V/Q Lung Scan (Ventilation-Perfusion)

The ventilation-perfusion (V/Q) scan compares regional pulmonary blood flow with regional ventilation to detect pulmonary embolism. Tc-99m MAA particles lodge transiently in pulmonary arterioles to map perfusion, while an inhaled radioaerosol or radioactive gas maps ventilation. A perfusion defect with normal ventilation (mismatch) in a segmental distribution is the hallmark of PE. It is the preferred test when CT pulmonary angiography is contraindicated, particularly in pregnancy and iodinated-contrast allergy, and delivers a lower breast/fetal dose.

Indications

Suspected acute pulmonary embolism when CTPA is contraindicated or undesirable
Pregnant or young patients where reduced breast and fetal radiation dose is advantageous
Iodinated contrast allergy or significant renal impairment
Diagnosis and mapping of chronic thromboembolic pulmonary hypertension (CTEPH)
Pre-operative quantification of regional lung function before lung resection or volume reduction

Contraindications & Cautions

Right-to-left cardiac shunt or severe pulmonary hypertension are relative contraindications for MAA (systemic embolisation of particles) — reduce particle number
Pregnancy is not a contraindication; a perfusion-only (half-dose MAA) protocol is often used first to limit fetal dose
Breastfeeding: interrupt or discard milk for ~12 h after Tc-99m aerosol/MAA per local guidance owing to free pertechnetate/aerosol
Known severe allergy to human albumin products (MAA)
Reduce MAA particle number in children, pregnancy and pulmonary hypertension to minimise capillary occlusion

Patient Preparation

Obtain a current chest radiograph (ideally within ~24 h) for correlation and interpretation
No fasting required; explain the breathing manoeuvre for the ventilation component
Confirm pregnancy status; counsel on the perfusion-only reduced-dose option where appropriate
Inject MAA with the patient supine to distribute particles evenly and reduce gravitational gradient
Ensure the patient can perform a tidal-breathing inhalation of the ventilation agent

Technique & Parameters

Perfusion: Tc-99m MAA ~40-150 MBq (~200,000-500,000 particles; fewer in pregnancy/pulmonary hypertension) injected slowly IV with the patient supine
Ventilation: Tc-99m Technegas or DTPA aerosol, or Xe-133 / Kr-81m gas, inhaled during tidal breathing
Acquire planar images in 6-8 projections (anterior, posterior, both laterals, and obliques) for both phases
When ventilation follows perfusion, use a higher ventilation dose to overcome existing Tc-99m activity (or vice versa)
SPECT or SPECT/CT improves segmental localisation and reduces indeterminate studies

Systematic Review

Correlate with the chest radiograph to identify matching parenchymal abnormalities
Assess perfusion images segment by segment for defects and their shape (wedge/segmental versus non-segmental)
Compare each perfusion defect with the corresponding ventilation image to classify as matched or mismatched
Count the number and size (large/moderate/small) of mismatched segmental defects
Apply modified PIOPED/PISAPED probability criteria to assign a diagnostic category

Key Findings & Significance

Segmental or larger perfusion defect with normal ventilation (V/Q mismatch) is the hallmark of acute PE
High probability: two or more large mismatched segmental perfusion defects without matching radiographic abnormality
Matched V/Q defect with corresponding radiographic opacity suggests parenchymal disease (e.g. pneumonia, COPD), not PE
Normal perfusion effectively excludes clinically significant PE
Multiple bilateral mismatched defects with mosaic perfusion in a stable patient suggest chronic thromboembolic disease (CTEPH)

Differential Considerations

V/Q mismatch: acute PE, chronic thromboembolic disease, extrinsic vascular compression, prior embolism/scarring
Matched defect: pneumonia, COPD/emphysema, atelectasis, effusion, bullae
Reverse mismatch (ventilation defect > perfusion defect): mucus plugging, consolidation, atelectasis
Non-segmental perfusion defect: cardiomegaly, mediastinal mass, pleural effusion
Diffuse heterogeneous perfusion: pulmonary hypertension, small-vessel disease

Pearls & Pitfalls

A perfusion-only study is often diagnostic in young patients with a normal chest radiograph and halves the dose
Interpret against a current chest radiograph — matched radiographic opacity reclassifies a defect away from high probability
The "stripe sign" (peripheral rim of preserved perfusion around a defect) argues against embolic occlusion
Inject MAA supine and avoid drawing blood back into the syringe to prevent hot-spot clumping artefacts
SPECT/CT reduces the intermediate/indeterminate category compared with planar imaging

Structured Report

State tracers, activities, projections and whether SPECT/CT was performed
Report the number, size and distribution of matched and mismatched defects with radiographic correlation
Assign a probability category (normal, very low, low, intermediate/indeterminate, high) using modified PIOPED
For CTEPH, describe the bilateral segmental mismatch pattern
Give a clear conclusion on the likelihood of PE and any recommended further imaging

References

Bajc M, et al. EANM guideline for ventilation/perfusion single-photon emission computed tomography (SPECT) for diagnosis of pulmonary embolism. Eur J Nucl Med Mol Imaging 2019
PIOPED Investigators. Value of the ventilation/perfusion scan in acute pulmonary embolism. JAMA 1990
SNMMI Procedure Standard for Lung Scintigraphy 4.0
Parker JA, et al. SNM practice guideline for lung scintigraphy. J Nucl Med Technol 2012

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.

V/Q Lung Scan (Ventilation-Perfusion)

📋 Indications

⛔ Contraindications & Cautions

🧑‍⚕️ Patient Preparation

⚙️ Technique & Parameters

🔎 Systematic Review

🩻 Key Findings & Significance

🧠 Differential Considerations

💡 Pearls & Pitfalls

📝 Structured Report

📚 References