Effective Half-Life (Tₑ)
Combine physical and biological half-lives to find the effective half-life of a radionuclide in the body or organ.
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Example: Tc-99m ≈ 6 hours, I-131 ≈ 8 days.
This depends on the organ and patient — e.g. for iodine, the thyroid clears slowly; for renal tracers, more quickly.
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What’s happening under the hood?
The effective half-life combines both physical decay and biological clearance:
1/Tₑ = 1/Tₚ + 1/Tᵦ → Tₑ = (Tₚ·Tᵦ)/(Tₚ + Tᵦ).