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What is "rescale intercept" and "rescale slope" in DICOM image (CT)? How to calculate window width and window center with that?

waldyrious
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Raj
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2 Answers2

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The rescale intercept and slope are applied to transform the pixel values of the image into values that are meaningful to the application.

For instance, the original pixel values could store a device specific value that has a meaning only when used by the device that generated it: applying the rescale slope/intercept to pixel value converts the original values into optical density or other known measurement units (e.g. hounsfield).

When the transformation is not linear, then a LUT (lookup table) is applied.

After the modality transform has been applied (rescale slope/intercept or LUT) then the window width/center specify which pixels should be visible: all the pixels outside the values specified by the window are displayed as black or white.

For instance, if the window center is 100 and the window width is 20 then all the pixels with a value smaller than 90 are displayed as black and all the pixels with a value bigger than 110 are displayed as white.

This allow to display only portions of the images (for instance just the bones or just the tissues).

Hounsfield scale: http://en.wikipedia.org/wiki/Hounsfield_scale

How to apply the rescale slope/intercept: final_value = original_value * rescale_slope + rescale_intercept

How to calculate the pixels to display using the window center/width:

  • lowest_visible_value = window_center - window_width / 2
  • highest_visible_value = window_center + window_width / 2
Tom Pohl
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Paolo Brandoli
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  • is there relation between Rescale slopt,intercept and window width,center...? – Raj Jan 09 '12 at 04:07
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    How to calculate window width and window center with rescale intercept and slope.? – Raj Jan 09 '12 at 04:07
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    @Raj No, there isn't any relation. Once the slope/intercept have been applied the values of the pixels are in Hounsfield units, optical density or other units. The Window center/width is used only to present the image to the end user. A single dicom file may define several center/widths (one to see only the tissue, one for the bones, etc) and the user select the right one when looking at the image. The end user may also define its onw center/width to see what he wants. – Paolo Brandoli Jan 09 '12 at 14:40
  • I am very new to medical domain, thats why having lots of doubts..please help me out. i have one IMA file(CT image). if i view that file using dicom viewer, i can see pixel datas, rescale intercept,slope,window width and center. my doubt is now that pixel datas are converted one(using rescale intercept slope) or original data ? – Raj Jan 10 '12 at 07:43
  • My task is to reduce row and column of image matrix if it is more than 1024. i am using interpolation formula to do that. consider the same CT image, image is 1200x1200. now output is 1024x1024 which is i converted. now do i need to calculate intercept and slope, window width and center values for this new pixel data ? – Raj Jan 10 '12 at 07:52
  • The data visualized by Dicom viewer depends from the implementation of the software, but my guess is that is display the values after the rescale slope/intercept has been applied. – Paolo Brandoli Jan 10 '12 at 22:03
  • You don't need to recalculate the rescale and the window width/center – Paolo Brandoli Jan 10 '12 at 22:04
  • I have a file that does not have width/center specified. One viewer shows the file completely white. Other viewers seem to calculate the width/center and it shows with correct gray scale. It also has a PhotometricInterpretation value of MONOCHROME1. Can you tell what is happening with the viewer that is showing all white? – user2568374 Nov 27 '18 at 14:34
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Rescale intercept and slope are a simple linear transform applied to the raw pixel data before applying the window width/center. The basic formula is:

NewValue = (RawPixelValue * RescaleSlope) + RescaleIntercept

BitBank
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