When a patient presents with a sudden, severe headache, clinicians immediately consider a subarachnoid hemorrhage (SAH). This type of bleeding occurs in the space between the brain and the thin tissues that cover it, and it represents a neurological emergency. A non-contrast computed tomography (CT) scan is the primary imaging tool used to rapidly identify blood in the subarachnoid space, providing crucial information for diagnosis and immediate management.
Understanding Subarachnoid Hemorrhage
Subarachnoid hemorrhage refers to bleeding into the subarachnoid space, which is the area between the arachnoid membrane and the pia mater surrounding the brain. This space is filled with cerebrospinal fluid (CSF), and blood mixing with CSF is a hallmark of the condition. The most common cause is the rupture of a cerebral aneurysm, although trauma, arteriovenous malformations, and other vascular pathologies can also be responsible. The clinical presentation is often dramatic, characterized by a “thunderclap” headache, neck stiffness, photophobia, and potential loss of consciousness.
The Role of Non-Contrast CT
In the hyperacute phase, a non-contrast CT head is the investigation of choice due to its speed, widespread availability, and high sensitivity for detecting acute blood. Within the first 6 hours of symptom onset, the sensitivity of a non-contrast CT can approach 95%. The scan visualizes fresh blood as hyperdense (bright white) areas within the CSF spaces, typically in the basal cisterns, sulci, or ventricles. This initial finding triggers a cascade of urgent interventions to prevent re-bleeding and manage complications.
Technical Protocol and Image Quality
Optimal detection relies on a standardized protocol. A slice thickness of 1 to 5 mm is preferred over thicker slices to avoid missing blood in narrow spaces. The scan should extend from the vertex to the foramen magnum to evaluate the entire subarachnoid space. Bone windows are also assessed to identify associated skull fractures, particularly in trauma cases. Modern multidetector CT scanners provide high-resolution images that allow for multiplanar reconstructions if needed, although the non-contrast axial images remain the primary diagnostic tool.
Interpreting the Findings: What the Radiologist Looks For
Radiologists do not merely look for “white spots.” They systematically evaluate specific cisterns and sulci to grade the amount of blood present. The Fisher scale is commonly used to describe the distribution and density of the blood, which correlates with the risk of subsequent complications like cerebral vasospasm. Locations of interest include the suprasellar cistern, basal cisterns, sylvian fissures, and fourth ventricle. Clots appearing in unusual locations may提示 alternative diagnoses or associated injuries.
