A Computed Tomography (CT) scan uses X-rays and computer processing to create cross-sectional images of the brain. A stroke occurs when blood flow to an area of the brain is disrupted, either by a blockage (ischemic) or a ruptured blood vessel (hemorrhagic). This interruption deprives brain cells of oxygen and nutrients, leading to cell death. CT scans play an immediate role in the initial assessment of acute stroke symptoms, helping doctors quickly determine the stroke type and guide time-sensitive treatment decisions.
The Critical Role of CT in Acute Stroke Diagnosis
When a patient arrives with signs of a stroke, time is essential for diagnosis and treatment. The speed and availability of a non-contrast head CT make it the initial imaging test of choice in the acute setting. This rapid imaging is performed primarily to triage the patient and determine the immediate course of action, which is important for potential clot-busting therapies. The goal is to quickly rule out a hemorrhagic stroke.
Administering clot-dissolving medications, such as intravenous tissue plasminogen activator (tPA), to a patient who is actively bleeding in the brain can worsen the outcome. The CT scan provides this distinction within minutes, enabling doctors to proceed with appropriate treatment or pursue further imaging. Speed in diagnosis is important because delayed treatment during an ischemic stroke leads to greater neuronal damage.
The CT scan can also show early signs of an ischemic stroke, though these changes are often subtle in the first few hours. The absence of visible bleeding, coupled with the patient’s symptoms, strongly suggests an ischemic event, allowing the medical team to consider time-dependent treatments.
Distinguishing Between Stroke Types on a CT Scan
The appearance of a stroke on a CT scan is based on the density of the tissue or fluid. Hemorrhagic strokes are easy to identify immediately because fresh blood is denser than brain tissue. The blood appears as a bright white area, or hyperdensity, on the non-contrast CT image. This hyperdense finding allows for rapid confirmation of bleeding within the brain tissue, known as an intracerebral hemorrhage, or around the brain, such as a subarachnoid hemorrhage. The location and size of this bright area are immediately visible, which is necessary for managing blood pressure and determining if surgical intervention is needed.
In contrast, an acute ischemic stroke, caused by a blockage, often appears subtle or normal during the first three to six hours after symptoms begin. As the brain tissue starts to die, it swells and takes on water, which makes it less dense than healthy tissue. Over time, this area of dead tissue will begin to appear darker, or hypodense, on the CT scan.
Other subtle early signs of ischemia can include a loss of the normal boundary between the gray and white matter, or an effacement of the sulci (the grooves on the brain’s surface) due to swelling. Occasionally, a clot in a major artery, like the middle cerebral artery, can be directly visualized as a bright line, referred to as the dense artery sign. However, the definitive hypodense area characteristic of dead tissue usually takes several hours to fully develop and become clearly visible on the scan.
Identifying Signs of a Previous Stroke
A CT scan is effective for identifying damage from a stroke that occurred long ago. The findings from a healed or chronic stroke are distinct from the subtle or dense changes of an acute event, representing the long-term residual effects on the brain tissue.
The most common sign of a previous, healed ischemic stroke is a clear area of tissue loss replaced by fluid, which appears distinctly dark on the CT scan. This condition is formally called encephalomalacia, describing the softening and eventual loss of brain tissue following an injury. The tissue dies and is eventually replaced by cerebrospinal fluid.
This older damage appears as a well-defined dark cavity, or significant hypodensity, that is clearer and more pronounced than the early darkening seen in an acute ischemic event. The area of tissue loss is often bordered by gliosis, which is the brain’s natural scarring process. These chronic changes on the CT provide definitive evidence that a stroke occurred in the past.