Download Vessel Imaging by Interferometric Phase-Contrast X

Vessel Imaging by Interferometric Phase-Contrast
X-Ray Technique
Tohoru Takeda, MD, PhD; Atsushi Momose, PhD; Jin Wu, MD, PhD; Quanwen Yu, PhD;
Tsutomu Zeniya, MS; Thet-Thet-Lwin, MD; Akio Yoneyama, MS; Yuji Itai, MD, PhD
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Background—Phase-contrast x-ray imaging using an x-ray interferometer has great potential to reveal the structures inside
soft tissues, because the sensitivity of this method to hydrogen, carbon, nitrogen, and oxygen is ⬇1000 times higher than
that of the absorption-contrast x-ray method. Imaging of vessels is very important to understand the vascular distribution
of organs and tumors, so the possibility of selective angiography based on phase contrast is examined with a
physiological material composed of low-atomic-number elements.
Methods and Results—Phase-contrast x-ray imaging was performed with a synchrotron x-ray source. Differences in
refractive index, d␦, of physiological saline, lactated Ringer’s solution, 5% glucose, artificial blood such as
pyridoxylated hemoglobin–polyoxyethylene conjugate, and perfluorotributylamine were measured. Because the d␦ of
physiological saline has highest contrast, it was used for the phase-contrast x-ray imaging of vessel, and this was
compared with absorption-contrast x-ray images. Vessels ⬎0.03 mm in diameter of excised liver from rats and a rabbit
were revealed clearly in phase-contrast x-ray imaging, whereas the vessel could not be revealed at all by the
absorption-contrast x-ray image. Absorption-contrast x-ray images with iodine microspheres depicted only portal veins
⬎0.1 mm in diameter with nearly the same x-ray dose as the present phase-contrast x-ray imaging.
Conclusions—Phase-contrast x-ray imaging explored clear depiction of the vessels using physiological saline with small
doses of x-rays. (Circulation. 2002;105:1708-1712.)
Key Words: imaging 䡲 vessels 䡲 liver 䡲 angiography 䡲 contrast media
P
resent clinical x-ray images are generated by the difference in x-ray absorption determined by the linear attenuation coefficient. The difference in the x-ray absorption of
the blood against the vessel wall and surrounding tissue is
very small; thus, the vessel structure must be imaged by
filling the vessel with a contrast agent containing a heavy
element such as iodine. Iodine contrast agent, however, has
various disadvantages, such as high osmolarity, high viscosity, and allergic reaction.
X-rays have the nature of wave, so the wave shifts when
x-rays pass through an object, and this is called x-ray phase
shift. Phase-contrast x-ray imaging is advantageous because a
cross section of the phase shift is ⬇1000 times larger than that
of absorption for low-atomic-number elements, such as hydrogen, carbon, nitrogen, and oxygen.1,2 Several x-ray imaging techniques for detecting the x-ray phase shift, such as an
interferometric method3 using a crystal x-ray interferometer,4
a Schlieren-like method using crystal diffraction,5 and a
holography-like method,6 are being developed. Among these
methods, the interferometric method using a crystal x-ray
interferometer is the most sensitive method for detecting the
minute difference of refractive index in soft tissue. With the
x-ray interferometer, slices of a rat cerebellum1 and a human
metastatic liver tumor2 have been imaged without contrast
agents. Furthermore, phase-contrast x-ray CT3 demonstrated a
rabbit cancer lesion7 and rat brain8 and human cancer lesions.9 –11
By use of phase-contrast x-ray imaging with the x-ray
interferometer, the possibility of demonstrating vessels by
blood itself was suggested12 and the vessels in excised livers
of a mouse and a rat were revealed.13 For clinical purposes,
however, the investigation of particular vessels of interest is
indispensable. Therefore, selective contrast enhancement
with some new contrast agent is necessary even when the
phase contrast is used. An important advantage is that one
does not need to use contrast agents containing heavy
elements; rather, the present new materials cause the x-ray
phase to shift sufficiently.
The purpose of this study was to investigate the potential
candidates for contrast agents in phase-contrast x-ray imaging
and to image the vessels of rat and rabbit by one agent. The
Received November 29, 2001; revision received January 28, 2002; accepted January 29, 2002.
From the Institute of Clinical Medicine, University of Tsukuba, Tsukuba-shi, Ibaraki (T.T., J.W., Q.Y., T.Z., T.-T.-L., Y.I.); the Department of Applied
Physics, School of Engineering, University of Tokyo, Bunkyo-ku, Tokyo (A.M.); and the Advanced Research Laboratory, Hitachi, Ltd, Hatoyama,
Saitama (A.Y.), Japan.
Correspondence to Tohoru Takeda MD, PhD, Institute of Clinical Medicine, University of Tsukuba, Tsukuba-shi, Ibaraki 305-8575, Japan. E-mail
[email protected]
© 2002 American Heart Association, Inc.
Circulation is available at http://www.circulationaha.org
DOI: 10.1161/01.CIR.0000012752.35225.6C
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Interferometric Phase-Contrast X-Ray Imaging
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Figure 1. Schematic of experiment setup.
Large monolithic x-ray interferometer had 4
parallel x-ray half-mirrors (a beam splitter [S],
2 mirrors [M1, M2], and an analyzer [A]) in a
skewed symmetric alignment. By means of
Laue diffraction (silicon 220) on beam splitter, incident radiation is divided into 2 coherent beams. These beams are spatially separated at mirror (M1, M2), where they are
again reflected in the Laue case. Two convergent beams generated by mirror, 1
coherent beam and another beam stamped
x-ray phase shift by an object placed in one
of the beam paths, are overlapped on
entrance surface of analyzer. Analyzer combines these beams and an x-ray interference
pattern appears in outgoing beams.
vessel images with phase contrast were compared with those
with absorption contrast at nearly the same x-ray dose.
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Methods
Phase-Contrast X-Ray Imaging System
The phase-contrast x-ray imaging system consisted of a silicon
asymmetrical cut crystal to make a 2D beam, a monolithic x-ray
interferometer, a phase shifter, an object cell, and an x-ray–sensing
pickup tube.14 To obtain phase-contrast x-ray images, a monolithic
x-ray interferometer having a 25⫻25-mm field of view was made
using a commercially available highly perfect single-crystal silicon
ingot 10 cm in diameter.15 The size of the beam on the sample,
however, was 25⫻15 mm, because the height (15 mm) was limited
by the maximal beam height available. Because the pixel size of the
x-ray–sensing pickup tube was 0.012⫻0.018 mm, the view size was
9.2⫻13.7 mm. A sample cell filled with water was inserted into a
beam path between M1 and A of the interferometer (Figure 1).
The synchrotron x-ray has excellent properties, such as natural
beam collimation and high x-ray flux for obtaining phase-contrast
x-ray images, so this system was installed at a vertical wiggler beam
line of the Photon Factory in Tsukuba, Japan. The x-ray energy was
set at 17.7 keV by the monochromator, and x-ray flux in front of the
sample cell was ⬇7.5 mR/s at a beam current of 350 mA with
2.5-GeV energy.
Phase-Contrast X-Ray Projection Image
(Phase Map)
The phase shift, ⌽, is given as ⌽⫽(2␲/␭)兰␦dr, where ␦, r, and ␭ are
refractive index decrement of sample, position along x-ray beam, and
x-ray wavelength, respectively. The phase map that describes spatial
distribution of the phase shift within a sample cannot be obtained
directly by use of the interferometer. The technique of phase-shifting
interferometry is used to generate a phase map.3,13
The phase map of the sample was obtained from several interference patterns (5 in this experiment), which were measured sequentially by changing the phase of the reference beam. Each interference
pattern was acquired at 3-second exposure, and the total exposure
time to obtain for a phase map was 15 seconds. The thickness of
sample cell was 12 mm and the total x-ray exposure was 113 mR to
obtain a phase map. The spatial resolution of the phase map was
⬇0.03 mm because of the diffraction phenomenon within the crystal
wafers in the analyzer.
Phase-Contrast X-Ray CT
Phase-contrast x-ray CT images mapped the variation of the d␦
between the tissue and water at 17.7 keV. Because the x-ray beam
was fixed, the specimens had to be rotated inside the cell filled with
water (the thickness of cell was 15 mm). The total beam exposure
time was 60 seconds per projection. The number of projections was
200 for 180°. Phase-contrast x-ray CT images were reconstructed
with a voxel size of 0.018⫻0.018⫻0.012 mm. The d␦ of specimens
was evaluated in 5 regions of the phase-contrast x-ray CT image.
Absorption-Contrast X-Ray Images
The absorption-contrast image was obtained by the x-ray–sensing
pickup tube without the use of the interferometer at the same cell
thickness as the phase map. The experimental setup of the phasecontrast x-ray system requires 1 day for installation. In view of this,
samples were prepared individually to obtain a phase-contrast x-ray
image and a corresponding absorption-contrast x-ray image. The
entrance surface x-ray exposure was 122 mR and nearly comparable
to the phase map.
In the imaging with iodine microspheres, the mass attenuation
coefficient of iodine is 35 cm2/g at 17.7 keV. This value is almost
same as that just above the K-edge energy of 33.2 keV (34 cm2/g)
used for synchrotron radiation angiography with iodine.16
Candidate Contrast Agents for Phase-Contrast
X-Ray Imaging
The d␦ of physiological saline, lactated Ringer’s solution, 5%
glucose, artificial blood such as 2% pyridoxylated hemoglobinpolyoxyethylene conjugate (Fe-C), and perfluorotributylamine (FC43) was measured as the candidate contrast agent. The mass densities
(␳) of these liquid samples are 1.006, 1.006, 1.019, 1.020, and 1.105
g/mL, respectively. The wedge cell filled with various liquid samples
was used to determine the difference of refractive index, d␦, against
water.12
Animal Preparations
Ten Wistar rats and a rabbit were anesthetized with pentobarbital,
and cannulation to the portal vein was performed surgically. Among
the liquid samples, the physiological saline was used for contrast
enhancement. The physiological saline or iodine contrast agent was
injected from the portal vein, and whole blood in the liver was
replaced. After infusion, the portal vein, hepatic artery, and hepatic
vein were ligated to prevent air contamination.
Phase maps (5 rats) and absorption-contrast x-ray images (3 rats)
of the excised liver infused with physiological saline were obtained.
The liver was placed in a square cell filled with physiological saline
to reduce the effect of the thickness variation of the object.
For phase-contrast x-ray CT, a rabbit liver perfused with physiological saline was fixed in formalin, because the data acquisition
time of phase-contrast x-ray CT required ⬎6 hours. A column
specimen 10 mm in diameter was cut from rabbit liver, because the
effective size of the detector was 13.8 mm.
Absorption-contrast x-ray vessel images of excised liver of 2 rats
were obtained by use of iodine microspheres. These images were
compared with phase-contrast x-ray images with physiological saline
under the same x-ray dose. The iodine-loaded acrylic microspheres,
with a mean diameter of 0.015 mm and an iodine concentration of
37% (1 to 2⫻108), were injected into the portal vein. Then portal
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April 9, 2002
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Figure 2. Comparison of measured difference of refractive index
d␦ between candidate contrast agent and water (cross-hatched
bar) and theoretical value (solid bar). d␦ of FC-43 is lower than
theoretical value (P⬍0.05). Glu indicates glucose; Lactic R, lactated Ringer’s solution.
veins ⬎0.015 mm were filled with the radiopaque microspheres
retrogradely.
The present experiment was approved by the Medical Committee
for the Use of Animals in Research of the University of Tsukuba, and
it conformed to the guidelines of the American Physiological
Society.
Statistical Analysis
Data of the d␦ were expressed as mean⫾SD. A 1-sample t test was
used to compare the measured mean values of the sample to
theoretical values. Statistical significance was defined at the level of
P⬍0.05.
Results
Measured Values of the Candidate Contrast Agents
The d␦ values of the liquid samples are shown in Figure 2.
The values of d␦ were 0.44⫻10⫺8⫾0.12⫻10⫺8, 0.46⫻10⫺8⫾
0.10⫻10⫺8, 1.29⫻10⫺8⫾ 0.13⫻10⫺8, 1.28⫻10⫺8⫾0.17⫻10⫺8,
and 5.86⫻10⫺8⫾0.18⫻10⫺8 for physiological saline, lactated
Ringer’s solution, 5% glucose, artificial blood such as Fe-C, and
FC-43, respectively. The measured d␦ values, except for that of
FC-43, correspond to theoretical d␦ values calculated from mass
density (see Appendix 1).
Phase Map and Absorption-Contrast X-Ray Image
Vessels in the rat liver were clearly shown as dark gray in the
phase map (Figure 3A), because physiological saline has a
low refractive index compared with that of normal liver
tissue. The vessel ⬇0.03 mm in diameter was imaged clearly.
The image obtained by absorption contrast, however, could
not reveal the vessel structure at all (Figure 3B). Absorptioncontrast x-ray images with iodine-loaded acrylic microspheres demonstrated only large portal veins 0.1 mm in
diameter at nearly the same x-ray dose as the phase map
(Figure 3C).
Figure 3. Phase map (A) and absorption-contrast x-ray image
(B) of a rat liver filled with physiological saline, and absorptioncontrast x-ray image with iodine-loaded acrylic microspheres
(C). Vessels of liver are shown as gray in phase map, and minimal diameter is ⬇0.03 mm. Absorption-contrast x-ray image
with physiological saline does not reveal vessel at all, and that
with iodine-loaded acrylic microspheres only depicted large portal veins 0.1 mm in diameter. Edge of liver is not shown in B
and C because excised liver is set in a cell filled with physiological saline.
Phase-Contrast X-Ray CT Images
The vessels in the rabbit liver were visualized as voids on the
phase-contrast x-ray CT image shown in Figure 4A. The d␦
of liver tissue and vessel filled with physiological saline was
3.83⫻10⫺8⫾0.21⫻10⫺8 and 0.10⫻10⫺8⫾0.23⫻10⫺8, respec-
Takeda et al
Interferometric Phase-Contrast X-Ray Imaging
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observed between the measured values of d␦ and theoretical
values. Liquid samples, except for FC-43, had lower d␦
values compared with the d␦ of liver tissue (3.5⫻10⫺8). The
measured d␦ of FC-43 was lower than the theoretical value.
We speculate that the density of FC-43 decreased during
stabilization of the interferometer (⬇5 to 10 minutes) because
of the nature of its rapid precipitation. Because the difference
in d␦ between the physiological saline and the liver was
highest among the liquid samples, the physiological saline
was used as a contrast agent for phase-contrast vessel
imaging.
The vessel of liver filled with physiological saline was
revealed clearly both in phase-map and phase-contrast x-ray
CT images, whereas the absorption-contrast x-ray image
could not depict the vessel structure at all. Although the
vessel 0.05 mm in diameter was depicted by the blood itself,13
the vessel 0.03 mm in diameter was revealed clearly by
physiological saline. This significant image contrast produced
by physiological saline will enable us to select the vascular
branches. Conversely, the absorption-contrast x-ray image
with iodine-loaded acrylic microspheres demonstrated only
large vessels 0.1 mm in diameter at almost the same x-ray
dose as the phase map. To depict the vessel 0.03 mm in
diameter by the absorption-contrast method, an x-ray exposure 17 times higher than the present phase map (113 mR)
was required (see Appendix 2).
In addition, the use of physiological saline for vessel
imaging has significant merits, because the side effects of the
iodine contrast agent used here, such as high osmolarity, high
viscosity, and allergic reaction,17,18 will be completely eliminated. The osmolarity and viscosity will be reduced by one
half to one third and one thirteenth to one fourth, respectively,
by use of physiological saline, so a more physiological
examination of the vessel would be possible.
Present Limitations and Future Plans
Figure 4. Phase-contrast x-ray CT image (A) and 3D image of
vessel (lateral view [B] and 20° anterior oblique view [C]) of a
rabbit liver. Hepatic vessels are demonstrated as rounded structures with gray in phase-contrast x-ray CT image. 3D image
depicts vessel trees, and minimal vessel diameter is ⬇0.03 mm.
tively. Three-dimensional renderings of hepatic vessel are
shown in Figure 4, B and C.
Discussion
Various liquid samples for phase-contrast x-ray imaging were
investigated quantitatively, and good correspondence was
Field of View
With the use of a monolithic x-ray interferometer made from
a silicon ingot, the maximal image size achievable is limited
by its diameter. Therefore, another type of interferometer, a
nonmonolithic one, currently is being developed to obtain a
large field of view, ⬎25⫻25 mm.19 In addition, the monolithic x-ray interferometer has another problem: The heat
from live objects causes its instability. The nonmonolithic
configuration can solve the heat problem, because an object
can be placed apart from the half-mirror, and it will be
possible to perform phase-contrast x-ray imaging of live
animals.
High X-Ray Energy
The x-ray energy in this study was 17.7 keV. The use of
higher x-ray energy is suitable, however, in phase-contrast
x-ray imaging of thick objects. Then, x-ray energy of 30 to 40
keV is planned to be used for imaging objects 4 cm thick.
Cinematic Image Acquisition
In this study, phase-contrast x-ray images were obtained
under stable conditions, because the x-ray flux was insufficient to acquire the image at video rates. The flow condition
would be revealed cinematically, however, by use of the
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April 9, 2002
high-flux-density x-ray beam available at the most advanced
synchrotron x-ray facilities.
We are planning to perform live-animal experiments for
visualizing tumor vessels using physiological saline
dynamically.
Conclusions
The hepatic vessels of rats and a rabbit could be revealed
clearly by physiological saline in phase-contrast x-ray imaging. This study suggests that physiological imaging of vessels
might be obtained without the side effects of the currently
used iodine contrast agents and with small doses of x-rays.
Appendix 1
Calculation of Refractive Index
The x-ray complex refractive index, n, is described as n⫽1⫺␦⫺i␤,
where ␦ and ␤ are related to the x-ray phase shift and x-ray
absorption. ␦ is given by
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冉 冊冘
␦⫽
re␭2
2␲
Nk(Zk⫹f rk)⬵(4.49⫻10⫺16)␭2Ne ,
k
r
where Nk, Zk, and f k are atomic density, atomic number, and real part
of the anomalous atomic scattering factor of element k, respectively.
The Ne, ␭, and re are electron density, wavelength, and classic
electron radius, respectively. The above equation suggests that the
value for ␦ is approximately proportional to the mass density (␳), and
d␦ is estimated with the formula d␦⫽(␳⫺1)␦w, where the refractive
index of water is 1⫺␦w (␦w⫽7.4⫻10⫺7 at 17.7 keV).
Appendix 2
Estimation of Photon Number in
Absorption-Contrast X-Ray Image
The signal-to-noise ratio (S/N) is defined as the ratio of the image
signal to the statistical noise associated with the detection of the
number of x-ray photons without scattering radiation.20 By use of the
S/N of the image, the required photon number, F, before the object
to obtain images for vessels of various diameters is calculated from
the following formula:
F⫽
{1⫹exp[⫺d(␮I*⫺␮0)]}(S/N)2
,
{1⫺exp[⫺d(␮I*⫺␮0)]}2[a⑀exp(⫺b␮0)]
where a is the area of each pixel (0.012⫻0.018 mm), ⑀ is the
detection efficiency of the pickup tube (70%), ␮0 is the x-ray linear
attenuation coefficient of water (1.1 cm⫺1), ␮I* is the x-ray linear
attenuation coefficient of iodine contrast agent at 17.7 keV (16.9
cm⫺1), b is the square cell thickness (12 mm), and d is the diameter
of the vessel (0.03 mm). When we defined the S/N ratio as 30.5, the
photon number in front of the object is calculated as 6.9⫻107
photons/mm2 (1.97 R).
Acknowledgments
This research was partially supported by Special Coordination Funds
from the Ministry of Education, Culture, Sports, Science, and
Technology of Japan. Experiments were performed under proposal
number 99S2-002, approved by the High Energy Accelerator Research Organization. We thank Keiichi Hirano, PhD, Xiaowei
Chang, PhD, and Kazuyuki Hyodo, PhD, for technical support at the
Photon Factory; Syounosuke Matsushita, MD, Donepudi V. Rao,
PhD, Yoshinori Tsuchiya, PhD, and Kouzou Kobayashi for preparation of experimental apparatus; Noboru Chiba, RT, for technical
support; and Yukiko Kawata for help in preparing this article.
References
1. Momose A, Fukuda J. Phase-contrast radiographs of nonstained rat cerebellar specimen. Med Phys. 1995;22:375–379.
2. Takeda T, Momose A, Itai Y, et al. Phase-contrast imaging with synchrotron x-rays for cancer lesion. Acad Radiol. 1995;2:799 – 803.
3. Momose A. Demonstration of phase-contrast X-ray computed
tomography using an x-ray interferometer. Nucl Instrum Methods. 1995;
A352:622– 628.
4. Bonse U, Hart M. An X-ray interferometer. Appl Phys Lett. 1965;6:
155–156.
5. Davis TJ, Gao D, Gureyev TE, et al. Phase-contrast imaging of weakly
absorbing materials using hard X-rays. Nature. 1995;373:595–598.
6. Wilkins SW, Gureyev TE, Gao D, et al. Phase-contrast imaging using
polychromatic hard x rays. Nature. 1996;384:335–338.
7. Momose A, Takeda T, Itai Y, et al. Phase-contrast x-ray computed
tomography for observing biological soft tissue. Nat Med. 1996;2:
473– 475.
8. Beckmann F, Bonse U, Busch F, et al. X-ray microtomography using
phase contrast for the investigation of organic matter. J Comput Assist
Tomogr. 1997;21:539 –553.
9. Momose A, Takeda T, Itai Y, et al. Phase-contrast tomographic imaging
using an x-ray interferometer. J Synchrotron Rad. 1998;5:309 –314.
10. Takeda T, Momose A, Ueno E, et al. Phase-contrast x-ray CT image of
breast tumor. J Synchrotron Rad. 1998;5:1133–1135.
11. Takeda T, Momose A, Hirano K, et al. Human carcinoma: early experience with phase-contrast x-ray CT with synchrotron radiation: comparative specimen study with optical microscopy. Radiology. 2000;214:
298 –301.
12. Momose A, Takeda T, Itai Y. Contrast effect of blood on phase-contrast
x-ray imaging. Acad Radiol. 1995;2:883– 887.
13. Momose A, Takeda T, Itai Y, et al. Blood vessels: depiction at phasecontrast x-ray imaging without contrast agents in the mouse and rat:
feasibility study. Radiology. 2000;217:593–596.
14. Suzuki Y, Hayakawa K, Usami K, et al. X-ray sensing pickup tube. Rev
Sci Instrum. 1989;60:2299 –2302.
15. Takeda T, Momose A, Yu Q, et al. Phase-contrast x-ray imaging with a
large monolithic x-ray interferometer. J Synchrotron Rad. 2000;7:
280 –282.
16. Graeff W, Dix WR. Nikos-non-invasive angiography at HASYLAB. In:
Ebashi S, Koch M, Rubenstein E, eds. Handbook on Synchrotron
Radiation. Vol 4. Amsterdam, Netherlands: North-Holland; 1991:
407– 430.
17. Shehadi WH, Toniolo G. Adverse reactions to contrast media: a report
from the Committee on Safety of Contrast Media of the International
Society of Radiology. Radiology. 1980;137:299 –302.
18. Katayama H, Yamaguchi K, Kozuka T, et al. Adverse reactions to ionic
and nonionic contrast media: a report from Japanese committee on safety
of contrast media. Radiology. 1990;175:621– 628.
19. Yoneyama A, Momose A, Seya E, et al. Operation of a separated-type
X-ray interferometer for phase-contrast imaging. Rev Sci Instrum. 1999;
70:4582– 4586.
20. Motz JW, Danos M. Image information content and patient exposure.
Med Phys. 1978;5:8 –22.
Vessel Imaging by Interferometric Phase-Contrast X-Ray Technique
Tohoru Takeda, Atsushi Momose, Jin Wu, Quanwen Yu, Tsutomu Zeniya, Thet-Thet- Lwin,
Akio Yoneyama and Yuji Itai
Circulation. published online March 25, 2002;
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