Which term describes the physiological rate of elimination of a substance, independent of its physical decay?

• A. Biologic half-life
• B. Attenuation
• C. Detective Quantum Efficiency
• D. Spatial resolution

Answer: (A)

Biologic half-life is the time it takes for half of a substance to be eliminated from the body by biological processes, independent of its radioactive decay. It describes how the body clears the substance—through metabolism, excretion, and other physiological pathways—without regard to how quickly the substance would physically decay. This makes it the term that precisely captures the physiological rate of elimination separate from physical decay.

In nuclear medicine and pharmacokinetics, the overall clearance you observe also involves physical decay; the combined effect is described by the effective half-life, with 1/Teff = 1/Tphys + 1/Tbiol. But for the question’s focus on physiological elimination independent of decay, biologic half-life is the correct concept.

Other terms refer to different imaging or tissue properties. Attenuation describes signal loss as radiation passes through tissue; Detective Quantum Efficiency is about how efficiently a detector converts incoming photons into a usable signal; spatial resolution concerns how finely an imaging system can distinguish two separate points.