Compact system for high resolution X-ray transmission
radiography, in-line phase enhanced imaging and micro
CT of biological samples
J. Jakubek*, J. Dammer, C. Granja, T. Holy, S. Pospisil, J. Uher
Institute of Experimental and Applied Physics, Czech Technical University in Prague,
Horska 3a/22, CZ-12800 Prague 2, Czech Republic
Work carried out within the CERN Medipix Collaboration
Abstract
Conventional transmission radiography is based on beam intensity attenuation. A fraction of the
beam has to be absorbed by the sample in order to obtain an image of its structure. The sensitivity of this
method is limited for weakly absorbing objects such as biological samples consisting of soft tissue. Phase
contrast imaging uses phase shift of photons which passed the sample (these photons don’t contribute to
radiation dose). Variations in the phase result from changes in the refractive index across the sample which
can be visualized.
There are several approaches of the phase shift visualization. An interferometric approach uses
interference of the transmitted beam with the primary beam. A diffractometric method (DEI) uses an
analyzer crystal to separate transmission and phase image. And a so called in-line method separates these at
a large object to detector distance.
Although phase sensitive X-ray imaging presents many advantages it is not routinely used in
biological research. Realization of the method currently demands high intensity and highly coherent
(monochromatic and well collimated) X-ray beam which is accessible mainly at synchrotron facilities. In
addition the use of perfect crystal is usually required both to produce such highly coherent beams and to
analyze the transmitted images. Avoiding the need for such complex instruments and large scale facilities,
it has been already shown that in-line phase enhanced imaging is possible and advantageous also with
polychromatic microfocus X-ray tubes. The main problem of such systems is low beam intensity which
prolongs exposure time to such an extent that common digital imaging detectors (CCD, Flat panels) are
disadvantageous due to low efficiency, dark current and noise.
This contribution presents a compact phase enhanced imaging system based on a microfocus X-ray
tube and position sensitive single photon counting pixel detector Medipix2. Each pixel of the detector is
provided with preamplifier, pair of discriminators and counter. Discriminators allow full suppression of the
noise and selection of energy range of interest. Combined with high detection efficiency (100% at 10keV),
this feature makes the device ideal for in-line phase enhanced imaging. The spectral sensitivity of the
detector together with the polychromatic nature of the beam allows to distinguish a transmission
(attenuation) image from a phase (refractive) image. Spatial resolution of the system can be on the
submicrometer level, measuring times less than a minute and doses of order of mGy.
Applications of the system such as the investigation of internal structure of several biological
samples (termites, mouse kidneys and lungs) including their 3D reconstructions will be presented. The
simplicity of the system allows for routine laboratory work including in-vivo and time dependent studies.
*
Corresponding author: Jan Jakubek, Institute of Experimental and Applied Physics of the Czech Technical University in Prague,
Horska 3a/22, CZ-12800 Prague 2, Czech Republic, Tel.: +420-603-589854, jan.jakubek@utef.cvut.cz.
Abstract (reduced)
Conventional transmission radiography is based on beam intensity attenuation. The sensitivity of
this method is limited for weakly absorbing objects such as biological samples. Phase imaging visualizes
phase shift of photons which passed the sample. Although phase sensitive X-ray imaging offers many
advantages it is not routinely used in biological research due to demands of high intensity and highly
coherent X-ray beam which is accessible mainly at synchrotron facilities. In-line phase enhanced imaging
is possible also with microfocus X-ray tubes but low beam intensity of such systems prolongs exposure
time to such an extent that common digital imaging detectors (CCD, Flat panels) are disadvantageous due
to low efficiency, dark current and noise. This contribution presents a compact phase enhanced imaging
system based on a microfocus X-ray tube and single photon counting pixel detector Medipix2. Each
detector pixel is provided with pair of discriminators allowing full noise suppression and selection of
energy range of interest. The spectral sensitivity of the detector together with the polychromatic nature of
the beam allows to distinguish a absorption image from a phase image. Spatial resolution of the system can
be on the submicrometer level and measuring times less than a minute. Several applications of the system
for biological samples including 3D reconstructions will be presented. The simplicity of the system allows
for routine laboratory work including in-vivo studies.