Window and level explained: why a CT looks different

A CT image holds far more brightness values than a screen can show, so the viewer picks a range and stretches it across black to white. Window width is the size of that range and level (also called centre) is its midpoint. Change either one and the same slice looks completely different, even though the data has not changed at all.

Why a window is needed

A CT pixel is stored as a number. After a standard conversion, the numbers are in Hounsfield units, where water is 0 and air is about -1000. Dense bone can reach well above +1000. That is a range of thousands of steps.

A typical display shows 256 grey levels. Even if your screen could show more, your eye cannot tell thousands of greys apart. So the viewer has to decide which part of the range gets those 256 greys. That choice is the window.

Level and width

Think of the window as a frame placed over the number line.

• Level is where the frame is centred.
• Width is how wide the frame is.

Everything below the bottom edge of the frame is shown as pure black. Everything above the top edge is shown as pure white. Only values inside the frame get shades of grey.

A worked example: level 40 and width 400 gives a frame from -160 to +240. A value of -500 is black, a value of 300 is white, and a value of 40 is mid grey.

What each control does

• Raising the level shifts the frame up the scale, so the image gets darker overall.
• Lowering the level shifts it down, so the image gets brighter.
• A narrow width spreads the greys over a small range, so small differences look stronger, and everything outside the range is lost to black or white.
• A wide width includes more of the range, so you see more of it at once, with less contrast between close values.

Nothing is added or removed from the file. You are only choosing which values to look at.

Why presets exist

Radiology software offers preset windows for common ranges. Common names are soft tissue, lung, bone and brain. Each is a stored pair of level and width. As an example of the idea, lung presets are usually centred on a low value with a wide window, because lung tissue is mostly air, while bone presets sit much higher.

Exact numbers differ between programs and hospitals, so treat any preset as a starting point. A preset is not a diagnosis and does not tell you what a structure is. It only brings a certain range of values into view.

The window can be stored in the file

A DICOM file may include a suggested window centre and width as tags. A viewer can use them as the first view of the image. If the file has no such tags, software may calculate a window from the pixel data instead. You can look at these values in the tag list. See DICOM tags and metadata.

The tags only describe a starting view chosen by the scanner or the hospital. You are free to change the window.

MRI is different

MRI has no Hounsfield scale. The numbers depend on the sequence and the scanner, so the same tissue can have different values in two studies. Windowing still works the same way, but there are no standard preset values to reuse. Level and width are relative to the image in front of you, and the usual approach is to adjust them until the image is comfortable to view. For the difference between the two kinds of study, see CT and MRI scans: studies, series and slices.

Using windowing on a phone

Pick a preset, then fine-tune with level and width. If a slice seems to be missing detail, it may only be outside the current window. Try another preset or move the sliders before drawing any conclusion about the file.

Radix DICOM has window/level presets for soft tissue, lung, bone, brain and abdomen, plus sliders for centre and width. When a file has no preset, it sets a window automatically. It is not a medical device and is not for diagnosis.

Doctors read the images

How a structure looks under a given window is not a reliable guide to what it is. Radiologists use the windows suited to the question being asked, and only they and the treating doctor can interpret the images. For any question about what a scan means, ask the patient’s doctor.