V/Q (ventilation/perfusion) scan is a scintigraphic examination of the lung that evaluates pulmonary vasculature perfusion and segmental bronchoalveolar tree ventilation.
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Indications
- diagnosis of suspected pulmonary embolism
- monitor pulmonary function following lung transplant
- provide preoperative estimates of lung function in lung cancer patients, where pneumonectomy is planned
Technique
A chest x-ray should be performed within 12 to 24 hours and reviewed prior to lung scintigraphy 2 as there are other causes of perfusion defects such as atelectasis.
The ventilation scan performed using radio-isotope labelled aerosols like technetium-99m DTPA delivered to the patient through a non-rebreathing mask, with the patient supine. The micro-aerosol particles are small enough to reach the distal tracheobronchial tree and reflect regional ventilation 1. The patient is then imaged in the upright position in three phases: initial breath, equilibrium and washout.
The perfusion lung scan involves injecting Tc-99m MAA intravenously in the supine position. The MAA particles are just small enough to get lodged in the pre-capillary arterioles. A high resolution, large field of view gamma camera is used to image the lungs.
Interpretation
Interpretive criteria developed from the PIOPED, PIOPED II, or PISAPED trials can be used to determine the probability of a pulmonary embolism on a V/Q scan. For the diagnosis of PE, a V/Q scan has a reported sensitivity of 77.4% and specificity of 97.7% when compared to CT pulmonary angiography (CTPA) or digital subtraction angiography (DSA) 3.
A normal ventilation scan (even distribution of radionuclide throughout the lung fields) with multiple, bilateral perfusion defects are the classical findings of pulmonary embolism 1.
Perfusion defects will be present in regions of acute or chronic airway obstruction due to reflex vasoconstriction in the same region. As result, abnormalities of perfusion may be classed as ventilation matched (airway obstruction) or mismatched (unlikely to be due to airway obstruction) 4.
Causes of ventilation/perfusion mismatch include:
- acute pulmonary embolism
- chronic pulmonary embolism
- obstruction of an artery by a tumour 2
- radiation therapy 2
The presence of a right-to-left shunt can be assessed by searching for evidence of renal or brain activity.
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