> 'High Resolution Soft Tissue Tomographic Imaging using Single X-ray Photon Counting Pixel Device Medipix2'
High Resolution Soft Tissue Tomographic Imaging using Single X-ray Photon Counting Pixel Device Medipix2
Conf.: New Technics in Lab Animal Science
High resolution X-ray transmission
The common methods of structure imaging of small animals include
optical microscopy, electron microscopy and fluorescence microscopy.
These techniques provide detailed information about external and
internal anatomy. However, the sample preparation is usually laborious,
time consuming and often also expensive. Moreover, the samples, after
being examined with one of the mentioned procedures, cannot be used for
further purposes. Therefore, new approaches are needed and are being
developed. Among them high resolution X-ray radiography, which is often
exploited also in tomographic mode to provide 3D images, appears as a
promising technique for real time and nondestructive visualization.
This method enables as well in-vivo analysis of time dependent
physiological processes in living specimen.
X-ray 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 imaging of biological samples
consisting of soft tissue is particularly difficult due to low
differences in X-ray attenuation. New X-ray imaging detectors like
Medipix2 with unlimited dynamic range allow to reach good contrast and,
in combination with advanced point like X-ray sources, it is possible
to refine spatial resolution of the method below 1m.
This contribution describes an arrangement for X-ray transmission
radiography with the state-of-the-art semiconductor hybrid pixel
detector Medipix2 (512 x 512 pixels with 55μm pitch) and a commercial
X-ray point source. The main advantages of the Medipix2 pixel detector
include: high sensitivity to low energy X-ray photons; position
sensitive and noiseless single photon detection with preselected photon
energies; photon counting in each pixel performed by digital counter,
therefore with no dark current; digital integration providing unlimited
dynamic range and absolute linearity in device response to number of
photons; high speed digital communication and data transfer.
X-ray phase sensitive imaging
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 sensitive imaging on the other
hand uses phase shift of the photons passing the sample. Variations in
the phase result from changes in the refractive index across the
medium. A substantial advantage of this method is that X-ray photons
used for phase shift imaging penetrate through a sample and therefore
they do not contribute to radiation dose.
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 beams at a large object to detector
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 X-ray beams which
are accessible mainly at large scale and costly synchrotron facilities.
In addition the use of perfect crystals 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 shown that in-line phase enhanced imaging can
be carried out also with polychromatic microfocus X-ray tubes. The main
problem of such systems is the low beam intensity which prolongs
exposure time to such an extent that common digital imaging detectors
(CCD, Flat panels) are disadvantageous due to their low efficiency,
dark current and noise.
This contribution presents a compact table-top phase contrast enhanced
imaging system based on a microfocus X-ray tube and the 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 its high detection
efficiency, 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 an
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 3D reconstructions will be presented. The simplicity
of the system allows for routine laboratory work including in-vivo and
time dependent studies.
Cite article as:
J. Jakůbek, J. Dammer, T. Holý, S. Pospíšil, J. Uher, "High Resolution Soft Tissue Tomographic Imaging using Single X-ray Photon Counting Pixel Device Medipix2", Conf.: New Technics in Lab Animal Science (2006)