Stroke vs Aneurysm on Scans: How Doctors Spot the Difference Under Pressure

Your complete guide to Stroke vs Aneurysm on Scans: How Doctors Spot the Difference Under Pressure, including in-depth facts.

On a plain non-contrast CT, the difference between these pathologies comes down to Hounsfield units, the quantitative scale used by radiologists to measure radiodensity. Acute blood typically measures between 50 and 70 Hounsfield units, appearing as an unmistakable bright, hyperdense fluid. In a ruptured brain aneurysm, this blood escapes under arterial pressure, filling the basal cisterns, Sylvian fissures, and cortical sulci in a characteristic starburst or branched pattern known as a subarachnoid hemorrhage.

Ischemia looks entirely different. When an embolus lodges in a cerebral vessel, it abruptly halts downstream cerebral blood flow. Deprived of oxygen and glucose, cellular sodium-potassium pumps fail, causing cytotoxic edema as water rushes into swollen neurons. Water possesses a low density (zero Hounsfield units), which causes ischemic brain tissue to appear dark and hypoattenuating. However, this parenchymal hypodensity takes hours to fully manifest on a standard CT. In the earliest phase of an ischemic stroke, radiologists must hunt for subtle indirect indicators:

The "dense middle cerebral artery sign" appears when a fresh, red blood cell-rich clot sits directly within the horizontal segment of the arterial vessel, appearing hyperdense against normal circulating blood. Radiologists also look for the loss of the insular ribbon, where the crisp boundary between gray matter and white matter at the lateral cortex blurs into a uniform gray smudge due to cellular swelling. If the scan shows bright branching fluid spanning the basal cisterns rather than regional gray-white blurring, the diagnosis is a ruptured intracranial aneurysm, not an arterial occlusion.