CSEM Scientific and Technical Report 2008

Applications of X-ray Phase Contrast Imaging
C. Kottler, V. Revol, R. Kaufmann, C. Urban, P. Niedermann, F. Cardot, A. Dommann, N. Blanc
An X-ray imaging facility for phase contrast imaging has been set-up. On the one hand, the system is dedicated, as part of the CCMX analytical
platform [1] , as a station for applied sample measurements on a user service basis. On the other hand, it serves as a laboratory set-up for research
activities heading for the industrialization of that technique in specific application fields. These activities imply developments in instrumentation as
well as experimental studies in order to explore the potential for applications, such as in medicine or non-destructive testing.
X-ray phase contrast imaging (PCI) has an inherent potential
for a significant dose reduction combined with an image
quality enhancement in X-ray radiography as well as in
computed tomography (CT) measurements. Not only the
absorption of X-rays in the measurement object is probed, as
in classical X-ray measurements, but also the difference in
velocity X-rays undergo while travelling through an
inhomogeneous object. The resulting (small) wavefront
deviations can be measured by interferometry, in a Talbot
interferometer set-up. X-rays do not need to be absorbed in
the object at all in PCI, which opens the possibility to optimize
X-ray measurements with respect to image quality and
radiation dose at the same time, which is not possible for
classical X-ray imaging. Though the potential of PCI has been
recently demonstrated experimentally [ 2, 3] , the realization for
specific applications is not at all straight forward, essentially
for the following two reasons: First, how to reveal and interpret
features obtained by the new modality is basically unknown
and still needs to be established. This assessment is a
prerequisite for the successful application of PCI for example
in medical imaging. Secondly, industrial or medical
applications set very stringent requirements to the
instrumentation. These requirements relate in particular to the
size of the field-of-view, the mechanical stability and/or the
flexibility, measurement time and sample throughput.
The most crucial issue with regard to the instrumentation
relates to the X-ray energy. In order to achieve sufficient
sample transmission, many applications require relatively high
energies for which both the fabrication and the operation of
micro structured diffraction gratings become very challenging.
It is thus the main scope of CSEM activities to tackle the
accessibility of higher X-ray energies.
Figure 1: View of the PCI measurement station seen through the
opened door of the shielding hutch: X-ray tube, sample handling
station, grating interferometer and digital image sensor.
Figure 1 shows a view of the PCI set-up through the open
door of the shielding hutch. The set-up contains a high power
X-ray tube that can be run up to 160 kV. The field-of-view of
the digital X-ray camera is 50x100 mm 2 (HxW) with a pixel
36
size of 50 µm. Exposure and acquisition of a single image
frame typically takes less than a few seconds.
The sample handling station allows for the positioning and
scanning of large samples, as well as tomography
measurements. It is designed so that it provides high flexibility
for the applied sample measurements: large samples of lateral
size up to 50x50 cm2 can be measured in scanning mode. For
tomography, however, the transverse sample size is limited to
≈ 8 cm.
Examples of measurements (performed with tube operated at
40 keV) are shown below: Figure 2 shows the absorption and
the phase-contrast image of the tip of a plastic syringe
equipped with a metal needle. As can be seen in Figure 3, in
fact all three images that are simultaneously obtained by the
PCI measurement of a grape reveal complementary details of
the grape’s internal structure: the radiograph, the phasecontrast
image and the third image, which reveals the X-ray
scattering characteristics of the sample.
a) b)
Figure 2: a) Absorption radiograph and b) phase-contrast image of a
syringe’s tip.
a) b) c)
Figure 3: PCI measurement of a grape: a) Absorption radiograph, b)
phase-contrast image and c) image of characteristic X-ray scattering.
[1] NMMC Analytical Platform, www.ccmx.ch
[2] F. Pfeiffer, et al., “Phase retrieval and differential phase-contrast
imaging with low brilliance X-ray sources”, Nature Physics 2
(2006), 258
[3] DIXI partners: CERN, Comet AG, CSEM, EMPA, ETHZ, PSI,
UNINE, USZ, XCAN AG