[Rtk-users] About Kaiser-Bessel Filter

[Chao Wu]

Hi Guangming,

To reconstruct a volume from a series of tiff files you have to let the
algorithm know the geometrical relationship among the source, the volume
and the projections stored in the tiffs. The idea is the same: RTK uses
projection matrices and image coordinates to understand the positioning of
those projections. The projection matrices are based on a lot of
geometrical parameters like gantry angle, source position, detector
position, in-plane and out-of-plane rotation angles of the detector etc. I
won't go though all since most of them have no differences if you use tiff
files or whatever format as input. I focus on some tiff tips.

In the simplest situation, the tiff files contain correct resolution (DPI)
information, then you need to do nothing on these files. You have to
specify the correct detector offset though, since in the tiff image
coordinates the origin is the first pixel (corner pixel) which is
apparently not the intersection of the central ray and the detector. The
detector offsets in x and y actually mean the position of the image origin
(the first pixel) in the coordinate system centred at the intersection.
Since the direction cosines is an identity matrix and pixel sizes are
positive, the detector offsets in x and y will be negative.

If the resolution information of the tiff files is not correct, you have to
rewrite this information. You can of course perform a pre-processing to
change the DPI of all tiff files with a third-party software, or you can
alter the RTK code to allow such changes after reading the images, by using
a itk::ChangeInformationImageFilter filter.

Another non-geometrical issue with tiff files is that when their integer
pixel values are converted into attenuation, the maximum possible integer
is used as the reference value. This may not be correct in practice, e.g.
in your 16-bit tiffs you use 62000 to represent air (no attenuation)
leaving the integer range from 62001 to 65535 for statistical fluctuation.
If for instance you only use the lower 14 bits for your data then this
problem is more severe. For sure you can correct this by changing the code
and specify the correct reference value.

Hope these can help.

Regards,
Chao

2014-11-18 15:09 GMT+01:00 Guangming Zang <guangming.zang at kaust.edu.sa>:

> Thanks
>
> *Guangming Zang (Alex)*
> *King Abdullah University of Science and Technology(KAUST)*
> *University of Chinese Academy of Sciences(UCAS)*
>
>
> 2014-11-18 16:46 GMT+03:00 Guangming Zang <guangming.zang at kaust.edu.sa>:
>
>> Hi,
>> I am using the sart reconstruction in RTK, it works well.Thanks for your
>> great work.
>> and i was wondering do you have any plan to add other filters except
>> Joseph, such as Kaiser-Bessel???  ( described here
>> http://wscg.zcu.cz/wscg2003/papers_2003/herman.pdf)
>> BTW, i have read the messages sent by other users(
>> http://public.kitware.com/pipermail/rtk-users/2014-October/000573.html)
>> but i still can not make it. can u help me and explain again about using
>> a series of .tiff images ans sart algorithms to get the volume.
>> Thanks in advance
>> Best
>> *King Abdullah University of Science and Technology(KAUST)*
>> *University of Chinese Academy of Sciences(UCAS)*
>>
>>
>
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