A ventilation/perfusion scan or V/Q scan is a radiological procedure used to study the circulation of gasses in the lungs. It is usually performed to look for a pulmonary embolism, clot or other abnormal blood flow. It has largely been replaced by the use of a CT scan with radiocontrast. However, when radiocontrast is inappropriate, such as in cases of kidney failure, or there is a concern about radiation exposure a V/Q scan is a safer alternative with a lower radiation dose that can be more carefully controlled.