(HU) [CT scanner display unit] The arbitrary scale is defined by air, which has a CT number of -1000 HU, and water, with a CT number of 0 HU. It is named after Sir Godfrey Hounsfield, who developed the first clinical CT scanner.

See also Hounsfield Scale.

(Computed tomography number) The CT number is a selectable scan factor based on the Hounsfield scale. Each elemental region of the CT image (pixel) is expressed in terms of Hounsfield units (HU) corresponding to the x-ray attenuation (or tissue density).
CT numbers are displayed as gray-scale pixels on the viewing monitor. White represents pixels with higher CT numbers (bone). Varying shades of gray are assigned to intermediate CT numbers e.g., soft tissues, fluid and fat. Black represents regions with lower CT numbers like lungs and air-filled organs.

Contrast is the relative difference of intensities in two adjacent regions of an image. When referring to computed tomography (CT), contrast is defined as a difference in Hounsfield units between structures. The measurement of contrast resolution in CT imaging involves determining how easy it is to differentiate tissues whose CT density is similar to that of their surroundings. An image lacks contrast when there are no sharp differences between black and white. Brightness refers to the overall lightness or darkness of an image.
The contrast between air, soft tissue, and bones in x-ray and CT images is based on their different absorption of x-rays. Differences in tissue density, thickness and changes of the x-ray spectrum have consequences for image contrast, image noise as well as patient dose.
Optimized tube current, collimation, pitch and image reconstruction improves the contrast. Higher image contrast is produced by increased slice thickness, smaller matrix, and large field of view which results in large voxel size; high mAs to reduce noise; low pass filter.

See also Contrast Enhanced Computed Tomography.

(H) The Hounsfield scale displays radiodensity in a linear scale of gray shades expressed in Hounsfield units (HU). The Hounsfield scale is a quantitative transformation of the attenuation coefficient.
The Hounsfield value -1000 is defined as the radiodensity of air, 0 H that of distilled water at standard pressure and temperature, and denser tissues like for example cranial bone can reach 2000 H. The radiation attenuation of dental fillings or artificial implants depends on atomic number of the elements used. Titanium usually has an amount of +1000 HU. Iron steel can have a density greater than the highest range (traditional 3095 H) covered by the standard Hounsfield scale of a CT scanner. Areas with attenuation coefficients that exceed the scale's maximum are white areas in which no detail is visible.
Some CT machines are relatively tolerant, precise representing regions with very high densities. Sometimes, an option is available to select an extended CT number scale.

Linearity is a property of a system, characterized by output that is directly proportional to the input.
In computed tomography (CT), linearity describes the amount to which the CT number of a material is exactly proportional to the density of this material (in Hounsfield units). This accuracy between the linear attenuation coefficient and the CT number is also utilized to describe the performance of a CT scanner.
The linearity of a gamma camera is a measure of the geometrical correctness of the images.