[Rtk-users] FDK for planar ct

[Simon Rit]

Hi,
It's still not clear to me but what is helpful is to think in terms of
source trajectory wrt the object.
Trajectory 1: if I understand, you move the source on two circles plus one
point. I don't know of a FBP algorithm to reconstruct this, but there might
be one. I would consider iterative reconstruction first.
Trajectory 2: your trajectory is a point, the source does not move with
respect ot the object since it lies on the rotation axis. So each
projection contains exactly the same information up to a simple 2D
projection deformation. So it's hopeless to reconstruct from one projection
only.
To create the correct geometry, I would suggest using the function
AddProjection
<https://github.com/SimonRit/RTK/blob/master/code/rtkThreeDCircularProjectionGeometry.h#L92>
for which you provide the source and detector positions plus the 3D
coordinates of the two axes of the coordinate system of the projection.
I hope this helps
Simon

On Tue, Oct 10, 2017 at 5:43 PM, "Robert Calließ" <Robert.Calliess at gmx.de>
wrote:

> Hello,
> thank you for the fast reply.
> To answer your questions first.
> In this case the abbrevation pcb stands for printed circuit board.
> Next point is the trajectory we are currently handling with.
> Please see the attached image "trajectory.png". There are two schematics
> showing the side view and top view for trajectory type 1
> and a side-view for trajectory type 2.
>
> For type 1:
> The xray source is fixed. The pcb is clamped within a transport, so the
> pcb and the detector are moveable with in the xy plane.
> As you can see at the image, the pcb moves along a circular path but the
> pcb itself is not rotating. And let's assume that the iso ray
> always passes through the centre of the pcb and the centre of the detector.
>
> For type 2:
> The xray source is fixed and the detector is tilted. The pcb lies centred
> in the middle of a table. So that the pcb rotates around its centre
> around the z-axis.
>
>
> I hope this makes clear what trajectory i'm dealing with. Thank you.
>
> Kind regards,
> Robert C.
>
>
>
>
>
>
> *Gesendet:* Dienstag, 10. Oktober 2017 um 15:31 Uhr
> *Von:* "Cyril Mory" <cyril.mory at creatis.insa-lyon.fr>
> *An:* "Robert Calliess" <robert.calliess at gmx.de>,
> rtk-users at public.kitware.com
> *Betreff:* Re: [Rtk-users] FDK for planar ct
>
> Dear Robert,
>
> Your description of the trajectory is very obscure to me. Maybe you have a
> very unusual X-ray system. Could you make the following points clear :
>
> - what is a PCB ?
>
> - what is fixed/moving in your system (we need this information for the
> object, the source and the detector), and what kind of trajectories have
> the moving parts ?
>
> - can you re-draw your sketch with just 2 or 3 positions (ideally, on
> similar but separate drawings), each one with the object, the source and
> the detector ?
>
> If you do that, we should have a clear understanding of how your
> acquisition goes, and be able to give you appropriate advice.
>
> Best regards,
>
> Cyril
>
> On 10/10/2017 15:02, Robert Calliess wrote:
>
> Hello rtk users,
>
> I have question to the RTK FDK Filter. As far as I understand from to the
> fourier slice theorem the object to be reconstructed needs a circular
> trajectory and needs to rotate its own centre.
>
> Please have a look at the attached sketch. With this planar trajectory
> (Object, a PCB, is moved on a circle trajectpry  “in-plane”, PCB itself is
> not rotating) do I need
>
> a special filtering if I want to use FDK for planar CT with respect to the
> sketched trajectory ? I tried a circular in-plane trajectory where the PCB
> is centred and rotates
>
> around its centre point. And with 100 projections I get good results. But
> with the trajectory I described (sketch, attached image) the results are
> not so good.
>
> Because of the row-wise ramp filter It looks like there is a directional
> dependency. My assumption is, and with respect to fourier slice theorem,
> that the missing object
>
> rotation (rotation around itself) causes there directional effects.
>
>
>
> So my questions to the experts are. Do I need to apply a special filtering
> before backprojecting with FDK or is it just the wrong
>
> algorithm for this kind of trajectory ?
>
>
>
> kind regards,
>
> Robert C.
>
>
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