Hounsfield Unit Values for Blood on CT
Fresh, non-sedimented blood typically measures between 40-88 HU on CT, while acute intracerebral hemorrhage demonstrates mean attenuation around 50-58 HU.
Specific HU Ranges by Blood Type and Clinical Context
Fresh Blood
- Non-sedimented blood: 40-88 HU 1
- Acute intracerebral hemorrhage: 50-58 HU (mean 57.5 ± 3.3 HU) 2
- This range reflects the density of red blood cells and hemoglobin concentration in fresh hemorrhage
Sedimented Blood
- Upper serum layer: 13-38 HU (overlaps with serous fluid range) 1
- The separation occurs due to red blood cell settling, with the cellular component remaining hyperdense and the serum layer becoming hypodense
Minimum Threshold for Blood
- ≥15 HU represents the lower limit for subdural hematoma 3
- This corresponds to approximately 0.5 × 10⁴ red cells/mm³ or 15 mg/dL hemoglobin
- Below this threshold, fluid appears xanthochromic or translucent and should be classified as effusion rather than hematoma
Important Clinical Considerations
Time-Dependent Changes
The HU value of blood changes over time due to clot evolution:
- Lower HU values (55.7 HU) correlate with hematoma expansion risk versus stable hematomas (58.6 HU) 2
- This reflects the clot contraction process—tighter clot contraction increases RBC density and HU values
- Serial CT measurements show progressive changes in heterogeneity over hours to days 4
Heterogeneous Hematomas
- HU difference (dHU) >3.25 between hyperdense and hypodense regions predicts poor outcomes 5
- Blend signs indicate active bleeding with mixed attenuation patterns
- Higher heterogeneity correlates with neurological deterioration
Acute Pulmonary Embolism Context
- Most acute PE demonstrates ≤125 HU 6
- Values >125 HU favor artifact rather than acute thrombus (97% sensitivity below this threshold)
- This is notably higher than intracranial blood due to different vascular and tissue contexts
Key Pitfalls to Avoid
Temperature and technical factors have minimal impact: Variations in corpse temperature (4-40°C) and CT beam energy (80-130 kV) cause only minor HU fluctuations and should not impede blood characterization 1.
Putrefaction confounds measurements: In decomposed bodies, putrefaction fluid exhibits wide HU ranges (80 to -130 HU), making differentiation from blood impossible 1.
Protein and minerals contribute minimally: While hemoglobin is the primary determinant of blood attenuation, protein, iron, and calcium have negligible effects on HU values in hematomas 3.
Practical Algorithm for Blood Identification
- HU ≥40: Consider fresh, non-sedimented blood
- HU 15-39: May represent sedimented blood (serum layer) or serous fluid—correlate with clinical context
- HU <15: Unlikely to be blood; consider effusion, CSF, or other body fluids
- Heterogeneous appearance: Measure dHU between regions; >3.25 suggests active bleeding with poor prognosis
The hemoglobin concentration directly determines CT attenuation, with the minimum bloody appearance requiring at least 0.5 g/dL hemoglobin, corresponding to the 15 HU threshold 3.