Re: Normalization question - ROI only or whole image - CT/CBCT

Posted by Michael Doube-4 on Dec 09, 2013; 2:39pm
URL: http://imagej.273.s1.nabble.com/Normalization-question-ROI-only-or-whole-image-CT-CBCT-tp5005843p5005855.html

On 09/12/13 03:12, Kenneth Sloan wrote:
> aren't CT images "born normalized"?

Most medical CT scanners output Hounsfield unit (HU) calibrated images. Sometimes the pixel value = HU, but more usually there's a linear calibration curve applied to raw pixel values. The main data points are air (-1000 HU) and water (0 HU). They're also usually at least 10-bit, often 12-bit or 16-bit data, so you can get a better visualisation by changing the brightness and contrast or window and level. There's no need to 'Apply' these to the data, just fiddle with the controls until you see what you want. Remember, most displays are 8-bit which matches well human dynamic response, but CT data have a greater bit depth which can be stretched to fit into that range.

If you are using X-ray microtomography to image your jaw bones, it's likely that your images aren't HU calibrated. However, the above still holds. Don't normalise, just adjust the window and level so you can see the contrast you need in the 16-bit data. But don't 'Apply'!

Michael

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Dr Michael Doube
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Lecturer, Comparative Biomedical Sciences
The Royal Veterinary College, University of London
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United Kingdom

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