Why is my reconstruction so bad ?
This question often arises when the resulting image (or volume) is so disappointing... Specially when you are working with experimental projections, new artifacts and defects are appearing never met before in this series Learning Tomography where all the images are always perfect.
Thus, it is time to show some of these defects... to understand how to fix them.
Non-centered Projections
One of the first problem is the rotation center... because in tomography, we assume that the center of rotation − used during the back-projection process − is superimposed to the image center.Why is it so important? Just look at this example, the sinogram of Fig. 1 was calculated from a 256x256 8-bit Lena with a rotation center located at (136, 132) corresponding to her right eye (the modified script is here [Link]). That means that during the sinogram computation, Lena is rotated about her right eye and then vertically projected.
Fig.1: Sinogram of Lena (256x256 8-bit image) calculated with a center of rotation (136, 132) roughly corresponding to her right eye. This sinogram was filtered with a ramp filter. |
Fig.2: Reconstruction with the script backProjImproved.js. In this script, we assume that the center of rotation is located at (128,128). |
This defect is characterized by a specific pattern.: Each dotted feature in the image (the eyes, for example) appears as an arc. Thus, if you see this artifact, a pre-processing of your projections is required to align them along a vertical centered axis (in the case of a equatorial collection scheme).
Hope this helps and thank you for reading these lines.
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