A computed tomography (CT) scanner is a device which is capable of mapping the variation in linear attenuation coefficient in a slice through an object. This is achieved by the multiple measurement of the attenuation of an X-ray beam at various positions and angles through the body. In medical diagnostic imaging using CT, contrast agents are administered to patients resulting in increased attenuation of the beam in the areas where the contrast agent resides. The increased contrast results in the easier and more accurate visualisation of abnormalities. In contrast-enhanced CT, iodine is almost universally used as the contrast agent when imaging the heart and associated arteries / veins. This is due to its low toxicity and high enhancement. It has been used extensively in traditional diagnostic radiology prior to the introduction of CT. A study was performed to determine whether iodine was the optimum element, in terms of the minimum concentration needed for visualisation, to use in contrast-enhanced CT scanning of the myocardium / heart wall. The results of this study show that gadolinium, and not iodine, is the optimum element to use as a CT contrast agent. Gadolinium, chelated to DTPA, is presently used as a contrast agent in MRI. The above study concentrated only on the particular case of imaging the myocardium. A theoretical study was undertaken to determine the minimum concentration of any element when scanned using two different imaging methods. The situation studied was that of administering the contrast agent / analyte to a cylinder, which is itself contained inside another cylinder, the space between filled with some matrix. By varying the size of the inner cylinder, administration of a contrast agent to various organs or arteries can be simulated. By varying the size of the outer cylinder, various object / patient sizes can be studied. In the first imaging method, two scans are performed at any energy, one with and one without the analyte present. These scans are subtracted to yield an image of the analyte alone. In the second method two scans are performed; one on the high side and one on the low side of the K absorption edge of the analyte. Again these are subtracted to yield an image of the analyte since the variation in the attenuation of the matrix across the K-edge is minor compared to that of the analyte. The equations were verified by both computer simulations and experimental scans. Two important results were obtained. As the relative size of the inner cylinder decreases, firstly the optimum element shifts towards higher atomic number transition elements and secondly, the ratio of the minimum concentration of the optimum elements to the minimum concentration of iodine needed decreases making the case for using the transition elements as contrast agents stronger when imaging low relative size objects.