TH-AB-204-08: Phase Retrieval in Grating-Based X-Ray Differential Phase Contrast CT with Twin-Peaks in Phase-Stepping Curves

Abstract

Purpose: As a promising new technique with high potential for translation into clinical applications, x-ray differential phase contrast (DPC) CT implemented with Talbot interferometry has drawn increasing interest. Usually, the x-ray DPC-CT employs a phase-stepping procedure to extract the phase signal. Since the fabrication processes may cause defects in analyzer grating G₂, the actual period of G₂ may double the nominal period of G₂, and twin peaks with remarkably different heights can be within a period of the experimental determined phase-stepping curve (PSC). For such a DPC-CT system with twin-peak PSCs, we develop an approach to retrieve the phase contrast for imaging. Methods: Based on the paraxial Fresnel-Kirchhoff theory, an analytical formula is derived to characterize the PSCs of an x-ray Talbot interferometry with flawed analyzer grating. In addition, an experimental investigation into the phase retrieval in x-ray DPC-CT with twin-peak PSCs is conducted by utilizing an x-ray Talbot interferometry with detector cell dimension and nominal period of G₂ being 96×96 micron2 and 4.6 micron, respectively. Results: Theoretical analysis demonstrates that the twin-peak feature of PSCs depends on not only the flaw in G₂, but also the mismatch between the phase grating and the spectrum of x-ray source. For a DPC-CT system with twin-peak PSCs (Figure 1), experimental results show that in comparison with scanning G₂ over its nominal period g₂, stepping the analyzer grating over its actual period 2g₂ can provide data to enable a significantly improved reconstruction of the phase-contrast CT images (Figures 2(b) and 2(d)). Conclusion: Our theoretical analysis and experimental investigation show that for an x-ray DPC-CT imaging system with twin-peak PSCs, the PSCs should be determined by scanning G₂ over the double of its nominal period g₂; and then the PSCs can be utilized to retrieve the phase signal for x-ray phase contrast imaging. Research Grant: W81XWH-12-1-0138 (DoD).