[Rtk-users] FDK for planar ct
Cyril Mory
cyril.mory at creatis.insa-lyon.fr
Wed Oct 25 03:34:04 EDT 2017
Dear Robert,
Could you send two datasets of projections, one for each case ? We would
have a look at them and it would help us understand your trajectories.
The drawings you sent do not seem to be sufficient to remove all
ambiguities.
Best regards,
Cyril
On 25/10/2017 09:03, "Robert Calließ" wrote:
> Hello,
> the object is moving on a circular path. There are arrows between the
> different detector positions showing the
> moving direction. The source is static.
> Kind regards,
> Robert
> *Gesendet:* Dienstag, 24. Oktober 2017 um 16:47 Uhr
> *Von:* "Simon Rit" <simon.rit at creatis.insa-lyon.fr>
> *An:* "Robert Calließ" <Robert.Calliess at gmx.de>
> *Cc:* "rtk-users at public.kitware.com" <rtk-users at public.kitware.com>
> *Betreff:* Re: Re: [Rtk-users] FDK for planar ct
> Hi,
> I see one drawing only, not two. And the object does not seem to be
> moving on your drawing, is it? If not and the source is also static
> (as it seem), this is equivalent to one large projection.
> Simon
> On Tue, Oct 24, 2017 at 3:58 PM, "Robert Calließ"
> <Robert.Calliess at gmx.de <mailto:Robert.Calliess at gmx.de>> wrote:
>
> Hello,
> I suppose there are still misunderstandings with respect to the
> trajectory.
> Attached you can find the two difference trajectories. I also had
> a closer look to
> the off centered fdk ( the paper you suggested). But I don't think
> it is in my case.
> The iso ray passes object center and detector center at each view.
> Off centered fdk
> has a different preweighting scheme.
> You said that the RTK ramp filter is along the u axis (orthogonal
> to rotaion axis). For planar_ct_1 trajectory that
> should fit. As you can see at the picture, the object is moving on
> a circular path but not rotating around the
> center point (red cross in the image).
> Kind regards,
> Robert C.
> *Gesendet:* Donnerstag, 12. Oktober 2017 um 07:14 Uhr
> *Von:* "Simon Rit" <simon.rit at creatis.insa-lyon.fr
> <mailto:simon.rit at creatis.insa-lyon.fr>>
> *An:* "Robert Calliess" <robert.calliess at gmx.de
> <mailto:robert.calliess at gmx.de>>
> *Cc:* "rtk-users at public.kitware.com
> <mailto:rtk-users at public.kitware.com>"
> <rtk-users at public.kitware.com <mailto:rtk-users at public.kitware.com>>
>
> *Betreff:* Re: [Rtk-users] FDK for planar ct
> Hello,
> No. The filter should be orthogonal to the rotation axis. The RTK
> ramp filter is along the u axis of the projection.
> Trajectory 2: if you take photos by rotating the cameras, they are
> photographies of the same point-of-view. This is what I meant.
> Cheers,
> Simon
> On Wed, Oct 11, 2017 at 8:58 PM, Robert Calliess
> <robert.calliess at gmx.de <mailto:robert.calliess at gmx.de>> wrote:
>
> Hello,
>
> thanks for the link to the paper but I dont have access to it.
> Aside from how the trajectory is interpreted within in RTK. My
> actual question was
>
> if any of those two trajectories would need another
> reconstruction filter than the FDK Filter. From my point of
> understanding a specific rotation around
>
> the object is necessary for fbp/fdk (like c-arm bow, standard
> circular cone-beam trajectory). That’s why I asked If the
> first trajectory needs some other reconstruction
>
> filter because the object itself doesn’t rotate around itself.
> It actually gets translated on a circular path. So I was more
> expecting a “yes” or “no” to the fdk filter
>
> or a hint to another filter (except iterative reconstructions)
> I should use for these trajectories.
>
> To trajectory 2: I think the projections are different. The
> object rotates and each projection shows a different view.
>
> Kind regards,
>
> Robert C.
>
> *Von:*simon.rit at gmail.com <mailto:simon.rit at gmail.com>
> [mailto:simon.rit at gmail.com <mailto:simon.rit at gmail.com>] *Im
> Auftrag von *Simon Rit
> *Gesendet:* Dienstag, 10. Oktober 2017 20:29
> *An:* Robert Calliess
>
>
> *Cc:* Cyril Mory; rtk-users at public.kitware.com
> <mailto:rtk-users at public.kitware.com>
> *Betreff:* Re: [Rtk-users] FDK for planar ct
>
> Hi,
>
> Let me try to clarify what I mean by "source trajectory wrt
> the object." In tomography, you need to determine the source
> trajectory in the object coordinate system, we don't really
> care about the source trajectory in the room coordinate
> system. For example, rotating the source on a circular
> trajectory or rotating the object makes no difference for the
> reconstruction algorithm. That's why we call diagnostic
> scanners "helical scanners".
>
> So for trajectory 1, it seems that the source trajectory
> (again, wrt to the object) is a circle but the object is
> offset. This is somewhat similar to
> https://doi.org/10.1109/TNS.2006.880977 except that the
> detector is not tilted so FDK would be the only FBP algorithm
> I could think of. But the situation is really not good, data
> are missing and iterative reconstruction should give better
> results.
>
> Trajectory 2: what I said in my previous email is true, it's
> useless I believe, all projections are similar up to a 2D
> transform of the projection.
>
> Simon
>
> On Tue, Oct 10, 2017 at 8:07 PM, Robert Calliess
> <robert.calliess at gmx.de <mailto:robert.calliess at gmx.de>> wrote:
>
> Hello,
>
> I try to clarify the both trajectories.
>
> Trajectory 1:
>
> No, i dont move the source on two circles. The xray source is
> fixed. Only the object and the detector moves. Both move on a
> circular path so that the iso-ray
>
> always passes through the pcb centre and the detector centre.
> There is one orthogonal view and the others are the ones
> moving on the circular path.
>
> (Object is not rotating around its own axis).
>
> Trajectory 2:
>
> Yes, the xray source lies in the rotation axis and only the
> object rotates around its z-axis. Detector and xray source are
> fixed and the detector is tilted.
>
> It’s almost like this trajectory here
> https://www.ikeda-shoponline.com/engctsoft/wp-content/uploads/2016/06/Oblique-View-CT1.jpg
>
> except that the xray source lies on the rotation axis.
>
> I hope this helps to understand the trajectories I have to
> deal with.
>
> Kind regards,
>
> Robert
>
> *Von:*simon.rit at gmail.com <mailto:simon.rit at gmail.com>
> [mailto:simon.rit at gmail.com <mailto:simon.rit at gmail.com>] *Im
> Auftrag von *Simon Rit
> *Gese**ndet:*Dienstag, 10. Oktober 2017 19:06
> *An:* Robert Calließ
> *Cc:* Cyril Mory; rtk-users at public.kitware.com
> <mailto:rtk-users at public.kitware.com>
>
>
> *Betreff:* Re: [Rtk-users] FDK for planar ct
>
> 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 <mailto: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
> <mailto:cyril.mory at creatis.insa-lyon.fr>>
> *An:* "Robert Calliess" <robert.calliess at gmx.de
> <mailto:robert.calliess at gmx.de>>, rtk-users at public.kitware.com
> <mailto: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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