[Rtk-users] RTK calculating attenuation from Varian projection

[Andy Shieh]

Hi Simon,

Thank you. At the moment I'm just doing something similar to what you did
for the Elekta system - using a log_ref (I'm using HND_INTENSITY_MAX) and
minus the log of intensity plus one (I assume the plus one is to avoid
taking the log of zero?).

Another few question:
1. Are you doing any normalization after reconstruction or forward
projection? Are the negative values in the reconstructed images simply a
result of the ramp filtering?
2. If I want to compare a reprojection from an image reconstructed using
RTK with the original projection, how should I make sure the normalization
is correct? i.e. should I plus/minus or multiply the  reprojected
attenuation before I use exponential and make it back to intensity values?
3. The rtkforward projections doesn't seem to work correctly when I'm
projecting using a geometry file containing several Projection (Angle). The
output mha file seems to "repeat" after a couple of projections instead of
showing all projections. I'm testing this on a Varian Half Fan geometry -
it works fine if I break the geometry file down into one each for each
projection angle and do the forward projection separately. I can send you a
reprojection mha file if my description is unclear to you.

Thanks Simon!!

Cheers,
Andy.



2012/10/24 Simon Rit <simon.rit at creatis.insa-lyon.fr>

> Hi Andy,
> Yes you're right, I have actually noticed recently that but I have not
> worked a lot on Varian images because we don't have a Varian CBCT in
> Lyon. I only had a sub-sampled acquisition from Greg of a Catphan that
> I used to roughly check the geometry. As far as I can remember, when I
> wrote this piece of code, I tried to use the same code as Plastimatch
> but I could well have done it wrongly. I'm actually waiting for a new
> complete Catphan acquisition from Greg to correct for this but if you
> already have a patch to suggest or a set of images to send, feel free
> to do it.
> Thanks,
> Simon
>
> On Tue, Oct 23, 2012 at 2:48 PM, Andy Shieh <hsieandy at gmail.com> wrote:
> > Hi Simon,
> >
> > I was looking at "rtkVarianObiRawImageFilter.h", and realized that the
> > attenuation is calculated from the projection image simply via a negative
> > transformation (1-Intensity/HND_MaxIntensity).
> > Is it usually the way this is done, and is there any reason for doing
> this?
> > I would have thought attenuation should be calculated from intensity via
> > logarithm (since I=I_0 exp(-mu x)).
> > Thanks!!
> >
> > Cheers,
> > Andy
> >
> > _______________________________________________
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> >
>
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