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

[Jerome Lesaint]

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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