# Pulmonary embolism: ventilation/perfusion imaging

Source: https://nuclide.cc/indications/pulmonary-embolism-vq/  
Nuclide record `pulmonary-embolism-vq` (Indication). Data CC BY-NC 4.0, attribute "Data from Nuclide (nuclide.cc)"; commercial use needs a licence.

## TL;DR

To check whether a blood clot has lodged in the lung's blood vessels, a scan compares which parts of the lung get air (ventilation) with which parts get blood flow (perfusion); a clot shows up as an area that is ventilated but not perfused.

## Summary

Ventilation/perfusion (V/Q) scintigraphy diagnoses or excludes acute pulmonary embolism by comparing two maps of the lung obtained in the same sitting: a perfusion map, made by injecting Tc-99m-labelled macroaggregated albumin (MAA) particles that lodge in pulmonary capillaries in proportion to blood flow, and a ventilation map, made with an inhaled agent such as Technegas or krypton-81m gas. A pulmonary embolus blocks a branch of the pulmonary artery, so the perfusion map shows a wedge-shaped defect in that lung segment while the corresponding ventilation map remains normal - a mismatch that is the scintigraphic hallmark of embolism, as opposed to a matched defect where both are reduced together, typical of parenchymal lung disease rather than embolism.

The 2019 EANM guideline for V/Q SPECT strongly recommends tomographic (SPECT) rather than planar imaging wherever available, since SPECT has higher sensitivity and specificity, and defines a positive study as a mismatch of at least one segment or two subsegments conforming to pulmonary vascular anatomy; a normal perfusion study, respecting the anatomical boundaries of the lungs, excludes pulmonary embolism outright regardless of any other finding. Combining V/Q SPECT with a low-dose CT further improves specificity, particularly in patients with coexisting chronic obstructive pulmonary disease, heart failure or pneumonia, all of which can otherwise complicate interpretation.

V/Q scanning has a substantially lower radiation dose than CT pulmonary angiography, no iodinated contrast requirement, and virtually no absolute contraindications, which makes it the preferred first-line test in pregnancy, in patients with renal impairment or contrast allergy, and in younger patients where lifetime radiation exposure is a bigger consideration. ACR appropriateness criteria and SNMMI/EANM/ASH/ACCP/ACEP appropriate use criteria both support V/Q scanning or CT pulmonary angiography as appropriate first-line tests for suspected pulmonary embolism with intermediate or high pre-test probability, with the choice between them driven by patient factors (pregnancy, renal function, chest radiograph findings, local availability) rather than one modality being universally superior.

## Fields

- Kind: Indication
- Last checked: 2026-09-22
- Tags: pulmonary; SPECT; emergency; cardiopulmonary
- Group: pulmonary
- Burden: Pulmonary embolism has an estimated annual incidence in the United States of roughly 60-115 cases per 100,000 people, is part of a broader venous thromboembolism burden affecting up to 900,000 Americans a year, and is the third most common cause of cardiovascular death after coronary artery disease and stroke.

## Standard of care

- Suspected acute pulmonary embolism, intermediate or high pre-test probability, especially pregnancy or contraindication to iodinated contrast: Ventilation/perfusion SPECT (or SPECT/CT), interpreted per EANM criteria: mismatch of at least one segment or two subsegments confirms PE, normal perfusion excludes it. ([Technegas (technetium Tc-99m labelled carbon inhalation aerosol)](https://nuclide.cc/drugs/technegas/), [Technetium Tc-99m albumin aggregated (MAA)](https://nuclide.cc/drugs/tc-99m-maa/), [Krypton-81m](https://nuclide.cc/isotopes/kr-81m/))
- Suspected acute pulmonary embolism, general population without a specific reason to avoid contrast: CT pulmonary angiography or V/Q scanning, either considered appropriate first-line by ACR and SNMMI/multi-society appropriate use criteria, chosen by local availability and patient factors.

## State of the art

- The 2019 EANM guideline for ventilation/perfusion SPECT for diagnosis of pulmonary embolism and beyond is the current European reference standard, recommending Technegas or krypton gas over Tc-99m-DTPA aerosol in patients with COPD and SPECT over planar imaging throughout.
- The SNMMI-led multi-society (with EANM, ASH, STS, ACCP, ACEP) appropriate use criteria for V/Q imaging in pulmonary embolism, most recently updated 2020, give scenario-by-scenario appropriateness ratings endorsed by both the American Society of Hematology and American College of Emergency Physicians.
- V/Q SPECT/CT combines the mismatch read with low-dose anatomical correlation, improving specificity in patients with coexisting COPD, heart failure or pneumonia that would otherwise confound interpretation.

## Open problems

- Probabilistic (PIOPED-style) interpretation categories are now considered flawed by EANM and have been replaced by a simpler binary/nondiagnostic scheme, but older planar-based probabilistic reporting persists in some centres.
- A 'nondiagnostic' or intermediate-probability result still occurs in a meaningful minority of studies and usually requires a second test (CT pulmonary angiography or clinical follow-up) to resolve.
- Real-world uptake of SPECT over planar V/Q imaging, and of Technegas over older ventilation agents, remains uneven internationally, driven by equipment and radiopharmaceutical availability rather than by guideline preference alone.

## Sources

- EANM guideline for ventilation/perfusion SPECT for diagnosis of pulmonary embolism and beyond (2019): https://pubmed.ncbi.nlm.nih.gov/31410539/
- SNMMI appropriate use criteria for ventilation/perfusion (V/Q) imaging in pulmonary embolism: https://snmmi.org/Web/Web/Clinical-Practice/Appropriate-Use-Criteria/Articles/Appropriate-Use-Criteria-for-Ventilation-Perfusion--V-Q--Imaging-in-Pulmonary-Embolism.aspx
- CDC: data and statistics on venous thromboembolism: https://www.cdc.gov/blood-clots/data-research/facts-stats/index.html

## Connected records

- drugs: [Technegas (technetium Tc-99m labelled carbon inhalation aerosol)](https://nuclide.cc/drugs/technegas/), [Technetium Tc-99m albumin aggregated (MAA)](https://nuclide.cc/drugs/tc-99m-maa/)
- isotopes: [Krypton-81m](https://nuclide.cc/isotopes/kr-81m/), [Technetium-99m](https://nuclide.cc/isotopes/tc-99m/)
- companies: [Cyclopharm](https://nuclide.cc/companies/cyclopharm/)
- technologies: [SPECT & bone scan](https://nuclide.cc/technologies/spect/), [SPECT/CT](https://nuclide.cc/technologies/spect-ct/), [Split-function perfusion scintigraphy before lung resection](https://nuclide.cc/technologies/split-function-lung-perfusion/)
- trials: [CYC-009: Technegas versus xenon-133 ventilation imaging](https://nuclide.cc/trials/nct03054870/)
- indications: [Non-small cell lung cancer](https://nuclide.cc/indications/nsclc/)

---
JSON: https://nuclide.cc/api/v1/entities/pulmonary-embolism-vq.json