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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 normally see the pixel value in Hounsfield units.

On any other type of image, you will often see a pixel value without units.

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:
Post-VOI LUT: This is the value after having been further transformed by the VOI window as selected by the Window/Level Tool.
Raw: This is the raw pixel value exactly as stored in the pixel data of the image, without accounting for the rescale function, such as the one used to compute Hounsfield units on CT images.

Technical Notes

The value reported by the probe tool will only have units if the image defines the function to transform the stored pixel values to a standardized scale.

For CT images, the value will typically be in Hounsfield units (HU).
For PT images, the value may have one of a number of possible units, from physical units like becquerels/millilitre (Bq/ml) to an abstract unit that is proportional to event counts.
There is currently no generally accepted standard unit for other modalities.


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