Journal of X-Ray Science and Technology 23 (2015) 141–146 DOI 10.3233/XST-150477 IOS Press

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Monochromatic X-ray imaging using a combination of doubly curved crystal and polycapillary X-ray lens Tianxi Suna,b,c and C.A. MacDonaldd,∗ a The

Key Laboratory of Beam Technology and Materials Modification of Ministry of Education, Beijing Normal University, Beijing, China b College of Nuclear Science and Technology, Beijing Normal University, Beijing, China c Center for X-Ray Optics, University at Albany, SUNY, Albany, NY, USA d Department of Physics, University at Albany, SUNY, Albany, NY, USA Received 6 April 2014 Revised 23 January 2015 Accepted 27 January 2015 Abstract. A monochromatic X-ray imaging setup based on a combination of a doubly curved crystal and a polycapillary focusing X-ray lens was designed. In this setup, the bent crystal optic was used not only to monochromatize but also to focus the divergent X-ray beam from a conventional X-ray source to form a monochromatic X-ray focal spot with a size of 426 × 467 µm2 at 17.5 keV. The beam expanding from this focal point was focused by the polycapillary optic to obtain a focal spot which was then used as the monochromatic X-ray imaging virtual source. The output focal spot size of the focusing polycapillary optic at 17.5 keV was 97 µm. Compared with the beam expansion after the focal spot of the bent crystal optic, the beam expansion after the focal spot of the focusing polycapillary optic was relatively large. This was helpful for magnifying the X-ray image of the sample. The focused beam was helpful to decrease the exposure time for imaging small samples. Keywords: Monochromatic X-ray imaging, doubly curved crystal, polycapillary X-ray lens

1. Introduction Monochromatic X-ray imaging can increase contrast and reduce dose compared with polychromatic X-ray imaging [1,2]. The monochromatic X-ray imaging setup can be based on synchrotron radiation in order to obtain adequate intensity [3,4]. However, for some applications, the necessity of moving the sample to the synchrotron facility is inconvenient or impractical. If a conventional X-ray source is used with a flat monochromator crystal, only the small fraction of the beam within the angular bandwidth of the crystal will be diffracted. This results in low power density on the sample. Further, even with the conventional source with a divergent X-ray beam, the beam from a flat crystal is nearly parallel in the diffraction plane, so that there is no image magnification. This lack of magnification results in a need for an improved spatial resolution of the detector system [4]. ∗ Corresponding author: C.A. MacDonald, Department of Physics, University at Albany, SUNY, Albany, NY 12222, USA. E-mail: [email protected].

c 2015 – IOS Press and the authors. All rights reserved 0895-3996/15/$35.00 

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Fig. 1. Negative image of the X-ray beam after the focal spot of the bent crystal optic. The optic consisted of three segments, with small breaks between the segments.

A doubly curved crystal monochromator (toroidally bent crystal optic) can collect and focus X rays from a large solid angle to produce a focused monochromatic X-ray beam [5,6], which makes a more efficient use of a conventional X-ray source. A doubly cured crystal used to perform X-ray imaging can provide magnification. However, the shape of the beam after focal spot of the bent crystal optic is a crescent, as shown in Fig. 1. This will deform the image of the sample. Therefore, a bent crystal optic alone may not be convenient for X-ray imaging. A polycapillary focusing X-ray lens is based on total external reflection. The global divergence of the beam after the focal spot of the polycapillary optic is relatively large. This is helpful in magnifying the image of small samples [7]. Moreover, the expanding beam from the focusing polycapillary optic will not deform the image of the sample [8]. A focusing polycapillary optic alone will not monochromatize the X rays, so a monochromator must be used. Previously, a monochromatic X-ray imaging system using synchrotron radiation and a plane monochromator followed by a polycapillary half focusing X-ray lens had been demonstrated [4]. In this system, the sample was placed after the focal spot of the polycapillary optic. The resultant magnification increased the resolution of the imaging setup. However, as mentioned above, such facilities based on the synchrotron radiation are not always readily available in laboratory or clinical settings. In this work, a combination of a bent crystal optic and a focusing polycapillary optic were used to monochromatize and focus X-rays from a conventional source to perform monochromatic X-ray imaging.

2. Measurements A monochromatic X-ray imaging setup based on a conventional X-ray source and the combination of the bent crystal optic and focusing polycapillary optic is shown in Fig. 2. The source was an Oxford Ultrabright Mo X-ray tube. The bent crystal optic was manufactured by XOS. A similar optic is shown in Fig. 3. The optic used silicon (220) diffraction, which gave a Bragg angle of 10.6◦ at the 17.5 keV Mo Kα [9]. The in-plane bending radius and the out-of-plane bending radius were 1028 and 35 mm, respectively. The size of the bent crystal optic was 11.5 × 45 mm2 . The input and output focal distances were both approximately 193 mm. The focal spot size of the bent crystal optic was 426 × 467 μm2 at 17.5 keV. The focusing polycapillary optic was placed at the confocal position with the bent crystal optic, with its axis aligned with the output axis of the bent crystal. The input and output focal distances of the focusing polycapillary optic were 51 and 48 mm, respectively. The full width at half maximum (FWHM) of the output focal spot from the polycapillary optic, measured with a knife edge scan, was 97 μm at 17.5 keV. This output focal spot from the polycapillary optic was the virtual imaging source. The length, input diameter and output diameter of the focusing polycapillary optic were 80, 2.1 and

T. Sun and C.A. MacDonald / X-ray imaging using

Detector

o

Bent Crystal

Polycapillary Optic

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Sample i

Source Fig. 2. Monochromatic imaging set up based on a doubly curved crystal optic followed by a focusing polycapillary optic. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/XST-150477)

Fig. 3. Photo of a torroidally bent crystal. (Colours are visible in the online version of the article; http://dx.doi.org/10.3233/ XST-150477)

Fig. 4. Image of X-ray beam a few centimeters after the focusing polycapillary optic when its input is from the bent crystal optic.

1.8 mm, respectively. The channel diameter of the monocapillaries composing the focusing polycapillary optic was 6.2 μm. This optic was not optimized for imaging. An optic with a shorter output focal length would have a smaller focal spot and could produce higher magnification. The imaging system employed a Fuji computed radiography plate read with a pixel size of 50 μm. The magnification M of the monochromatic imaging setup based on the a combination of the bent crystal optic and the focusing polycapillary optic is given by M=

i o

(1)

where o and i (Fig. 2) were the object-virtual source and image-virtual source distances, respectively. The image of the X-ray beam at a distance about 170 mm from the output focal spot of the focusing polycapillary optic is shown in Fig. 4. Figure 5 shows the image and the intensity profile through the image of the resolution phantom with a magnification of 2.1. Because the phantom was fairly large, it was placed relatively far, 240 mm, from the output focal spot of the focusing polycapillary optic. The image plate was 500 mm from the focal point. The exposure time was 220 seconds with a current and voltage of 400 μA and 28 kV, respectively. A smaller sample could be placed closer to the focal point and still be fully illuminated, so that the exposure time could be reduced. The contrast of the bar pattern was defined as C=

Imax − Imin , Imax

(2)

where the intensities Imax and Imin were the intensity of peak and neighbouring valley, respectively, from the profile as shown in Fig. 4(c). The contrasts computed from the profiles at different distances

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Fig. 5. (a) Photograph of the resolution phantom. The circle shows the approximate area used in the measurement. (b) Image of the resolution phantom. (c) Intensity along a vertical profile through the image.

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Sample-to-detector distance (mm) Fig. 6. Contrast of the image of the resolution phantom as a function of distance between the sample and the detector. The black squares are the measured data. The red dashed line is the computed effect of the increasing magnification on the resolution due to the finite pixel size. The blue dotted line shows the expected effect of the increasing geometrical blur. The product of these two effects is shown as the solid purple line in the figure. (Colours are visible in the online version of the article; http://dx.doi. org/10.3233/XST-150477)

between the sample and the detector, with a fixed phantom-to-focal spot distance o = 24 cm are plotted in Fig. 6. As shown in Fig. 6, when the phantom-to-focal spot distance is 24 cm, the contrast increased with the increasing sample-to-detector distance within a range with a smallest and largest magnification of 1.3 and 3.9, respectively. The contrast is lower than expected at small sample-to-detector distances. This may be due to scatter from the resolution phantom which consists of a series of lead strips on a glass substrate, as shown in Fig. 5(a). The scatter fraction is reduced as the detector is moved away from the sample, so that the contrast rises with increasing distances. A rise in contrast with increasing distance in Fig. 6 would also be consistent with the fact that the

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effect of the finite pixel size of the detector is diminished as the image is magnified, as shown in the red dashed line in Fig. 6. The line was computed from F (k) =

sin(kp/2) , kp/2

(3)

where P is the 50 μm pixel readout size for the image plate, k = 2π/λ, λ is the measured wavelength of the sinusoidal measured intensity profile, as shown in Fig. 5. Because the profiles on the different images were not taken at the same points on the phantom, the spacing λ was taken from a fit to the actual image profile. The deduced spacing on the phantom, after dividing by the image magnification, ranged from 3.9 to 4.9 lp/mm for the 60–700 mm detector distances, the region shown within the circle in Fig. 5. The measured line spacings are large compared with the pixel size, so the effect of increasing magnification is not very significant. While the scatter reduction and magnification should contribute to increased contrast with increasing distance, the rise in geometric blur would be expected to reduce the contrast. Geometric blur increases with detector distance as blur = Dα,

(4)

where D is the object-to-detector distance (Fig. 2) and α is the virtual source divergence, s α= , o

(5)

o is the focal spot-to-object distance (Fig. 2), and s is the Gaussian source size. The Gaussian size is

FWHM , s=  2 2 ln(2)

(6)

so that s = 41 μm given the measured FWHM of 97 μm for the output focal spot size of the polycapillary optic. The effect of the geometrical blur can then be estimated as G(k) = e−

k2 ·blur2 2

.

(7)

The computed G(k) is shown as a dashed blue line in Fig. 6. The measured contrast with a fixed phantom-to-focal spot distance of 24 cm and a magnification range of 1.3 to 3.9 does not fall as expected with increasing sample-to-detector distance. The reason for this might be that the divergence of the beam was less than would be seen from an isotropic source of the same size as the focal spot of the polycapillary optic. This could arise from of the way that the spot from the bent crystal optic fed into the polycapillary. For the crystal optic there is a correlation between the direction and location of the ray which is not the case for an isotropic source [6]. 3. Conclusions The images show good contrast and resolution. Compared with the plane monochromator, the bent crystal optic had a small focused beam with a relative high gain in power density. The monochromatic Xray imaging setup, using the bent crystal optic and the focusing polycapillary optic to focus the divergent X-ray beam from the conventional source, has potential application for imaging small samples.

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Acknowledgements This research was supported by the Breast Cancer research program DoD BCRP # W81XWH-041-0752 and the National Institutes of Health, # 7 R01 EB009715, and also the National Natural Science Foundation of China (11375027 and 11075017), and the Fundamental Research Funds for the Central Universities (2014kJJCA03), and the Program for Excellent Talents by Beijing Government (2010C009012000005). References [1] [2] [3] [4] [5] [6] [7] [8] [9]

M. Hoheisel, R. Lawaczeck, H. Pietsch and V. Arkadiev, Advantages of monochromatic X-rays for imaging, Proc SPIE 5745 (2005), 1087–1095. F.R. Sugiro, D. Li and C.A. MacDonald, Beam collimation with polycapillary X-ray optics for high contrast high resolution monochromatic imaging, Med Phys 31 (2004), 3288–3297. J. Baruchel, P. Bleuet, A. Bravin, P. Coan, E. Lima, A. Madsen, W. Ludwig, P. Pernot and J. Susini, Advances in synchrotron hard X-ray based imaging, C R Physique 9 (2008), 624–641. T. Sun, M. Zhang, Z. Liu, Z. Zhang, G. Li, Y. Ma, X. Du, Q. Jia, Y. Chen, Q. Yuan, W. Huang, P. Zhu and X. Ding, Focusing synchrotron radiation using a polycapillary half-focusing X-ray lens for imaging, J Synchrotron Radiat 16 (2009), 116–118. Z. Chen and W.M. Gibson, Doubly curved crystal X-ray optics and applications, Powder Diffr 17(2) (2002), 99–103. A. Bingolbali and C.A. MacDonald, Curved crystal X-ray optics for monochromatic imaging with a clinical source, Med Phys 6(4) (2009), 1176–1183. S. Han, H. Yu, J. Cheng, C. Gao and Z. Luo, Contrast and resolution in direct Fresnel diffraction phase-contrast imaging with partially coherent X-ray source, Rev Sci Instrum 75(10) (2004), 3146–3151. H. Yu, P. Zhu, S. Han, Z. Luo and C. Gao, In-line phase-contrast imaging using partially coherent hard X-ray, Chin Phys Lett 20(2) (2003), 220–222. A. Bingöbali and C.A. MacDonald, Quality assessment system for curved crystal X-ray optics, Nucl Instrum Methods Phys Res B 267 (2009), 832–841.

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Monochromatic X-ray imaging using a combination of doubly curved crystal and polycapillary X-ray lens.

A monochromatic X-ray imaging setup based on a combination of a doubly curved crystal and a polycapillary focusing X-ray lens was designed. In this se...
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