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=== Combining images and projection data === When you are forward projecting an image or reconstructing projection, you need to know what the relation between the 2 conventions is. Unfortunately, STIR does not support different "bed positions" yet. For backwards compatibility, the following convention is used. :'''For images with zero offset, the middle of the scanner is assumed to coincide with the middle plane of the image.''' So, for the generate_image example above, the cylinder would be located in the centre of the scanner. :<strong>WARNING: the combination of these conventions means that if you change the number of planes in the image, you also have to change the "origin" of the shape such that it would forward project into the same projection data.</strong> You can see this clearly from the formulas used in the generate_image example above. However, this convention is confusing and therefore might be changed in the future. Together with a current limitation of the STIR projectors, this leads us to the following: :'''We recommend that you use images which have 2*(num_rings-1) planes with z-voxel-size equal to ring_spacing/2.'''
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