Hi Louis,
You first need to get the projections right. RTK has a filter
"rtkProjectionsReader" to read projections exported from a scanner, and
apply all the necessary processing, including log-transform, to obtain
line-integrals. The reconstruction methods (rtkfdk, rtksart, ...) all
use line integrals as inputs, but they embed an rtkProjectionsReader,
which can do the conversion at runtime. However, if rtkProjectionsReader
does not recognize your data and applies the wrong processing, the
reconstruction methods will return garbage. So the safest way is to
first call the rtkProjectionsReader (using the command line tool
rtkprojections), look at its output, and make sure it looks like line
integrals: it must have near-zero values in air, and higher values in
dense objects. Only then can you start reconstructing.
The diagram available here
http://www.openrtk.org/Doxygen/classrtk_1_1ProjectionsReader.html shows
which processing is applied to which kind of data. The files in .mha
format are assumed to be line integrals already, and bypass all
processing, so it is probably not the format you should use. I do not
know which way .tif are processed, especially unsigned short .tif, but
I've had a look at your data, and it looks as if no log-transform is
applied on your projections. The borders of the projections, where
probably the rays have only gone through air, have high values.
Therefore, you're reconstructing a cylinder, as large as your
projections, which probably isn't what you want.
Are you getting these images from a custom device built at the
university, or from a commercially available machine ?
Do you have an acquisition where the object is clearly visible in the
projections ? If so, can you send over one of these projections ?
Hope that helps,
Cyril
On 25/05/2018 16:46, louis.go...@student.uliege.be wrote:
Hi,
I am currently using RTK for my Electrician Master's thesis and I am finding
some difficulties...
We observed that reconstructed values range using fdk algorithm and sart are
different. How can this range be interpreted depending on the algortihm used in
order to compare the image quality between both algorithms?
Furthermore, some voxels have negative value, is it possible to obtain negative
values with rtkfdk or rtksart application? does it make sens and how can that
be?
And finally, last question, is there a specific projection file type (.tif,
.mha, ...) and/or pixel type (unsigned short, float, ...) to use when using
rtkfdk or rtksart?
Here are some informations that may be useful:
Object scanned:
Rock sample and homogeneous polymer cylinder
Projection are:
-type: .tif (unsigned short) or .mha (float)
-projection set: 360 seperated (.tif) files or 360 seperated (.mha) files or
one single (.mha) file
Reconstructed image:
-type: .mha (float)
Algorithm used:
-rtkfdk (without ramp filter)
-rtksart (3~5 iterations, 1 projections processed between each update of the
reconstructed volume (1 for SART))
Here is a link to download some projection and reconstructed image:
https://www.dropbox.com/sh/sgrzrylphe4dar2/AAAFW3hJYzyZGbPevf_CYteka?dl=0
Thanks in advance for your help or advices.
Best regards,
Louis
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