No, hypodense (low-attenuation) areas on non-contrast CT do NOT indicate hemorrhage—they indicate the opposite.
Hypodense areas on non-contrast CT represent low-attenuation regions that are LESS dense than normal brain tissue or clotted blood, not hemorrhage itself. In fact, acute hemorrhage appears as hyperdense (high-attenuation) on non-contrast CT, not hypodense 1.
Understanding CT Density in Hemorrhage
Acute Hemorrhage Appearance
- Acute blood appears hyperdense (bright/white) on non-contrast CT due to the high protein content of hemoglobin
- Acutely thrombosed vessels show homogenous hyperdensity 1
- This is the primary sign used to diagnose acute intracranial hemorrhage
Hypodensity Within Hemorrhage: A Critical Warning Sign
When hypodense areas appear WITHIN a hyperdense hematoma, this is paradoxically a marker of ACTIVE bleeding and hematoma expansion, not absence of hemorrhage 2, 3, 4:
- Hypodensities within acute hematomas represent uncoagulated, actively extravasating blood 5
- These low-attenuation areas indicate ongoing hemorrhage that hasn't yet clotted 3, 6
- The "swirl sign" describes hypodense blood swirling within hyperdense clotted blood—a marker of acute extravasation 6
- The "Black-&-White sign" (hypodensity on CT co-localized with contrast extravasation) predicts substantial hematoma expansion with 100% specificity 2
Clinical Significance
Hypodensities within hematomas are independent predictors of hemorrhagic expansion 4:
- When onset-to-imaging time is ≤2 hours, hypodensities predict hematoma expansion with 80% positive predictive value 4
- These areas typically have densities <50 Hounsfield Units 5
- 85.7% of early "spot signs" (active contrast extravasation) originate from hypodense areas 5
Common Pitfall to Avoid
Do not confuse hypodensity WITHIN a hematoma with hypodensity in brain parenchyma:
- Hypodensity in brain tissue = ischemia/infarction (decreased density from edema)
- Hypodensity WITHIN a hyperdense hematoma = active uncoagulated bleeding
- Both represent pathology, but have opposite implications regarding hemorrhage presence
Subacute/Chronic Hemorrhage Evolution
As hemorrhage ages, the entire hematoma becomes isodense or hypodense compared to brain tissue 1:
- This occurs in the subacute-to-chronic stage (days to weeks after bleeding)
- At this point, contrast-enhanced CT or MRI is needed for accurate diagnosis 1
In summary: Acute hemorrhage = hyperdense. Hypodensity within acute hemorrhage = active bleeding. Isolated hypodensity without surrounding hyperdensity = NOT acute hemorrhage.