Normal: Symmetric skeletal uptake, kidneys and bladder visible, mild growth plate activity in children.
Patterns and Interpretation
| Pattern | Differential / Interpretation | Key Pearls |
|---|---|---|
| Focal hot spot (delayed) | Metastasis, fracture, osteomyelitis, Paget's disease, arthritis — non-specific | Most common pattern; CT/MRI correlation required for specificity |
| 3-phase positive (↑ flow + ↑ blood pool + ↑ delayed at same focal site) | Osteomyelitis — all 3 phases elevated at infection site | Sens ~90%, Spec ~73%; compare with cellulitis: flow + blood pool elevated but delayed normal/mildly ↑ |
| Flow + blood pool ↑, delayed normal or mildly ↑ | Cellulitis — no osseous involvement | Key distinction from osteomyelitis; if delayed becomes focally hot, consider underlying bone infection |
| Cold / photopenic defect | Early AVN, aggressive lytic tumor, multiple myeloma, radiation field | Early AVN: cold on bone scan — MRI gold standard; myeloma: cold or normal (unlike most metastases) |
| Superscan — diffusely increased skeletal uptake; absent or faint renal/soft tissue activity | Diffuse bone metastases (prostate, breast), metabolic bone disease (renal osteodystrophy, hyperparathyroidism) | Can appear deceptively "normal" — absent kidney activity is the key finding; correlate with PSA/tumor markers |
| Flare phenomenon | Transient worsening of bone scan 2–6 months after starting effective chemotherapy — represents healing, NOT progression | Correlate with tumor markers and CT; serial scans show improvement after 6 months |
Superscan pitfall: Diffuse metastatic disease can look deceptively "normal" — the skeleton is uniformly hot and the kidneys are absent or faint. Always assess renal activity. If kidneys are not visible on a whole-body bone scan, consider superscan.
Osteomyelitis vs. cellulitis: The key is the delayed phase. Both elevate flow and blood pool. Osteomyelitis elevates all three phases at the same focal site. Cellulitis shows diffuse soft-tissue phase uptake that does NOT focally concentrate on delayed images. When in doubt, MRI provides definitive soft tissue and marrow characterization.