Online calibration of focal spot drift for a high-resolution micro-CT system

Micro-CT (Micro-Computed Tomography) is a high-resolution, non-destructive three-dimensional imaging technology widely applied in biomedical research. However, the long scanning time of micro-CT and its higher sensitivity to small-scale perturbations make focal spot drift a more likely and non-negligible source of geometric artifacts. Focal spot drift is typically stochastic and unrepeatable, which may make offline calibration inaccurate, while conventional online calibration tends to be time-consuming due to iterative operations.

This paper presents a fast, accurate, and convenient online calibration method that overcomes the limitations commonly associated with existing online calibration approaches. To the best of our knowledge, this work is the first to explicitly and quantitatively describe the relationship between 3D focal spot drift and the resulting 2D projection shifts in micro-CT systems. By leveraging the prior geometric information of a specific feature point on the marker, its true spatial location can be precisely determined, which enables the tracking of its ideal trajectory and corresponding ideal projection positions across all views. Consequently, artifacts induced by focal spot drift can be effectively corrected by compensating for the offsets between the measured and ideal projection positions of the point. The entire correction process requires only a single pass of forward and backward projection.

The effectiveness and applicability of the method were validated through numerical simulation, physical experiments, and supplementary experiments. In numerical simulations, the method remained effective with even fivefold the normal perturbation level. In physical experiments without ground truth, this method achieved the highest level score according to BRISQUE (Blind/Referenceless Image Spatial Quality Evaluator) and completed the correction in the shortest time, making it the most efficient among all methods. Finally, the supplementary experiments were conducted to verify the applicability of the algorithm under certain assumptions and errors, further demonstrating its practicality.

The proposed method preserves the high correction accuracy and strong applicability of online calibrations while avoiding the need for frequent iterations or forward/backward projections, thereby achieving high computational efficiency. It is particularly well-suited for micro-CT systems with a large number of scan views and high projection resolution.