Bone Scintigraphy: Three-Phase Interpretation

This pattern is called a 'superscan' - characterised by diffusely increased skeletal uptake with absent or faint renal visualisation. The kidneys are not seen because essentially all tracer is taken up by bone. In this patient with prostate cancer, the most likely cause is diffuse skeletal metastatic disease. However, the differential includes: (1) Diffuse metastases (prostate, breast most common), (2) Metabolic bone disease - hyperparathyroidism, osteomalacia, renal osteodystrophy. To differentiate, I would look for heterogeneous uptake pattern (favours metastases) versus uniform uptake (favours metabolic). Correlate with PSA level, calcium, PTH, and vitamin D. Plain radiographs may show blastic metastases.

A three-phase bone scan has: Phase 1 (flow/angiogram, 0-60 seconds) showing arterial blood flow, Phase 2 (blood pool, 2-5 minutes) showing soft tissue hyperaemia, and Phase 3 (delayed, 3-4 hours) showing bone uptake. For a stress fracture, I would expect: Phases 1 and 2 may be mildly positive or negative, Phase 3 positive with focal intense uptake at the fracture site. For osteomyelitis, I would expect: All three phases positive with focal bone uptake. The key difference is that stress fractures typically have less prominent early phases compared to osteomyelitis. However, bone scan cannot reliably differentiate acute fracture from osteomyelitis - clinical correlation and potentially MRI would be needed.

Bone scan interpretation around arthroplasty is challenging. Normal healing causes increased uptake for 1-2 years post-surgery, so uptake at 3 years is abnormal. However, bone scan cannot reliably differentiate aseptic loosening from prosthetic joint infection - both cause increased uptake. The scan has low specificity in this setting. For improved specificity, I would recommend combined labelled white cell scan and sulphur colloid marrow scan. In infection, WBC uptake occurs WITHOUT corresponding marrow uptake (spatial mismatch). In normal post-operative change or aseptic loosening, WBC and marrow uptake are matched. This combined technique has greater than 90% accuracy for prosthetic joint infection.