The hypodense vessel sign, or hypodense artery sign, is a radiological sign appreciated on non-contrast CT brain in acute ischemic stroke due to the rare entity of fat macroembolism causing large vessel occlusion. Notably, this is a distinct sign to the hyperdense vessel sign.
The sign describes a focal hypoattenuation, with a radiodensity in-keeping with fat (variable, but often within -60 to -120 HU), within an intracranial blood vessel 1-5. The sign has most commonly been described in the context of fat macroembolisation to the middle cerebral artery, but has been described in other intracranial arteries and can theoretically occur in any intracranial artery 1-5.
While fat macroembolism causing large vessel occlusion, and thus the hypodense vessel sign, shares etiologies with fat embolism syndrome, namely most commonly following surgery (cardiac or orthopedic) or trauma, the two entities are notably distinct 1-5. In contrast to fat macroembolism causing large vessel occlusion, which is not associated with other organ involvement, cerebral fat embolism syndrome is caused by fat microembolism and is additionally associated with respiratory and dermatological complications 5. Thus, the hypodense vessel sign is typically not seen in fat embolism syndrome 5.