A theoretical and experimental study has been performed to determine which element, when used as a contrast agent in CT, requires the lowest concentration when delivered to various sized organs. Iodine is at present the primary contrast agent used in contrast enhanced CT imaging. The results presented show that iodine is not the optimum element to use in terms of concentration needed for visibility. When administered to very small organs, the use of gadolinium reduces the concentration required for visibility by at least a factor of 3 over that required when using iodine.