Why in the v1.3 instructions on CT/CBCT systems does it say to add half the size of a detector element when setting the source-detector distance? Why would there be a need to do that?

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# Do not forget to add half size of detection element !!!
cbct_system.set_source_detector_distance(1500.5, 'mm')

In GGEMS v1.3, the detector elements have a finite thickness, and the detector position is defined using the centre of the detection element along the source–detector direction.

Therefore, if the intended source–detector distance corresponds to the entrance surface of the detector, half of the detection element depth must be added in order to place the centre correctly.

For example, with a detector element depth of 1.0 mm and a desired distance of 1500.0 mm to the detector entrance surface:

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cbct_system.set_source_detector_distance(1500.0 + 0.5, 'mm')
This correction concerns the detector element depth along the source–detector axis, not its in-plane pixel dimensions.