Role of MRI in comprehensive diagnostics
Although MRI has revolutionized imaging, it is used in inseparable connection with traditional radiography. X-ray remains the first-line method, but has significant limitations:
- Tissue superposition: three-dimensional structures overlap each other. This is usually resolved by imaging in two perpendicular projections, but complex bones (e.g., the scaphoid) require 4–6 views. If the scaphoid radiograph is normal, but suspicion of a fracture persists, early limb MRI is indicated.
- Bone mass loss: for destruction to become visible on a radiograph, the loss of local bone substance must exceed 50%. MRI allows visualization of bone marrow edema long before cortical destruction occurs.
- Avulsion fractures (avulsion fractures): paradoxically, injuries at ligament attachment sites with thin bone fragments ("flecks") are better visualized on X-rays. On MRI, they can be easily missed due to the partial volume effect and mild bone edema.
Protocols and scanning sequences
A high-quality extremity examination requires a high-resolution scanner, local coils, and proper positioning (e.g., the axial plane is optimal for assessing fascial compartments).
| Consistency | What we assess |
|---|---|
| T1 SE | Bone anatomy, tendon integrity, subcutaneous tissue structure. |
| T2 / STIR / PD-FS | Detection of bone marrow edema, fluid collections, and active inflammation zones. |
Intravenous contrast is not used routinely. It is prescribed when it is necessary to clearly visualize abscess margins, track fistulous tracts, or when the non-contrast study is inconclusive and may alter management.
MRI semiotics of infectious lesions
Using diabetic foot complications as an example, MRI clearly demonstrates its advantages. The classic radiological triad of osteomyelitis (osteolysis, periosteal reaction, destruction) develops only after 10–20 days. On MRI, the pathology is visible much earlier:
- Bone marrow edema: hyperintense (bright) signal on T2 is obligatorily accompanied by a hypointense (dark) signal on T1.
- Abscess: intraosseous fluid collection, which may be indicated by the penumbra sign (penumbra sign) — an intermediate signal zone on T1 surrounding the central focus.
- Fistulous tracts (sinus tracts): linear fluid structures. The walls of the fistula actively accumulate contrast, creating the "tramline appearance" (tramline appearance).
- Cellulitis: inflammatory edema of the adipose tissue causes a decrease in signal from normal fat on T1 and an increase on fat-suppressed T2 sequences.
- Skin ulcers: skin signal defects with indurated edges (low signal on T2) surrounded by an edema zone.
Assessment of systemic diseases
MRI and ultrasound help detect soft tissue pathology and arthropathies that are invisible on standard images due to the absence of bone destruction.
- Dermatomyositis and polymyositis: MRI objectively demonstrates changes in skeletal muscles depending on the stage of the process (inflammatory edema, fatty infiltration, atrophy).
- Systemic lupus erythematosus (SLE): the most common finding is synovitis. SLE is characterized by deforming non-erosive arthropathy. Due to ligamentous laxity (ligamentous laxity), ulnar subluxations occur, which spontaneously reduce when the patient's limbs are positioned on the X-ray table. Therefore, the joints on the image may look completely normal.
To remember the CREST syndrome in scleroderma: Calcinosis (soft tissue calcinosis, often amorphous), Raynaud's (Raynaud's phenomenon), Esophageal dysmotility, Sclerodactyly, Telangiectasia. Acro-osteolysis of the terminal phalanges is often visible on images, but joint erosions are almost absent.
Class and exam questions
How to distinguish osteomyelitis from osteitis on MRI?
In osteitis, there is an isolated absence of the corresponding low signal on T1 characteristic of bone marrow edema in true osteomyelitis.
Why is the STIR sequence used during scanning?
STIR superbly visualizes edema and occult fluid (bright signal), but can sometimes lead to overdiagnosis, as the high signal from inflammation blends with normal surrounding soft tissue.
Can soft tissue calcifications be seen on MRI?
Virtually impossible. Soft tissue calcification and ossification (e.g., in dermatomyositis) are obvious on conventional radiographs, but "blind" to magnetic resonance imaging.
What else to study on this topic
- MRI semiotics of tumors of the infrapatellar fat pad (Hofa's fat pad)
- Radiological criteria for the differential diagnosis of lipoma and well-differentiated liposarcoma
- Differential diagnosis of Charcot foot and diabetic osteomyelitis
- The role of power Doppler (ultrasound) in predicting the malignancy of masses
- Radiological stress tests for indirect evaluation of the ligamentous apparatus
Master the entire topic in the "Diagnostic Radiology" course
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Other topics in the "Musculoskeletal System" section
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