Posted by
Harry Parker on
Jun 10, 2007; 12:26am
URL: http://imagej.273.s1.nabble.com/Plugin-announcement-MosaicJ-tp3699175p3699179.html
Hello, M. Nur Heriawan.
I do not know much about CT imaging, but I am a physicist and a photographer.
First, the relation between CT values and rho is dependent on many factors such as the exposure level and energy of the X-rays in the beam. So we cannot tell what is the correct equation. That depends on your setup. If the material density is too high for your current settings, the exposure needs to be increased one way or another. Perhaps different exposures will be required to get good data for different parts of the sample. This is one way to increase the dynamic range of the data.
Perhaps I can help with your second problem, the "cupping" effect. Regular camera images also have similar nonuniformities due to "lens shading" effects, sometimes called "vignetting".
In digital photography, images do not have negative values. What do
they indicate in CT? The "zero" /"no signal" value needs to be determined for your system. The constant offset must be subtracted out. This technique also assumes that the signal level is linear to exposure. So if your recorded values are nonlinear (e.g. logarithmic) this needs to be corrected as well.
Basically, less light reaches the sides and corners of an image than reaches the center. So an image of a uniform scene will not be uniform, but darker in the corners.
This can be corrected by multiplying each pixel by a factor that is the inverse of the falloff. This factor is 1.0 near the center of the image ( = no correction) and increases towards the edges and corners.
This effect can be measured and eliminated by taking images of a blank uniform scene. Getting a really uniform scene is the hardest part of this technique. Many blank images should be averaged together to minimize noise. Then convert the image to 32 bit (floating point) if it is not already in that format.
Then divide the whole image by its center intensity. Now you have a "gain image" where each pixel's value is its relative gain coefficient. Use the Image Calculator function to divide your object images by this gain image to correct them.
If the falloff is large in the corners, then the gain will be great in the corners, greatly increasing the noise in that part of the image. This limits the practical value of this technique.
Does this help?
--
Harry Parker
Senior Systems Engineer
Digital Imaging Systems, Inc.
----- Original Message ----
From: M. Nur Heriawan <
[hidden email]>
To:
[hidden email]
Sent: Friday, June 8, 2007 7:21:37 AM
Subject: CT value and cupping effect
Dear ImageJ`ers,
Firstly, I would like to make sure...is it correct
that the CT values resulted from X-ray CT scanner is
only ranging from -1000 to 2000? So, if we consider
the relationship between CT value and bulk density of
material (rho) is:
CT value = 1000*rho - 1000
then only material with bulk density from 0 to
3 g/cc can be detected by using X-ray CT scanning.
In case our material contains a small material with
bulk density more than 3 g/cc (for example: mineral
pyrite has density around 5 g/cc), how we can detect
it from the CT image?
Secondly, after I did thresholding using ImageJ for my
CT image, I found that it contains of `cupping effect`
with darker color presents around the center part of
image. Cupping effect is one of the artificial effect
on CT image due to the beam hardening effect during
scanning process. Does anyone have experience to
remove this cupping effect by image processing?
Any comments and suggestions are very welcome.
Thank you.
---------------
M. Nur Heriawan
Laboratory of Applied Geoscience and Technology
Graduate School of Science and Technology
Kumamoto University, JAPAN
http://www.civil.kumamoto-u.ac.jp/tansa
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