes: axial oblique*, sagittal oblique* (option)

Some indications might benefit from the application of contrast media as e.g. inflammatory conditions, tumors or in some conditions ankle impingement.

• T1-weighted C+ (fat-saturated)
  • purpose: for inflammatory  conditions, suspected tumors
  • technique:  T1 fast spin echo
  • planes: axial, sagittal, coronal depending on the pathology

(*) indicates optional planes or sequences

Practical points

• in the ankle, the protocol can and should be tailored to the specific indication or clinical question
• as with the other joints, the examination will benefit if every plane is imaged
• a typical native protocol will consist of 4-6 sequences
• an alternative to the intermediate,-weighted (fat-saturated) image stacks in three planes would be an intermediate weighted Dixon variant, which includes a fluid sensitive fat-saturated and a non-fat-saturated image stack, without additional scanning time, this can help in the assessment of ligament and tendon pathology
• pathologies of the Achilles tendon might require an increase of the field of view in the cranial direction to include the myotendinous junction, likewise, the axial image stack has to be adjusted in that respect
• on the downside pathologies of the plantar fascia might require an increase of the field of view in the distal direction and an increased coronal image stack