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The probe tool allows you to peek at the pixel coordinates of the location under your cursor and the actual value of that pixel.

1.Select the Probe Tool.

2.Click and hold the left mouse button at the point on the image that you wish to probe. The image pixel coordinates and value are shown in a little information box on the image – although what you see depends on the modality of the image.

On a CT image, you will probably see the pixel value in Hounsfield Units.

On any other type of image, you will see the raw pixel value as stored in the image.

3.Clicking on the drop-down beside the probe button allows you to customize the values it displays.  The options presented essentially allow you to choose pixel values sampled at different stages in the image rendering/LUT pipeline to be shown by the probe tool.

Technical Notes

The probed pixel value will typically be given in Hounsfield Units (HU) for CT images.  This is because most CT scanners will embed a LUT in the image headers that defines the transformation from the raw pixel values into the Hounsfield Scale.  There is currently no equivalent common unit for other modalities, which is why you will only see the raw pixel value by default on such images.


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