HU is like cm for length, it is a measure of density within CT.
Higher HU = hyperdense image on CT.
Taking water as standard HU = 0
HU -1000 = Air
HU -100 = Fat
HU 0 = Water (note a chronic bleeding can have HU = 10)
HU 10-20 = CSF
HU 30 = White Mater
HU 40 = Gray Mater
HU 50 = Subacute bleeding
HU 100 = Acute bleeding
HU 1000 = Bone
From Radiopaedia.org, blood age in CT
– The older the bleeding, the less denser it appears on CT