# Point on 2D CT slice image to RAS coordinate transformation

**URL:** <https://discourse.slicer.org/t/point-on-2d-ct-slice-image-to-ras-coordinate-transformation/35770>\
**Category:** Support\
**Tags:** coordinates, ras, transform\
**Created:** [April 27, 2024, 12:57pm UTC](https://discourse.slicer.org/t/point-on-2d-ct-slice-image-to-ras-coordinate-transformation/35770 "2024-04-27T12:57:38Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![joanne40226](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.slicer.org/joanne40226/32/14681_2.png) [@joanne40226](https://discourse.slicer.org/u/joanne40226)\
**Post date:** [April 27, 2024, 12:57pm UTC](https://discourse.slicer.org/t/point-on-2d-ct-slice-image-to-ras-coordinate-transformation/35770/1 "2024-04-27T12:57:38Z")

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Hi community,  
I encounter problems when doing point on 2D CT slice image to RAS coordinate.

I have screen shot an 2D CT image as shown below. i check some imformation for example the actual size of the image in mm is : 649.092624007295, 314.6698141861267(mm) and the sliceToRAS is [-1.0 0.0 0.0  
0.0 -0.6712189288498026 -0.7412591649036953  
0.0 -0.7412591649036953 0.6712189288498026 ]  
also, the offset of the slice is -184.6262(mm)，and the offset of the 1st image of the slice is 424.6262(mm). also the Image Origin: (204.5mm, 200mm, -361mm)  
now on the 2D CT image, i want to transfer the point (716, 473) in pixel to {RAS}. the answer is R=−9.352mm,A=41.234mm, 𝑆=−229.525mm bu measurement.  
can anyone tell me how the calculation should be when doing this kind of 2D-3D coordinate transformation?

 ![screenshot1](https://us1.discourse-cdn.com/flex002/uploads/slicer/original/3X/2/9/29de6aba2834e08a06510c4b36c359288bede7d2.jpeg)

thank you for the time.