MRI Ankle and Foot

MRI of the ankle and foot is the best test for the ligaments, tendons, cartilage, bone marrow and soft tissues of the hindfoot, midfoot and forefoot. It is used when radiographs and clinical examination do not explain persistent pain, swelling or instability.

The study uses a 1.5 or 3 tesla scanner with a dedicated ankle and foot coil. The patient lies supine with the foot in a neutral position or slight plantar flexion (about 20 degrees), which relaxes the Achilles tendon and reduces artefact.

Slices are 3 to 4 mm thick. The examination takes 20 to 30 minutes.

The core sequences are sagittal, coronal and axial proton density or T2-weighted fat suppressed images, and T1-weighted images in the sagittal or coronal plane. Oblique planes along the tendons and the long axis of the foot are used for the forefoot.

Contrast is added for infection, tumour, inflammatory disease and neuropathic foot. Direct arthrography is used in selected cases for osteochondral lesions and ligament injury.

Ankle sprains account for about 25% of sports injuries, and most settle with treatment. Chronic ankle pain after sprain occurs in about 20 to 30% of patients, and MRI shows associated osteochondral lesions, tendon tears, impingement and occult fractures.

MRI has a sensitivity and specificity of over 90% for anterior talofibular and calcaneofibular ligament tears and for Achilles tendon rupture.

In diabetic foot disease, MRI has a sensitivity of 90 to 95% and a specificity of about 80 to 90% for osteomyelitis, and it shows abscess and the extent of infection.

Indications.

• Persistent ankle or foot pain, swelling or instability after injury with normal radiographs.
• Suspected ligament injury, including lateral ankle ligaments, syndesmosis and medial deltoid ligament.
• Tendon disorders: Achilles, peroneal, tibialis posterior, flexor hallucis longus, tibialis anterior.
• Osteochondral lesions of the talus, and osteonecrosis.
• Occult, stress and avulsion fractures.
• Impingement syndromes (anterolateral, anteromedial, posterior), sinus tarsi syndrome and tarsal coalition.
• Plantar fascia disease and heel pain.
• Forefoot pain: stress fractures, interdigital neuroma, plantar plate injury, sesamoid disorders and bursitis.
• Infection: osteomyelitis, septic arthritis and abscess, particularly in diabetes.
• Neuropathic arthropathy and its differentiation from infection.
• Inflammatory arthritis, crystal disease, tumours and soft tissue masses.
• Post-operative assessment and painful hardware.

Not for acute ankle injury with clear fracture on radiographs, or for uncomplicated sprain that is settling.

Contraindications and precautions.

• The usual MRI safety screen. Orthopaedic hardware is mostly safe but causes artefact.
• Gadolinium: renal precautions apply, which is important in diabetic patients with renal impairment.
• Pregnancy: non-contrast MRI is acceptable.
• Claustrophobia: feet first positioning helps.
• Patients with a diabetic foot ulcer: dressings are removed and the wound is cleaned and covered to protect the coil.

Preparation and technique.

No preparation is needed. The foot is placed in the coil, held neutral or slightly plantar flexed, with the toes pointing up.

For the ankle, a field of view covering the distal tibia to the mid-foot is used. For the forefoot, a dedicated field with thin slices, short axis and long axis to the metatarsals is used. Both sides are scanned if bilateral disease is suspected.

Fat suppressed sequences show marrow oedema and fluid. Gradient echo images show blood products and metal. Post-contrast fat suppressed T1 images are taken for infection and tumour.

Normal appearances.

The tibia, fibula, talus and calcaneus have bright fatty marrow on T1, and a thin dark cortex. The talar dome has smooth hyaline cartilage of 1 to 2 mm. A small amount of fluid in the ankle joint and tendon sheaths is normal.

The lateral ligaments (anterior talofibular, calcaneofibular, posterior talofibular) are dark bands of fibres. The deltoid ligament is a thick, striated, dark ligament on the medial side. The syndesmosis (anterior and posterior tibiofibular ligaments, interosseous membrane) is dark and continuous.

The Achilles tendon is a dark, oval structure 4 to 6 mm thick in the anteroposterior dimension. Peroneal tendons run behind the lateral malleolus, and tibialis posterior and flexor tendons run behind the medial malleolus. All are dark and continuous.

The plantar fascia is a dark band 3 to 4 mm thick.

Interpretation.

• Lateral ligament sprain: grade 1 (periligamentous oedema, intact fibres), grade 2 (partial tear, thickening or thinning with fluid), grade 3 (complete discontinuity, wavy, retracted ends). Anterior talofibular is injured first, then calcaneofibular.
• Syndesmosis injury (high ankle sprain): widening of the tibiofibular interval, tibiofibular ligament disruption and interosseous oedema, often with fibular fracture.
• Deltoid ligament tear: thickening, loss of striations and fluid, often with a medial malleolar fracture.
• Achilles tendon: tendinosis (fusiform thickening, intermediate signal), partial tear (longitudinal split, focal fluid), complete rupture (gap, retraction, haematoma, usually 2 to 6 cm proximal to the insertion). Insertional disease shows calcaneal spur, bursitis and pre-Achilles fat pad oedema.
• Peroneal tendons: tenosynovitis, longitudinal split tears, and subluxation with tear of the retinaculum. Look for a low lying muscle belly and an accessory tendon.
• Tibialis posterior: tenosynovitis, tendinosis, partial or complete tear with a collapsing arch (flatfoot). Early detection of tears prevents deformity.
• Osteochondral lesion of the talus: a fragment of bone and cartilage at the medial or lateral talar dome with a rim of fluid or cyst underneath (instability), or adherent and stable. Subchondral oedema and cysts show activity.
• Occult and stress fractures: linear low signal with surrounding oedema in the calcaneus, navicular, metatarsals, tibia, and talus. Avulsion fractures at ligament attachments.
• Osteonecrosis: serpiginous margin with a double line in the talus or other tarsal bones, typically after talar neck fracture or in sickle cell disease.
• Impingement: anterior osteophytes with synovitis, soft tissue scar in the anterolateral gutter, posterior os trigonum with oedema, and sinus tarsi fat replacement by fluid and fibrosis.
• Tarsal coalition: osseous, cartilaginous or fibrous bridging between tarsal bones (calcaneonavicular or talocalcaneal), with oedema and altered alignment.
• Plantar fasciitis: thickening over 4 mm, intermediate T2 signal, perifascial oedema and calcaneal marrow oedema. Rupture shows a gap and fluid.
• Forefoot: stress fractures of the metatarsal shafts and necks, interdigital neuroma (a fusiform mass, low to intermediate signal between metatarsal heads, enhancing), plantar plate tears with toe deformity, sesamoid fractures, osteonecrosis, and bursitis.
• Osteomyelitis: low T1 marrow signal that is confluent, replacing bright marrow, with high T2 signal and enhancement, in contact with an ulcer or sinus tract. Cortical destruction, abscess and sinus tract support the diagnosis.
• Septic arthritis and soft tissue infection: effusion with thickened enhancing synovium, cellulitis (subcutaneous oedema), abscess (rim enhancing collection), and tenosynovitis.
• Neuropathic arthropathy: fragmentation, subluxation and joint destruction in the midfoot with marrow oedema, not contiguous with an ulcer, and usually without a sinus tract. It overlaps with infection.
• Inflammatory arthritis and gout: synovitis, erosions, tophi and marrow oedema.
• Tumours: ganglion and tendon sheath giant cell tumour, lipoma, plantar fibromatosis, and sarcoma. Features of malignancy are size over 5 cm, deep location and necrosis.
• Post-operative: ligament and tendon repair integrity, fusion, hardware loosening and infection.

Critical values and findings needing immediate action.

• Osteomyelitis or abscess in a diabetic foot: antibiotics, debridement and vascular assessment.
• Septic arthritis or deep space infection: urgent drainage.
• Acute Achilles tendon rupture: early surgical or functional treatment.
• Displaced or unstable fracture, unstable osteochondral fragment, or tarsometatarsal injury: orthopaedic referral.
• Acute compartment syndrome of the foot or leg: urgent fasciotomy.
• Aggressive bone or soft tissue tumour: specialist sarcoma centre referral before biopsy.
• Suspected critical limb ischaemia with a non-healing ulcer: vascular review.

Limitations and artefacts.

• Magic angle effect causes false high signal in the peroneal, tibialis posterior and Achilles tendons at about 55 degrees to the field. Longer echo sequences resolve it.
• Findings are common in people without symptoms: peroneal splits, tendon sheath fluid, small osteochondral changes and os trigonum are seen in many asymptomatic people.
• Marrow oedema is non-specific: fracture, stress reaction, infection, neuropathic change, inflammation and tumour all cause it. The pattern and clinical context decide.
• Distinguishing neuropathic arthropathy from osteomyelitis is difficult. Ulcer contact, sinus tract, abscess and confluent marrow replacement favour infection.
• Metal artefact from hardware limits assessment. Metal artefact reduction sequences help.
• Dynamic instability and weight bearing alignment cannot be assessed with a static non-weight-bearing study.
• Ligament injury grading is subjective.
• Contrast is of limited help in end stage renal disease.

Special groups.

• Children and adolescents: physeal fractures, tarsal coalition, osteochondroses, and infection. Sedation may be needed.
• Athletes: ankle ligament and osteochondral injuries, stress fractures, and Achilles and peroneal tendon injuries.
• Patients with diabetes: infection and neuropathic arthropathy, with contrast when renal function permits.
• Older adults: tibialis posterior dysfunction, insufficiency fractures and tendon degeneration.
• Patients with inflammatory disease: synovitis and erosions.
• Regions: diabetic foot infection, trauma, tuberculous osteomyelitis, mycetoma, and podoconiosis occur. MRI is scarce, so radiographs, ultrasound and clinical probing are used. Early debridement and antibiotics matter more than imaging detail.

Follow up tests.

• Weight bearing radiographs of the ankle and foot.
• Ultrasound for tendons, plantar fascia, neuroma, abscess, and injection under ultrasound.
• CT for fracture, coalition and surgical planning.
• Bone scan or white cell scan if MRI is not possible.
• Bone biopsy and culture for osteomyelitis.
• Ankle brachial index, toe pressure and arterial duplex for ulcers.
• Blood tests: full blood count, inflammatory markers, glucose and glycated haemoglobin, uric acid, rheumatoid factor.
• Referral to orthopaedics, podiatry, vascular surgery, sports medicine or infectious diseases.