In fluoroscopic imaging, what does Hounsfield Unit (HU) refer to?

Hounsfield Unit (HU) is a specific quantitative measure used in computed tomography (CT) to represent the radiodensity of a specific tissue compared to the radiodensity of water. Radiodensity refers to how much a substance can attenuate X-rays or gamma rays; it provides an indication of how dense or how much mass per volume a material has. The scale is set such that the radiodensity of water is defined as zero Hounsfield Units, while air is defined as -1000 HU and denser materials like bone can have values exceeding +1000 HU.

This standardized measuring system allows practitioners to evaluate and differentiate between various tissues and materials in the body based on their composition and density, which is critical for accurate diagnosis and treatment planning in medical imaging.