Phase-contrast imaging with x-rays is a developing field for imaging weakly absorbing materials. In this work, two phase-contrast imaging methods, grating- and speckle-based imaging, that measure the derivative of the phase shift, have been implemented with a laboratory source and compared experimentally. It was found that for the same dose conditions, the speckle-tracking differential phase-contrast images have considerably higher contrast-to-noise ratio than the grating-based images, but at the cost of lower resolution. Grating-based imaging performs better in terms of resolution, but would require longer exposure times, mainly due to absorption in the grating interferometer

Burvall, A.

Hertz, H. M.

Koch, F. J.

Romell, J.

Sala, S.

Zdora, M.

Zhou, T.

Journal of Physics Conference Series

Phase-contrast imaging with x-rays is a developing field for imaging weakly absorbing materials. In this work, two phase-contrast imaging methods, grating- and speckle-based imaging, that measure the derivative of the phase shift, have been implemented with a laboratory source and compared experimentally. It was found that for the same dose conditions, the speckle-tracking differential phase-contrast images have considerably higher contrast-to-noise ratio than the grating-based images, but at the cost of lower resolution. Grating-based imaging performs better in terms of resolution, but would require longer exposure times, mainly due to absorption in the grating interferometer.</p

Southampton (e-Prints Soton)

Comparison of laboratory grating-based and speckle-tracking x-ray phase-contrast imaging

J. Romell

T. Zhou

M. Zdora

S. Sala

F.J. Koch

H.M. Hertz

A. Burvall

Crossref

KITopen

This content has been downloaded from IOPscience. Please scroll down to see the full text.Download details:IP Address: 129.13.72.197This content was downloaded on 07/08/2017 at 10:50Please note that terms and conditions apply.Comparison of laboratory grating-based and speckle-tracking x-ray phase-contrast imagingView the table of contents for this issue, or go to the journal homepage for more2017 J. Phys.: Conf. Ser. 849 012035(http://iopscience.iop.org/1742-6596/849/1/012035)Home Search Collections Journals About Contact us My IOPscienceYou may also be interested in:Reconstruction method for grating-based x-ray phase-contrast images without knowledge of thegrating positionsG. Pelzer, J. Rieger, C. Hauke et al.Visibility in differential phase-contrast imaging with partial coherence sourceLiu Xin, Guo Jin-Chuan, Peng Xiang et al.Reconstruction of In-Line Holograms Using Phase Retrieval AlgorithmsYan Zhang, Giancarlo Pedrini, Wolfgang Osten et al.High-sensitivity phase-contrast tomography of rat brain in phosphate buffered salineFranz Pfeiffer, Christian David, Oliver Bunk et al.Correcting Intensity Drift in X-ray Grating-based Phase   Contrast ImagingP. Zhang, Q. Shao, R. Hu et al.X-ray phase-contrast tomosynthesis of a human ex vivo breast slice with an inverse Compton x-raysourceE. Eggl, S. Schleede, M. Bech et al.Low-dose phase-contrast X-ray imaging: a comparison of two methodsT Zhou, U Lundström, D H Larsson et al.Application of Bi Absorption Gratings in Grating-Based X-ray Phase Contrast ImagingYaohu Lei, Yang Du, Ji Li et al.Soft-tissue phase-contrast tomographyMartin Bech, Torben H Jensen, Robert Feidenhans'l et al.1Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd1234567890X-Ray Microscopy Conference 2016 (XRM 2016)  IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 849 (2017) 012035  doi :10.1088/1742-6596/849/1/012035Comparison of laboratory grating-based andspeckle-tracking x-ray phase-contrast imagingJ. Romell1, T. Zhou1, M. Zdora2,3, S. Sala2,3, F.J. Koch4, H.M.Hertz1, A. Burvall11 Biomedical and X-ray Physics, KTH Royal Intsitute of Technology, AlbaNova UniversityCenter, SE-106 91, Stockholm, Sweden2 Diamond Light Source, Harwell Science & Innovation Campus, Didcot, OxfordshireOX11 0DE, UK3 Department of Physics & Astronomy, University College London, London WC1E 6BT, UK4 Institute of Microstructure Technology, Karlsruhe Institute of Technology, 76344Eggenstein-Leopoldshafen, GermanyE-mail: jenny.romell@biox.kth.seAbstract. Phase-contrast imaging with x-rays is a developing field for imaging weaklyabsorbing materials. In this work, two phase-contrast imaging methods, grating- and speckle-based imaging, that measure the derivative of the phase shift, have been implemented with alaboratory source and compared experimentally. It was found that for the same dose conditions,the speckle-tracking differential phase-contrast images have considerably higher contrast-to-noise ratio than the grating-based images, but at the cost of lower resolution. Grating-basedimaging performs better in terms of resolution, but would require longer exposure times, mainlydue to absorption in the grating interferometer.1. IntroductionTo overcome the limitations of low resolution and poor contrast when imaging weakly absorbingobjects with x-rays, different phase-sensitive imaging techniques are being developed. Whileconventional absorption is readily measured, phase shift must be detected by finding the intensityvariations caused by the shift. The well-established propagation-based imaging method (PBI)measures the second-order derivative of the phase, while grating-based imaging (GBI) [1] andthe relatively new speckle-based imaging (SBI) [2, 3] measure the refraction angle. By trackingthe shift of a known intensity pattern, a map of the refraction angle, which is proportional tothe derivative of the phase shift, can be constructed. GBI and SBI use a Talbot pattern [4] anda near-field speckle pattern, respectively. These two methods have been implemented with alaboratory micro-focus x-ray source, and have been used to image different samples under thesame dose conditions, with the goal of comparing the quality of the obtained images.2. MethodThe experimental arrangements of grating- and speckle-based imaging are shown in Fig. 1. Theliquid-metal-jet source, similar to the source described in [5], was run at 40 kVp with an emissioncurrent of 0.6 mA, generating an x-ray beam with spot size 5.3 × 7.7 µm2. To keep the dosecontrolled, the source-to-object distance was kept constant at 65.6 cm at all times. The grating21234567890X-Ray Microscopy Conference 2016 (XRM 2016)  IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 849 (2017) 012035  doi :10.1088/1742-6596/849/1/012035Figure 1. Experimental arrangements. (a) GBI. (b) SBI. (c) The intensity of a pixel duringphase stepping in the GBI method, where deviations from the fit indicate photon noise.interferometer used in GBI was designed with grating periods p1 = 4.08 µm of the pi-shiftingphase grating G1 and p2 = 2.4 µm of the absorption grating G2. Si with a thickness of 200 µmwas used as substrate for both gratings, with 9.2 µm of Ni on G1 and 40 µm of gold on G2.Both G1 and G2 had a duty cycle of 50%. The distance between the gratings was d = 12.4cm, and the detector was placed as close as possible after G2. For SBI, the speckle patternwas generated by a diffuser, here a piece of P800 sandpaper, placed after the sample. To reachsufficient speckle visibility [6], the detector was placed 1.5 m away from the source.In GBI, G1 was scanned laterally over one period of the Talbot intensity pattern [1]. For eachstep, two images were recorded: one with a sample in the beam path, and one reference without.Processing of the stacks of images was carried out pixel-wise. The intensity variation of one pixelduring the phase stepping can be seen in Fig. 1c, and can be described in terms of a cosine series.The superscripts s and r denote sample and reference, respectively. Differential phase contrast(DPC), which has been the main interest in this work, is proportional to φ = φs1 − φr1. For thegrating interferometer, the angle of refraction α is related to the phase shift Φ(x, y) caused bythe object, and to φ as [7]αx =∂Φ∂xλ2pi=φp22pid. (1)The parameters a0, a1 and φ1, for both reference and sample, were extracted from the rawimages by fast Fourier transform.In x-ray speckle-tracking imaging, the type of SBI considered here, two images were recorded:one with only the diffuser in the beam path, and one with the diffuser and the sample. Thearrangement is shown in Fig. 1b. Since no scanning process was carried out, the images werecompared region-wise to find the shift of the speckle pattern, the absorption, and the loss ofspeckle visibility. Least-square minimisation of a cost function [8] gave the sample-induced localshift (δx, δy) around each pixel, in both horizontal and vertical directions. For comparison withGBI, only the horizontal shift has been considered. From the shift, the angles of refractioncould be calculated as αi = δi/L for i = x, y, where L is the diffuser-to-detector distance.The resolution in the final images was mainly affected by the window size chosen for thereconstruction.3. ResultsFor a plastic wedge with a small angle, specifically a LEGO R© sword (Fig. 2), the contrast-to-noiseratios (CNR) of the DPC images were evaluated as CNR = |IA− IB|/σ. Here, σ is the standarddeviation of the background and I the mean intensity. The figure was imaged with GBI andSBI for an exposure time of 540 s each, and for the SBI reconstruction a 24× 24 pixel window31234567890X-Ray Microscopy Conference 2016 (XRM 2016)  IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 849 (2017) 012035  doi :10.1088/1742-6596/849/1/012035Figure 2. DPC images, expressed as refraction angle αx ∝ ∂Φ/∂x. (a) SBI, with regions usedin the CNR calculation. (b) GBI. The scale bar represents 2 mm in both figures.Figure 3. (a) Shrimp with marked region used for the DPC images (b)−(g). (b) GBI differentialphase contrast. (c) Low-pass filtered GBI. (d)−(g) Speckle-based images, reconstructed witha window of N × N pixel2, where N is (d) 4, (e) 8, (f) 12 and (g) 24. The white scale barcorresponds to 1 mm and the black 500 µm. The grey scale represents refraction angle in µrad.was chosen for optimal noise [8]. Partly due to the inherent filtering in the speckle-trackingreconstruction method, the CNR was here 9.6, compared to 0.83 of GBI.To evaluate the resolution, some small structures on a shrimp (see Fig. 3a) were imaged.The exposure time was 780 s. Since the window size of the SBI reconstruction greatly impactsnot only noise but also resolution, different window sizes N × N pixel2 were used, N rangingfrom 4 to 24. Shown in Fig. 3d-g are reconstructions for N = 4, 8, 12 and 24. It is clear fromconsidering Fig. 3g that choosing a large window will filter out small features, while the hairscould be clearly seen, however noisy, when a window of 8 × 8 or 4 × 4 pixel2 was used. TheGBI DPC image, acquired during the same exposure time (Fig. 3b), showed strong backgroundnoise which made it difficult to distinguish the hairs at all. Comparing the methods from a noiseperspective, by low-pass filtering the GBI image until the same background noise as in the 4× 4speckle image was reached (Fig. 3c), showed that the hairs were then blurred out entirely.41234567890X-Ray Microscopy Conference 2016 (XRM 2016)  IOP PublishingIOP Conf. Series: Journal of Physics: Conf. Series 849 (2017) 012035  doi :10.1088/1742-6596/849/1/012035Figure 4. (a) Background noise of SBI DPC reconstructions of the shrimp (Fig. 3a) as afunction of the window size. (b) Background RSP for SBI. (c) Background RSP for GBI.Noise is clearly limiting the detectability of small features in the grating-produced image,while the reconstruction window, and hence resolution, is the main limiting factor for thespeckle-tracking method. The noise dependence on the reconstruction window was evaluated bydetermining the background noise for different window sizes (Fig. 4a).Finally, to appreciate the impact of the reconstruction method on the images, the averageradial spectral power (RSP) of the background was evaluated for the four different window sizesused in Figs. 3d-g, and the unfiltered and filtered GBI from Figs. 3b and 3c. The RSP is shownin Figs. 4b and 4c. The effect of the rectangular window can clearly be seen in the fluctuationsin Fig. 4b and the filtered GBI in Fig. 4c.4. Conclusion and outlookTo summarise, the quality of the DPC images largely depends on both experimental arrangementand reconstruction method. The speckle-tracking method gives considerably higher CNR thanGBI, but at the cost of lower resolution. With GBI, smaller details can be resolved, but itrequires a larger dose, mainly due to the absorption in the interferometer.As a continuation of this work, image quality in GBI could be compared to variations of SBIwhich can give higher resolution, such as speckle-scanning phase-contrast imaging.AcknowledgmentsWe thank William V˚agberg and Jakob Larsson for assistance in the lab, and Karlsruhe Instituteof Technology (KIT) for providing the gratings for the interferometer.References[1] Pfeiffer F, Weitkamp T, Bunk O and David C 2006 Phase retrieval and differential phase-contrast imagingwith low-brilliance X-ray sources Nat. Phys. 2 pp 258−61[2] Morgan K S, Paganin D M and Siu K K W 2012 X-ray phase imaging with a paper analyzer App. Phys.Lett. 100 124102[3] Be´rujon S, Ziegler E, Cerbino R, Peverini L 2012 Two-Dimensional X-Ray Beam Phase Sensing Phys. Rev.Lett. 108 158102[4] Talbot H F 1836 Facts related to optical science Philos. Mag. 9 pp 401−7[5] Larsson D H et al. 2011 A 24 keV liquid-metal-jet x-ray source for biomedical applications Rev. Sci. Instrum.82 123701[6] Zhou T et al. 2015 Speckle-based x-ray phase-contrast imaging with a laboratory source and the scanningtechnique Opt. Lett. 40 pp 2822−5[7] Bech M 2009 X-ray imaging with a grating interferometer, PhD thesis, University of Copenhagen[8] Zanette I et al. 2014 Speckle-based x-ray phase-contrast and dark-field imaging with a laboratory sourcePhys. Rev. Lett. 112 253903

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Comparison of laboratory grating-based and speckle-tracking x-ray phase-contrast imaging

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