Tc-99m MAA

Last revised by Dr Patrick J Rock on 17 Mar 2021

Tc-99m MAA (macroaggregated albumin) is one of the technetium radiopharmaceuticals used in lung perfusion imaging.

Characteristics

  • photon energy: 140 KeV
  • physical half life: 6 hours
  • biological half life: 2-3 hours
  • normal distribution: lungs, liver, spleen, kidneys
  • pharmacokinetics:
    • >90% particles trapped in lung capillaries during first pass
    • particles <10 μg phagocytosed by liver and spleen
    • ~100,000 of 280 billion capillaries occluded during normal lung perfusion scan (= 0.00003% capillaries occluded)
    • particles eventually enter systemic circulation and are phagocytosed by the reticuloendothelial system (RES)
  • excretion: renal, RES
  • particle size: 5-100 μm
  • critical organ: lung

Uses, doses and timings

  • V/Q perfusion
    • adult dose: 74-185 MBq (2-5 mCi) IV (37 MBq (1 mCi) if before ventilation)
    • timing: immediate
    • reduced injection of 100,000-200,000 particles advised in:
      • pulmonary hypertension, R-L shunt, single lung transplantation
      • pregnant patients
      • pediatric patients: weight-related dosing advised
    • blood should not be drawn back into the syringe to ensure needle is in vein before injection (in contrast, to other studies). Blood causes MAA aggregation which results in hot pulmonary microemboli
    • brain uptake is diagnostic of R-L shunt

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