After a Mo-99/Tc-99m generator is eluted, approximately how many hours does it take for the Tc-99m activity to reach a maximum level?

The correct answer is 24 hours.

In a Mo-99/Tc-99m generator system, Molybdenum-99 (Mo-99) decays into Technetium-99m (Tc-99m). After the generator is eluted, the activity of Tc-99m increases as it is harvested from the decay of Mo-99. The process of elution does not yield Tc-99m instantaneously at a maximum level; instead, it follows a specific kinetics in relation to the decay of Mo-99, which has a half-life of approximately 66 hours.

The peak activity of Tc-99m typically occurs around 24 hours post-elution. This timing aligns with the fact that, after initial extraction, the activity will be affected by the decay of the parent isotope while being replenished from the decay of remaining Mo-99 in the generator. After about 24 hours, the Tc-99m activity will begin to plateau and ultimately will start to decline again due to its own short half-life of about 6 hours.

This timeline is important in a clinical setting, where patient dosing and imaging schedules need to consider the optimal timing for maximum radiopharmaceutical effectiveness.