kidneys (a parenchymal phase), and a CT scan taken while the contrast travels through the renal tract (an excretory phase). Risks A person may have an allergy to the contrast dye. When this occurs, it is usually mild, causing symptoms such as an itch or a rash, however rarely causes a serious reaction such as anaphylaxis that may impair breathing. The contrast dye may not all go inside the vein at the cannula site, and if it extravasates, it may cause pain or bruising to the local area. The scan involves radiation, which may increase the risk of future cancers by a very small amount, or prove damaging to a pregnancy. Additionally, the dye used can damage kidney function. References Urologic imaging