Is acute blood hyperdense on a computed tomography (CT) scan?

Is Acute Blood White (Hyperdense) on CT?

Yes, acute blood appears hyperdense (bright/white) on non-contrast CT scans, typically with attenuation values ranging from 50-90 Hounsfield Units, appearing brighter than surrounding brain tissue. 1

Imaging Characteristics of Acute Hemorrhage

Acute blood demonstrates homogenous hyperdensity on CT that is easily and accurately identified when imaging is performed within the first week after bleeding onset. 2 The hyperdense appearance results from:

• High protein content and clot formation in fresh blood, creating attenuation values typically between 50-90 HU 1
• Homogenous bright signal that clearly contrasts with adjacent brain parenchyma 2
• Optimal detection when CT is performed ideally within 1 week of symptom onset to reliably demonstrate high density consistent with recent hemorrhage 2

Specific Manifestations by Location

Intracranial Hemorrhage

• Intracerebral hemorrhage appears as a focal hyperdense mass within brain parenchyma 1
• Subarachnoid hemorrhage demonstrates hyperdensity in the subarachnoid spaces, with CT showing high sensitivity for detection 2, 1
• Subdural/epidural hematomas typically show hyperdensity corresponding to clotted blood 3

Vascular Thrombosis

• Hyperdense vessel sign indicates acute thrombus within an artery or vein 2
• Hyperdense MCA sign occurs in one-third to one-half of angiographically proven middle cerebral artery thrombosis, with typical attenuation around 61 HU 2, 4
• Cerebral venous thrombosis shows homogenous hyperdensity filling the vein or sinus, best visualized when CT slices are perpendicular to the vessel 2, 5

Evolution of Blood on CT Over Time

The appearance changes predictably as blood ages:

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

Critical Diagnostic Considerations

Hounsfield Unit Thresholds

• Acute blood typically measures 50-90 HU 1
• Values >40-42 HU are optimal for detecting basilar artery thrombosis 1
• Values >90 HU poorly predict hemorrhagic transformation with low sensitivity (23%) but high specificity (94%) 6

Important Caveats and Pitfalls

Non-contrast CT can be normal in early or small hemorrhages, with only 30% of cerebral venous thrombosis cases showing abnormalities on initial imaging. 5, 1 Additional pitfalls include:

• Anemia can cause acute subdural hematomas to appear isodense rather than hyperdense, creating a diagnostic trap 7
• Hyperacute unclotted blood may show mixed hyperdensity and isodensity, indicating ongoing active bleeding or coagulopathy 3
• Beam hardening artifact in the posterior fossa limits detection of posterior circulation hemorrhages 1
• Partial volume averaging with adjacent calcium, CSF, or fat can alter apparent density of small thrombi 2

Clinical Application

If clinical suspicion for hemorrhage remains high despite normal non-contrast CT, proceed immediately to CT angiography or MRI, as CT angiography can detect active bleeding at rates as low as 0.3 mL/min. 1 This is particularly important because:

• CT has limited sensitivity for detecting acute and small cortical or subcortical infarctions, especially in the posterior fossa 2
• MRI with gradient-echo sequences may be at least as accurate as CT for detecting acute intracranial hemorrhage and can identify microbleeds not visible on CT 2
• The "empty delta" sign on contrast-enhanced CT is characteristic of cerebral venous thrombosis but may not appear for several days after symptom onset 2, 5