The workhorse of nuclear medicine. Produced from the Mo-99/Tc-99m generator (molybdenum cow) via beta decay of Mo-99 (t½ 66 hours), allowing on-site supply without a cyclotron. The 140 keV energy is near-ideal for gamma cameras — high enough to exit tissue, low enough to be collimated efficiently.
Tc-99m itself has no biological activity; utility depends entirely on the labeled compound, which determines biodistribution:
| Radiopharmaceutical | Mechanism | Use |
|---|---|---|
| MDP | Chemisorption to hydroxyapatite | Bone scan |
| DTPA | GFR filtration | Renal function, cisternography |
| MAG3 | Tubular secretion | Renal drainage (preferred in impaired GFR) |
| Mebrofenin | Hepatocyte uptake, biliary excretion | HIDA scan |
| Sestamibi / Tetrofosmin | Myocardial uptake proportional to flow | MPI |
| MAA | Trapped in pulmonary capillaries | Lung perfusion |
| Sulfur colloid | Phagocytosed by RES | Liver/spleen, GI bleed, sentinel node |
| DMSA | Binds renal tubular cells | Cortical imaging, scarring |
| HMPAO / ECD | Lipophilic, crosses BBB | Brain perfusion SPECT |
| Pertechnetate | Trapped in thyroid and gastric mucosa | Thyroid scan, Meckel’s |
| Labeled RBCs | Stays in vascular space | GI bleed, cardiac blood pool |