How Blood Appears on CT Scan
Acute blood appears as hyperdense (bright white) areas on non-contrast CT, with attenuation values typically ranging from 50-90 Hounsfield Units (HU), appearing brighter than surrounding brain tissue. 1
Acute Phase Appearance (Hours to Days)
Blood in the acute phase demonstrates characteristic hyperdensity due to the high hemoglobin content of retracted clot or sedimented blood 2. This hyperdensity can manifest in several distinct patterns:
Common Patterns of Acute Hemorrhage
- Homogeneous hyperdensity: Uniform bright appearance throughout the lesion, seen in approximately 22% of acute hematomas 2
- Heterogeneous hyperdensity: Mixed density patterns with varying degrees of brightness 2
- Linear shadows: Streak-like hyperdense areas within the hemorrhage 2
- Fluid-fluid levels: Dependent hyperdensity with layering of blood products 2
- Hyperdense filling defects: Bright areas surrounded by lower-density fluid 2
Location-Specific Appearances
Intracerebral hemorrhage appears as a focal hyperdense mass within the brain parenchyma 1. Subarachnoid hemorrhage demonstrates hyperdensity in the subarachnoid spaces, following the contour of sulci and gyri, often concentrated in basal cisterns and the Sylvian fissure 3. Subdural and epidural hematomas may show mixed density patterns when active bleeding is ongoing, with areas of hyperdensity (clotted blood) admixed with isodensity (liquid unclotted blood) 4.
Temporal Evolution of Blood on CT
Subacute Phase (Days to Weeks)
As the hematoma ages, mixed attenuation occurs as the clot begins to lyse 1. The hemorrhage may become isodense to brain tissue during this phase, making detection more challenging 1, 5. Blood products may layer in dependent locations, commonly seen in the occipital horns of lateral ventricles, dorsal margins of Sylvian fissures, and sulci of cerebral convexities 3.
Chronic Phase (Weeks to Months)
Chronic hemorrhage demonstrates low attenuation (hypodensity) as tissue breakdown occurs and hemosiderin deposits form 1. CT becomes significantly less sensitive in this phase, and MRI is superior for detecting chronic blood products 3.
Diagnostic Thresholds and Measurements
The optimal Hounsfield Unit cut-off for detecting acute thrombosis is 40-42 HU 1. In one study examining post-thrombolysis imaging, metallic hyperdensity with maximum CT values exceeding 150 HU was associated with subsequent significant hemorrhagic transformation in 30% of cases 6. However, the predictive value of HU measurements has limitations, with an HU >90 showing only 23% sensitivity but 94% specificity for hemorrhagic transformation 7.
Critical Diagnostic Considerations
Sensitivity and Timing
CT should ideally be performed within 1 week of symptom onset to reliably demonstrate high density consistent with recent hemorrhage 8. For subarachnoid hemorrhage specifically, non-contrast CT has sensitivity approaching 100% within the first 3 days after onset 3. However, only 30% of cerebral venous thrombosis cases show abnormalities on initial non-contrast CT 1.
Important Pitfalls
- Early or small hemorrhages may not be visible on non-contrast CT 1
- Beam hardening artifact in the posterior fossa can limit detection of posterior circulation hemorrhages 1
- Isodense hematomas in the subacute phase may be missed without contrast enhancement 5
- Partial volume averaging with adjacent calcium, cerebrospinal fluid, or fatty material can alter the apparent density of small thrombi 8
When CT is Insufficient
If clinical suspicion remains high despite normal non-contrast CT, proceed to CT angiography or MRI 1. CT angiography can detect active bleeding at rates as low as 0.3 mL/min and has sensitivity of 79-85% and specificity of 92-95% for detecting gastrointestinal bleeding 9. For intracranial hemorrhage, MRI with gradient echo (GRE) sequences is particularly helpful for identifying small hemorrhages that CT may miss 8.
Distinguishing Blood from Other Hyperdensities
In a study of 73 patients with recent hematomas, 75% exhibited regions of hyperdensity, while only 1.25% of control lesions (neoplasms or abscesses) showed relative hyperdensity that could not be explained by calcification, bone fragments, or diminished organ density 2. This high specificity makes hyperdensity a useful diagnostic sign of recent hemorrhage, though the pattern and distribution must be carefully evaluated in clinical context 2.