[Gate-users] Voxelised Source Placement Confusion

Thu Jun 16 15:27:07 CEST 2022

Hi all

I raised this query a while (or one similar) but unfortunately, I've not managed to get back around to my project for a while. In short, I can't get good alignment between my CT and PET images. My understanding is that the voxelised PET source image should be offset by half of its size to account for it being placed in the corner of the world instead of the centre where the CT image is place. My world is 1 1 2 m and my CT data is 780 780 693 mm (512x512x213 matrix). My PET data is 814.568 814.568 639 mm (200x200x213 matrix).

I think I was a bit confused at first, and was moving the PET source by half of its own size but then I re-read the ReadTheDocs and I think it is suggesting that I actually need to move the PET image by half of the world size (setPosition -500 -500 -1000 mm) but this results in a large offset. I've tried various combinations of half the size of the world, half the size of the PET data etc. and I think I'm just going around in circles now. Logically, I feel like the solution is that I move half of the world size at first to move the corner of the PET data into the centre of the world, then back by half of the PET image to put the centre of the PET in the centre of the world, but it is still offset. Any suggestions, as I'm tearing my hair out a little now.

Additionally, in order to test where the positioning is correct, I'm relying on the generation of secondary electrons within the water phantom in my image and looking at the NbofHits file produced. This works fine when the difference is obvious between the two, but I wonder if there is a better way to see the isocentre in order to 100% confirm it is correct? The reason I use secondary electrons is that positrons and gammas flood the image making it impossible to see the underlying CT data, plus I expect the secondary electrons to largely be created in the water and not air.

Thank you in advance!

Mark Baker
Principal Clinical Scientist
Imaging Physics (Ionising)
The Clatterbridge Cancer Centre NHS Foundation Trust
(he, him)

CCCW Tel:  0151 556 5030
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Honorary Lecturer, Dept. of Physics
University of Liverpool

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