Blood is a complex mixture of cells, fluid, and protein. The appearance on MR and CT varies with time as the chemistry of blood products evolves. Intracellular deoxyhemoglobin and methemoglobin (in RBCs) and hemosiderin (in macrophages) cause loss of MR signal on T2 and T2* images - largely due to susceptibility effects. Methemoglobin in solution, after red cell lysis, produces a high signal on both T1 and T2 weighted images.