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It is important to realize when looking at blood on both CT and MR imaging, that the qualities of clotted blood will change over time. On CT, acute hemorrhage is usually higher density than normal blood in vessels, but over time, it may become closer to water density. On MR, the magnetic properties of blood change in a complex manner that can lead to decreased or increased signal. On CT, white matter is lower in density (darker) than grey matter because it consists of both nerve tissue and myelin, a fatty substance.
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