[Rtk-users] Reconstruction Artifact

[Simon Rit]

Yes, there is no such limitation as far as I know, you can use negative
numbers and value above 1.
Your result is really strange, it it supposed to be a square? I don't know
what is the problem but that's clearly a geometry issue. We can always have
a look if you're able to share some data.
Simon

On Wed, Jun 3, 2020 at 1:56 PM Andreas Andersen <andreasga22 at gmail.com>
wrote:

> 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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