# Non-uniform background

**URL:** <https://castor-project.discourse.group/t/non-uniform-background/466>\
**Category:** General Discussions\
**Created:** [February 6, 2023, 4:18pm UTC](https://castor-project.discourse.group/t/non-uniform-background/466 "2023-02-06T16:18:54Z")\
**Posts on this page:** 4\
**Page:** 1

<div class="post-metadata">

**Author:** ![mipeac](https://avatars.discourse-cdn.com/v4/letter/m/53a042/32.png) [@mipeac](https://castor-project.discourse.group/u/mipeac)\
**Post date:** [February 6, 2023, 4:18pm UTC](https://castor-project.discourse.group/t/non-uniform-background/466/1 "2023-02-06T16:18:54Z")

</div>

Dear Castor users:  
I am currently simulating with GATE an IQ phantom using two scanners. One scanner has a 4x larger axial FOV than the other. While the image from the smaller scanner (A) is correct, the larger scanner (B) results in a non-uniform background, particularly around the central spheres, it can be seen in the attached images.  
Please, if someone has managed to do it and can help me, I would appreciate it.  
Best Regards!  
Miriam

 ![im](https://global.discourse-cdn.com/free1/uploads/castor_project/original/1X/24aa6d130205a9e9ead0e24dcf4559e80098f7f0.png)

---

<div class="post-metadata">

**Author:** ![Xin\_Zhao](https://avatars.discourse-cdn.com/v4/letter/x/f05b48/32.png) [@Xin\_Zhao](https://castor-project.discourse.group/u/Xin_Zhao)\
**Post date:** [February 8, 2023, 5:19am UTC](https://castor-project.discourse.group/t/non-uniform-background/466/2 "2023-02-08T05:19:38Z")

</div>

Hello Miriam,  
Have you applied Normalization in CASToR recon? It is introduced to correct artifacts caused by the nonuniformity of sensitivity. If you do not apply normalization, ring & star artifacts may appear.  
Best,  
Xin

---

<div class="post-metadata">

**Author:** ![mipeac](https://avatars.discourse-cdn.com/v4/letter/m/53a042/32.png) [@mipeac](https://castor-project.discourse.group/u/mipeac)\
**Post date:** [February 13, 2023, 10:38am UTC](https://castor-project.discourse.group/t/non-uniform-background/466/3 "2023-02-13T10:38:33Z")

</div>

Hello Xin!  
Thank you very much for your answer. In fact no, I did not apply the normalization in the CASToR recon. I have been reading about how to introduce the normalization factors but it is still not clear to me.  
Please, if anyone has managed to perform a normalization correction from a GATE simulation and can give me some guidance on what steps I have to follow, I would appreciate it.  
Best Regards!  
Miriam

---

<div class="post-metadata">

**Author:** ![Xin\_Zhao](https://avatars.discourse-cdn.com/v4/letter/x/f05b48/32.png) [@Xin\_Zhao](https://castor-project.discourse.group/u/Xin_Zhao)\
**Post date:** [February 16, 2023, 3:24am UTC](https://castor-project.discourse.group/t/non-uniform-background/466/4 "2023-02-16T03:24:27Z")

</div>

Hi Miriam,  
You can follow the methods of normalization as introduced in " Pepin, Audrey et al. “Normalization of Monte Carlo PET data using GATE.” _2011 IEEE Nuclear Science Symposium Conference Record_ (2011): 4196-4200."

You will find details about normalization factor calculation in the article. When you have normalization factors for each LOR, you can provide a normalization data file into CASToR recon.

Best!  
Xin