Blood that lies outside the arteries and veins within the brain and skull is abnormal. The ability to recognize hemorrhages on computed tomography (CT) is important since hemorrhages are frequently a sign of significant underlying disease.
So, let’s review the typical appearance of several different types of hemorrhages on brain CT scans and how to determine their location!
Anatomy of the meninges
Let’s begin with some basic anatomy. If we were to drill down through the skin over the skull, then pass from the skull towards the brain, we would first encounter a dense and fibrous layer called the dura. Next, we would find a much finer membrane called the arachnoid, and then a layer called the pia that covers the surface of the brain.
As a result of these well-defined meningeal layers, there are three potential spaces for blood to collect outside the brain:
- The epidural space—outside the dura but within the skull
- The subdural space—between the dura and arachnoid layers
- The subarachnoid space—between the arachnoid and pia
Identifying subdural hematomas on brain CT scans
When the patient has a subdural hematoma, the hemorrhage lies between the dura (outer layer of the meninges) and the arachnoid layer. Just as submarine means below the water, the term subdural indicates that the hemorrhage lies below the dura.
Subdural hematomas are usually the result of significant head trauma, but occasionally appear after a seemingly trivial fall or injury in older patients. Keep in mind that subdural hematomas can be difficult to see on CT—particularly on axial images when the subdural blood is at the top or bottom of the skull.
For example, a tentorial subdural hematoma may be much more evident on a coronal scan than on an axial scan. Normally, the tentorium is difficult to see and should be pencil-thin and symmetric from left to right.
Identifying an acute subdural hematoma
So, how do you identify an acute subdural hematoma on a CT image? Well, there are two main findings to look for:
The attenuation of an acute subdural hematoma on a CT image is between the brain cortex and the skull. In other words, an acute subdural hematoma has a higher attenuation than the normal brain due to the clot retraction of the blood products.
You can rely on the shape of the blood collection to predict which space it resides in. For example, a crescent moon shape is the typical contour of blood in the subdural space.
Identifying a chronic subdural hematoma
A subdural hematoma will appear to have lower attenuation than the brain on follow-up scans at about four- to six-weeks post-injury. This is caused by the breakdown of red blood cells and an influx of water, which represents the normal evolution of blood products in the subdural space. In this phase, it is called a chronic subdural hematoma.
Distinguishing between a chronic subdural hematoma and a wide subarachnoid space
A chronic subdural hematoma can be easily confused with a wide subarachnoid space in both older adults and children. One visual clue on CT that can help determine whether the patient has a chronic subdural hematoma, or a wide subarachnoid space is the shape and location of the cortical veins.
A subdural fluid collection will displace the cortical veins away from the inner table of the skull. The larger cortical veins will usually be visible on non-contrast CT scans.
Identifying an isodense subdural hematoma
At some point in time between the acute and chronic stages—usually about two- to three-weeks after the injury—the subdural hematoma will have the same attenuation as normal brain tissue. This is called an isodense subdural hematoma and can be particularly difficult to perceive on CT scans.
For example, in the following CT scan, there is subtle high attenuation along the margin of the right hemisphere of the brain representing blood in the subdural space. But if you keep looking, you will notice that the cortex appears unusually thick on the other side of the brain.
The patient’s magnetic resonance imaging (MRI) revealed bilateral, subacute subdural hematomas. The reason that the hematoma on the patient’s left is so difficult to see is because it is isodense, meaning that it has nearly the same attenuation as the brain. So, when you see an unexplained mass effect on the brain, look carefully at the thickness of the cortex!
Identifying epidural hematomas on CT scans
Epidural hematomas are less common than subdural hematomas and are usually seen with a skull fracture. They are the result of blood accumulating at a relatively high pressure, which pulls the dura away from the inner table of the skull.
Epidural hematomas can enlarge rapidly since they are often the result of arterial injury accompanying the skull fracture. The shape of the blood collection in the epidural space on CT differs from a subdural hematoma since it will have a biconvex, or lens shape, rather than a crescent moon shape.
When epidural hematomas enlarge rapidly, there can be significant displacement and compression of the adjacent brain. A rapidly enlarging acute epidural bleed may require emergency brain surgery to drain the blood collection.
After timely surgery for an epidural hematoma, the patient’s brain usually appears normal on subsequent scans. This differs from cases needing surgery for a traumatic subdural hematoma. In these patients there is frequently a traumatic injury of the underlying brain as well.
Identifying a subarachnoid hemorrhage on CT
When blood collects between the arachnoid layer and the brain, it is called a subarachnoid hemorrhage. Trauma is a common cause of a subarachnoid hemorrhage, but it can also be caused by a ruptured intracranial aneurysm or another vascular abnormality.
The appearance of subarachnoid intracranial bleeding differs from both epidural and subdural hematomas. This is because the blood in the subarachnoid space conforms to the shape of the cortical sulci.
Detection of small subarachnoid hemorrhages will require careful evaluation of all reconstruction planes of the CT scan; it may be helpful to review sections thinner than the usual 5 mm to make the hemorrhages more visible.
But not all subarachnoid hemorrhages found on a CT scan are the result of trauma. When the findings are out of proportion to the magnitude of the trauma, you should consider the possibility of an underlying vascular cause.
For example, a CT was obtained in the emergency room after a patient had an unwitnessed car accident. It demonstrated a small subarachnoid hemorrhage. A subtle area of high attenuation was visible between the left and right frontal lobes.
A subsequent CT image then showed subarachnoid hemorrhage within the interpeduncular cistern. A hemorrhage there takes on the cistern’s triangular shape, so it is worthwhile to look carefully for blood in the interpeduncular cistern whenever a subarachnoid hemorrhage is suspected.
In this patient, an anterior communicating aneurysm was later demonstrated on a computed tomography angiogram (CTA) and confirmed on a digital subtraction angiogram (DSA).
Digital subtraction angiogram requires the navigation of a small catheter through the arterial system, accessed through blood vessels in the leg or arm so that contrast can be injected within a cerebral artery. Digital subtraction angiogram allows high-resolution imaging of the brain arteries and veins.
In patients with equivocal findings of subarachnoid blood on CT after trauma, consider reviewing the thinnest possible reconstruction thickness of the CT scan data. Another option is to obtain magnetic resonance imaging (MRI) and carefully review the fluid-attenuated inversion recovery (FLAIR) images since they often demonstrate subtle abnormalities in the subarachnoid space. Blood products are much more evident on FLAIR since they appear white while the cerebrospinal fluid normally appears black.
It is critical to recognize a hemorrhage in a brain CT of trauma patients. With this information, you should be able to both identify hemorrhages on brain CT scans and correctly predict the compartment where it resides!
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