Both, CT scans and VQ scans, are used by doctors to look for pulmonary embolism. The most common reason to order a VQ scan is to avoid the IV dye. The IV dye used for CT scans can cause kidney problems in some patients, called contrast-induced nephropathy or "CIN." This is a kidney problem that usually does not make patients feel any differently or change how they urinate. Most of the time, it can only be found by testing blood several days later. This kind of kidney problem can be very mild and some patients will never have any symptoms, rarely these problems can be severe. Some patients can also have similar kidney problems for many other reasons (reactions to medications, blood pressure problems, etc.) and can even happen in patients that do not get IV dye. That is why doctors are not sure exactly who will have these problems or if using a test that does not use IV dye can prevent this kidney problem. The VQ scan uses a different medication through the IV that is not IV dye and has not been linked to kidney problems. The purpose of this study is to learn if using the test that does not use IV dye (the "VQ scan") instead of a CT scan in some patients can help to prevent kidney problems.