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Synchrotron-based X-ray Tomographic Microscopy atthe Swiss Light Source for Industrial ApplicationsF. Marone a , R. Mokso a , J.L. Fife a , S. Irvine a b , P. Modregger a b , B.R. Pinzer a , K.Mader a c , A. Isenegger a , G. Mikuljan a & M. Stampanoni a ca Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerlandb University of Lausanne, Medical School, Lausanne, Switzerlandc Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Available online: 03 Jan 2012

To cite this article: F. Marone, R. Mokso, J.L. Fife, S. Irvine, P. Modregger, B.R. Pinzer, K. Mader, A. Isenegger, G. Mikuljan& M. Stampanoni (2011): Synchrotron-based X-ray Tomographic Microscopy at the Swiss Light Source for IndustrialApplications, Synchrotron Radiation News, 24:6, 24-29

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Synchrotron-based X-ray Tomographic Microscopy at theSwiss Light Source for Industrial Applications

F. MARONE,1 R. MOKSO,1 J.L. FIFE,1 S. IRVINE,1,2 P. MODREGGER,1,2 B.R. PINZER,1 K. MADER,1,3

A. ISENEGGER,1 G. MIKULJAN,1 AND M. STAMPANONI1,3

1Swiss Light Source, Paul Scherrer Institut, Villigen, Switzerland2University of Lausanne, Medical School, Lausanne, Switzerland3Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Introduction

The potential of X-rays for the non-invasive investigation of the

interior of bulky samples has long been recognized and their pene-

tration depth is presently widely exploited [1]. Although initially

third generation synchrotron sources were almost exclusively the

realm of academic scientific experiments, the use of synchrotron

light for industrial research is becoming increasingly important and

relevant for both technology development and non-destructive test-

ing, in particular when the requirements in terms of spatial, tem-

poral, and density resolution are exceptional.

We begin by introducing X-ray tomographic microscopy, one of

the most versatile techniques particularly suited to address impor-

tant industrial issues with emphasis on the tremendous advantages

of using synchrotron light. In the second part of this article, we

discuss a few cutting-edge examples relevant for the materials and

biomedical industry.

X-ray tomographic microscopy

Technique

X-ray tomographic microscopy is a powerful technique used to

visualize and study the three-dimensional (3D) internal structure and

material properties of a variety of optically opaque samples in a non-

destructive manner. It was introduced in 1973 by Houndsfield [1] and

initially mostly applied in the medical field, where it is now well

established. With time it became clear that X-ray tomographic micro-

scopy is an excellent tool for the investigation of a wider range of

specimens. Currently, most academic and industrial research depart-

ments in fields such as materials, food, pharmaceutical, environmen-

tal, and earth sciences are equipped with laboratory-based microCT

systems, which are routinely used for the non-destructive analysis of

the internal structure of the most diverse samples.

In absorption contrast tomographic microscopy, radiographic

projections are acquired, showing the selective attenuation of the X-

ray beam traveling through the sample. For a given beam energy, the

number of absorbed photons depends on the sample material (Z

number and electron density): the higher these parameters are, the

more photons will be absorbed. In phase contrast imaging, the dif-

fraction of the beam and the resulting interference phenomena are

instead exploited. Radiographic projections, however, only provide

2D cumulative information on the structure along the beam path. 3D

internal structural details can be ascertained by taking radiographies

at different sample orientations and combining those using sophisti-

cated algorithms for tomographic reconstructions based on Fourier

analysis (e.g. filtered back-projection) or iterative methods.

Knowledge provided by X-ray tomographic microscopy on the

interior of optically opaque objects is immense. In addition to the mere

visualization of internal structural details in 3D, extraction of quantita-

tive information is now possible thanks to the ever-increasing computa-

tional power. Number, size, shape, orientation, spatial distribution,

connectivity, and packing of features of interest in the analyzed volume

are just a few of the possible quantitative metrics that can be extracted

from tomographic datasets. If adequate calibration measurements are

performed,X-ray tomographicmicroscopy canalso provide insight into

composition (e.g. bone mineralization). In addition, volumetric infor-

mation on the real specimen structure can provide drastically more

realistic results in simulations than the often used idealized models.

Advantages of synchrotron radiation

At third generation synchrotrons, thanks to a very intense and

coherent beam, X-ray imaging has experienced a true revolution.

The tremendous photon density reached by these novel sources

brings huge advantages with respect to traditional X-ray laboratory

instruments. The high brilliance of synchrotron light provides

increased spatial and temporal resolution: detection of details as

small as 1 micron in millimeter-sized samples is routinely possible

within only a few minutes. In addition, the monochromaticity of the

X-ray beammakes quantitativemeasurements ofmaterial properties

possible and vastly simplifies the identification of different phases,

since beam hardening artifacts, distinctive for laboratory set-ups,

can be avoided. Increased contrast and reduced noise are also pro-

moted by the monochromatic beam and the high photon flux.

Moreover, the almost parallel beam geometry usual at tomographic

microscopy endstations at synchrotron sources permits the accurate

reconstruction of tomographic volumes without cone beam artifacts.

All of these factors together result in images of astonishing quality.

Furthermore, thanks to the coherence of the beam, different

imaging modalities can be exploited, enabling optimal investigation

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of both low- and high-absorbing samples. In fact, in addition to

absorption tomographic microscopy, phase contrast imaging can

also be exploited, enhancing the contrast in low-absorbing samples

or in specimens composed of materials with similar Z numbers.

Finally, the latest detector generation based on CMOS technol-

ogy [2] coupled with the highly brilliant synchrotron radiation has

made sub-second temporal resolution a reality. In this way, pre-

viously unimaginable new science and experiments, where fast

dynamic processes can for the first time be captured in 3D through

time [3, 4], are now possible.

Beamline overview

At the Swiss Light Source, the beamline for TOmographic

Microscopy and Coherent rAdiology experimentTs (TOMCAT) [5]

(Figure 1) offers cutting-edge technology and know-how for best

exploiting the distinctive peculiarities of synchrotron radiation for

fast, non-invasive, high-resolution, quantitative, volumetric investi-

gations on diverse samples. Absorption and phase contrast imaging

with an isotropic voxel size ranging from 0.37 up to 14.8 microns

(field of views from 0.75 · 0.75 mm2 up to 30 · 30 mm2, respectively)

are routinely performed in an energy range of 8–45 keV. Phase

contrast is obtained either with simple edge-enhancement,

propagation-based techniques [6, 7] or grating interferometry [8].

Typical acquisition times are on the order of a fewminutes. A sample

exchanger and a package of automation tools [9] are available for

performing high throughput studies in a fully automatic manner.

Custom-specific devices for in-situ experiments can easily be

installed on the sample stage. A cryo-chamber, a compression-tensile

device, and a laser-heated furnace [10] are currently (or will soon be)

available for such experiments.

New cutting-edge experiments are now possible thanks to the

latest efforts towards improving temporal resolution. Dynamic pro-

cesses can be followed in 3D for the first time thanks to the new

ultrafast tomographic endstation offering sub-second temporal reso-

lution [3]. Specifically, it is equipped with a CMOS detector coupled

to different high numerical aperture magnifying objectives suitable

for white-beam operation to achieve the highest temporal resolution.

3D tomographic datasets are reconstructed from 2D projections

using highly optimized software [11, 12] based on Fourier methods

and a user-friendly web interface.

Applications

Material research

Characterization of the microstructure of materials (e.g. porosity

and spatial distribution of chemical components) is essential for under-

standing their properties and performance. Synchrotron-based X-ray

tomographic microscopy is particularly suited for obtaining this kind

of quantitative volumetric information in a fast, non-invasive manner

[13] when the scale of interest is in the micrometer range. While the

investigation of static samples and extraction of relevant morphologi-

cal information is relatively well established, we discuss here two

cutting-edge applications dealing with dynamic processes, aiming at

understanding microstructure formation and evolution.

Foam is a substance that is formed by trapping gaseous bubbles in a

liquidor solid.Liquid foamsareubiquitous; foodproducts (e.g. chocolate

mousse, bread, and beer) and cosmetic goods (e.g. shaving foam and

shampoos) are only a few examples. More technical applications also

benefit from the particular properties of liquid foams, such as nuclear

decontamination,oil extraction,and fire retardation. Inaddition, theyare

often the key element for manufacturing solid foams, such as car bum-

pers, floatation devices, packing and thermal insulating material.

Such versatile and widespread use of foams relies on their distinct

properties, frequently described by theoretical models, which, how-

ever, are only partially validated. In fact, despite the pervasive applica-

tion of foams, our understanding of their rheological and flow

behavior as well as their coarsening characteristics, particularly in

3D systems, is still poor, mainly due to the lack of appropriate experi-

mental data. In particular, liquid foams are rapidly evolving complex

systems and until recently could not be studied in 3D without artifi-

cially slowing down (and thus altering) their fast dynamics [14, 15].

At the new cutting-edge TOMCAT endstation devoted to ultra-

fast tomographic microscopy, 3D reconstructions—free of motion

artifacts—of naturally evolving foams consisting of thousands of bub-

bles, including their fast initial phase, can now be obtained [3]

(Figure 2). Individual bubbles can be tracked through time, making

detailed characterization of the flow and rheological behavior possible.

Neighborhood relations, volumes as well as bubble deformation or

coalescence can be observed in-situ in 3D. This new information will

enable researchers to extend existing theoretical models (e.g. [16]) until

now supported exclusively by 2D experimental data. In the industrial

environment, this new know-how is essential for improving the per-

formance of existing products. It is also critical for a better control in

designing new foamware with custom-tailored advanced properties.

Figure 1: Automatic sample mounting at TOMCAT (Image: Paul ScherrerInstitut).

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Dendrites are the general growth morphology during the solidi-

fication of metallic alloys. These topologically complex microstruc-

tures develop due to a morphological instability at the solid-liquid

interface. If the solid-liquid mixture is held above the eutectic tem-

perature for any length of time, the dendrites undergo coarsening.

During this process, the morphology, distribution of chemical com-

ponents, and length scale of the microstructure change considerably,

affecting many properties of the end material. Clear evidence also

shows that the initial solidification procedures have a significant

impact on the evolution of the microstructure [17]. Due to the fast

dynamics of early-stage coarsening and solidification in general, the

3D visualization and in-situ investigation of the microstructure evolu-

tion during these phenomena have been limited, hampering a deep

understanding of how small changes in processing affect the end

result. Ultrafast X-ray tomographic microscopy as offered at the

TOMCAT beamline [3], coupled to a newly developed laser-heated

furnace [10], provides an ideal environment for 3D, real-time exam-

ination of the dynamics of microstructure formation and evolution

(Figure 3). Furthermore, this approach is also extremely valuable for

the in-situ observation and investigation of defect formations

(e.g. porosity and hot tearing), which are important industrial issues

significantly affecting the properties and performanceof end products.

Use of a compression-tensile device is instead useful for in-situ

mechanical testing of a variety of materials (e.g. cement, wood,

bone) where structural deformation all the way to crack formation

can be captured in 3D as it happens [18–20].

Biomedical and pharmaceutical research

X-ray imaging in the biomedical field, in particular in the clinical

environment for the detection of different pathologies, is well

(a)

(b)

0 s

1 mm

1 mm

180 s 210 s

0 s 180 s 210 s

Figure 2: Evolving liquid foam (standard dishwasher) captured at 3 different time steps at the ultrafast TOMCAT endstation. Each single tomogram wasacquired in 0.5 s (X-ray energy: 20 keV, voxel size: 11 microns). (a) The individual bubbles are color-coded according to their diameter, illustrating foamcoarsening and providing understanding of the evolution of the bubble-size distribution. (b) Single bubble tracking through time thanks to their labeling,enabling a better characterization of foam flow and rheological behavior.

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established. The peculiar advantages provided by synchrotron light

over clinical and laboratory instruments show a great potential for

the application of X-ray tomographic microscopy to address typical

issues encountered in the biomedical and pharmaceutical industry.

Synchrotron-based X-ray tomographic microscopy is routinely

used for the analysis of a wide variety of biopsies (e.g. bone, carti-

lage, teeth, lung, brain, skin, eye tissue). The investigation scope is

usually very diverse and ranges from functional and diagnostic 3D

microstructural studies (e.g. osteocyte lacunae morphology and dis-

tribution in bones) to the developmental aspects of early tumor

growth. High-resolution X-ray tomographic microscopy has also

proven extremely useful for optimizing tissue engineering; for exam-

ple, by closely monitoring porosity and mineralization in scaffolds

while simultaneously tracking the distribution and proliferation of

cells within these constructs.

Sub-micrometer structural details in mm-sized 3D volumes can

be visualized with synchrotron light withinminutes. Full automation

of data acquisition, thanks to a sample exchanger and a package of

automation tools as integrated at the TOMCAT beamline [9], facil-

itates the realization of high-throughput studies often necessary in

the pharmaceutical sector in early drug discovery research or during

initial clinical trials with small animals (e.g. mice). Automation is not

limited to data acquisition. Often data analysis and extraction of the

relevant information can also be performed in a fully automated

manner, making large-scale studies with hundreds, even thousands,

of samples feasible in a robust, repeatable manner in a reasonable

time frame (Figure 4).

An additional advantage of synchrotron light lies in its coher-

ence enabling phase contrast imaging, particularly suited for study-

ing biological soft tissues in their native state, without excessive

fixation or additional contrast agents. At the TOMCAT beamline,

several phase contrast techniques are available [21], optimized for

the required spatial and density resolution. These cutting-edge meth-

odologies provide comparable information to histological

approaches. Results for large volumes can be obtained in a fraction

of the time required for histological studies without requiring sec-

tioning and its accompanying artifacts (Figure 5).

Other applications

Although most applications of synchrotron-based tomographic

microscopy are so far focused on materials and biomedical research,

its versatility and flexibility make it an optimal technique for addres-

sing a much broader range of industrial issues.

In the energy industry, synchrotron-based tomographic micro-

scopy has proven extremely valuable for in-situ corrosion studies

pertinent to security issues for nuclear waste disposal [22, 23], for

improving the performance of lithium ion batteries [24], and for

gaining in-depth knowledge on the working mechanisms of fuel cells

to be used in future electric cars [25]. Although the oil industry has

been benefiting from X-ray tomographic microscopy for a while, the

use of synchrotron radiation in this field would enable shading light

on porosity unsolved questions at the sub-micrometer level as well as

gaining a deeper understanding of the dynamics of multi-phase flow

in porous media, extremely relevant issues for a better exploitation of

oil fields. Finally, a branch that is increasingly discovering the assets

of these synchrotron-based techniques is the food industry. The spa-

tial and size distribution of the different ingredients in the end product

(e.g. ice cream and chocolate) can contribute to manufacturing more

Figure 3: First 3D solidification and growth of dendrites in an Al-20wt%Cualloy obtained using the laser-heated furnace at the ultrafast TOMCATendstation. The solid-liquid interface, encompassing the Al-rich dendrites,is shown and colored by its velocity calculated using two subsequent experi-mentally determined 3D microstructures. The voxel size is 1.1 microns. Forthe experiment polychromatic X-ray radiation has been used. The samplewas manually cooled from above the eutectic temperature at approximately0.5o/s. For more details, please see [10].

Figure 4: Characterization example of the ultra-structure in a mouse femur(data acquired at the TOMCAT beamline, X-ray energy: 17.5 keV, voxelsize: 1.4 microns). (a) 3D rendering of the tomographic volume, showing inred the canals and in yellow the osteocyte lacunae. (b) Segmented datasetcolor-coded according to the distance to the nearest canal.

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palatable goods, while the high temporal resolution made possible

with synchrotron radiation enables monitoring of the actual produc-

tion in-situ as it happens, providing invaluable information on each

single step, critical for product optimization. Finally, manufacturing

processes such as granulation and tableting used in the food and

pharmaceutical industries for preparing fast-dissolving dried food

and drugs are handicapped by highly empirical and expensive trial-

and-error performance optimization approaches. X-ray tomographic

microscopy, in particular with its high temporal resolution, could

provide a more scientific understanding of the effects of the different

parameters involved in the preparation of the end product.

Conclusions

X-ray tomographic microscopy is an ideal tool for the non-

destructive, volumetric, quantitative characterization of a variety of

bulky materials in situ at the micrometer scale. The advantages of

synchrotron light make this technique even more powerful and parti-

cularly suited for addressing multiple industrial issues, ranging from

the deep understanding of dynamic processes in 3D to unraveling

micrometer details of biological samples in their native state. The

versatility and flexibility of the method coupled to the penetration

depth of X-rays provide nearly unlimited room for new developments

adapting to emerging customer needs and exploring previously

uncharted problems. For example, one of the current areas of research

in our group aims at making high-resolution in-vivo studies of small

animals a reality, enabling biological processes such as breathing and

vascular circulation to be studied for the first time on live animals on

the micron scale with sub-second time resolution.

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