Residency · Residency · Diagnostic Radiology
Ventilation-Perfusion Scintigraphy
Introduction
Ventilation-perfusion (V/Q) scintigraphy remains a valuable diagnostic tool for the evaluation of pulmonary embolism (PE), particularly in patients with contraindications to CT pulmonary angiography (CTPA).
Radiopharmaceuticals
Perfusion Agents
Technetium-99m macroaggregated albumin (Tc-99m MAA) is the standard perfusion agent. The particles range from 10-90 micrometers in diameter and lodge in precapillary arterioles. Approximately 200,000-700,000 particles are injected, occluding less than 0.1% of pulmonary arterioles. The patient should be supine during injection to ensure uniform distribution, as upright injection causes a gradient artifact. In patients with known right-to-left shunts, the particle count should be reduced to approximately 100,000-150,000. In pregnancy, both particle count and activity are reduced.
Ventilation Agents
Xenon-133 gas provides single-breath, equilibrium, and washout phases that detect air trapping. Its 81 keV energy is lower than Tc-99m, so ventilation must be performed first. Technetium-99m DTPA aerosol is widely available but may show central deposition (clumping artifact) in patients with obstructive airway disease. Technetium-99m Technegas consists of ultrafine carbon particles labeled with Tc-99m that achieve excellent peripheral penetration and are less affected by obstructive airways disease, though it is not available in the US. Krypton-81m gas has a 13-second half-life and allows simultaneous V/Q imaging but has limited availability.
Acquisition Protocol
Perfusion Imaging
Eight standard views are acquired: anterior, posterior, right lateral, left lateral, right posterior oblique, left posterior oblique, right anterior oblique, and left anterior oblique. Each view is acquired for approximately 500,000-750,000 counts. SPECT/CT acquisition is increasingly used for improved sensitivity and specificity.
Ventilation Imaging
Ventilation imaging is performed before or after perfusion depending on the agent. Xenon-133 is acquired in a single posterior projection through single-breath, equilibrium (3-5 minutes), and washout (2-3 minutes) phases. Tc-99m aerosol or Technegas allows multiple projections matching the perfusion views.
Chest Radiograph Correlation
A recent chest radiograph (within 12-24 hours) is essential for interpretation because parenchymal opacities can cause matched V/Q defects that mimic PE. A normal chest radiograph increases the diagnostic value of the V/Q scan.
Interpretation Criteria
PIOPED II Modified Criteria
| PIOPED II Category | Findings | PE Likelihood |
|---|---|---|
| Normal | No perfusion defects | Excludes PE |
| Very low probability | Non-segmental defects (cardiomegaly, elevated diaphragm) | <10% |
| Low probability | Small subsegmental defects or matched V/Q with CXR abnormality | ~15% |
| Intermediate probability | Difficult to categorize; 1 moderate mismatched defect | ~30% |
| High probability | >=2 large segmental mismatched defects | ~97% specificity |
A normal scan shows no perfusion defects and excludes PE. A very low probability scan shows non-segmental perfusion defects such as those from cardiomegaly, elevated hemidiaphragm, or costophrenic angle defects. A low probability scan shows small subsegmental perfusion defects or matched V/Q defects with a corresponding chest radiograph abnormality. An intermediate probability scan is difficult to categorize and often requires further workup. A high probability scan shows two or more large segmental perfusion defects (or equivalent) with normal ventilation (mismatch), with a specificity for PE of approximately 97%.
PISAPED Criteria (Perfusion-Only Interpretation)
The PISAPED criteria focus on the shape and distribution of perfusion defects. Wedge-shaped, segmental perfusion defects are considered PE-positive, while non-wedge-shaped defects (such as the stripe sign or non-segmental defects) are considered PE-negative. This approach eliminates the need for ventilation imaging in some protocols.
Key Interpretive Signs
The stripe sign is a stripe of preserved perfusion between the defect and the pleural surface, which argues against PE and suggests parenchymal disease. Only segmental defects conforming to pulmonary arterial anatomy suggest PE, while non-segmental defects do not. A triple match -- matched ventilation, perfusion, and chest radiograph abnormality in the lower lobes -- is classified as intermediate probability because it could represent PE or atelectasis/pneumonia.
SPECT V/Q Imaging
SPECT and SPECT/CT improve sensitivity and specificity compared to planar imaging. They reduce the proportion of indeterminate studies from approximately 30-40% to less than 5%. The CT component provides anatomic correlation and may identify alternative diagnoses. European guidelines recommend SPECT V/Q as first-line, and it is increasingly adopted in North America. Automated quantitative analysis software is available for standardized reporting.
Clinical Applications Beyond PE
Quantitative Lung Perfusion
Quantitative lung perfusion is used for pre-operative assessment before lung resection (pneumonectomy or lobectomy). Differential perfusion to each lung is calculated as a percentage of total counts. Predicted post-operative FEV1 is calculated by multiplying the pre-operative FEV1 by the percentage of perfusion to the remaining lung. A minimum predicted post-operative FEV1 of greater than 800 mL or greater than 40% predicted is generally required for surgery.
Chronic Thromboembolic Pulmonary Hypertension (CTEPH)
The V/Q scan is the screening test of choice for CTEPH. One or more segmental or larger mismatched perfusion defects are highly sensitive for CTEPH, and a normal V/Q scan effectively excludes it. CTPA may miss subsegmental chronic thrombi that V/Q scintigraphy detects.
Right-to-Left Shunt Quantification
Tc-99m MAA particles that traverse a right-to-left shunt lodge in systemic capillary beds. Brain and kidney uptake on whole-body images quantifies the shunt fraction, which is normally less than 5%.
Clinical Pearls
Two or more large segmental mismatched perfusion defects constitute a high-probability scan with approximately 97% specificity for PE. V/Q scan is preferred over CTPA in patients with contrast allergy, renal insufficiency, or pregnancy (though institutional protocols vary for pregnancy). SPECT V/Q significantly reduces indeterminate results compared to planar imaging. V/Q scintigraphy is the screening test of choice for CTEPH and is more sensitive than CTPA for this diagnosis.
References
- PIOPED II Investigators: Diagnostic Pathways in Acute Pulmonary Embolism. JAMA. 2006;295(2):172-179.
- EANM Guidelines for Ventilation/Perfusion Scintigraphy. Eur J Nucl Med Mol Imaging. 2019;46(12):2429-2451.
- V/Q SPECT and SPECT/CT in the Diagnosis of Pulmonary Embolism. Semin Nucl Med. 2017;47(6):584-598.
- Chronic Thromboembolic Pulmonary Hypertension: Role of Imaging. Radiographics. 2020;40(4):1046-1068.