Purpose: We investigate an analyzer-less x-ray interferometer with a spatially modulated phase grating (MPG) that can deliver three modalities (attenuation image, phase image, and scatter images) in breast computed tomography (BCT). The system can provide three x-ray modalities while preserving the dose to the object and can achieve attenuation image sensitivity similar to that of a standard absorption-only BCT. The MPG system works with a source, a source-grating, a single phase grating, and a detector. No analyzer is necessary. Thus, there is an approximately 2x improvement in fluence at the detector for our system compared with the same source–detector distance Talbot–Lau x-ray interferometry (TLXI) because the TLXI has an analyzer after the object, which is not required for the MPG.
Approach: We investigate the MPG BCT system in simulations and find a clinically feasible system geometry. First, the mechanism of MPG interferometry is conceptually shown via Sommerfeld–Rayleigh diffraction integral simulations. Next, we investigate source coherence requirements, fringe visibility, and phase sensitivity dependence on different system parameters and find clinically feasible system geometry.
Results: The phase sensitivity of MPG interferometry is proportional to object–detector distance and inversely proportional to a period of broad fringes at the detector, which is determined by the grating spatial modulation period. In our simulations, the MPG interferometry can achieve about 27% fringe visibility with clinically realistic BCT geometry of a total source–detector distance of 950 mm and source–object distance of 500 mm.
Conclusions: We simulated a promising analyzer-less x-ray interferometer, with a spatially sinusoidal MPG. Our system is expected to deliver the attenuation, phase and scatter image in a single acquisition without dose or fluence detriment, compared with conventional BCT.
Phase-contrast X-ray provides attenuation, phase-shift and small-angle-scatter in tissue in same scan yielding multi-contrast information about object, which has greatly benefitted breast-imaging, pre-clinical lung-imaging and bone imaging. A primary barrier for clinical adaptation of interferometric X-ray/CT for torso imaging is the manufacturing difficulty of large gratings. Large-gratings have to be stitched from smaller elements introducing errors such as gaps, errors in pitch, phase-jumps, tilts, causing imaging artifacts. Removing these artifacts will be an advancement towards clinical adaptability this multi-contrast modality. In this work we focus on the Talbot-Lau X-ray Interferometer and investigate effects of different grating defects in 1-D simulations. The grating spot defects include gaps, pitch errors, phase-height errors. We quantify the sum-squared error in reconstructed phase for different types of defects, showing most egregious artifacts for the pitch-errors. We developed two artifact correction methods in interference fringe patterns (i.e. before reconstruction) – an analytical and a neutral network approach. The analytical method (SWFT) uses Short- Window-Fourier-Transform to estimate the local phase-shift and attenuation due to the defect in the blank scans and then applies the correction for the with-objects scans. We also proposed a Regression Convolution Neural Network (R-CNN) to learn these errors and correct for them. Distinct sets of pitch artifacts were used each for training (300 datasets) and testing (300 datasets) with variety of levels of severity of artifacts for three different objects – sphere, ramp and slab. The algorithms performed well, reducing the artifacts from initial average normalized-mean-squared-error of 44.7% to 6.3% for SWFT and 7% for SWFT+R-CNN.
Phase contrast X-ray not only provides attenuation of tissue, but two other modalities (phase and scatter) in same scan. Scatter (dark-field) images provided by the technology are far more sensitive to structural and density changes of tissue such as lungs and can identify lung disease where conventional X-ray fails. Other areas poised to benefit greatly are mammography and bone joint imaging (eg. imaging arthritis). Of the various interferometer techniques, the two at the forefront are: Far-field Interferometry (FFI) (Miao et al, Nat. Phy. 2015) and Talbot-Lau interferometry (TLI) (Momose JJAP 2005, Pfeiffer Nature 2006). While the TLI has already made clinical strides, the newer FFI has advantage of not requiring an absorption grating (“analyzer”) and provides few-fold higher scatter sensitivity. In this work, a novel 2D single phase-grating (not requiring the analyzer), near-field phase contrast system was simulated using Sommerfeld- Rayleigh diffraction integrals. We observed 2D fringe patterns (pitch 800nm) at 50mm distance from the grating. Such a pattern period of 0.05mm, can be imaged by the LSU-interferometers with CT detector resolution (0.015mm) or Philips mammography detector resolution (0.05mm) making this practical system. Our design has a few advantages over Miao et al FFI system. We accomplish in one X-ray grating the functionality that requires 2-3 phase-grating in their design. And our design can also provide a compact system (source to detector distance < 1m) with control over the fringe pattern by fine tuning grating structure. We retain all the benefits of far-field systems -- of not requiring analyzer and high scatter sensitivity over Talbot-Lau interferometers.