Hounsfield Units (HU) are the standardized measurement scale used in computed tomography (CT) scanning to represent tissue density. Named after Sir Godfrey Hounsfield, who invented the first CT scanner, these units are essential for interpreting medical images and diagnosing various conditions. A CT HU value provides doctors with crucial information about the composition and density of different tissues and materials in the body.
The Hounsfield scale is a quantitative scale of radiodensity used in CT imaging. It's calibrated using two universal reference points:
This standardization ensures consistent measurements across different CT scanners and medical facilities worldwide. The scale typically ranges from -1000 to +3000 HU, though modern scanners can detect even higher values.
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Here's a comprehensive Hounsfield unit chart showing typical ranges for different tissues and materials:
| Tissue/Material | HU Range |
|---|---|
| Air | -1000 |
| Lung | -700 to -600 |
| Fat | -120 to -90 |
| Water | 0 |
| Blood (unclotted) | +13 to +50 |
| Muscle | +35 to +55 |
| Soft Tissue | +100 to +300 |
| Bone | +700 to +3000 |
| Metal | >3000 |
Blood HU values are particularly important in medical diagnosis. The CT HU value of blood varies depending on its state:
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CT HU values help medical professionals:
Metal objects create distinctive patterns in CT images due to their high density. Common metal HU values include:
Pacemakers and other medical devices present unique challenges in CT imaging:
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A measurement of 12 Hounsfield units is particularly significant in medical imaging as it falls within the range of several important tissue types:
To interpret CT HU values effectively:
Q: What is the normal range of Hounsfield units for blood? A: Normal blood typically measures between +13 and +50 HU, with variations depending on whether it's clotted or unclotted.
Q: How accurate are CT HU measurements? A: CT HU measurements are highly accurate when properly calibrated, with variations of ±4 HU considered normal.
Q: Can Hounsfield units help differentiate between benign and malignant tumors? A: Yes, HU values can help characterize tumors, particularly in organs like the adrenal glands, where values <10 HU often indicate benign lesions.
Q: How do metal implants affect CT HU measurements? A: Metal implants can create artifacts and scatter in CT images due to their very high HU values (>3000), potentially affecting nearby tissue measurements.
Reviewed by: Mathias Engström on March 1, 2025