On the Characteristics of Helical 3D X-Ray Dark-Field Imaging
The X-ray dark-field can be measured with a grating interferometer. For oriented structures like fibers, the signal magnitude depends on the relative orientation between fiber and gratings. This allows to analytically reconstruct the fiber orientations at a micrometer scale. However, there currently exists no implementation of a clinically feasible trajectory for recovering the full 3D orientation of a fiber. In principle, a helical trajectory can be suitable for this task. However, as a first step towards dark-field imaging in a helix, a careful analysis of the signal formation is required. Towards this goal, we study in this paper the impact of the grating orientation. We use a recently proposed 3D-projection model and show that the projected dark-field scattering at a single volume point depends on the grating sensitivity direction and the helix geometry. More specifically, the dark-field signal on a 3D trajectory always consists of a linear combination of a constant and an angular-dependent component.
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