What is the Hounsfield Unit (HU) value of blood in a Computed Tomography (CT) scan?

Hounsfield Unit Values of Blood on CT Scans

On unenhanced CT scans, blood typically measures between 40-70 Hounsfield Units (HU), with variations depending on hematocrit levels and whether the blood is acute, sedimented, or clotted. 1, 2

Normal Blood Attenuation Values

• Fresh, non-sedimented blood in living patients typically measures between 40-88 HU on unenhanced CT scans 2
• Blood attenuation has a strong positive correlation with hematocrit levels (correlation coefficient of 0.72), with higher hematocrit resulting in higher HU values 3
• In postmortem CT studies, non-sedimented blood shows HU values of 40-88 HU within 1-4 days after death 2

Variations in Blood HU Values

Based on Blood Condition:

• Acute blood clots/thrombi typically show higher density, with cerebral venous sinus thrombi averaging 66.2±5.3 HU compared to normal venous blood at 47.2±4.5 HU 4
• Sedimented blood shows layering with:
  • Upper serum layer: HU values overlap with serous fluids (13-38 HU) 2
  • Lower cellular layer: Higher HU values due to concentrated cellular components 2

Based on Patient Factors:

• Anemia significantly lowers blood HU values:
  • Blood with hemoglobin <10 g/dL typically measures <35 HU 3
  • A threshold of 35 HU has 76% sensitivity and 81% specificity for detecting anemia 3
• Gender differences exist in the correlation between HU and hemoglobin (stronger correlation in men at 0.81 vs. women at 0.52) 3

Technical Considerations Affecting HU Measurements

• CT scanner model and manufacturer can cause variations of 7-56 HU for the same material 5
• kVp settings affect measured HU values, with higher kVp settings generally resulting in slightly lower HU values 5, 2
• Temperature changes can slightly affect HU measurements, with increasing temperature leading to slightly decreased HU values 2

Clinical Applications of Blood HU Measurements

• Diagnosis of venous thrombosis:
  • HU values >58 and HU:hematocrit ratios >1.4 have shown 100% sensitivity for diagnosing cerebral venous sinus thrombosis in pediatric patients 4
  • In adults, attenuation >60.4 HU has 71.4% sensitivity and 100% specificity for cerebral venous thrombosis 6
• Assessment of anemia:
  • Quantitative HU measurements perform significantly better than visual assessment for detecting anemia 3
  • A threshold of 35 HU provides 76% sensitivity and 81% specificity for detecting anemia (hemoglobin <10 g/dL) 3

Common Pitfalls in Blood HU Interpretation

• Visual assessment of blood density is unreliable with poor interobserver agreement (kappa 0.0906-0.2128) 3
• Contrast enhancement significantly alters blood HU values and makes standardized interpretation impossible 1
• Scanner-specific variations mean that absolute HU thresholds should be used with caution across different CT machines 5
• Putrefaction fluids in decomposed bodies show wide HU ranges (-130 to 80 HU) that can overlap with blood values 2