Mid- and forefoot protocol (MRI)

The MRI mid- and forefoot protocol encompasses a set of MRI sequences for routine assessment of the mid- and forefoot.

Note: This article aims to frame a general concept of an MRI protocol for the assessment of the fore- and midfoot. 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 fore and midfoot pain as in:

• stress fractures
• plantar plate injury/turf toe
• Morton neuroma
• adventitial bursitis
• osteomyelitis / Charcot joint

1.5 vs 3 tesla

Musculoskeletal examinations are generally done on both 1.5 and 3.0 tesla. They profit from the improved spatial and contrast resolution of 3.0 tesla. Postoperative examinations in patients with metallic implants, however, should be done on 1.5 tesla with metal artifact reduction sequence.

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

Patient positioning

There are the following options:

• the patient in a prone position (favorable for suspected Morton neuroma)
• supine position

Technical parameters

Coil

Multi-phased-array coils are recommended.

• flexible small extremity coil
• dedicated ankle 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 oblique images:
  • angulation: parallel to the long axis of the 1st metatarsal bone
  • volume: from dorsal skin to plantar skin 
  • slice thickness: ≤3mm
• coronal oblique images*:
  • angulation: perpendicular to the long axis of the 1st metatarsal bone  
  • volume: depends on the question but should include at least the proximal interphalangeal joints up to the midtarsal (Chopart) joint
  • slice thickness: ≤3 mm
• coronal images*: (supine position, ankle coil)           
  • angulation: parallel to the tibial axis  
  • volume: depends on the question but should include at least the proximal interphalangeal joints up to the midtarsal (Chopard) joint
  • slice thickness: ≤3 mm
• sagittal images:
  • angulation: parallel to the axis of the 2nd and 3rd metatarsal bones and the rearfoot  
  • volume: from medial skin to lateral skin
  • slice thickness: ≤3 mm

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.

Many indications of an MRI of the forefoot benefit from contrast media.

Standard sequences

A native protocol can be performed just without the suggested postcontrast sequences.

• intermediate-weighted (fat-saturated)
  • purpose: bone and/or soft-tissue characterization, detailed anatomy, including ligament and tendon anatomy and depiction of the plantar plate
  • technique: IM fast spin-echo
  • planes: sagittal, axial oblique
• T1 weighted: angulation depends on suspected pathology
  • purpose: bone and/or soft-tissue characterization
  • technique:  T1 fast spin echo
  • planes: depends on the pathology
    • axial oblique* (option in stress fractures)
    • coronal oblique* (option in Morton neuroma)
    • sagittal* (option in osteomyelitis or Charcot-arthropathy)
• T2 weighted
  • purpose: bone and soft tissue characterization 
  • technique: T2 fast spin echo
  • planes: coronal oblique

Optional sequences

• T1 weighted C+
  • purpose: for inflammatory  conditions, tumors
  • technique:  T1 fast spin echo
  • planes: in same, the plane of the T1 weighted image without contrast  
• T1 weighted C+ (fat-saturated)
  • purpose: for inflammatory  conditions,  tumors
  • technique:  T1 fast spin echo
  • planes: coronal oblique or sagittal (option in osteomyelitis or Charcot arthropathy)

(*) indicates optional planes

Practical points

• in the fore and midfoot the protocol can and should be tailored to the specific indication or clinical question, still, as with the other joints, the examination will benefit if every plane is imaged
• a typical protocol will contain native 3-5 sequences and two T1-weighted image stacks after contrast with or without fat saturation
• in case of suspected Charcot joint or osteomyelitis, it might be worthwhile to examine the foot with a dedicated ankle coil and an increased field of view, which also includes the ankle and rearfoot, whereas in suspected turf toe the examination can be confined and angulated to the first digit and confined to the forefoot
• in suspected Morton neuroma the protocol might profit from the prone position if appropriate coils are available