[Rtk-users] Parker short scan test for enough over scan

[Simon Rit]

Hi,
Looking at the CPU code (which is more recent), it's because the width of
the detector is computed at the isocenter. You can have a look at function
ToUntiltedCoordinateAtIsocenter
<https://github.com/SimonRit/RTK/blob/master/code/rtkThreeDCircularProjectionGeometry.cxx#L519>
(in the standard case, you have sx=px=cosa=0). This complication comes to
deal with tilted detector, see this paper
<http://dx.doi.org/10.1118/1.4945418>.
There seems to be a bug in the GPU code, I'll submit an issue that it
should be fixed by making the two codes similar.
Simon

On Mon, Sep 25, 2017 at 2:01 PM, Fredrik Hellman <fredrik.hellman at gmail.com>
wrote:

> Hi,
>
> In rtkParkerShortScanFilter.hxx (line 113) and
> rtkCudaParkerShortScanFilter.cxx (line 95), the overscan angle is checked
> to be large enough for the used beam fan angle. (There is a difference
> between how this check is performed in the two cases. It seems like the
> CUDA-version assumes that the piercing point is in the middle of the
> detector. That's not what this question is about though.)
>
> The beam fan angle seems to be computed as
>
> arctan( halfDetectorWidth / sourceToIsocenterDistance )
>
> Why sourceToIsocenterDistance? Shouldn't it be sourceToDetectorDistance?
> If the detector is far away from the isocenter, the beam fan angle should
> also be small as far as I understand. This is not reflected in this
> computation.
>
> Best regards,
> Fredrik Hellman
>
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