Bragg magnifier optics for dose-efficient X-ray phase contrast imaging

Abstract

Propagation-based X-ray phase contrast imaging (PB-PCI) enables the visualization of soft materials and tissues by exploiting the coherent self-interference of the diffracted wavefield behind the sample, which evolves into intensity contrast as the propagation distance between the sample and the detector increases. While phase contrast imaging allows significantly reducing the dose compared to conventional X-ray absorption imaging, the ionizing nature of X-rays still induces radiation damage. The dose therefore needs to be further reduced both for high, micrometer resolution $\textit{in vivo}$ and $\textit{in situ}$ imaging of biological or radiation-sensitive samples, as well as for imaging at moderate resolution of tens to hundreds of micrometers, e.g., in (bio)medical research and diagnostics. However, both resolution regimes face severe constraints. On the one hand, conventional high-resolution detectors suffer from decreasing efficiency with increasing resolution. On the other hand, PB-PCI at moderate resolution requires propagation distances of hundreds of meters to generate sufficient image contrast. The main objective of this work is to push the limits of dose-efficient X-ray imaging by optimizing the entire imaging process of PB-PCI with respect to the deposited dose. In a first part, high-resolution imaging with highest dose efficiency is realized by combining PB-PCI, asymmetric Bragg crystal optics, and a single photon counting detector, thereby operating close to the theoretical limit of dose efficiency for PB-PCI. The superior imaging performance of the developed system compared to conventional detector systems is demonstrated theoretically and experimentally, and in particular, a substantial increase in dose efficiency is shown for high spatial frequencies, which comprise the relevant high-resolution components of the image. The technique’s potential is exemplified by a pilot $\textit{in vivo}$ study of submillimeter-sized parasitoid wasps inside their hosts with unprecedentedly long observation times. Second, for imaging large, centimeter-sized samples at moderate resolution, a new technique is introduced that allows achieving high propagation-based image contrast within a meter-scale setup, thereby eliminating the need for very long wavefield propagation distances. Simultaneously, the technique reduces image blur caused by the finite size of the X-ray source. The strong increase in image contrast is demonstrated in a proof-of-concept experiment, realized by asymmetric Bragg crystal optics with reversed optical path. This approach paves the way for low-dose studies of large radiation-sensitive specimens, with potential applications ranging from biomedical soft tissue and small animal $\textit{in vivo}$ imaging up to medical diagnostics, e.g., the early detection of breast cancer