What color does blood typically appear as on a computed tomography (CT) scan?

Blood Appearance on CT Imaging

Acute blood appears bright white (hyperdense) on non-contrast CT scans, with attenuation values typically ranging from 50-90 Hounsfield Units, making it significantly brighter than surrounding brain tissue. 1

Acute Hemorrhage Characteristics

• Acute bleeding demonstrates homogenous hyperdensity that is distinctly brighter than adjacent brain parenchyma 1
• The hyperdense appearance results from high hemoglobin content in retracted clot or sedimented blood 2
• Approximately 75% of acute hematomas exhibit regions of localized or diffuse hyperdensity on pre-contrast CT 2
• The optimal cut-off for detecting acute thrombosis is 40-42 Hounsfield Units 1

Evolution of Blood Over Time

The appearance of blood on CT changes predictably as the hematoma ages:

• Acute phase (hours to days): High attenuation/hyperdensity due to clot formation and retraction 1
• Subacute phase (days to weeks): Mixed attenuation occurs as clot begins to lyse; may become isodense (same density) to brain tissue, making it difficult to visualize 1
• Chronic phase (weeks to months): Low attenuation/hypodensity develops as tissue breakdown occurs and hemosiderin deposits form 1

Morphologic Variations

Blood can present with different patterns on CT depending on the stage and location:

• Predominant hyperdensity throughout the lesion (48% of cases) 2
• Linear shadows representing layering 2
• Hyperdense filling defects surrounded by fluid 2
• Dependent positioning of hyperdense fragments due to gravity 2
• Fluid-fluid levels with dependent hyperdensity 2

Location-Specific Appearances

• Intracerebral hemorrhage: Focal hyperdense mass within brain parenchyma 1
• Subarachnoid hemorrhage: Hyperdensity in the subarachnoid spaces 1
• Cerebral venous thrombosis: Acutely thrombosed veins and sinuses appear as homogenous hyperdensity, best visualized when CT slices are perpendicular to the vessel 1
• Hyperdense vessel sign: High-density thrombus/embolus visible within an artery, indicating arterial occlusion 3

Critical Diagnostic Pitfalls

• Non-contrast CT can be normal in early or small hemorrhages 1
• Only 30% of cerebral venous thrombosis cases show abnormalities on initial non-contrast CT 1
• Beam hardening artifact in the posterior fossa can limit detection of posterior circulation hemorrhages 1
• If clinical suspicion remains high despite normal non-contrast CT, proceed immediately to CT angiography or MRI 1
• Partial volume averaging with adjacent calcium, cerebrospinal fluid, or fatty material can alter the apparent density of small thrombi 1
• Certain medications can cause unexpected intraluminal hyperdensities that may be mistaken for acute hemorrhage 4

Distinguishing Blood from Contrast Staining

After endovascular procedures, hyperdensity on CT may represent either hemorrhage or contrast staining:

• Higher density measurements (Hounsfield Units >90) suggest hemorrhage with high specificity (94%) but low sensitivity (23%) 5
• Hemorrhagic hyperdensities are associated with significantly higher mortality (50% versus 5.9%) compared to contrast staining alone 6
• CT angiography can detect active bleeding at rates as low as 0.3 mL/min 1

Optimal Imaging Timing

• CT should ideally be performed within 1 week of symptom onset to reliably demonstrate high density consistent with recent hemorrhage 1
• MRI with gradient echo (GRE) sequences is particularly helpful for identifying small hemorrhages that CT may miss 1