Lung Ventilation-Perfusion Scan
The lung ventilation perfusion (V/Q) scan compares the distribution of air with the distribution of blood in the lungs. It is used mainly to diagnose pulmonary embolism, and to measure the share of function of each lung or lobe before lung surgery.
Perfusion uses technetium-99m macroaggregated albumin (100 to 200 MBq, 200 000 to 700 000 particles) injected intravenously with the patient supine. The particles lodge in the pulmonary capillaries in proportion to flow.
The number of particles is reduced (to 100 000) in pregnancy, pulmonary hypertension and right-to-left shunt, and in children.
Ventilation uses a tracer inhaled as an aerosol or gas: Technegas (ultrafine technetium carbon particles), technetium-99m DTPA aerosol, or the gases krypton-81m or xenon-133. Planar images in 6 to 8 views, or SPECT or SPECT-CT, are taken. The study takes 30 to 45 minutes.
The effective dose is about 1 to 2.5 mSv, with a low foetal dose.
Pulmonary embolism affects about 1 in 1000 adults each year, and is fatal in 5 to 10% of those who present with it, and untreated it recurs. A normal V/Q scan excludes pulmonary embolism, with a negative predictive value of over 97%.
The scan is the preferred test in pregnancy, in young women, in renal impairment, in contrast allergy and for chronic thromboembolic disease.
Indications.
- Suspected acute pulmonary embolism, particularly when the chest radiograph is normal, in younger patients, in pregnancy, and when iodinated contrast is contraindicated.
- Suspected chronic thromboembolic pulmonary hypertension: mismatched defects that persist after 3 months of anticoagulation. V/Q is the preferred screening test and is more sensitive than CT.
- Unresolved pulmonary embolism after treatment.
- Pre-operative lung function: share of each lung or lobe in the prediction of post-operative lung function before resection or volume reduction.
- Right-to-left shunt (quantified by uptake of macroaggregated albumin in the brain and kidneys).
- Pulmonary arteriovenous malformations and congenital heart disease.
- Lung transplant: function of each lung and rejection (bronchiolitis obliterans).
- Cystic fibrosis, emphysema and other lung diseases in selected cases (regional function).
- Pulmonary vasculitis and pulmonary artery sarcoma or stenosis.
Contraindications and precautions.
- Pregnancy: not contraindicated for suspected pulmonary embolism. The perfusion study is done first with a reduced dose, and the ventilation is added only if perfusion is abnormal.
- Breast feeding: a brief interruption (about 12 hours) as the local rule requires.
- Severe pulmonary hypertension or right-to-left shunt: reduce the number of particles to 100 000 or fewer, to avoid blocking too many capillaries.
- Allergy to human albumin is very rare.
- Patients must be able to cooperate with breathing.
- A recent chest radiograph (within 24 hours) is needed to interpret the scan.
- Active bronchospasm interferes with the aerosol distribution. A bronchodilator is given beforehand.
Preparation and technique.
No fasting is needed. A chest radiograph is checked. The perfusion agent is injected slowly during supine breathing, with the patient supine, and the suspension is mixed gently, with no blood drawn back into the syringe. In patients with known pulmonary hypertension, the injection is slow with fewer particles.
Ventilation is done with the patient breathing the aerosol through a mouthpiece and nose clip for 3 to 5 minutes, or with Technegas inhaled to a target count. Images are taken in anterior, posterior, both posterior oblique and lateral views, or by SPECT and low dose CT.
SPECT and SPECT-CT improve accuracy and cut the rate of non-diagnostic studies. Quantitative lung function is calculated from counts in regions of each lung or lobe, with geometric means of anterior and posterior counts.
