Avascular Necrosis (Osteonecrosis)⁹⁹ᵐTc-MDP/HDP

Quick Answer

Avascular necrosis (AVN / osteonecrosis) is bone death from interrupted blood supply — commonly the femoral head (steroids, alcohol, trauma, sickle cell, the dysbarism/Caisson disease). Its bone-scan appearance evolves: very early the infarcted bone is photopenic (cold); as reactive revascularization and repair begin, a rim of increased uptake surrounds the cold center — the "doughnut" sign. MRI is the most sensitive and specific test (earliest changes, marrow edema, double-line sign), so bone scan/SPECT is now a secondary tool, useful when MRI is contraindicated or for whole-body survey (e.g. multifocal infarcts).

The evolving scan

AVN's scintigraphic hallmark is evolution: acutely the dead bone has no perfusion or osteoblastic activity → a cold (photopenic) region; with repair, a surrounding rim of increased uptake develops (the "doughnut" sign — hot rim, cold center). Catching the early cold phase requires suspicion, because later reactive uptake can mask it. SPECT improves detection in the femoral head over planar imaging.

Where MRI leads

MRI is the reference standard for AVN — it detects the earliest marrow changes, shows the double-line sign, and stages disease before collapse. Use bone scan/SPECT when MRI is unavailable or contraindicated, or to survey the whole skeleton for multifocal osteonecrosis/bone infarcts (sickle cell, steroids). In children, Legg-Calvé-Perthes disease shows early femoral-head photopenia.

High-Yield Pearls

• AVN evolves cold → hot rim: early photopenia, then the "doughnut" sign (hot rim, cold center) with repair.
• MRI is most sensitive/specific (earliest changes, double-line sign) — bone scan/SPECT is secondary.
• Bone scan/SPECT is useful for whole-body survey of multifocal infarcts and when MRI is contraindicated.
• Common causes: steroids, alcohol, trauma, sickle cell, dysbarism.

Common Pitfalls

• Missing the early cold phase because later reactive uptake dominates.
• Assuming a normal planar scan excludes femoral-head AVN — add SPECT or MRI.

Related Pages

• Protocol: Bone scintigraphy; pattern: Photopenic/cold lesions.

Differential Diagnosis

• Aggressive lytic metastasis or other tumor causing a photopenic defect — a cold focus is not specific to AVN.
• Osteomyelitis/septic arthritis, which can be cold early (devascularized) then hot with repair, overlapping the AVN evolution.
• Transient osteoporosis of the hip / bone-marrow-edema syndrome — diffuse increased uptake that is self-limiting rather than a fixed doughnut.
• Subchondral insufficiency or stress fracture of the femoral head, which can mimic early osteonecrosis.
• Degenerative/osteoarthritic periarticular uptake around the hip.
• Attenuation/artifact from a hip prosthesis or overlying structures producing an apparent cold region.

Reporting Checklist

• Describe the phase/pattern (photopenic defect vs hot-rim "doughnut") and the site/laterality.
• Add SPECT/SPECT-CT of the femoral head; note that a normal planar scan does not exclude AVN.
• Recommend MRI when clinical suspicion persists or the scan is equivocal (reference standard, pre-collapse staging).
• Survey for multifocal infarcts when risk factors are present (steroids, sickle cell).
• Correlate with radiographs for femoral-head collapse and with risk factors.

Self-Check

Key References

• Reviews of scintigraphy and MRI in osteonecrosis staging; SNMMI bone scintigraphy guideline.