# Why Are There Negative Voxel Values in My RTKFDK X-ray Reconstruction?
**URL:** https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498
**Category:** Algorithms
**Created:** [April 21, 2025, 2:06am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498 "2025-04-21T02:06:47Z")
**Posts on this page:** 9
**Page:** 1
### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 21, 2025, 2:06am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/1 "2025-04-21T02:06:47Z")
I’m working on 3D reconstruction using cone-beam X-ray projection images with the **FDK algorithm in RTK (Reconstruction Toolkit)**. After reconstruction, I noticed that **some voxel values in the output volume are negative**.
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 21, 2025, 2:07am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/2 "2025-04-21T02:07:43Z")
What I reconstruct is a head phantom.
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 21, 2025, 2:10am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/3 "2025-04-21T02:10:00Z")
@simon.rit
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 21, 2025, 2:37am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/4 "2025-04-21T02:37:10Z")
This is 3D reconstruction of a vertebra phantom.
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 21, 2025, 3:30am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/5 "2025-04-21T03:30:23Z")
I didn’t use -Hann -radius -multiplier .
my code is : .\rtkfdk.exe -g -o -p -r “.\*.dcm” -v “on” --neworigin “-214.76,214.76,0” --newdirection “1,0,0,0,-1,0,0,0,1” --hardware “cuda” --newspacing “0.56,0.56,1” --spacing 0.28 --dimension 240 --origin “-40,-80,-30”
and
.\rtkfdk.exe -g -o -p -r “.\*.dcm” -v “on” --neworigin “-214.76,214.76,0” --newdirection ‘1,0,0,0,-1,0,0,0,1’ --newspacing “0.56,0.56,1” --hardware “cuda” --spacing 0.28 --dimension 560
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### Author:  [@simon.rit](https://discourse.itk.org/u/simon.rit)
#### Post date: [April 21, 2025, 7:40pm UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/6 "2025-04-21T19:40:46Z")
It’s not unusual to have negative values as FDK does not enforce positivity. For example, noisy measurements in air oscillate around zero.
I don’t know why the head and vertebra phantoms seem have negative values without looking at the projection images, you should probably check for negative values in them or truncation artefacts.
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 22, 2025, 1:42am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/7 "2025-04-22T01:42:06Z")
thank you for your help.
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### Author:  [@zhangxiang](https://discourse.itk.org/u/zhangxiang)
#### Post date: [April 22, 2025, 2:02am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/8 "2025-04-22T02:02:54Z")
The voxel values of my input 2D images are 0 for black (phantom) and 255 for white(nothing or air). Do I need to inverse it?
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### Author:  [@simon.rit](https://discourse.itk.org/u/simon.rit)
#### Post date: [April 22, 2025, 7:45am UTC](https://discourse.itk.org/t/why-are-there-negative-voxel-values-in-my-rtkfdk-x-ray-reconstruction/7498/9 "2025-04-22T07:45:47Z")
Yes, it would make sense as the linear attenuation coefficient of air is ~0 and positive for tissues. Are you using RTK to forward project your phantom?