For purposes of MR imaging, the foot is divided into two exams:
1. Ankle/Hindfoot/Midfoot (everything behind metatarsals)
2. Forefoot (metatarsals & toes)
MRIs for tendons & ligaments use "Ankle" protocol unless AOI is specifically in the forefoot.
"Foot" MRIs should be treated as Ankle/Hindfoot/Midfoot MRIs until proven otherwise.
Exceptions: For neuroma, mass, stress fracture,
or osteomyelitis in the forefoot- do a dedicated forefoot (metatarsals
and toes) exam.
• Plantar fibroma & LisFranc fractures- use forefoot protocol but center
over area of interest in the midfoot.
-only image one foot at a time
Sagittal T1 & STIR
Axial T1 & fat sat T2 (or T2 without fat sat if low field)
Coronal fat sat T2 (or STIR)
-Contrary to coil design, the ankle tendons and ligaments are best imaged with the foot extended 15-30°. So don't use the chimney in the coil. Position the calcaneus in the middle of the coil and the relaxed foot will fit comfortably in the coil at the correct degree of extension.
-Prescribe Sagittals off an axial scout at the level of the tibiotalar joint (see figure below). Angle slices to bisect the Achilles tendon and middle of tibia:
-Inferiorly, axial sections through the ankle must include the fifth metatarsal base.
-Superiorly, axial images should go about 3 cm above the level of the ankle joint. If the exam is for evaluation of the Achilles tendon, make sure to include the Achilles myotendinous junction.
-Axial images can be angled slightly with the tibiotalar joint, but it's better to do straight axials than to oblique too much (see figure below).
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-Use the Ankle/Hindfoot/Midfoot protocol to image the hindfoot and midfoot.
- Place patient prone to image forefoot whenever possible
- Forefoot images should be prescribed off the level of the MTP joints.
- Prescribe longitudinal and sagittal slices off a transverse scout.
- Look at your sagittal or longitudinal STIR before you prescribe the transverse sections so that you can be sure you've covered all the pathology (on STIR: white stuff = edema = pathology).
Angle along the plane of the joints. Longitudinal Rx should bisect the 2nd & 4th MT heads (see figures below):
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- Always mark region of concern with a Vitamin E tablet.
- Use small FOVs
Longitudinal T1 & STIR (3/0.5mm)
Transverse T1 & STIR (4/1 mm)
Sagittal T1 & STIR (4/1 mm)
If the patient has had prior neuroma surgery then add a transverse post gad T1W (with fat sat if available)
If the area of concern is isolated to the midfoot, such as plantar fibroma, then center the coil over the area of interest & use forefoot protocol.
MATRIX:
If you have enough signal, use 512 x 256 for T1W sequences. STIR & T2 fat sat can be 256x192.
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