[Rtk-users] Reconstruction Artifact

[Andreas Andersen]

I don't think there are any restrictions technically.
You should be able to use negative values, as the main arithmetic is just a
sum
<https://github.com/SimonRit/RTK/blob/master/include/rtkBackProjectionImageFilter.hxx#L106>
and a multiplication
<https://github.com/SimonRit/RTK/blob/master/include/rtkBackProjectionImageFilter.hxx#L115>
.

The only restriction I can see is that this sum and multiplication should
not overflow (or underflow for negative values) the underlying type of the
output image, as that would be undefined behaviour. Over- and underflow is
unlikely for float, unless you have extremely high values (see wikipedia
for floating point range
<https://en.wikipedia.org/wiki/Floating-point_arithmetic#Range_of_floating-point_numbers>
).

/Andreas

__________________________________

Andreas Gravgaard Andersen

Danish Center for Particle Therapy,

Aarhus University Hospital

Palle Juul-Jensens Blvd. 99,

8200, Aarhus

Mail:     agravgaard at protonmail.com

Cell:      +45 3165 8140


On Wed, 3 Jun 2020 at 03:34, Benjamin W. Maloney <
Benjamin.W.Maloney.TH at dartmouth.edu> wrote:

> Hi,
>
> I thought the same in regards to trying to rotating in the other
> direction. Unfortunately, that has a similar artifact but with the
> reconstruction flipped.
> Interestingly the overlap happens in a similar place but the internal
> structures are flipped
>
> ​1. Thanks!
> 2. I should have worded that better. My projection images will be
> preprocessed in a float format. I wanted to check if there were
> restrictions on these float values. Can input image data have negative
> values or high values? Or are they expected to have values between 0 and 1?
> I ask because some of the tools I have used to do this preprocessing
> (outside of RTK) have given negative values or 'stretched' the data from 0
> to 255 before saving.
>
> Ben
>
> ------------------------------
> *From:* Simon Rit <simon.rit at creatis.insa-lyon.fr>
> *Sent:* Tuesday, June 2, 2020 5:28 PM
> *To:* Benjamin W. Maloney <Benjamin.W.Maloney.TH at dartmouth.edu>
> *Cc:* rtk-users at public.kitware.com <rtk-users at public.kitware.com>
> *Subject:* Re: [Rtk-users] Reconstruction Artifact
>
> Hi,
> Sometimes rotating in the wrong direction gives this kind of artefacts.
> It's quite possible that we don't use the same convention as other toolkits
> regarding this.
> For other questions:
> 1. Yes, you can use pixel as the unit. Then the image spacing should be 1
> obviously and indeed, sdd and sid should be in pixels.
> 2. I don't fully understand. If your data is the output of a count
> detector, then you either rely on RTK to guess the counts without object to
> compute the line integral, or your preprocess your projections to pass line
> integrals in a float format.
> Simon
>
> On Tue, Jun 2, 2020 at 4:51 PM Benjamin W. Maloney <
> Benjamin.W.Maloney.TH at dartmouth.edu> wrote:
>
> Hi all,
>
> Not sure if this is the right group to post to but here's my question:
>
> I have a code that pulls in my own projection images and uses
> the FDKConeBeamReconstructionFilter for reconstruction
>
> The reconstruction I am getting has an artifact where it looks like there
> are two overlapping objects rotated.
> I have used other reconstruction toolboxes (mainly TIGRE in MATLAB) with
> the similar geometry inputs and not had this issue. I suspect the
> difference is in the parts of the geometry that are set by default. My
> question is if anyone has seen this before and what input I should look
> into?
>
> I have a few more questions I may or may not be related:
> 1. I assume that since I set the origin etc of my images in pixel, sid and
> sdd should be in pixels as well?
> 2. Are there restrictions related the scalar values of the projection
> data? My data will be in detector counts rather than linear attenuation
> coefficients, is that okay?
> I have attached an image to show this issue. It is supposed to be a
> rectangular mammography phantom. It is a slice in XZ plane
>
> Thanks for the help!
> Ben
>
>
>
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