What causes the dense MCA sign in the brain on non-contrast CT?
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Answer:
The hyperdense MCA sign (dense MCA sign, HMCAS) is intraluminal
thromboembolus in the M1 segment of the middle cerebral artery, visible on non-contrast CT
because clot attenuates more than brain. It counts as a true sign only when the affected artery is
denser than its counterpart by a ratio above 1.2, roughly 46 HU or more [1,2,3].
Why is it called so?
The sign is named for the focal hyperattenuation of the middle cerebral artery against adjacent
brain. Gács and colleagues reported it in 1983 in eight patients with presumed acute intracranial
occlusion, and proposed the rule still used today: the suspect artery must look diffusely dense
and denser than the other visualised vessels [1]. Hence the alternative eponym, Gács sign.
Pathophysiology
Fresh thrombus is denser than brain mainly because of its packed erythrocytes and their
haemoglobin, so an occluded artery becomes conspicuous on unenhanced CT before any parenchymal change
appears. The detail that matters at the workstation is which measurement tracks that composition: in
a thrombectomy series with retrieved-clot histology, the relative density (clot HU minus contralateral
artery HU) correlated with the erythrocyte fraction, while the absolute clot HU did not [4]. A
red-cell-rich clot is dense, a fibrin-rich clot can be isodense, and that is why the criteria below
are ratios rather than a single number.
Alternative names: Hyperdense MCA sign (HMCAS), Hyperdense artery sign, Gács sign
Other associated named signs:
Insular ribbon sign
(loss of grey-white differentiation in the insular cortex, another early CT sign of MCA territory
infarction); MCA dot sign (a hyperdense distal MCA branch in the sylvian fissure, from a more distal
embolus); hyperdense basilar artery sign (the posterior circulation equivalent).
What counts as a true hyperdense MCA sign?
Visual impression alone is unreliable, which is why objective criteria exist. Measure an ovoid ROI
over the suspect M1 and an identical one over the contralateral M1, then compare.
| Criterion | Threshold | Source |
|---|---|---|
| Qualitative rule | Artery diffusely dense and denser than every other visualised vessel | Gács 1983 [1] |
| MCA density ratio (denser side to less dense side) | Above 1.2 in every true positive; mean affected MCA 54.0 HU vs 41.3 HU in controls | Koo 2000 [2] |
| Attenuation plus ratio, ovoid ROI | 46 HU or more and ratio above 1.2 (100% sensitivity in that cohort) | Abul-Kasim 2009 [3] |
| Attenuation plus ratio, pixel-sized ROI | 50 HU or more and ratio above 1.4 | Abul-Kasim 2009 [3] |
| Relative density used in thrombectomy cohorts | Clot HU minus contralateral artery HU of 4 HU or more | Ye 2021 [4] |
| Slice thickness for measuring clot length | 1.25 to 2.5 mm reconstructions; 5 mm is unreliable | Riedel 2010 [5] |
Mimics, and why a normal CT proves nothing
On millimetre-sliced non-contrast CT the sign detected large vessel occlusion with a sensitivity of
62% and an accuracy of 86%, and it was present in only 21% of suspected strokes [6]. So a negative
non-contrast study excludes nothing: close to four in ten occlusions show no dense artery at all. In
the other direction, several findings are read as a dense MCA when they are not.
| Mimic | What separates it |
|---|---|
| Intracranial atherosclerotic calcification | Much higher density than clot, often bilateral and curvilinear, unchanged on any prior CT |
| Calcified cerebral embolus | Discrete, very high attenuation; look for a calcified valve or arch plaque as the source |
| Mature or chronic infarct with a dense vessel | Established encephalomalacia rather than a normal-looking hemisphere; a recognised false-positive group [2] |
| Raised haematocrit or polycythaemia | All vessels dense, including the contralateral M1, so the ratio stays at or near 1.0 |
| Subarachnoid blood in the sylvian fissure | Follows the fissure and extends beyond the vessel course; not tubular |
| Residual contrast from a recent study | Arteries and veins both dense; check the scan history |
What it means for treatment
The prognostic meaning of this sign reversed when thrombectomy arrived, and that is the single most
useful thing to know about it. Under intravenous thrombolysis alone it marked a worse outcome. With
mechanical thrombectomy, a dense (red-cell-rich) clot is the one the device engages best.
| Treatment context | Effect of a positive sign | Evidence |
|---|---|---|
| Intravenous thrombolysis alone | Risk of poor 3-month outcome 1.56-fold higher (RR 1.56, 95% CI 1.50 to 1.62); no significant difference in symptomatic haemorrhage | Meta-analysis, 11 studies, 11,818 patients [7] |
| Mechanical thrombectomy | Successful recanalisation 92.1% vs 74.5%; independent predictor of a favourable 3-month outcome (OR 2.48, 95% CI 1.10 to 5.58); symptomatic haemorrhage 2.9% vs 17.6% | 191 patients [8] |
| Clot length and position | Favourable 30-day outcome in 39% with a distal sign, 11% proximal and 0% when the sign ran the full M1 length; absence of the sign predicted nothing | 126 patients treated intra-arterially [9] |
Practical points when you report one:
- Confirm it with an ROI ratio before calling it, and say which side and which segment.
- State the extent (dot, partial M1 or full-length M1), because length carries the prognosis [9].
- Use thin reconstructions, not 5 mm axials, if anyone will measure clot length [5].
- Never let a negative non-contrast CT stand in for vascular imaging [6].
The current AHA/ASA guideline for the early management of acute ischemic stroke, published in 2026,
replaced the 2018 guideline and its 2019 update [10]. The triage logic it inherits has not changed:
because the sign misses close to four in ten occlusions, selecting a patient for thrombectomy needs
vascular imaging, not the unenhanced study [6].
Frequently asked questions
What does a hyperdense MCA sign mean?
It means thromboembolic material is sitting in the middle cerebral artery, usually the M1 segment,
and is dense enough to see on a non-contrast CT. In the right clinical setting it is a direct marker of
acute large vessel occlusion and should trigger immediate vascular imaging [1,6].
What is the HU cut-off for a hyperdense MCA sign?
There is no single number, because clot density varies with red cell content. Use the ratio: above
1.2 between the two M1 segments. Combining 46 HU or more with a ratio above 1.2 on an ovoid ROI gave
100% sensitivity in one series, and 50 HU with a ratio above 1.4 for a pixel-sized ROI [2,3,4].
Can a patient have a large vessel occlusion with a normal non-contrast CT?
Yes, and often. On millimetre slices the sign was only 62% sensitive for large vessel occlusion and
was seen in 21% of suspected strokes, so roughly four in ten occlusions have no dense artery. CT or MR
angiography is what excludes an occlusion [6].
Is a hyperdense MCA sign a bad prognostic sign?
It depends on the treatment. With intravenous thrombolysis alone it raised the risk of a poor
3-month outcome 1.56-fold. In patients taken for mechanical thrombectomy the opposite held: higher
recanalisation rates, less symptomatic haemorrhage and an independent association with a favourable
outcome [7,8].
References
- Gács G, Fox AJ, Barnett HJ, Vinuela F. CT visualization of intracranial arterial thromboembolism. Stroke. 1983;14(5):756-62. PMID: 6658961.
- Koo CK, Teasdale E, Muir KW. What constitutes a true hyperdense middle cerebral artery sign? Cerebrovasc Dis. 2000;10(6):419-23. PMID: 11070370.
- Abul-Kasim K, Selariu E, Brizzi M, et al. Hyperdense middle cerebral artery sign in multidetector computed tomography: definition, occurrence, and reliability analysis. Neurol India. 2009;57(2):143-50. PMID: 19439843.
- Ye G, Cao R, Lu J, et al. Histological composition behind CT-based thrombus density and perviousness in acute ischemic stroke. Clin Neurol Neurosurg. 2021;207:106804. PMID: 34304067.
- Riedel CH, Jensen U, Rohr A, et al. Assessment of thrombus in acute middle cerebral artery occlusion using thin-slice nonenhanced computed tomography reconstructions. Stroke. 2010;41(8):1659-64. PMID: 20595670.
- Jodaitis L, Ligot N, Chapusette R, et al. The hyperdense middle cerebral artery sign in drip-and-ship models of acute stroke management. Cerebrovasc Dis Extra. 2020;10(1):36-43. PMID: 32344421.
- Sun H, Liu Y, Gong P, et al. Intravenous thrombolysis for ischemic stroke with hyperdense middle cerebral artery sign: a meta-analysis. Acta Neurol Scand. 2020;141(3):193-201. PMID: 31598961.
- Merlino G, Tereshko Y, Pez S, et al. Hyperdense middle cerebral artery sign predicts favorable outcome in patients undergoing mechanical thrombectomy. J Thromb Thrombolysis. 2023;55(2):312-21. PMID: 36434302.
- Man S, Hussain MS, Wisco D, et al. The location of pretreatment hyperdense middle cerebral artery sign predicts the outcome of intraarterial thrombectomy for acute stroke. J Neuroimaging. 2015;25(2):263-8. PMID: 24707921.
- Prabhakaran S, Gonzalez NR, Zachrison KS, et al. 2026 guideline for the early management of patients with acute ischemic stroke: a guideline from the American Heart Association/American Stroke Association. Stroke. 2026;57(8):e316-e436. PMID: 41582814.
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