DICOM Window Width and Level Explained

This guide explains why windowing exists, how the numbers work for CT, and how to adjust them in practice. It's an explanation of the display controls, not guidance on reading scans.

Why Medical Images Need Windowing

A monitor can show roughly 256 levels of grey at once. A CT image typically stores values spanning several thousand units. If a viewer squeezed the entire range onto the screen, subtle differences, such as between two types of soft tissue, would be compressed into the same few grey shades and become invisible.

Windowing solves this by choosing a narrower range to display at full contrast. The rest is clipped to black or white. Different windows are used to look at different structures in the same data.

How the Numbers Work

With window level L and width W:

• The lowest displayed value is L − W/2, shown as black.
• The highest displayed value is L + W/2, shown as white.
• Values in between are mapped proportionally to grey.

For example, a window of level 40 and width 400 displays values from −160 to +240. Anything below −160 is black; anything above +240 is white.

A narrow width gives high contrast over a small range, so small differences are exaggerated. A wide width gives low contrast over a large range, so very different structures are all visible at once but subtle differences flatten out.

Hounsfield Units on CT

CT pixel values are calibrated in Hounsfield units (HU), a scale defined so that water is 0 HU and air is −1000 HU. Approximate values for common materials:

Values vary between scanners, protocols and individuals, so treat these as rough guides. Because the values have physical meaning, CT windows are fairly standardised.

Many CT files store raw pixel values plus a rescale slope and intercept, which the viewer applies to convert them into HU before windowing. That's why the raw number in a pixel may not look like an HU value at all.

Common CT Window Presets

These are common starting points; departments and textbooks use slightly different numbers. The DICOM viewer on SimpleTools24 uses these exact values for its four CT presets.

Windowing MRI and Other Images

MRI values aren't calibrated like Hounsfield units. The numbers depend on the sequence and scanner, so there are no universal MRI presets. Viewers start from the window stored in the file, or from the image's own value range, and you adjust visually. The same applies to many X-ray, ultrasound and nuclear medicine images.

Default Windows Stored in the File

DICOM files can include Window Center (0028,1050) and Window Width (0028,1051) tags with the window the scanner or radiographer chose. Some files store several pairs, with explanations such as "BRAIN" or "BONE". A viewer usually opens the image with the first stored window. If an image opens all white or all black, the stored window may not suit the data, or there may be none. Tags like these are covered in what a DICOM file is.

How to Adjust Window and Level

In most DICOM viewers:

1. Select the window/level tool (often the default).
2. Drag horizontally to change the width (contrast).
3. Drag vertically to change the level (brightness).
4. Click a preset for a standard window, or reset to the file's default.

Direction conventions vary between viewers, so try a small drag first. In the online DICOM viewer, the current level and width are shown as you drag, and the CT presets are one click away. Files are decoded in your browser, and if you haven't opened your images yet, how to open DICOM files covers the options.

Common Mistakes

• Judging an image in one window only. A structure that looks absent in a soft-tissue window may be obvious in a bone or lung window, and vice versa.
• Exporting with the wrong window. Screenshots and PNG exports capture only the current window. The full range of values is lost.
• Comparing images with different windows. Two scans can look very different simply because they were displayed differently.

Summary

Level sets the middle of the displayed range; width sets its size. On CT, Hounsfield units make windows predictable, which is why presets for soft tissue, lung, bone and brain are so common.