Imaging in osteomyelitis
- Finding
Bone infection takes one to two weeks to show on an X-ray; MRI shows it within days, which is why it is the key test when the X-ray is normal.
What it is
Osteomyelitis is infection of bone. It reaches the bone through the blood (children's long bones, adults' spine), from a nearby wound or ulcer (diabetic foot, pressure sores) or directly after a fracture or surgery. The plain X-ray is the first study, but it changes late: a large part of the bone mineral must be lost before destruction shows, usually 10 to 14 days into the illness. At first only soft-tissue swelling and blurred fat planes are seen; then periosteal reaction, ill-defined lytic areas and cortical loss. MRI is the most useful test: it shows marrow oedema and abscesses early, and a normal MRI makes osteomyelitis very unlikely. CT shows dead bone and gas well, nuclear medicine helps when metal or MRI limits the picture, and ultrasound finds fluid in children. The diagnosis is confirmed by bone culture or biopsy.
What it looks like
- Early X-ray: often normal; soft-tissue swelling and loss of fat planes first, bone changes after about 10 to 14 days.
- Later X-ray: periosteal reaction, ill-defined lytic destruction, cortical loss; it can mimic an aggressive tumour.
- Chronic osteomyelitis: sequestrum (a dense fragment of dead bone), involucrum (a shell of new bone around it), cloaca (a hole draining pus) and sinus tract.
- Brodie abscess: subacute infection seen as a lucent lesion with a sclerotic rim, usually in a child's metaphysis.
- MRI: marrow dark on T1 and bright on fluid-sensitive (T2 or STIR) images; contrast shows abscesses as rim-enhancing collections.
- Spine: infection usually destroys the disc and both adjacent endplates; tumours tend to spare the disc. Tuberculosis spreads under the ligaments with large paraspinal abscesses.
Why it looks that way
- Pus and oedema replace the fat of the marrow, so MRI sees the change in days, long before enough mineral is lost to show on X-ray.
- Pressure inside the rigid bone cuts off blood supply; a piece of cortex dies and becomes a sequestrum, and the lifted periosteum lays down new bone (involucrum).
- In children, slow blood flow in the vessel loops of the metaphysis lets bacteria settle there, which is why acute haematogenous osteomyelitis starts at the metaphysis.
| Study | Strength | Weakness |
|---|---|---|
| X-ray | First study; shows fractures, gas, foreign bodies, chronic changes | Normal for the first 1 to 2 weeks |
| MRI | Most sensitive and specific; marrow, abscess and extent | Less available; metal artefact; Charcot foot can look similar |
| CT | Sequestrum, cortical destruction, gas; guides biopsy | Poor for early marrow change; radiation |
| Three-phase bone scan | Very sensitive, whole skeleton | Not specific: fracture, surgery and Charcot also light up |
| Labelled white cell scan or FDG-PET | Helps with metal implants and the diabetic foot | Limited availability; slower |
| Ultrasound | Subperiosteal abscess and joint effusion in children; guides aspiration | Cannot see inside bone |
Differential and pitfalls
- A normal early X-ray does not exclude osteomyelitis; if suspicion remains, go to MRI rather than wait.
- Diabetic foot: osteomyelitis sits under an ulcer (toes, metatarsal heads, heel); Charcot neuroarthropathy usually affects the midfoot without an ulcer.
- Destruction of a disc and the two endplates beside it suggests discitis-osteomyelitis rather than metastasis.
- Infection can look like Ewing sarcoma or lymphoma on X-ray; tissue settles the question.
- Bone that can be touched with a sterile probe through an ulcer strongly suggests osteomyelitis.
Next step
- Suspected osteomyelitis: X-ray of the area first, with blood cultures and inflammatory markers (CRP, ESR).
- Normal or doubtful X-ray with ongoing suspicion: MRI with contrast; if MRI is not possible, CT or a labelled white cell scan.
- Ask for bone biopsy or culture (image-guided or surgical) to identify the organism; in a stable patient, discuss with the team before starting antibiotics.
- Abscess, sequestrum, spinal cord compression or neurological signs: urgent call to orthopaedics or spine surgery.
- Child with a limp, fever and bone pain: ultrasound for effusion or subperiosteal abscess and early MRI; call a senior if septic arthritis is possible.
Test yourself
Think of the answer, then tap the question to see it.
How long after onset does osteomyelitis usually start to show bone changes on X-ray?
About 10 to 14 days, because a large share of bone mineral must be lost first.
What is the imaging test of choice when osteomyelitis is suspected and the X-ray is normal?
MRI, ideally with contrast: it shows marrow oedema and abscesses early, and a normal study makes osteomyelitis very unlikely.
What are a sequestrum and an involucrum?
A sequestrum is a fragment of dead bone, dense on imaging; an involucrum is the shell of new bone that the lifted periosteum lays down around it.
How does spinal infection usually differ from spinal metastasis on imaging?
Infection destroys the disc and both adjacent endplates; metastases usually involve the vertebral body and pedicles and spare the disc.
Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Osteomyelitis Imaging (StatPearls (NCBI Bookshelf), 2023); Osteomyelitis (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.