Nuclear medicine

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Nuclear medicine/Radiographic bone scan/Scintigraphy

Radioactive radionuclide isotopes are slowly injected into the body and its distribution and accumulation is analysed by a gamma camera. Generally has a high sensitivity, but low specificity. Useful as, in general, the metabolic changes in bone can be detected before the anatomic changes.

‘Hot spot’ – area of increased uptake of the isotope (eg fracture; infection; metastases; bone tumours; aseptic inflammatory process; metabolic bone disease)
‘Cold spot’ – area of no isotope uptake (eg avascular necrosis; very early osteomyelitis)

Advantages:
• highly sensitive to increases in bone turnover
• good survey tool for entire body
• good role to initially locate bone lesions for further investigation by other modalities

Disadvantages:
• poor spatial resolution
• false negatives for some bone lesions (eg osteoclastic metastases)
• relatively high radiation dose

Indications:
Diagnosis, detection and staging of malignancy; screening for skeletal metastases; differentiating osteomyelitis from cellulitis (bone changes of osteomyelitis may take time to show up on x-ray); diagnosing bone infarction, necrosis and viability; diagnosis of complex regional pain syndrome; evaluating bony healing following fracture or surgery; evaluating unidentified ‘bone pain’ when plain x-ray is normal; early evaluation of possible stress fracture.

Isotopes:
Technetium 99m – uptakes in areas of bony turnover  identifies areas of increased bone metabolism and increased blood flow; used in 3 phase bone scans
Gallium-67 (Ga-67) – binds to white blood cells, plasma proteins  will travel to site of inflammation  localises in neoplastic and inflammatory processes; not a bone scan
Indium 111 (In-111) – bind to white blood cell membrane – may be more accurate at detecting acute infections – Ga-67 may be more sensitive for chronic/subacute infections.

Contraindications:
Poor vascular supply; kidney disease; immunocompromised

Phases of Bone Scan:
Phase 1 - blood flow stage – depends on blood flow to bone  shows perfusion to lesion
Phase 2 - blood pool stage or tissue phase
depends on blood flow to particular area or lesion prior to bone uptake
1 to 6 minutes post injection
Phase 3 - bone imaging phase
represents relative bone turnover
2 to 4 hours post injection
Phase 4 - delayed phase
represents tissue clearance
24 hours post injection

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