Pulmonary Nuclear Medicine — Ventilation/Perfusion Imaging
- Method: EANM (2019) recommends tomographic V/Q (SPECT), read holistically and reported as PE present or absent; it is positive with a mismatch of at least one segment or two subsegments.
- Accuracy: in over 5000 patients, V/Q SPECT had sensitivity 96–99%, specificity 96–98% and only 1–4% non-diagnostic reports.
- Dose: about 2 mSv effective dose, and under 1 mSv to the breast — far less than CTPA.
- CTEPH: V/Q is the screening test of choice (sensitivity ≈96% vs ≈51% for CTPA); a normal perfusion scan effectively excludes it.
Introduction
Ventilation/perfusion (V/Q) scintigraphy maps regional airflow and pulmonary arterial blood flow. Its main use is the diagnosis of acute pulmonary embolism (PE), where it has no contraindications and is preferred in pregnancy, contrast allergy, renal impairment and young women. It is also the reference screening test for chronic thromboembolic pulmonary hypertension (CTEPH), and it quantifies regional lung function before surgery.
Radiopharmaceuticals
| Component | Agent | Key facts |
|---|---|---|
| Perfusion | ⁹⁹ᵐTc-MAA | Particles 15–100 µm lodge in precapillary arterioles; about 400 000 particles usually injected (60 000 suffice for imaging) |
| Ventilation | ⁹⁹ᵐTc-Technegas | Graphite particles 0.005–0.2 µm behave almost like a gas; use within 10 min of generation; preferred in COPD |
| Ventilation | ⁹⁹ᵐTc-DTPA aerosol | Droplets 1.2–2 µm; central airway deposition ("hot spots") in obstructive disease |
| Ventilation | ⁸¹ᵐKr gas | Half-life 13 s, 190 keV — ventilation and perfusion can be acquired simultaneously; ⁸¹Rb generator lasts only one day (T½ 4.6 h) |
| Ventilation | ¹³³Xe gas | 81 keV, so ventilation must precede perfusion; allows wash-out imaging; now rarely used |
Acquisition protocol and safety
- One-day protocol: aerosol ventilation SPECT (25–30 MBq deposited) followed immediately by perfusion SPECT (140–160 MBq ⁹⁹ᵐTc-MAA), without moving the patient, general-purpose collimator, 64 × 64 matrix, 120–128 projections, iterative reconstruction (EANM 2019).
- Shake the vial; never draw blood back into the syringe (it clumps MAA into hot spots); inject over about 30 s with the patient supine.
- Low-dose CT (adds about 1–2 mSv) improves specificity by showing emphysema, pneumonia and other causes of defects.
- Pregnancy: two-day protocol — perfusion-only SPECT with 50 MBq MAA on day 1; a normal study excludes PE. If abnormal, anticoagulate and add ventilation on day 2. Fetal dose is ≤0.12 mGy for both V/Q and CTPA, but the maternal breast dose is 0.5–0.8 mSv for V/Q versus 5–20 mSv for CTPA.
Clinical indications
- Suspected acute PE after clinical probability scoring (Wells or revised Geneva) and D-dimer, when imaging is needed.
- Follow-up of PE and suspected recurrence — V/Q separates old from new defects when a baseline exists.
- Screening for CTEPH in unexplained or persistent breathlessness after PE, or in pulmonary hypertension.
- Quantifying regional perfusion before lung resection or lung-volume reduction.
- Quantifying right-to-left shunt (for example hepatopulmonary syndrome).
Interpreting V/Q
| Pattern | Meaning |
|---|---|
| Mismatch — perfusion defect, normal ventilation, segmental or subsegmental and pleural-based | Pulmonary embolism |
| Matched defect | Airway or parenchymal disease (COPD, pneumonia, atelectasis, effusion, tumour) |
| Reversed mismatch — ventilation worse than perfusion | COPD, pneumonia, mucus plugging |
| Antigravitational perfusion (anterior more than posterior when supine) | Pulmonary congestion from left heart failure — a non-segmental mismatch that is not PE |
| Stripe sign — rim of preserved perfusion at the pleura beyond a defect | Argues against PE; typical of pneumonia |
| Normal perfusion | Excludes PE |
Diagnosing pulmonary embolism
- V/Q SPECT (EANM 2019): PE is present with a V/Q mismatch of at least one segment or two subsegments that conforms to pulmonary vascular anatomy. A single subsegmental mismatch is reported but does not meet the criteria.
- No PE: normal perfusion; matched or reversed-mismatch defects of any size; mismatch that does not follow lobar, segmental or subsegmental anatomy.
- Non-diagnostic: widespread abnormalities not typical of a specific disease (1–4% of SPECT studies).
- Planar studies were historically reported with PIOPED probability categories (normal, very low, low, intermediate, high); this probabilistic approach produced many non-diagnostic reports, and EANM recommends binary SPECT reporting instead.
- The extent of PE can be expressed as the percentage of lung with mismatched defects, which helps decide outpatient management and follow-up.
SPECT vs planar vs CTPA
- SPECT has higher sensitivity and specificity than planar imaging and far fewer non-diagnostic results.
- In a head-to-head study, V/Q SPECT with low-dose CT had sensitivity 97% and specificity 100%, versus 68% and 100% for multidetector CTPA (Gutte 2009).
- In PIOPED II, CTPA had sensitivity 83% and specificity 96%; 6% of studies were inconclusive because of image quality.
- CTPA is widely available around the clock, shows alternative diagnoses and right-heart strain, and is preferred in haemodynamically unstable patients — although a perfusion-only scan can also exclude massive PE.
Quantitative lung perfusion
Split perfusion from quantitative scintigraphy predicts post-operative lung function: predicted post-operative FEV₁ = pre-operative FEV₁ × (1 − fraction of total perfusion in the lung to be resected), and the same calculation applies to DLCO. ACCP 2013 classes a predicted value above 60% of normal as low risk, 30–60% as needing exercise testing and below 30% as high risk. Try it in the calculators.
CTEPH
V/Q scintigraphy is the imaging test of choice to exclude CTEPH: sensitivity 96–97% and specificity 90–95%, compared with a sensitivity of about 51% for CTPA (Tunariu 2007). A normal perfusion scan effectively excludes CTEPH; mismatched segmental defects prompt right-heart catheterisation and CT or conventional angiography to plan treatment. CTEPH follows about 0.1–4% of acute PE episodes.
Right-to-left shunt quantification
With a right-to-left shunt, MAA bypasses the lungs and lodges in systemic capillary beds such as the brain and kidneys. The shunt fraction is estimated from the proportion of whole-body counts outside the lungs, or from brain uptake. Use a reduced particle number and acquire promptly, because free pertechnetate from breakdown of MAA also appears outside the lungs and overestimates the shunt.
Pitfalls
Matched and non-segmental defects
- COPD, pneumonia, effusion, atelectasis, tumour, bullae, prior radiotherapy and surgery.
- Cardiomegaly, hilar nodes and mediastinal masses compressing vessels — non-segmental defects.
- Left heart failure — antigravitational, non-segmental mismatch.
Technical
- MAA hot spots from blood drawn into the syringe or poor mixing.
- Free pertechnetate (thyroid, stomach, kidneys) from poor labelling.
- Central aerosol deposition in COPD with DTPA — use Technegas.
- Patient movement between ventilation and perfusion, which prevents direct comparison.
Guideline recommendations
- EANM 2019: V/Q SPECT with holistic, binary reporting; positive with one segmental or two subsegmental mismatches; perfusion-only first in pregnancy.
- SNM lung scintigraphy 4.0 (2012): adult MAA 40–150 MBq; usually 200 000–700 000 particles, fewer in pulmonary hypertension, right-to-left shunt, infants and children.
- ESC 2019 (acute PE): V/Q scintigraphy is an accepted diagnostic test; in pregnancy, perfusion scintigraphy or low-dose CTPA should be considered to rule out PE.
Frequently asked questions
When is V/Q preferred over CTPA?
Pregnancy, young women, contrast allergy, renal impairment and follow-up of known PE — provided the patient is stable and the service is available.
Does a normal perfusion scan exclude PE?
Yes.
Why reduce MAA particles in a right-to-left shunt?
Particles bypass the lungs into the systemic circulation, including the brain; fewer particles keep this safe.
Summary
- V/Q SPECT: PE = mismatch of ≥1 segment or ≥2 subsegments.
- Normal perfusion excludes PE and CTEPH.
- Pregnancy: 50 MBq perfusion-only first.
- Stripe sign and reversed mismatch favour pneumonia, not PE.
- Never draw blood into the MAA syringe.
References
- Bajc M, et al. EANM guideline for ventilation/perfusion single-photon emission computed tomography (SPECT) for diagnosis of pulmonary embolism and beyond. Eur J Nucl Med Mol Imaging 2019;46:2429–51.
- Parker JA, et al. SNM practice guideline for lung scintigraphy 4.0. J Nucl Med Technol 2012;40:57–65.
- Konstantinides SV, et al. 2019 ESC Guidelines for the diagnosis and management of acute pulmonary embolism. Eur Heart J 2020;41:543–603.
- Stein PD, et al. Multidetector computed tomography for acute pulmonary embolism (PIOPED II). N Engl J Med 2006;354:2317–27.
- Gutte H, et al. Detection of pulmonary embolism with combined ventilation-perfusion SPECT and low-dose CT: head-to-head comparison with multidetector CT angiography. J Nucl Med 2009;50:1987–92.
- Tunariu N, et al. Ventilation-perfusion scintigraphy is more sensitive than multidetector CTPA in detecting chronic thromboembolic pulmonary disease. J Nucl Med 2007;48:680–4.
- Brunelli A, et al. Physiologic evaluation of the patient with lung cancer being considered for resectional surgery: ACCP evidence-based clinical practice guidelines, 3rd ed. Chest 2013;143(5 Suppl):e166S–e190S.