Mo-99/Tc-99m Generator

The Molybdenum-99/Technetium-99m generator is the “workhorse” of nuclear medicine, providing an on-site source of Tc-99m.

Learning Objectives

• Explain the parent-daughter relationship and transient equilibrium
• Describe the construction and operation of the generator
• Calculate elution efficiency and theoretic yield
• Understand Mo-99 and Alumina breakthrough limits

Principle

The generator relies on Transient Equilibrium, where the parent (Mo-99) half-life is longer than but not significantly greater than the daughter (Tc-99m).

• Parent: Molybdenum-99
  • Half-life: 66 hours
  • Source: Fission of U-235 (reactor produced)
  • Chemically mimics: Sulfate (MoO₄²⁻)
• Daughter: Technetium-99m
  • Half-life: 6 hours
  • Chemically mimics: Pertechnetate (TcO₄⁻)

Generator Construction

1. Column: Contains Alumina (Al₂O₃)
2. Loading: Mo-99 (as Molybdate) is adsorbed onto the alumina column.
3. Elution: Saline (0.9% NaCl) is passed through the column.
   • Mo-99 binds tightly to alumina.
   • Tc-99m (as pertechnetate) binds loosely and washes off with saline.

Elution Profile

• Typical elution efficiency is 75-85%.
• Maximum Tc-99m yield occurs roughly every 23-24 hours.

Quality Control (The “Moly Breakthrough” Test)

We must ensure that the eluate contains pure Tc-99m without contaminants from the column.

1. Radionuclidic Purity (Moly Breakthrough)

• Limit: < 0.15 µCi Mo-99 per 1 mCi Tc-99m (at time of administration).
• Test: Comparison shield method (Moly Assay Canister).
  • Place eluate in lead shield (blocks 140 keV Tc gammas, allows high energy Mo-99 gammas ~740/780 keV to pass).
  • Measure unshielded (total activity) and shielded (Mo-99 only).

2. Chemical Purity (Aluminum Breakthrough)

• Contaminant: Aluminum ion (Al³⁺) from the column.
• Limit: < 10 µg Al³⁺ per mL.
• Test: Colorimetric spot test (Aurintricarboxylic acid).
  • Compare color of drop to standard solution. Pink/red indicates excess aluminum.
• Consequence: Excess Al³⁺ causes Tc-99m colloid formation → liver/lung uptake.