X-ray microtomography using correlation of near-field speckles for material characterization

Abstract

Nondestructive microscale investigation of objects is an invaluable tool in life and materials sciences. Currently, such investigation is mainly performed with X-ray laboratory systems, which are based on absorption-contrast imaging and cannot access the information carried by the phase of the X-ray waves. The phase signal is, nevertheless, of great value in X-ray imaging as it is complementary to the absorption information and in general more sensitive to visualize features with small density differences. Synchrotron facilities, which deliver a beam of high brilliance and high coherence, provide the ideal condition to develop such advanced phase-sensitive methods, but their access is limited. Here we show how a small modification of a laboratory setup yields simultaneously quantitative and 3D absorption and phase images of the object. This single-shot method is based on correlation of X-ray near-field speckles and represents a significant broadening of the capabilities of laboratory-based X-ray tomography.X-ray imaging | near-field speckles | phase-contrast imaging | refractive index measurement | microtomography N ear-field speckles are observed when the granular diffraction pattern created by a random phase modulator (diffuser) is recorded in the near-field regime. This speckle intensity pattern has interesting properties: it is not dependent on the propagation distance if the near-field condition is satisfied (1), near-field speckles can be observed also with beams of low longitudinal coherence, and the speckle pattern reflects the spatial properties of the scatterers used to generate it (2).Although speckles are a well-known phenomenon especially in the far field and for different wavelengths, e.g., from radio waves to visible light, the first observation and characterization of nearfield speckles with X-rays was achieved in 2008 by Cerbino et al. who reported on measurements performed with synchrotron radiation (1). After this first experiment, X-ray near-field speckles have been used at synchrotron facilities for, among other applications, coherence measurements, optics characterization, and imaging (3-5).The principle of speckle-based imaging is to quantify the effect on the speckle pattern by the sample through a windowed correlation between a pair of images taken with and without sample. This correlation quantifies the distortion of the speckles caused by the sample and yields accurate information on its refraction and thus phase-shifting properties. Moreover, it simultaneously provides the complementary absorption image of the investigated object (4-6). Because near-field speckles exhibit sufficient contrast also when a beam with a low degree of temporal coherence is used, near-field speckle-based techniques are not limited to largescale synchrotron facilities, but can also be implemented with polychromatic laboratory X-ray sources. Such an experiment has been demonstrated using a high-brightness liquid-metal-jet source (7) making this imaging method available for widespread use (6).Up until now, near...

State of the Art of X-ray Speckle-Based Phase-Contrast and Dark-Field Imaging

“…At around the same time, a quantitative analysis of speckle tomography data was presented from XST measurements at a liquid metal-jet laboratory source [88]. Here, it was shown that the complementary quantitative absorption and refraction information from transmission and phase tomograms, respectively, can be combined for identifying and characterising different materials with similar refraction and absorption properties in a sample; see Figure 10V in Section 7.2.…”

“…It was demonstrated that XST tomography implemented at laboratory sources can be employed for the identification of different materials in a sample. Several types of plastic were successfully distinguished using the combined information from phase and absorption tomograms [88], as shown in Figure 10V. In another publication, the chip of a computer memory card was imaged using 1D XSS at a laboratory source, and the complementary phase and dark-field signals enabled identifying different components of the chip [136]; see Figure 10VI.…”

“…As the speckles have an irregular, random shape and a distribution of sizes, only an average speckle size can be estimated. This is done for example by determining the spatial frequency at the maximum of the power spectrum [86,87,123,129] or from a 2D auto-correlation of the speckle pattern [88,128,[130][131][132][133][134]. Generally, well-defined, small speckles that can be resolved easily and cover a few pixels in the detector plane are desired.…”

“…where σ I andĪ are the standard deviation and the mean intensity value, respectively, of the speckle pattern in a small region of interest (typically around 150 × 150 pixels), as, e.g., used in [87,88,133,135,136], or:…”

X-ray Multimodal Tomography Using Speckle-Vector Tracking

“…Indeed, without any a priori assumption on the samples, only a few images per projection are required for multimodal tomography when employing the appropriate scheme. In contrast to previous work [19,20], the XSVT approach provides 3D rendering of the dark-field signal, in addition to the absorption and phase signals, whilst the spatial resolution of the imagery is limited only by the detector used.…”