Residency · Residency · Nuclear Medicine

Ventilation-Perfusion (V/Q) Lung Scintigraphy

Overview

V/Q scintigraphy evaluates regional ventilation and perfusion to detect ventilation-perfusion mismatch, the hallmark of pulmonary embolism. It remains an important alternative to CT pulmonary angiography in specific patient populations. The study is non-invasive, delivers a low radiation dose, and does not require iodinated contrast.

Radiopharmaceuticals

Perfusion Agent

Tc-99m MAA (macroaggregated albumin) consists of particles 10 to 90 micrometers in diameter, with a target range of 20 to 40 micrometers. These particles undergo microembolization in the pulmonary capillary bed in proportion to regional blood flow. The typical dose is 1 to 4 mCi (37 to 148 MBq), delivering 200,000 to 700,000 particles. The particle count is reduced to 100,000 to 200,000 in patients with right-to-left shunt, severe pulmonary hypertension, or pediatric patients. The injection must be performed with the patient supine to ensure uniform distribution, and blood should not be drawn back into the syringe to avoid clot formation around the particles.

Ventilation Agents

Xenon-133 gas emits at 81 keV and must be performed before Tc-99m perfusion imaging due to its lower energy. It is acquired in single-breath, equilibrium, and washout phases, with washout abnormalities being particularly sensitive for small airway disease. Its disadvantages include limited image quality at the lower energy and the need for dedicated gas trapping systems. Tc-99m DTPA aerosol is nebulized and inhaled, but produces a central deposition pattern in patients with COPD that can limit interpretation. Tc-99m Technegas uses ultrafine carbon particles labeled with Tc-99m and offers superior peripheral penetration compared with DTPA aerosol, especially in COPD, but is not available in the United States. Krypton-81m gas has an ultra-short half-life of 13 seconds, allowing simultaneous ventilation and perfusion imaging, but is limited by availability and cost.

<image>Normal V/Q lung scan showing matched ventilation (Xe-133 washout phase) and perfusion (Tc-99m MAA) with uniform distribution in all lung segments bilaterally</image>

Acquisition Protocol

Perfusion Imaging

Eight standard views are acquired: anterior, posterior, right posterior oblique, left posterior oblique, right anterior oblique, left anterior oblique, right lateral, and left lateral. A minimum of 500,000 to 750,000 counts per view is obtained. SPECT/CT is increasingly used as an alternative, offering improved sensitivity and specificity.

Ventilation Imaging

When using xenon-133, single-breath, equilibrium (rebreathing), and washout phases are acquired in the posterior projection. With Tc-99m DTPA aerosol, the same eight views as perfusion are obtained. Ventilation is typically performed first to avoid downscatter from the perfusion agent.

Interpretation Criteria

PIOPED II (Modified) Criteria

A normal scan shows no perfusion defects, with perfusion matching ventilation throughout. Very low probability includes non-segmental perfusion abnormalities such as cardiomegaly, elevated hemidiaphragm, and costophrenic angle effusions. Low probability encompasses small subsegmental mismatches and matched V/Q defects in up to three segments. Intermediate probability includes patterns that do not fit high or low probability categories, such as a single moderate segmental mismatch or findings that are difficult to categorize. This intermediate category is problematic because it requires further testing. High probability requires two or more large segmental mismatches (greater than 75% of a segment) without corresponding ventilation or radiographic abnormality.

Modified PISAPED Criteria

The PISAPED approach uses a binary classification of PE present or PE absent. PE is diagnosed when one or more wedge-shaped perfusion defects at the segmental level or larger are identified. This approach does not require ventilation imaging, using a perfusion-only method. It achieves higher specificity than PIOPED and reduces the number of indeterminate results.

SPECT V/Q Approach

V/Q SPECT achieves improved sensitivity of approximately 97% and specificity of approximately 91% compared with planar V/Q scanning. It reduces the number of indeterminate studies to less than 3% and allows three-dimensional assessment with better segmental localization. V/Q SPECT/CT adds a low-dose CT for anatomic correlation and identification of alternative diagnoses.

<image>High-probability V/Q scan showing multiple large segmental perfusion defects in the right middle and lower lobes with preserved ventilation (mismatch pattern) consistent with pulmonary embolism</image>

Segmental Anatomy

The right lung has 3 lobes and 10 segments, while the left lung has 2 lobes and 8 segments, with the lingula corresponding to the right middle lobe. Recognizing segments in all eight views is essential for accurate defect sizing and localization. The fissure sign refers to a sharp linear perfusion defect along a fissure boundary, which suggests PE rather than parenchymal disease.

Stripe Sign

The stripe sign refers to preserved perfusion at the periphery of a lung segment, between a perfusion defect and the pleural surface. Its presence suggests that the defect is not caused by PE, since PE produces wedge-shaped defects that extend to the pleural surface. The stripe sign lowers the probability of PE.

Matched vs. Mismatched Defects

A mismatch, where a perfusion defect has preserved ventilation, indicates PE. A matched defect, where both ventilation and perfusion are abnormal, indicates parenchymal disease such as pneumonia, COPD, or atelectasis. A reverse mismatch, where there is a ventilation defect with preserved perfusion, indicates airway obstruction from a mucus plug or bronchial lesion.

Controversy: V/Q vs. CTPA

Advantages of V/Q

V/Q scanning requires no iodinated contrast and is safe in patients with renal insufficiency or contrast allergy. It delivers a lower radiation dose to breast tissue, which is important in young women. It is safe in pregnancy, with a lower fetal dose than CTPA, and provides functional information about regional perfusion.

Advantages of CTPA

CTPA offers rapid acquisition, wide availability, and high sensitivity. It provides alternative diagnoses such as pneumonia, aortic dissection, and malignancy. It is better suited for detecting large central PE and assessing right ventricular strain. Interpretation is less operator-dependent.

Preferred Populations for V/Q

V/Q scanning is preferred in young women (due to breast dose concerns), pregnant patients, patients with renal insufficiency (eGFR below 30 mL/min), patients with contrast allergy, and patients with a normal chest radiograph (which yields a higher diagnostic quality V/Q study).

<image>Side-by-side comparison of a V/Q scan and CTPA in the same patient with pulmonary embolism: V/Q showing segmental mismatch and CTPA showing the corresponding filling defect in the pulmonary artery</image>

Special Considerations

Right-to-Left Shunt

In patients with a right-to-left shunt, Tc-99m MAA particles bypass the lungs and deposit systemically in organs such as the brain and kidneys. The particle count is reduced to 100,000 to 200,000. Systemic activity is visible on imaging but clinically insignificant at diagnostic doses. Shunt fraction can be quantified.

Pulmonary Hypertension

A reduced particle count of 100,000 to 200,000 is used to avoid further hemodynamic compromise, though the risk is largely theoretical at standard diagnostic particle counts.

Pregnancy

V/Q scanning is safe in pregnancy. A half-dose of Tc-99m MAA (1 to 2 mCi) is used. A perfusion-only study may be performed if the chest radiograph is normal, skipping ventilation to further reduce the dose. The fetal radiation dose is approximately 0.1 to 0.5 mSv, well below the threshold for concern.

Clinical Pearls

A normal perfusion scan effectively excludes PE regardless of the ventilation result.

The chest radiograph must be reviewed before V/Q interpretation, as pleural effusions and consolidation affect the assessment.

SPECT V/Q significantly reduces indeterminate results compared with planar imaging.

In chronic thromboembolic pulmonary hypertension, V/Q scanning is more sensitive than CTPA for detecting chronic perfusion defects.

MAA must be injected with the patient supine. Upright injection causes a gravity-dependent perfusion gradient that mimics basal disease.

Triple-matched defects, where V/Q defects are matched and correspond to a chest radiograph abnormality, are low probability for PE.

References

• Defined Role of V/Q SPECT vs. Planar V/Q. Journal of Nuclear Medicine, 2017.
• PIOPED II Investigators. JAMA, 2006.
• Defined Role of V/Q Scanning in Pregnancy. European Journal of Nuclear Medicine, 2019.
• ACR Appropriateness Criteria: Suspected Pulmonary Embolism.
• EANM Guideline for V/Q SPECT. European Journal of Nuclear Medicine, 2019.

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