Spectral X-ray phase contrast imaging with a CdTe photon-counting detector

Abstract

The present study focuses on the implementation of two X-ray phase contrast imaging (XPCI) techniques: free-space propagation (FSP) and single mask edge illumination (SM-EI) with a microfocus polychromatic X-ray source and a Timepix3 photon-counting detector with a CdTe sensor. This detector offers high spatial resolution, high detection efficiency and it is able to simultaneously record information about Time-over-Threshold (ToT) and Time-of-Arrival (ToA) for each X-ray photon. All these features play a key role in enabling an improvement of XPCI image quality, especially through spectral analysis, since it is possible to measure the energy of each incident X-ray photon. Measurements of phase contrast and contrast-to-noise ratio (CNR) are presented for different energy bins within the typical spectrum of soft X-ray imaging. It is shown that a significant enhancement of XPCI image quality can be obtained, for both implemented techniques, by performing pixel clustering to correct for charge sharing and by introducing some degree of energy-weighting. Keywords. X-ray imaging; Phase-contrast; Photon-counting detector; Timepix; Image quality. Single Mask Edge Illumination (SM-EI) XPCISM-EI is based on the Edge-Illumination principle, originally described by Olivo et al. [23]. In the original set-up, two periodic masks are used: the sample mask, placed before the sample, and the detector mask, located in front of the detector. The sample mask reshapes the incoming beam into smaller beamlets that hit the apertures of the detector mask to create the Edge-Illumination condition, in which only a fraction of the beamlets impinges on the pixel active surface. The detector mask also serves to reduce the smoothing of the phase signal caused by non-ideal pixel Point Spread Functions (PSF) [24]. However, with recent photon-counting devices with a more defined PSF, such as those from the Medipix and Timepix families, the detector mask can be removed and the beamlets aligned with the boundary between two pixels columns (or rows, depending on the mask orientation). This allows for the detection of beam displacement after it traverses the sample, as shown in Fig. 1b [25]. By separating the odd and even pixel columns (or rows), this set-up allows for phase and attenuation signal separation with a single acquisition, since it gives two images with equivalent absorption information but opposite differential phase contrast signals. So long as the PSF is sufficiently sharp [26], the single mask arrangement can provide comparable phase sensitivity to the double-mask configuration, with a tradeoff between resolution and image: the spatial resolution is halved, and the image statistics is doubled. Besides, with a mask design like the one proposed by Krejci et al. [27], in which the beamlets are shaped so they hit the region between four pixels instead of two, the system can be extended to provide 2D sensitivity, even in CT [28]. XPCI with spectral photon-counting detectorsThe development of photon-counting detectors with energy-resolving ca...