There are several types of intracranial hemorrhage, all of them relatively readily diagnosed by CT.

First, basic principles: Normal brain tissue has CT numbers of about 30 HU (white matter) to about 40 HU (gray matter). Acute blood had a density of about 80-100 HU. With time, a collection of blood will become less dense, passing through a phase where it is isodense with brain tissue, and either becomes an area of encephalomalacia (density equal to CSF, or about 0 HU) or calcifies, and becomes heteregeneous and extremely dense (>200 HU).

Intracranial hemorrhage is roughly grouped into intra-axial and extra-axial.

Intra-axial (a.k.a. intraparenchymal) hemorrhage is bleeding within the brain tissue itself. The radiographic hallmark of this in the acute phase is bright blood which is separated from the inner table of the skull by brain tissue. The tissue surrounding a bleed is often hypodense relative to the rest of the brain due to edema. The most common cause is trauma (hemorrhagic contusion), but bleeding from rupture of an aneurysm or AVM (arteriovenous malformation) can also cause it, as can bleeding within a tumor.

Extra-axial hemorrhage falls into three subtypes.

• Epidural bleeding is caused by trauma, and results from laceration of an artery, most commonly the middle meningeal artery (branch of the external carotid). The typical appearance is "lentiform", like a lens, with a convex surface away from the skull. An epidural hematoma can never cross a skull suture. This is a very dangerous type of injury because the bleed is from a high-pressure system and increased intracranial pressure can result rapidly, with catastrophic consequences. This diagnosis requires a patient to be cared for in a facility with a neurosurgeon on call to decompress the hematoma if necessary and stop the bleed by ligating the injured vessel branches.
• Subdural hemorrhage results from shearing of the bridging veins between the dura and arachnoid mater. The typical shape is crescentic, with a concave surface away from the skull. Subdural hematomas can cross sutures. Subdural blood can also be seen as a layering density along the tentorium cerebelli. This can be a chronic, stable process, since the feeding system is low-pressure. In such cases, look for more subtle signs of bleeding such as effacement of sulci or displacement of the gray/white junction medially (remember, a chronic bleed can be isodense to brain).
• Subarachnoid hemorrage results from either trauma or from aneurysmal/AVM bleeds, like intraparenchymal hemorrhage. Blood is seen layering into the brain along sulci and fissures, or filling cisterns (most often the suprasellar cistern because of the presence of the vessels of the circle of Willis and their branchpoints within that space). The classic presentation of this entity is the sudden (like a light-switch) onset of the "worst headache of your life". This can be a very dangerous entity, and requires emergent neurosurgical evaluation, and sometimes urgent intervention.

Disclaimer: The information in this article is meant as a review for medical students and/or residents, or others interested in the subject. Do NOT attempt to make a diagnosis of intracranial hemorrhage from a head CT based on this article alone. Call your local friendly radiologist! Also, for the radiology crowd--this is not enough information for you to pass the boards with on this subject. Read at the least the Brant and Helms chapters on brain trauma, or better yet the sections in Kirkwood's or Osborne's books relating to the topic.