The Hounsfield Unit (HU) is a universal measurement scale used in computed tomography (CT) scans. It quantifies the radiodensity of tissues and materials in the body, allowing for consistent, quantitative analysis of CT images.
The Science Behind the Scale
The derivation of Hounsfield Units stems from the principle of linear attenuation, which describes how X-rays are absorbed or scattered as they pass through various materials. Denser materials attenuate more X-rays, while less dense materials allow more X-rays to pass through. The Hounsfield scale is standardized using two specific reference points to ensure consistency across different scanners and examinations.
Distilled water is assigned a value of 0 HU, serving as a neutral baseline. Air, with its very low density, is defined as -1000 HU. All other tissue values are calculated relative to these points, creating a spectrum of radiodensities. The scale is named after Sir Godfrey Hounsfield, who received the Nobel Prize for his role in the invention of the CT scanner.
The Hounsfield Unit Chart Explained
The Hounsfield Unit chart provides typical values for various tissues and substances, reflecting their X-ray attenuation. Air, the least dense, registers at approximately -1000 HU. Lung tissue, containing significant air, typically falls within -700 to -600 HU.
Fats, such as adipose tissue, exhibit values between -120 and -90 HU. Fluids like water are 0 HU, while cerebrospinal fluid (CSF) in the brain measures around +15 HU. Blood values vary by state; unclotted blood ranges from +13 to +50 HU, and acute hemorrhage often appears denser, between +40 and +60 HU.
Soft tissues, including muscle, kidney, and liver, commonly show values from +35 to +80 HU. Muscle tissue is often around +45 to +50 HU, and the liver typically measures +45 to +50 HU. Denser tissues like cortical bone register high positive values, generally from +700 to +3000 HU, with some bones exceeding +2000 HU. Medical contrast agents and metals, used in implants, can have even higher values, often surpassing +3000 HU.
Practical Application in Medical Imaging
Healthcare professionals, particularly radiologists, use the Hounsfield Unit scale for precise tissue identification and diagnostic purposes in CT imaging. Measuring the HU value of a region of interest on a CT scan helps ascertain tissue type, aiding differentiation of normal anatomy from pathology. This quantitative approach enhances diagnostic accuracy.
For instance, a fluid-filled cyst typically exhibits HU values close to 0, similar to water, distinguishing it from a solid tumor with higher soft-tissue values, often +40 to +80 HU. Acute hemorrhage in the brain is also detectable, as fresh blood has a distinct HU range, generally +40 to +60 HU, appearing brighter than surrounding brain tissue. Quantifying tissue density using Hounsfield Units allows for a more confident medical diagnosis.