Hounsfield units (HU) are a measure of density on CT. Water has a HU of 0. Free fluid, often having some cellular material within it, has a HU just above 0. Fresh liquid blood has a HU slightly lower (30-45) than blood that has clotted (45-70). The clot is often a clue to as the site of bleeding (sentinel clot). Following a period of red cell lysis and clot organisation, the HU measurement falls. A lower HU is also seen in anaemia.