Factors affecting the appearance  


In order to discretely identify tissues that are immediately adjacent to one another on CT, their densities must be different enough so that they appear as varying shades of gray on the CT image.

To aid in the differentiation of structures seen on CT, particularly if they are of similar density, intravenous contrast dye (x-ray dye) is often used. The dye is composed of iodine which is quite dense and therefore attenuates x-rays, thus appearing white on CT. The dye is administered to the patient by intravenous injection. When this is done, tissues in the body will take up the contrast and appear a lighter shade of gray than they would without the dye. The more dye that a tissue takes up, the lighter shade of gray it appears on CT; the more it is said to "enhance".