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Tilt correction

When the turn table and detector are not aligned there will be an error in the reconstructed data. For small deviations this error can be corrected by adjusting the center of rotation for each slice. In MuhRec you have two parameters for the tilt correction. The first is the axis tilt angle and the other set the pivot point relative to the image.

The pivot is used for long samples where the turn table is located far away from the detector. For a single data set this parameter is less important than when you have several scans that you want to merge after the completed reconstruction. It is not possible to correct for both detector and pivot tilt simultaneously in the current version. The figure below shows the two different detector rotations.

figures/axis_tilt.png

Possible acquisition axis tilts.

figures/tilt_sample.png

Tilt correction in MuhRec

The tilt correction in MuhRec is designed for small angles (less than 0.5°) under the assumption that the experimental setup is well aligned. It uses a simplified algorithm to speed up processing by shifting the center of rotation from slice to slice based on the estimated tilt angle. This approach avoids rotating the projections which was the only way to correct the tilt when single sinograms were reconstructed. Rotation is more accurate but requires more data and significantly increases processing time.

For tilts greater than 0.5°, it is recommended to rotate the projections prior to loading them into MuhRec. Otherwise, the simplified correction may introduce noticeable inaccuracies in the reconstruction.

The plot below illustrates how far the projection data is fetched from its correct position, measured in image lines. Ideally, this offset should be less than one line. Offsets up to 2.5 lines are still acceptable, but larger deviations will cause significant distortions in the reconstructed image. At first, these inaccuracies appear as blurring. As the correction angle increases, the artifacts can become more severe and visibly distort the reconstructed image. These distortions are especially apparent in fine details near the object's perimeter. The maximum tolerable correction angle also depends on the object's radius: compact objects near the rotation center are less affected than larger or off-center ones.

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