Introduction to windowing

Windowing is also known as gray-level mapping, contrast stretching, histogram modification, or contrast enhancement. This process manipulates the CT image in grayscale via the CT numbers. It changes the appearance of the picture to highlight particular structures. The brightness and contrast of the image are adjusted through the window level and the window width, respectively. We’ll talk more about window level and window width later.

Hounsfield units (HU)

Hounsfield units (HU) are dimensionless units universally used in Computed Tomography (CT) scanning. They express CT numbers in a standardized, convenient form. Hounsfield units are obtained from a linear transformation of the measured attenuationThe lessening in signal value over distance coefficients.

The attenuation coefficient measures how easily a material can be penetrated by an incident energy beam (for example, in an ultrasound or X-ray). It quantifies how weak the beam gets by passing through the material. This weakening is caused by the arbitrarily-assigned densities of air and pure water.

The radiodensity of distilled water and air at standard temperature and pressure (STP) is 0 HU and -1000 HU, respectively.

Note: STP is 00 °C, and pressure is 105 pascals at sea-level. It results in a scale running from 1000-1000 HU for air to  +2000~+2000 HU for very dense bone (such as the cochlea) and over 30003000 for metals. Generally, that’s why CT images use 12-bit images able to store values between 1024−1024 to 30713071.

Window width

The transition of dark to light structures occurs during a larger transition to a narrow window width (<1000<1000 HU). A significantly wider window (>2000> 2000 HU) will cause various attenuation between soft tissues to be darkened.

Wide window

Wide window (WW) is defined as 4002000400-2000 HU. It is best used in the areas of acute differing attenuation values. Good examples of these types of areas are the lungs or cortical tissue, where air and vessels are present side by side.

Narrow window

Narrow window is 5035050-350 HU. This window is excellent when examining areas of similar attenuation, like soft tissue.

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