Cyclotron-produced. Acts as a potassium analogue — taken up by viable myocardium via Na/K-ATPase in proportion to blood flow and cell viability. This makes it useful not only for perfusion but also for distinguishing viable from infarcted myocardium (redistribution on delayed images).
Redistribution: Initial uptake reflects perfusion; delayed images (3–4 hours) show redistribution into ischemic but viable myocardium — a unique property that Tc-99m agents lack. Severely reduced initial uptake that improves on delayed imaging = ischemia; fixed defect = scar.
Limitations compared to Tc-99m agents:
- Lower energy photons (68–80 keV) lead to more soft-tissue attenuation
- Longer half-life → higher patient radiation dose
- Cannot be used for first-pass cardiac studies
- No gating possible with same quality as Tc-99m
Parathyroid: historically used as the “blood pool” agent in dual-isotope subtraction with Tc-99m pertechnetate (thyroid only). Now largely replaced by Tc-99m sestamibi subtraction or 4D CT.