# Multi-Modal X-ray Imaging and Analysis for Characterization of Urinary Stones
###### Abstract
These imaging techniques are sensitive to small phase shifts of X-ray wave fronts upon their interaction with matter, in addition to the amplitude attenuation; hence, they can provide additional contrast channels, such as phase-contrast and dark-field channels, that reveal complementary information about the inner microstructures [8; 9]. The phase shift occurs due to the refraction of X-rays inside the matter and is considered the coherent effect of X-ray diffraction from the sample. The dark-field contrast is generated due to the ultra-small-angle scattering of X-rays from structures of a sub-pixel-length scale, which results in a diffused beam and reduced visibility of the recorded intensity, typically with respect to a empty-field reference image [10; 11; 12; 13]. Many versatile applications have been demonstrated for X-ray phase-sensitive methods during the last two decades, from material characterization [14; 15; 16; 17; 18] to biomedical virtual 3D histology [19; 20; 21; 22; 23; 24; 25; 26]. Various operational mechanisms have been implemented to extract such multi-contrast images by changing the configuration of X-ray radiography and tomography systems or introducing some optical elements [27]. Furthermore, most of them require a high degree of temporal and spatial beam coherence to provide efficient imaging. Therefore, most of them have been realized in synchrotron facilities. However, some of these techniques can be implemented with laboratory sources such as propagation-based imaging (PBI) [28], grating-based interferometry (GI) [29], edge-illumination imaging (EI) [30], and speckle-based imaging (SBI) [31; 32]. The differentiation of urinary stone types through dark-field imaging has been investigated by [PERSON] et al. [33] with the GI technique. The authors provided a classification based on the complementarity of the absorption and dark-field contrast images. Since scattering mainly takes place at boundaries with locally changing densities and rich and varying microstructures, the resulting scattering signal is strong, whereas homogeneous and less micro-structured compositions lead to smaller scattering intensities [33; 34; 35]. On the other hand, the relation is the opposite for the absorption intensities. It has been shown, for in vitro urinary stones, that the ratio of the scatter and absorption coefficient can be used as an indicator of the stone type [33; 35]. The sensitivity and specificity of the GI method, based on these preliminary results, seem to be excellent, superseding those based solely on absorption imaging (i.e., CT and DECT). However, the GI method has some limitations that makes its use for actual clinical applications challenging. First of all, the GI technique relies on the micro-structured absorption gratings which, despite other grating-based methods such as EI [36], need to be fabricated in high aspect ratios. Therefore, the gratings are currently difficult and expensive to make for high-energy full-field imaging such as pelvis and abdomen imaging. Moreover, GI needs an additional source grating in order to be used with high-power X-ray sources and to provide a sufficient spatial coherence. This would impose a beam flux loss and sub-optimal photon economy [36] and, consequently, an extra delivered dose to the pelvis and abdomen upon increasing the exposure time for achieving a sufficient signal-to-noise ratio (SNR). The time is also lengthened by a phase-scanning procedure, which is necessary to measure the interference modulation that further increases the scanning time and absorbed radiation dose. Another multi-modal technique is speckle-based imaging (SBI) [37], of which a schematic is provided in Figure 1. The main advantage of this technique is simplicity and cost-effectiveness, with low coherence requirements (see Section 2.1). This method allows multimodal imaging at high energies and with a large field of view (FOV), with essentially no stepping procedure with laboratory sources.