Lung Ventilation Perfusion Scan (VQ Scan)(Archived)

Pulmonary embolism is a treatable disease caused by thrombus formation in the lung vasculature, commonly from the lower extremity's deep veins, compromising the blood flow to the lungs. Undiagnosed massive pulmonary embolism can be fatal if not diagnosed and treated in a timely fashion. The diagnosis of pulmonary embolism is based on imaging. Computed tomography of pulmonary arteries (CTPA) and ventilation-perfusion (V/Q) scan are the 2 most common and widely practiced testing modalities to diagnose pulmonary embolism. Pulmonary ventilation (V) and Perfusion (Q) scan, also known as lung V/Q scan, is a nuclear test that uses the perfusion scan to delineate the blood flow distribution and the ventilation scan to measure airflow distribution in the lungs. The primary utilization of the V/Q scan is to help diagnose lung clots called pulmonary embolisms. V/Q scans help in clinical decision-making by evaluating scans that show ventilation and perfusion in all areas of the lungs using radioactive tracers. PIOPED I study by Vreim CE et al. showed that 65% of the V/Q scans were non-diagnostic for pulmonary embolism.  V/Q scan faced a setback because of this study from 1990 until later studies and EANM guidelines based upon holistic principles and modern imaging techniques for V/Q scintigraphy showed a rate of non-diagnostic pulmonary embolism equal to or less than 3% with excellent sensitivity and specificity. In the V/Q lung scan, an aerosol and injectable radioactive tracer are used to assess lung ventilation (V) and perfusion (Q) to identify V/Q mismatch areas. The most common clinical indication for a V/Q lung scan is to assess the likelihood of pulmonary embolism (PE) when contrast or radiation exposure is contraindicated. A subset of the patient population who can not tolerate the intravenous contrast, the radiation of the definitive diagnostic test (CT pulmonary angiography), have severe renal insufficiency (stage IV) or had a severe allergic reaction to contrast material then V/Q scan is the test of choice to diagnose PE. Usually, ventilation imaging is conducted before perfusion imaging. In conventional scintigraphy, a radiolabeled agent like technetium 99m-diethylenetriamine pentaacetic acid (99mTc-DTPA) in aerosol form and gamma-emitting 99mTc-macro aggregated albumin (MAA) in the intravenous form are given to the patient to assess ventilation and perfusion, respectively. Then, the gamma camera registers their distribution into the alveoli and pulmonary arteries. One segmental or 2 sub-segmental perfusion defects with a normally ventilated area (V/Q mismatch) is the definition of a high-probability scan for PE.