Tc-99m-MAA⁹⁹ᵐTc-MAA
Macroaggregated albumin for lung perfusion and pre-radioembolization mapping
Quick Answer
Tc-99m-MAA consists of albumin particles slightly larger than capillaries that lodge in the pulmonary (or hepatic-arterial) microvasculature on first pass, mapping regional blood flow. It images lung perfusion (the "Q" of a V/Q scan for pulmonary embolism), predicts postoperative lung function, and is central to pre-Y-90 radioembolization mapping, where it simulates microsphere distribution to measure lung-shunt fraction and detect extrahepatic deposition.
Mechanism
Temporary microembolization: particles (~10–90 µm) occlude a tiny, safe fraction of capillaries (a small fraction of the pulmonary bed) in proportion to blood flow, then degrade over hours. Because distribution follows flow, the same particle serves both a diagnostic (lung perfusion) and a planning/simulation (Y-90 mapping) role.
Biodistribution
Lungs (IV injection) or liver (hepatic-arterial injection during mapping); free pertechnetate → thyroid/stomach if labeling is poor. In a right-to-left shunt, particles reach the systemic circulation (brain, kidneys).
Clinical Indications
- V/Q scanning for pulmonary embolism (perfusion component), especially where CTPA is contraindicated.
- Quantitative lung perfusion before lung surgery (predicted postoperative FEV₁/DLCO).
- Pre-radioembolization mapping: lung-shunt fraction and extrahepatic-activity assessment.
Protocol Notes
- Lung perfusion: IV injection with the patient supine (to distribute particles evenly against gravity); ~200,000–500,000 particles typically. Paired with a ventilation study for V/Q.
- Y-90 mapping: catheter-directed injection into the intended hepatic-arterial territory, then SPECT/CT to compute lung-shunt fraction and check for GI deposition.
- Reduce particle number when a large right-to-left shunt is suspected (safety).
Interpretation
- V/Q: segmental perfusion defects with preserved ventilation (mismatch) suggest PE (see V/Q page).
- Mapping: a high lung-shunt fraction raises radiation-pneumonitis risk and limits deliverable Y-90 activity; extrahepatic GI deposition may require coil embolization before therapy.
Reporting Checklist
- V/Q: report perfusion pattern (segmental defects) with the ventilation study and a PISAPED/PIOPED-type read.
- Mapping: report the lung-shunt fraction (with the dose implication) and any extrahepatic deposition.
- Note labeling quality (free pertechnetate) and any systemic (shunt) activity.
Common Pitfalls
- Poor radiolabeling → free pertechnetate (thyroid/stomach) confounding images.
- Inadequate mixing → clumping and artifactual hot spots.
- Right-to-left cardiac shunt → systemic (brain/kidney) activity (reduce particle number).
- Injecting too few/too many particles (count matters for safety and image quality).
Board Pearls
MAA particles are slightly larger than capillaries, so on first pass they lodge in the microvasculature in proportion to blood flow — mapping lung perfusion (the "Q") when injected IV, and simulating Y-90 microsphere distribution when injected into the hepatic artery before radioembolization.
Before Y-90, MAA mapping measures the lung-shunt fraction (activity bypassing the liver to the lungs) and detects extrahepatic GI deposition — a go/no-go and dose-limiting step: a high shunt raises radiation-pneumonitis risk and limits deliverable activity.
Practical cautions: poor labeling leaves free pertechnetate (thyroid/stomach); clumping produces artifactual hot spots; and in a right-to-left shunt, systemic brain/kidney MAA activity appears — particle number is reduced for safety when a large shunt is suspected. For quantitative pre-op planning, regional perfusion predicts postoperative lung function.
Related Pages
- Protocol/disease: V/Q scan & PE, hepatocellular carcinoma; therapy: Y-90 radioembolization.
- Calculator: lung-shunt fraction tool.
- Pitfalls: Pearls, pitfalls & normal variants.
Figure / Diagram Suggestions
- A microembolization cartoon (particles lodging proportional to flow).
- A Y-90 mapping workflow (MAA simulation → lung-shunt gating → therapy).
- A right-to-left shunt teaching image (systemic brain/kidney MAA activity).
Self-Check (Board-Style)
Q1. How does the same MAA particle serve both lung perfusion imaging and Y-90 planning?
Answer: It lodges in the microvasculature proportional to blood flow — mapping lung perfusion when injected IV and simulating microsphere distribution when injected into the hepatic artery.
Q2. Why is the lung-shunt fraction a dose-limiting, go/no-go measurement before Y-90?
Answer: A high lung-shunt fraction means more Y-90 would reach the lungs, risking radiation pneumonitis — it caps the deliverable activity (or contraindicates therapy).
Q3. On a lung perfusion scan, unexpected brain and kidney activity appears. What does this indicate?
Answer: A right-to-left shunt — particles bypass the lungs to the systemic circulation; reduce particle number for safety when a large shunt is suspected.
Q4. Thyroid and gastric activity appears on an MAA study. Cause?
Answer: Free pertechnetate from poor radiolabeling — a labeling artifact, not shunt or disease.
Evidence & sources
https://nuc.med/maa/ Report an issue