An intracranial hemorrhage, which is bleeding that occurs anywhere within the skull, is a significant medical event. This condition accounts for up to 15% of all strokes and can be caused by trauma, high blood pressure, or abnormalities in the blood vessels. When a patient presents with symptoms like a sudden, severe headache, weakness, or decreased consciousness, doctors must make a quick and accurate diagnosis. The primary tool used for this initial evaluation is the computed tomography, or CT, scan.
The Role of CT in Diagnosis
The preference for computed tomography in diagnosing intracranial hemorrhages stems from its practical advantages in an emergency setting. The most significant of these is speed. A non-contrast CT scan of the head can be completed in just a few minutes, providing physicians with immediate information to guide treatment.
Accessibility is another major factor. Nearly every hospital emergency department has a CT scanner, ensuring diagnosis is not delayed by transferring a patient. While Magnetic Resonance Imaging (MRI) can be more sensitive for small bleeds, its limited availability and longer scan times make it less suitable for the initial assessment of an acute hemorrhage.
The high accuracy of CT in detecting acute bleeding further solidifies its role as the first-line diagnostic tool. Freshly leaked blood has a high density, which makes it easily visible on a CT scan. This allows for a confident diagnosis in most cases.
Interpreting CT Scan Images
The principle behind interpreting a CT scan for an intracranial hemorrhage lies in understanding radiodensity. A CT scanner takes a series of X-ray images from different angles to create cross-sectional images of the body. Different tissues and materials absorb the X-ray beams to varying degrees. Dense materials, like bone, block a significant portion of the X-rays and appear bright white on the final image.
Conversely, less dense substances, such as the cerebrospinal fluid that circulates around the brain, allow most X-rays to pass through and appear dark or black. Brain tissue itself has an intermediate density and is visualized as various shades of gray. This creates a grayscale map of the intracranial structures.
When a blood vessel within the skull ruptures, the escaping blood begins to clot. This collection of clotted blood is significantly denser than the surrounding brain tissue. On a non-contrast CT scan, this acute hemorrhage shows up as a bright white, or hyperdense, area.
Classifying Hemorrhage Types on CT
The location and shape of the hyperdense area on a CT scan are used to classify the specific type of intracranial hemorrhage. This classification is important for determining the likely cause of the bleeding and for planning subsequent treatment. There are four main types of hemorrhages.
An epidural hematoma occurs between the dura mater and the skull and is characterized by its lens-like, or biconvex, shape. Because the dura is tightly attached to the skull’s inner surface, the expanding blood is constrained, creating a shape that bulges inward like a lemon. This type is often associated with a skull fracture that tears an underlying artery.
A subdural hematoma forms between the dura mater and the arachnoid mater. Blood spreads more freely here, resulting in a crescent-shaped appearance that follows the curve of the skull, like a banana. These are frequently caused by the tearing of bridging veins and can occur with less forceful head trauma, especially in older adults.
A subarachnoid hemorrhage is bleeding into the space between the arachnoid and pia mater. This space contains cerebrospinal fluid and the brain’s major blood vessels. When bleeding occurs here, often from a ruptured aneurysm, the blood mixes with the fluid and spreads throughout the brain’s surface grooves. On a CT scan, this appears as fine, bright white lines tracing the brain’s folds.
An intraparenchymal hemorrhage is bleeding that occurs directly within the brain tissue. It appears as a well-defined, hyperdense collection of blood within the brain’s gray or white matter. These hemorrhages are often caused by uncontrolled high blood pressure leading to the rupture of small, deep arteries. The location of the bleed can provide clues to its cause.
Beyond Initial Detection
One of the most immediate concerns is the “mass effect” that a large collection of blood can have on the brain. The skull is a rigid, enclosed space, and a significant bleed can increase the pressure inside, compressing and displacing delicate brain structures. The CT scan can show evidence of this, such as a midline shift, where the brain is pushed from one side to the other.
This initial CT can also reveal complications such as hydrocephalus, which is a buildup of cerebrospinal fluid, or brain swelling, known as edema. The tissue surrounding a hematoma often becomes swollen with fluid, appearing as a darker area around the bright white of the hemorrhage. These findings indicate the patient’s prognosis and can signal the need for urgent neurosurgical intervention.
In some cases, a CT angiogram (CTA) is performed. This procedure involves injecting a contrast dye into the bloodstream, which makes the blood vessels appear bright on the scan. A CTA can help identify the source of the bleeding, such as a ruptured aneurysm or an arteriovenous malformation. By pinpointing the underlying vascular abnormality, the CTA provides a roadmap for surgeons or interventional radiologists to treat the cause of the hemorrhage.