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Window and Level: Why the Same CT Scan Can Show Lungs or Bones
A CT image stores far more shades of gray than a screen can show, so a viewer shows only a range of them. The level (window center) picks the middle of that range and the width sets how wide it is, both in Hounsfield units. A wide window centered high shows bone; a wide window centered low shows the lungs; a narrow one shows soft tissue. Everything below the window is black and everything above it is white.
Open a chest CT and you might see a white heart and black lungs. Change one setting and the lungs fill with fine gray vessels while the heart turns into a white blob. Nothing about the scan changed. What changed is the window.
Why a window at all
The chest CT in our viewer stores 12 bits per pixel: 4,096 possible values. A typical screen shows 256 shades of gray. Squeeze all 4,096 into 256 and the differences that matter, between liver and muscle, or between healthy and diseased lung, all but disappear.
So a viewer shows only a slice of the range and stretches it across the whole gray scale:
- Level (window center): the value shown as middle gray.
- Width: how far the range reaches either side. Everything below it is black, everything above it white.
A narrow window gives strong contrast over a small range. A wide window shows more, with less contrast.
Hounsfield units
CT values are measured in Hounsfield units (HU). Water is 0. Air is about −1000. Fat is around −100. Soft tissues sit a little above zero, and bone runs from the hundreds up.
A DICOM file stores raw numbers and tells the viewer how to convert them: in our sample CT, subtract 1,024. Point at the image in the viewer and the lower right corner shows the value under the pointer, in HU.
The usual presets
These are the presets in our viewer, written as level / width:
| Preset | Level / width (HU) | What it shows |
|---|---|---|
| Lung | −600 / 1500 | Airways, vessels and nodules in the lungs |
| Soft tissue | 40 / 400 | Organs, muscle, fat, contrast in vessels |
| Mediastinum | 50 / 350 | The heart and great vessels between the lungs |
| Bone | 400 / 1800 | The skeleton, fractures |
| Brain | 40 / 80 | Gray and white matter of the brain |
The file can suggest a window too: most CTs carry the one the scanner or the technologist chose. That’s what From the file uses.
Try it
- Open the DICOM viewer and pick Chest CT.
- Choose Lung, then Soft tissue, then Bone.
- Pick the Window tool and drag: left and right changes the width, up and down the level.
- Point at the aorta, then at the lung: the corner shows the Hounsfield units.
On an X-ray or an MRI the same tools work, but the numbers aren’t Hounsfield units: an MRI’s values have no fixed meaning from one scan to the next, which is why there are no presets for them.
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