windowlevel

Window level and window width are post-processing parameters used in medical imaging, especially computed tomography (CT), to control how voxel values are mapped to grayscale on a display. Window level (WL) is the central value of the range, while window width (WW) defines the span of values around that center that will be shown with detail.

The display mapping works by selecting a window of Hounsfield units (HU). Values within the window are displayed along a grayscale ramp, with shades depending on their distance from the center. Values below the lower limit are rendered as black, and values above the upper limit as white; values inside the window are displayed with graded intensity. Adjusting WL shifts the center of the range, effectively moving the overall brightness, while adjusting WW changes the contrast: a narrower WW increases contrast within the window, and a wider WW decreases it.

In practice, WL and WW are used to optimize visualization for different tissues and clinical questions. Radiologists tailor window settings to highlight structures such as soft tissue, air-filled lungs, or dense bone, often using predefined presets or auto-windowing features. Because different anatomic regions and pathologies require different contrasts, windowing is a common, essential tool for CT interpretation and can be adjusted on a per-image or per-study basis.

Technical note: Window Center and Window Width values may be stored in DICOM headers (among other metadata) and can be modified by viewing software or during image post-processing. They are part of the broader concept of windowing used to enhance diagnostic visibility across imaging modalities.