The hyperdense MCA sign, also known as Gács sign, is a type of hyperdense vessel sign and refers to focal hyperattenuation of the middle cerebral artery (MCA) on non-contrast brain CT and is due to intraluminal thromboembolic material. It is the earliest visible sign of MCA infarction and is seen within 90 minutes after the event ¹. The MCA dot sign and hyperdense basilar tip sign are due to clot at these respective locations.

Pathology

This sign is due to embolic red blood clot arising from the heart or arteries. The high attenuation is predominantly caused by the high globin concentration in the clot. The clot usually lodges in the proximal MCA.

By contrast, smaller white clot emboli typically lodge in the M1 or M2 segments and are not visible on non-contrast CT ⁹. These globin-poor clots have similar attenuation to blood.

Sensitivity and specificity

The sensitivity of this CT sign for embolism is only about 30% but its specificity is about 90% ⁶.

Radiographic features

CT

The hyperattenuating clot typically occludes the proximal MCA, often protruding into the terminal supraclinoid internal carotid artery. Thin sections are important to avoid partial volume averaging. If CT angiography (CTA) is performed, enhancement is typical and is due to the permeable nature of red clot. Delayed phase CTA may demonstrate the distal end of the clot allowing an estimate of length.

MRI

Susceptibility weighted imaging (SWI) may be exquisitely sensitive to the MCA clot (see susceptibility vessel sign) and distal embolic shower due to blooming artifact.

Treatment and prognosis

Red clot fragments easily and typically responds to recanalization therapy, both thrombolysis and endovascular clot retrieval. Proximal balloon occlusion helps to avoid distal embolization. Success is adversely affected by length ¹¹.

History and etymology

The hyperdense vessel sign was first described by Gyula Gács (fl. 2024), in 1983, a renowned, still active Hungarian neurologist-psychiatrist, who works in Budapest ⁷. His initial case series mainly included patients with a hyperdense MCA sign ⁷, hence, it is not uncommon to hear this sign referred to as the "Gács sign" by Hungarian physicians.

Differential diagnosis

• atherosclerosis can cause hyperattenuating MCA due to calcification

• polycythemia or hemoconcentration cause generalized intravascular hyperattenuation due to high globin concentration

• relative MCA hyperattenuation compared with temporal lobe occurs in mature cerebral infarction and HSV encephalitis ⁴

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