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Differential phase contrast (DPC) imaging, which utilizes phase shift information of X-ray, has the potential of
dramatically increasing the contrast in biological sample imaging compared to attenuation-based method that relies on
X-ray absorption information, since the X-ray phase is much more sensitive than the attenuation during transmission. In
a DPC imaging system, the phase stepping method is widely used to obtain DPC images: at each angle the phase grating
is shifted incrementally to produce a set of images and then the so obtained images are used to retrieve DPC image.
However, DPC imaging requires a high mechanical precision to perform phase stepping, which is generally one order
higher than the period of phase grating. Given that phase grating period is generally 2-4 um, the requirement of
mechanical accuracy and stability are very demanding (<0.5um) and difficult to meet in a system with rotating gantry. In
this paper, we present a method that is able to greatly relax the requirement of mechanical accuracy and stability by
stepping the source grating rather than the analyzer grating. This method is able to increase the system's mechanical
tolerance without compromising image quality and make it feasible to install the system on a rotating gantry to perform
differential phase-contrast cone beam CT (DPC-CBCT). It is also able to increase the grating shifting precision and as a
result improve the reconstructed image quality. Mechanical tolerance investigation and image quality investigation at
different phase stepping schemes and different dose levels will be carried out on both the original modality and the new
modality, the results will be evaluated and compared. We will deliberately create random mechanical errors in phase
stepping and evaluate the resulting DPC images and DPC-CBCT reconstructions. The contrast, noise level and sharpness
will be evaluated to assess the influence of mechanical errors. By stepping the source grating, the system is expected to
tolerate an error of 6-7 times bigger than that with analyzer grating stepping.
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