Quantitative Lung Perfusion (Split Function)⁹⁹ᵐTc-MAA

Quick Answer

Quantitative perfusion scintigraphy with ⁹⁹ᵐTc-MAA measures how much each lung (and region) contributes to total pulmonary blood flow, which stands in for regional lung function. Its main use is predicting postoperative lung function before lung-cancer resection: the predicted postoperative FEV₁ (ppoFEV₁) and ppoDLCO estimate what the patient will retain after surgery. A ppoFEV₁ or ppoDLCO below ~40% predicted marks high perioperative risk and prompts further testing (e.g. cardiopulmonary exercise testing). The same split-function data guide lung transplant, lung volume reduction surgery (LVRS), and differential-function questions.

Why it matters

A patient may have borderline whole-lung spirometry yet tolerate resection well if the diseased, to-be-removed lung contributes little function — or, conversely, be far riskier than spirometry suggests if the resected portion is doing most of the work. Perfusion quantifies that regional contribution, turning "how bad are the lungs overall" into "how much function remains after we take this part out."

Technique

• ⁹⁹ᵐTc-MAA perfusion imaging with anterior and posterior planar views; regional counts are combined as the geometric mean to correct for depth, and divided into lung halves or thirds (upper/middle/lower). SPECT improves regional (lobar) quantification.
• Each region's percentage of total perfusion counts is reported. Ventilation can be added, but perfusion is the standard for split-function prediction.

Predicting postoperative function

ppoFEV₁ = preoperative FEV₁ × (fraction of perfusion contributed by the lung/region that REMAINS). For a pneumonectomy, multiply preop FEV₁ by the % perfusion of the non-operated lung; for a lobectomy, use the fraction of remaining functional segments (segment-counting) or regional perfusion. Compute ppoDLCO the same way. ppoFEV₁ or ppoDLCO < ~40% predicted → high risk — proceed to cardiopulmonary exercise testing (VO₂max) rather than straight to surgery.

Express results as % predicted, not absolute liters — risk thresholds are defined against predicted values, and a raw ppoFEV₁ in liters can mislead across body sizes.

Worked example

A patient with preop FEV₁ 2.0 L (80% predicted) needs a right pneumonectomy; perfusion shows the left lung contributes 55%. ppoFEV₁ ≈ 2.0 × 0.55 = 1.1 L (~44% predicted) — above the ~40% high-risk threshold, so resection is likely tolerable pending exercise testing. Had the left lung contributed only 40%, ppoFEV₁ would fall to ~0.8 L (~32% predicted), flagging high risk.

Other uses

The same split-function data inform lung transplant (differential function; which native lung to replace or the contribution of a single transplanted lung), lung volume reduction surgery (targeting the most destroyed, least-perfused regions), and rare congenital/vascular questions. In transplant and LVRS the question shifts from "predict what remains after removal" to "which regions are functionally worthless and safe to remove or bypass."

High-Yield Pearls

• ppoFEV₁ = preop FEV₁ × fraction of perfusion to the remaining lung; the same math gives ppoDLCO.
• ppoFEV₁ or ppoDLCO < ~40% predicted = high perioperative risk → cardiopulmonary exercise testing.
• Use the geometric mean of anterior + posterior counts to correct for depth; SPECT sharpens lobar quantification.

Common Pitfalls

• Reporting absolute liters instead of % predicted against risk thresholds.
• Using perfusion fractions from planar without geometric-mean correction (anterior-only over-weights anterior structures).
• Forgetting that DLCO is an independent predictor — a normal ppoFEV₁ with a low ppoDLCO still flags risk.

Related Pages

• Perfusion agent: ⁹⁹ᵐTc-MAA; base study: V/Q scan & pulmonary embolism.

Self-Check

Key References

• ACCP / ERS-ESTS guidelines on physiologic evaluation before lung resection (ppoFEV₁, ppoDLCO, exercise testing).
• SNMMI/EANM guidance on quantitative lung perfusion scintigraphy and split-function calculation.