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When a VQ Scan (Ventilation-Perfusion Scan) is performed, it consists of two parts, a Ventilation Scan and a Perfusion Scan. The Ventilation Scan involves breathing a radioisotope gas through a face-mask, then being positioned at a scanning table. The lungs are scanned by the device, which picks up the distribution pattern of the inhaled radioisotope in the lungs. The Perfusion Scan involves injection of radioisotope solution into a vein, via an intravenous (IV) cannula. The lungs are scanned; this time the distribution of blood flow in the blood vessels of the lungs (pulmonary arteries) is detected.
Your doctor may request a VQ Scan (Ventilation-Perfusion Scan) when there is a reasonable suspicion of pulmonary embolism, a clot in the blood vessels of the lung. Symptoms of a possible pulmonary embolism include shortness of breath, pain on deep breathing (also known as pleuritic chest pain), and coughing up blood (haemoptysis). These symptoms each have many different causes, but in the setting of a blood clot in the deep veins of the leg (deep vein thrombosis or DVT) the combination of all three symptoms would be highly suspicious for a PE. Risk factors for DVT and PE include increasing age, smoking, surgery, and prolonged immobility. DVT and PE have a tendency to recur in the same person, so a previous history of DVT or PE would make it more likely that your doctor would do tests to exclude a PE.
Interpretation of the VQ Scan (Ventilation-Perfusion Scan) is performed by a Nuclear Medicine Specialist. The areas of uptake of radioisotope gas in the lungs are compared with the areas of blood flow in the lungs. The aim is to identify a ventilation-perfusion mismatch, where there is gas uptake but reduced blood flow, suggesting a clot in a blood vessel. The interpreting specialist applies standard criteria and provides one of four possible results:
The doctor who ordered the test then needs to determine the need for treatment or further testing, based on the combination of the VQ Scan result and the “clinical pre-test probability”. For example, a high-probability scan in a patient with a high pre-test probability of a Pulmonary Embolism means that treatment is definitely indicated.
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