Phase and absorption retrieval using incoherent X-ray sources

Abstract

X-ray phase contrast imaging has overcome the limitations of X-ray absorption imaging in many fields. Particular effort has been directed towards developing phase retrieval methods: These reveal quantitative information about a sample, which is a requirement for performing X-ray phase tomography, allows material identification and better distinction between tissue types, etc. Phase retrieval seems impossible with conventional X-ray sources due to their low spatial coherence. In the only previous example where conventional sources have been used, collimators were employed to produce spatially coherent secondary sources. We present a truly incoherent phase retrieval method, which removes the spatial coherence constraints and employs a conventional source without aperturing, collimation, or filtering. This is possible because our technique, based on the pixel edge illumination principle, is neither interferometric nor crystal based. Beams created by an X-ray mask to image the sample are smeared due to the incoherence of the source, yet we show that their displacements can still be measured accurately, obtaining strong phase contrast. Quantitative information is extracted from only two images rather than a sequence as required by several coherent methods. Our technique makes quantitative phase imaging and phase tomography possible in applications where exposure time and radiation dose are critical. The technique employs masks which are currently commercially available with linear dimensions in the tens of centimeters thus allowing for a large field of view. The technique works at high photon energy and thus promises to deliver much safer quantitative phase imaging and phase tomography in the future.

Fig. 1.

Schematic diagram of the system used obtain quantitative CAXPCI images. Note that the diagram is not to scale, for example, z_so = 1.6 m and W = 16 μm were used to obtain results for this paper.

Fig. 2.

Phantom images and profiles. (A) Raw images, I_R (i) and I_L (ii) of a phantom of filaments made of titanium (125 μm radius), sapphire (125 μm radius), aluminium (125 μm radius), PEEK (225 μm radius), and PEEK (100 μm radius) respectively, going from left to right. (B) Extracted DP (i) and absorption (ii) images. (C) Raw image profiles along the line indicated in images A, i and ii. (D) Absorption image profile. Note how the raw image profiles for titanium in C possess only positive peaks and yet the peaks are still nulled in the absorption image. This is due to a subtle asymmetry in the raw profiles and is explained further in the SI Text. (E) DP image profile (i) and blow up of the titanium profile (ii) comparing the theoretical effective DP profile. (F) Monochromatic DP profiles of titanium (125 μm radius, i) and PEEK (225 μm radius, ii) along with theoretical DP profiles at the synchrotron photon energy of 20 keV. It should be noted that the plots in F are the only results in thus paper that were not acquired using a conventional source. The horizontal units of plots C–F are in millimeters.

Fig. 3.

Plots of the sum and difference of the K(x, ± P/2) functions for a point source and a source with focal spot FWHM of 60 μm. The broken lines indicate the position of the projected presample aperture.

Fig. 4.

Differential phase and absorption images of a ground beetle. (A) The DP image () for a ground beetle. (B) The absorption image of the ground beetle. The smaller images along the lower part of the figure are blown up versions of the boxed regions in A and B. In both cases the image contrast has been saturated slightly to enhance the visibility. Note also that the highly absorbing region near the center of the beetle is a metal pin. (C and D) Small hairs attached to the leg are visible on the DP contrast image (C), yet completely invisible in the absorption image (D). Other fine details are much better defined in the DP image. (E and F) Structure and fine details are much better resolved in the DP image (E). (G and H) Internal structure of over laying features are much better defined in the DP image (G).

Fig. 5.

Synchrotron experimental setup. The beam extends 120 mm into the page and an image is generated by scanning the object vertically through the beam. The diagram is not to scale.