What is a VQ Scan Used For?
A VQ (ventilation-perfusion) scan is primarily used to diagnose acute pulmonary embolism (PE) and to screen for chronic thromboembolic pulmonary hypertension (CTEPH), serving as the preferred imaging modality in specific clinical scenarios including pregnancy, contrast allergy, renal failure, and young patients where radiation reduction is prioritized. 1, 2
Primary Clinical Indications
Acute Pulmonary Embolism Diagnosis
VQ scanning is the established diagnostic test for suspected acute PE, with a high probability scan correctly indicating PE in 86-92% of cases and a normal scan excluding PE with 96% accuracy. 1
The scan should be performed within 24 hours of clinical suspicion, as some scans revert to normal quickly and half normalize within one week. 1
VQ scanning is particularly indicated as first-line imaging in patients with contraindications to CT contrast (severe allergies, renal failure, myeloma), pregnancy, young patients (especially women) to minimize radiation exposure, and outpatients with low clinical probability and normal chest X-ray. 1, 2
The radiation exposure from VQ scan (1.1 mSv) is significantly lower than CT angiography (2-6 mSv). 1
Chronic Thromboembolic Pulmonary Hypertension Screening
VQ scanning is the mandatory first-line screening test for CTEPH in all patients with unexplained pulmonary hypertension, demonstrating 90-100% sensitivity and 94-100% specificity. 1, 3
A normal or low-probability VQ scan essentially excludes CTEPH as a diagnostic consideration. 1, 3
VQ scanning is superior to CT pulmonary angiography for CTEPH detection, with sensitivity of 96-97% versus only 51% for CT. 1, 3
The presence of at least one segmental or larger mismatched perfusion defect (wedge-shaped, segmental pattern) is the key criterion suggesting CTEPH. 3
Interpretation Framework
Three-Tier Classification System
Reports should categorize findings as: normal scan (excludes PE), high-probability scan (confirms PE in most patients), or non-diagnostic/indeterminate scan (requires further testing). 1, 2
High probability scans have >90% reader agreement, while indeterminate and low probability scans have only 70-75% agreement. 1
Non-diagnostic scans require further evaluation, not management decisions based on clinical grounds alone. 1
Technical Requirements
A current good quality chest radiograph must be available before performing and interpreting the VQ scan. 1, 2
Both ventilation and perfusion components should be performed for optimal diagnostic accuracy, though perfusion scanning alone may be acceptable when ventilation scanning is unavailable. 1, 2
Multiple views (minimum four: anterior, posterior, right and left posterior oblique) should be obtained to reduce indeterminate reports. 1, 2
Clinical Scenarios Where VQ Scan Interpretation is Difficult
Alternative imaging investigations are often more rewarding in these situations: 1, 2
- Previous pulmonary embolism (unless follow-up scan has been performed)
- Left heart failure (causes regional perfusion variations)
- Chronic obstructive airways disease (local ventilation variations and vascular constriction)
- Lung fibrosis (patchy unmatched defects in both ventilation and perfusion)
- Proximal lung cancer (vascular occlusion with preserved ventilation)
Additional Clinical Applications
- Evaluation of congenital heart disease and lung abnormalities 4, 5
- Pretreatment planning for lung cancer and emphysema 5
- Post-transplant imaging for bronchiolitis obliterans 5
- Assessment of childhood diseases including cystic fibrosis, bronchopulmonary dysplasia, and asthma 4
Integration with Clinical Probability Assessment
VQ scan results must be interpreted in conjunction with validated clinical probability assessment (Wells score or revised Geneva score) and clinical features. 1, 2
A negative highly sensitive D-dimer in low probability patients combined with normal VQ scan has a 3-month thromboembolic risk <1%. 6
Direct communication between the nuclear medicine physician and requesting clinician improves diagnostic value. 1