What are the CT and MRI features of hemorrhage?
On CT, nonenhancing high attenuation (30 to 70 HU) hemorrhagic fluid is typically seen with acute hemorrhage, whereas acute hemorrhage in patients with anemia or more chronic hemorrhage may have lower attenuation (0 to 30 HU). A hematocrit effect manifesting as fluid-fluid levels due to layering of cellular components of blood also may be seen in acute hemorrhage. Focal areas of extraluminal contrast material, when encountered, indicate the presence of active arterial hemorrhage. Over time, hematomas tend to decrease in size.
On MRI, acute hemorrhage has low signal intensity on T1-weighted and T2-weighted images due to deoxyhemoglobin, subacute hemorrhage has high T1-weighted and variable T2-weighted signal intensity due to methemoglobin, and chronic hemorrhage has very low signal intensity on T1-weighted and T2-weighted images due to hemosiderin and ferritin. The “concentric ring” sign, manifested by an inner rim of high signal intensity on T1-weighted images and an outer rim of very low signal intensity on T1-weighted and T2-weighted images, is specific for presence of a subacute hematoma.