[Insight-users] threshold levels for pixels in itkThresholdImageFilter

[Robert Haase]

Hi Daniel,

firstly it depends on what image you are trying to threshold. CT images for example usually have values between -1000 and +4000 (hounsfield units). If you are working with a 'crazy' PET (Bequerel per millilitre) or MRI image, your value range may be explainable. If you know what kind of image it is, you should find a definition of the value range in the documentation of the modality. You may also have a look at the tag "0054|1001" in the DICOM eader. It documents in what physical unit the image should be interpreted.

http://dicomlookup.com/lookup.asp?sw=Tnumber&q=%280054,1001%29

Secondly the range depends on what data type your image variable is. If you wish to have it as an unsigned char (range 0-255) you could easily resample any image to a range of 0 to 255 by using itkRescaleIntensityImageFilter.

Best,
Robert

-------- Original-Nachricht --------
> Datum: Wed, 18 Apr 2012 23:05:20 -0400 (EDT)
> Von: danieljohnsonusa at netscape.net
> An: insight-users at itk.org
> Betreff: [Insight-users] threshold levels for pixels in	itkThresholdImageFilter

> I am using  itkThresholdImageFilter to threshold a dicom image.  I am 
> using the ThresholdOutside method to specify the range of pixel to 
> leave unfiltered.  I initially set my upper and lower thresholds to 
> values between 0 and 255, but I kept getting that everything was above 
> the threshold.  I did some experimenting and  found the range of 
> intensities that filtered what I wanted ended up being 6000 to 300000.  
> Can anyone explain the intesity mapping range of values for a dicom 
> image in itk?  Is this intensity number RGB pixel values all multiplied 
> together, and if so, is there a way to specify thresholds individually 
> to each instead of all 3 multiplied together?  Is there a way to tell 
> how the pixels are formatted in your input image and convert between 
> formats like RGB to greyscale?  A reference to any type of 
> documentation that explains pixel intensities in the different formats 
> would be greatly appreciated.
> 
> Thanks,
> 
> Daniel
> 
> 
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