The tissue density in CT depends on several factors of which the first is the atomic number. Iodine contrast agents have such high atomic numbers that they appear white on pictures even at the highest window settings. Secondly, tissue density depends on the compactness or electron density. Thus, if the fluid element of blood is removed it appears more dense. Conversely, the presence of edema lowers the tissue density. We also know that tumors of a higher cell count may exhibit a higher density in the plain scan. Thirdly, it is known that tissue density is also energy-dependent. The importance of recording the kV used for a particular examination is therefore obvious. For the visibility of a dense structure in a CT-image the following facts are also relevant: the amount of the tissue of interest in the z-direction of the slice; the attenuation difference between this tissue and the adjacent tissue, the amount of noise in the reconstruced image.
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