[Rtk-users] Lateral blur in a FDK reconstructed volume

[Vincent Libertiaux]

On 19.07.22 19:21, Jerome Lesaint wrote:
> Hello all,
>
> The question of neglecting out-of-plane offsets if less than 2 degrees 
> is discussed in Yang et al, Med. Phys., 2006, section III.A.
> Regards,
> Jerome
>
> Le mar. 19 juil. 2022 à 18:23, Simon Rit 
> <simon.rit at creatis.insa-lyon.fr> a écrit :
>
>     Hi Vincent,
>     Thanks for the report. I don't believe that there is need for a
>     PR. It comes down to using a different parameterization which I
>     think you can always go around with one of the different versions
>     of AddProjection.
>     Did I mention that the out of plane angle has no effect below 2°?
>     If yes, I'm not sure you can trust this information... as I don't
>     know where it comes from.
>     Best regards,
>     Simon
>
>     On Tue, Jul 19, 2022 at 11:34 AM Vincent Libertiaux <vl at xris.eu>
>     wrote:
>
>         On 11.05.22 15:20, Vincent Libertiaux wrote:
>>         On 11.05.22 15:15, Simon Rit wrote:
>>>         Hi,
>>>         Yes, I think it's correct. To be sure you correctly
>>>         understand it, you can always do test cases with the source
>>>         and detector positions, u v vectors in the coordinate system
>>>         of your object.
>>>         http://www.openrtk.org/Doxygen/classrtk_1_1ThreeDCircularProjectionGeometry.html#a0fb1475ed76a28cde24fac85eae18e1e
>>>         and then check the resulting angles and distances.
>>>         Simon
>>>
>>>         On Wed, May 11, 2022 at 2:15 PM Vincent Libertiaux
>>>         <vl at xris.eu> wrote:
>>>
>>>             On 10.05.22 22:54, Simon Rit wrote:
>>>             > Hi Vincent,
>>>             > RTK can parametrize any orientation of the detector
>>>             with the three
>>>             > angles GantryAngle, InPlaneAngle, OutOfPlaneAngle.
>>>             0.025° seems very
>>>             > small indeed! I don't know how much you know about
>>>             software B but the
>>>             > easiest would be to have either the projection matrix
>>>             or the source
>>>             > position, detector position, u axis and v axis in
>>>             patient/object
>>>             > coordinates to derive the RTK parameters.
>>>             > Good luck with this!
>>>             > Simon
>>>
>>>             Hi Simon !
>>>
>>>             Unfortunately, I don't have access to B projection matrices.
>>>
>>>             As for the detector orientation in RTK, I have made this
>>>             picture to make
>>>             sure I understand properly how to use the gantry angle
>>>             to achieve my
>>>             desired geometry:
>>>
>>>             https://ibb.co/J3H8z9M
>>>
>>>             The cyan detector is the default configuration with a 0°
>>>             gantry angle.
>>>             The blue detector is at a gantry angle of alpha (largely
>>>             exaggerated for
>>>             the sake of clarity).  So in order to simulate an
>>>             out-of-plane rotation
>>>             of the detector around its vertical axis, I should
>>>             translate this blue
>>>             detector so that its center matches the coordinates of
>>>             the cyan one, and
>>>             translate the source accordingly (along the black
>>>             vectors on the
>>>             picture) ?  I assume that proj_iso_x/y and source_x/y
>>>             are expressed in
>>>             the gantry system of coordinates (local) ?
>>>
>>>
>>>             Thank you again for your feedback,
>>>
>>>             kindest regards,
>>>
>>>             V.
>>>
>>         Thanks Simon,
>>
>>         I'll investigate more and let you know.  Hopefully, it might
>>         be useful to someone else one day !
>>
>>         V.
>>
>         Hi Simon,
>
>         I finally got some time to investigate further this issue this
>         week.  I managed to get sharp edges everywhere now and it was
>         indeed the detector out-of-plane angle colinear with the
>         gantry angle that was the cause.  The value given by the other
>         software seems to have been in rad rather than degrees; the
>         angle I found was 1.15°.  This makes me wonder what were the
>         assumptions under which no effect was found for angles below
>         2°.  If you know the title of the seminal paper, I'd be
>         interested to read it.
>
>
>         As for the mean to include this angle in the geometry, no
>         extra code was indeed needed.  If we call this extra angle
>         "c", the following modifications have to be made in
>         rtksimulatedgeometry:
>
>         - first angle = c
>
>         - sdd = sdd_0 * cos(c)
>
>         - sid = sid_0 * cos(c)
>
>         - source_x = source_x0 - sid*sin(c)
>
>         - proj_iso_x = proj_iso_x0 + (sdd-sid)*sin(c)
>
>         I can't really promise I'll find time to do it, but if it is
>         the case, I'll submit a PR to include that in the matrices
>         computation.
>
>         Hopefully, it will help others on the list who encountered a
>         similar issue.
>
>         Best regards,
>
>         Vincent
>
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Thank you very much for the reference Jérôme !

Best regards,

Vincent
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