科研之友
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摘要
X-ray dark-field imaging using a grating interferometer has shown potential benefits for a variety of applications in recent years. X-ray dark-field image is commonly retrieved by using discrete Fourier transform from the acquired phase-stepping data. The retrieval process assumes a constant phase step size and a constant flux for each stepped grating position. However, stepping errors and flux fluctuations inevitably occur due to external vibrations and/or thermal drift during data acquisition. Previous studies have shown that those influences introduce errors in the acquired phase-stepping data, which cause obvious moire artifacts in the retrieved refraction image. This work investigates moire artifacts in x-ray dark-field imaging as a result of flux fluctuations. For the retrieved mean intensity, amplitude, visibility and dark-field images, the dependence of moire artifacts on flux fluctuation factors is theoretically derived respectively by using a first-order Taylor series expansion. Results of synchrotron radiation experiments verify the validity of the derived analytical formulas. The spatial frequency characteristics of moire artifacts are analyzed and compared to those induced by phase-stepping errors. It illustrates that moire artifacts can be estimated by a weighted mean of flux fluctuation factors, with the weighting factors dependent on the moire phase and different greatly for each retrieved image. Furthermore, moire artifacts can even be affected by object's features not displayed in the particular contrast. These results can be used to interpret images correctly, identify sources of moire artifacts, and develop dedicated algorithms to remove moire artifacts in the retrieved multi-contrast images.
