Pulmonary Embolism Support Group
By far the most common form of pulmonary embolism is a thromboembolism, which occurs when a blood clot, generally a venous thrombus, becomes dislodged from its site of formation and embolizes to the arterial blood supply of one of the lungs. Symptoms may include difficulty breathing, pain during breathing, and more rarely circulatory instability and death.
From what I understand, it's not great for your kidneys to have a lot of CT w/contrast scans. The VQ scan is another way of getting a picture of your lungs w/out the same risks. I had one when I had just had a CT w/contrast 3 weeks before and it was considered to soon to repeat that test.