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FEATURE ARTICLE
X-RAY OPTICS DEVELOPMENT FOR BIOMEDICAL
IMAGING APPLICATIONS AT THE CANADIAN LIGHT
SOURCE
BY
DEAN CHAPMAN, NAZANIN SAMADI, MERCEDES MARTINSON, BASSEY BASSEY, SHEILA BOIRE,
GEORGE BELEV AND TOMASZ WYSOKINSKI
he synchrotron provides an excellent source of
x-rays for a number of research applications
spanning a number of basic and applied science
research areas. The Biomedical Imaging and
Therapy beamlines [1,2] at the Canadian Light Source
(CLS) with associated laboratories form a research
facility that provides high intensity X-ray beams for
both imaging and therapy applications of a wide range of
biomedical systems from mice to humans to horses. A
diagram of the facility at the CLS is shown in Fig. 1.
Access to high intensity, tunable X-radiation allows a
number of synchrotron specific imaging methods to be
applied to a number of biomedical systems and problems.
Examples are a number of phase related imaging methods
such as analyzer based imaging or diffraction enhanced
imaging, in-line phase contrast imaging, Talbot or grating
interferometry based imaging, coded aperture imaging
and Shack-Hartmann imaging. Absorption based imaging
methods include K-edge subtraction imaging and conventional absorption based imaging.
T
The synchrotron’s unique broad band spectrum, intensity
and small source size make it an ideal environment for
testing new approaches to imaging and imaging optics. It
has been likened to an “X-ray wind tunnel” for testing
new ideas and to simulate possible clinical or laboratory
applications of methods that are developed or advanced
at the synchrotron. The X-Ray Imaging Group at the
University of Saskatchewan and the Canadian Light
Source are working on a number of imaging optics and
systems in support of the biomedical imaging program at
the biomedical beamlines. The purpose is to assist in
solving unique imaging problems and to ensure that the
SUMMARY
X-ray optical research and development at
the biomedical beamline at the Canadian
Light Source is providing new and better
systems for imaging of biological systems.
facility remains a world leader in biomedical applications
of synchrotron radiation research.
In this paper, we will discuss some of the activities of this
group and specifically some of our advances that will
provide new tools for biomedical imaging research.
K-EDGE SUBTRACTION
‘SPECTRALKES’
FROM KES TO
K-Edge Subtraction (KES)1 at the synchrotron was
originally developed for imaging of the human coronary
anatomy at the Stanford Synchrotron Radiation Laboratory by Robert Hofstadter, Nobel laureate in physics 1961,
and Edward Rubenstein, a cardiologist from the Stanford
School of Medicine. This method utilizes two crossing
line X-ray beams which were prepared to be just above
and below the iodine K-edge at 33.17 keV [36]. After
passing the subject located at the crossover, the two line
beams were detected in a dual line X-ray detector. A
simple algorithm allowed an image of the projected
amount of iodine and water to be determined. The original
application only required a resolution of a few hundred
microns for successful human vasculature imaging; however, most applications require a resolution of a few
microns to visualize smaller systems in greater detail. Our
goal for small animal vasculature and a gene expression
imaging program was to achieve a resolution in the range
of 10’s of microns. For this to be achieved a small focus
of the beam needed to be developed in order to sample an
appropriately small region of the subject. Bent Laue
(transmission X-ray case of diffraction) crystals had been
used for some time to prepare the imaging beams needed
for the simultaneous above and below K-edge beams
needed for living (and thus moving) systems. The focus
size of these systems was typically a few hundred microns
and is set by an interesting interplay between a geometric
focus caused by the overall bend of the crystal and a
single ray or polychromatic focus that each ray experiences in traversing the thickness of the crystal. The single
1. Also known as Digital Subtraction Imaging, K-Edge Dichromography,
Synchrotron Radiation Dichromographic Imaging for simplicity we
refer to this method as K-Edge Subtraction Imaging.
LA PHYSIQUE
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Dean Chapman,
Bdean.chapman@
usask.ca,
Anatomy & Cell
Biology,
Nazanin Samadi,
Division of
Biomedical
Engineering,
Mercedes Martinson,
Bmercedes.m@
usask.ca,
Bassey Bassey,
Bbassey.bassey@
usask.ca, Physics
and Engineering
Physics,
and
Sheila Boire,
Mechanical
Engineering,
University of
Saskatchewan,
Saskatoon, SK,
S7N 5E5
George Belev,
Bgeorge.belev@
lightsource.ca,
and
Tomasz Wysokinski,
BTomasz.
wysokinski@
lightsource.ca,
Canadian Light
Source, Inc.,
Saskatoon,
SK S7N 2V3
CANADA / Vol. 70, No. 1 ( 2014 ) + 19
X-RAY OPTICS DEVELOPMENT
Fig. 1
FOR
BIOMEDICAL IMAGING . . . (CHAPMAN
Layout of the Biomedical Imaging and Therapy beamlines facility at the Canadian Light Source which is comprised of two
beamlines (a bend magnet beamline and a superconducting wiggler sourced beamline). The bend magnet is used in POE-2 and the
wiggler beamline in POE-2 and SOE-1.
ray focus typically occurs before the geometric focus and thus
diverges. The crossing of these diverged beams at the
geometric focus gives the beam size at focus.
Bent Laue crystals have been successfully used to micro-focus
X-ray beams [7,8] and we adopted this method for preparing a
beam for a small animal imaging system (Fig. 2) [9]. We bent a
silicon (5,1,1) wafer to an approximate 3 m radius and utilized
the [3,1,1] reflection for our system. We measured the focal line
size at approximately 88 microns which is appropriate for our
application; this can be made smaller by preparing a specific
crystal plate to be bent. One interesting aspect of achieving a
small focus is that the angular energy dispersion of the crystal is
very good. Typically in a bent Laue system used for human
imaging the K-edge of iodine is blurred approximately by 1/3 of
the vertical size of the beam. Thus in these systems, fully 33% of
the vertical beam size must be blocked to avoid edge crossing
energies in the high and low energy beams. For the x-ray optic
we have, the part of the beam that is involved in edge crossing is
approximately 5% of the vertical beam size making blocking
this part of a beam impractical. An advantage of using most of
the beam is that the efficiency of the system is very high as most
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of the vertical profile of the beam is used and not blocked.
This allows this optic to be very effectively used on a bend
magnet beamline with good flux. A section of an image of the
beam with an iodine filter is shown in Fig. 3. This is an image
from the detector with an iodated filter placed in the beam for
energy calibration purposes. The energy span of this image is
approximately 500 eV which spatially corresponds to a vertical
beam size of approximately 4 mm.
Given the energy range covered and the spatially dispersed
energies measured, a multiple energy algorithm [10] was
implemented to extract composition information from the
energies. This method fully accounts for the energy dependent
shape of the absorption of materials.
An example of the use of this system for imaging of iodated
contrast material injected into the chest cavity of a mouse is
shown in Fig. 4.
Current research with this method is assessing methods of
eliminating artifacts associated with the fact that the imaging
beams are focused at the subject and that different energies
take slighty different paths through the object.
X-RAY OPTICS DEVELOPMENT
Fig. 2
FOR
BIOMEDICAL IMAGING . . . (CHAPMAN
ET AL.)
Schematic diagram of spectral-KES monochromator,
object and detector.
BENT LAUE BEAM EXPANDER
One of the emerging areas of synchrotron biomedical research
is full field dynamic imaging or movies that show how a
system behaves physiologically. The biomedical beamline at
the CLS could not be built with sufficient length to allow the
beam size to expand beyond about 5 mm on the bend magnet
beamline and 1cm on the insertion device beamline. We have
begun a program to explore whether bent Laue optics could be
used to vertically expand the imaging beam, effectively
making the beamline appear to be at least 5 times longer.
Bent Laue crystals behave somewhat like lenses, however,
with the added complication of the need to satisfy the Bragg
condition for the lattice planes used. A double crystal system
was fabricated with two matched bent Laue crystals with
appropriate bending radii and distances between them similar
to the schematic shown in Fig. 5. As shown in the schematic,
two bent Laue crystals are used to form the double crystal
monochromator expander. The crystals are arranged such that
the virtual focus of the first crystal becomes the source for the
second crystal with an appropriate choice of bending radius.
The virtual focus of the second crystal then becomes somewhat
farther away depending on the radii and the crystal-crystal
separation. Expansions beyond 5x have been achieved in
preliminary experiments [11]. As an example, a portion of the
beam from an expander system was used to acquire a computed
tomography data set of a seed pod shown in Fig. 6. The imaging
field was approximately 20 mm vertical 29 mm horizontal
Fig. 3
Image of beam at detector with 30 mg/cm3
iodine solution. Image is a negative logarithm of a flat and dark field normalized
image. Note the sharp transition in absorption at 33.17 keV corresponding to the
K-edge of iodine.
which corresponds to a beam whose vertical size is about
5 times larger than presently accessible using the BMIT bend
magnet beamline. The imaging energy was 17.5 keV and
(1,1,1) reflections were used for each crytal with a 19.5 degree
asymmetry angle. The imaging flux (intensity per unit area)
was approximately equivalent to that prepared by the double
crystal monochromator utilizing Si (2,2,0) crystals at the same
imaging energy. This level of flux is possible given greatly
enhanced reflectivity width afforded by the bent crystal system.
CONCLUSION
A number of X-ray optical instrumentation programs are being
pursued at the BMIT beamlines in support of the imaging
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X-RAY OPTICS DEVELOPMENT
Fig. 4
FOR
BIOMEDICAL IMAGING . . . (CHAPMAN
Example spectral-KES images of a mouse with
iodated contrast material injected in the chest cavity.
Figures a and c are water equivalent images with
figures b and d iodine images. Figures c and d are
projection images and figures a and b are computed
tomography images. The computed tomography
slices shown are taken near the location of the
dashed line in the projection images.
Fig. 6
Fig. 5
ET AL.)
Double crystal bent Laue beam expander monochromator schematic. Two bent Laue crystals are used
where the virtual focus of the first crystal becomes the
effective source for the second larger bending radius
crystal. The overall effect is to make the effective
location of the second crystal’s virtual focus to be H/h
times farther away than the real source.
programs there. We have active projects in multiple energy
imaging, variants of coded aperture imaging, Talbot interferometry imaging and a beam width doubling monochromator.
Images of a seed pod. Figure a shows a projection
image of the seed with a 20 29 mm field of
view. Figure b shows the matching computed
tomography sagittal section of the seed. All
images were acquired at 17.5 keV. Normally
such a large vertical acquisition would require
4 to 5 separate acquisition due to the limited
vertical beam size (5 mm).
These projects are directed to providing state of the art
imaging systems and capabilities to the wide range of imaging
programs. The two systems described here are in the process
of being made more robust and implemented in the bend
magent beamline.
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ET AL.)
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