[Rtk-users] Gantry rotation direction and artefacts

[Vincent Libertiaux]

On 28.08.20 14:13, Simon Rit wrote:
> Thanks for the illustration. Maybe the detector is not oriented as 
> intended by RTK? If you look at the first drawing of the geometry doc 
> <http://www.openrtk.org/Doxygen/DocGeo3D.html>, I would question the 
> direction of the vector v. You can probably just flip it to put it in 
> the right direction? e.g. with
> rtkfdk -p . -r ^proj.mha$ -g direct.xml --spacing 0.5 -d 300 
> --hardware cuda -o fdk.mha --newdirection 1,0,0,0,-1,0,0,0,1 
> --neworigin -140,151.6,0
> which comes down to flipping the y axis after reconstruction without 
> the last two options. I think that the RTK coordinate system becomes 
> indirect if you flip this v axis which is probably ignored by your 
> visualization tool. I admit I realized only recently that I often 
> reconstruct data which are like this.
> I hope I'm clear, if not that's probably because I don't master so 
> well all this...
> Simon
>
> On Fri, Aug 28, 2020 at 1:23 PM Vincent Libertiaux <vl at xris.eu 
> <mailto:vl at xris.eu>> wrote:
>
>     Hi Simon,
>
>     I am afraid I was no clear enough. Please find a picture of the
>     real object and the reko at that link:
>
>     https://www.dropbox.com/sh/ul0oy9kv3us4ey7/AABQ5Y4R1PR-jcRawGFKOUK4a?dl=0
>
>
>     So you can see that on the part, the serial number is on the
>     "head" side while it is on the "tail" side on the reconstruction,
>     using the "direct" geometry.  That is what I call the mirror
>     image.  The rotation axis is along the vertical direction of the
>     image.  I could easily reorder the reconstructed slice to get it
>     in the right orientation, but I was wondering where the issue
>     comes from.
>
>     Best regards,
>     Vincent
>
>     On 28.08.20 12:13, Simon Rit wrote:
>>     Mirror in which direction? Depending on the direction, it can
>>     also be a 180° offset of the angle. If it reconstructs well, I
>>     would assume that the direct direction is the correct one but
>>     there is something else you need to understand...
>>
>>     On Fri, Aug 28, 2020 at 10:44 AM Vincent Libertiaux <vl at xris.eu
>>     <mailto:vl at xris.eu>> wrote:
>>
>>         Hi Simon,
>>
>>         thank you for testing my dataset.
>>
>>         I get the same results you describe and I am quite happy with
>>         the first result. However, the reconstructed volume is a
>>         "mirror" view of the real object, and my guess was that the
>>         rotating plate was going in the opposite direction assumed by
>>         rtk. Is it the wrong assumption?
>>
>>         Thank you again for your help,
>>
>>         best regards,
>>
>>         Vincent
>>
>>
>>         Thanks for the dataset. When I run
>>           rtkfdk -p . -r ^proj.mha$ -g direct.xml --spacing 0.5 -d
>>         300 --hardware cuda -o fdk.mha
>>         The result looks good to me. Obviously, when I run
>>           rtkfdk -p . -r ^proj.mha$ -g inverse.xml --spacing 0.5 -d
>>         300 --hardware cuda -o fdk.mha
>>         the result is bad since the correct rotation direction seems
>>         to be the direct one. Did you expect the second line to
>>         produce the correct result? Or is the first line not
>>         producing a good enough result in your opinion?
>>
>
Hi Simon,

thanks for the explanation.  I'll have a go later today or Monday, but I 
will definitely let you know what was the result.


Have a nice week end,

Vincent

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