Abstract

Beam tracking X-ray Phase Contrast Imaging is a “Shack-Hartmann” type approach which uses a pre-sample mask to split the x-rays into “beamlets” which are interrogated by a detector with sufficient resolution. The ultimate spatial resolution is determined by the size of the mask apertures, however achieving this resolution level requires “stepping” the sample or the mask in increments equal to the aperture size (“dithering”). If an array of circular apertures is used (which also provides two-dimensional phase sensitivity) instead of long parallel slits, this stepping needs to be carried out in two directions, which lengthens scan times significantly. We present a mask design obtained by offsetting rows of circular apertures, allowing for two-dimensional sensitivity and isotropic resolution while requiring sample or mask stepping in one direction only. We present images of custom-built phantoms and biological specimens, demonstrating that quantitative phase retrieval and near aperture-limited spatial resolutions are obtained in two orthogonal directions.

                                                                                           

Introduction

Contrast in conventional x-ray images depends on the attenuation of x-rays passing through matter; conventional x-ray computed tomography (CT) provides information on the internal structure of materials in three-dimensions based on the attenuation signal¹. Both planar x-rays (radiography) and CT are routinely used in a variety of applications, including medicine and material science. However, they suffer from low contrast in cases where the sample is weakly attenuating (e.g., biological tissue) and/or consists of multiple materials with similar attenuation.

Overcoming the limitations of attenuation-based x-ray imaging has been the subject of extensive research during the past decades. One approach is to utilize, in image formation, the phase shift that x-rays experience while passing through matter, which gives rise to refraction effects (the refraction angle is proportional to the first derivative of the phase shift^2,3). X-ray phase contrast imaging (XPCI) and tomography (XPC-CT) are powerful techniques that have many advantages over attenuation-based methods; in particular, they enable significantly higher contrast⁴. This results in an increase in contrast-to-noise (CNR) ratio for the same detected x-ray statistics, ultimately allowing details that are invisible to conventional x-ray imaging to be detected, and different materials to be more easily discriminated. In addition,

phase-based contrast can be maintained at high x-ray energies, which reduces the amount of dose deposited in the sample^3,5, an advantage particularly useful in biomedical imaging.

Imaging techniques that exploit x-ray phase in image formation include propagation-based imaging methods⁶, analyzer-based imaging methods⁷, speckle-based imaging methods⁸, crystal-based interferometric methods⁹, grating-based interferometric methods¹⁰, and grating-based non-interferometric methods¹¹. These methods use different experimental setups to generate phase sensitivity, and consequently their requirements very in terms of x-ray beam spatial and temporal coherence. Some attempts to quantitatively compare different XPCI methods have been made in the past^12,13,14.