Re: Image Gradient

Posted by Dr. Ulrich Mletzko on Dec 22, 2005; 9:17am
URL: http://imagej.273.s1.nabble.com/Image-Gradient-tp3704168p3704176.html

At 15:07 21.12.2005, Dan Siler wrote:

>I am X-raying tangents of cones/cylinders with the following chord lengths
>taken every 0.100 inches.
>2.3967"
....
>7.3376"
>This gives intensities in the part that range from 3043 to 515
>What would be the best way to flatten the image gradient?

Do you want to flatten the 'image gradient' or the 'x-ray intensity gradient'?

The first question should be no problem, using contrast manipulation or
histogramm equalization tools, available in almost all image processing
software, including ImageJ.

The second question is more difficult.

Let's assume, that you have 'narrow beam' attenuation (i.e. lead or
tungsten or uranium iris collimators on the source side and on the detector
side) and a linear intensity response of the detector. Then the measured
x-ray intensity is depending on the chord length according to the well
known exponential intensity attenuation law. To flatten, you have three
possibilities:
(i) Use of a pre-filter (some millimeters of copper plate, e.g.) between
x-ray source and specimen, to attenuate the low or 'weak' energy parts of
the x-ray spectrum (to 'harden up' the spectrum).
(ii) Use of a higher x-ray energy (i.e. higher voltage), as up to some MeV
the attenuation coefficients are decreasing with increasing energy.
(iii) Use of a detector with non-linear response (or working near the
saturation of the detector).

If you have 'broad beam' attenuation according to a cone beam geometry
without  iris collimators, you could try the same measures. But the image
quality in general would be lower, as you have additional scattered
radiation to the detector from both the specimen and the environment.

Regards
Uli Mletzko
NDT Group, Materials Testing Institute,
University of Stuttgart (Germany)