Jiri Dammer

Spectrometric properties of TimePix pixel detector for X-ray color and phase sensitive radiography

The semiconductor pixel detector TimePix is a newly developed successor of the Medipix2 device. Each TimePix pixel is provided with preamplifier, discriminator and counter. Discriminators allow full suppression of the noise and selection of energy range of interest. Each counter can be configured to work in one of three principal operation modes: 1. counting of detected particles; 2. measurement of particle energy; 3. measurement of time of interaction. Possibility of per pixel energy measurement presents a substantial advantage for X-ray radiography with polychromatic X-ray sources (tubes). This feature allows to utilize normally not desirable beam-hardening phenomenon for material determination. If the radiographic system is equipped with a microfocus X-ray tube enabling phase sensitive imaging, the spectrometric properties of TimePix bring further advantages as the phase effects are energy dependent. This contribution presents a compact X-ray microradiographic phase sensitive system based on nanofocus X-ray tube and position sensitive single photon counting pixel detector TimePix (256 times 256 square pixels, pitch of 55 mum) with 300 mum thick silicon sensor. The spectral sensitivity of the detector together with the polychromatic nature of the beam allows material determination (color imaging). Moreover, in phase sensitive configuration it is possible to distinguish a transmission (attenuation) image from a phase (refractive) image. Spatial resolution of the system is on the submicrometer level and measuring times in order of seconds.

Compact System for High Resolution X-ray Transmission Radiography, In-line Phase Enhanced Imaging and Micro CT of 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. Phase sensitive imaging can be also carried out with microfocus X-ray tubes. However, the low beam intensity of such systems prolongs the exposure time to such an extent that common digital imaging detectors (CCD, Flat panels) are insufficient due to low efficiency, dark current and noise. This contribution presents a compact phase contrast enhanced imaging system based on a microfocus X-ray tube and the single photon counting pixel detector Medipix2. The spectral sensitivity of the detector together with the polychromatic nature of the beam allows distinguishing an absorption image from a phase image. Spatial resolution of the system can be on the sub micrometer level and measuring times less than a minute. Applications of the system for biological samples are presented. The simplicity of the system allows for routine laboratory work including dynamic in-vivo studies.

High resolution X-ray imaging of bone-implant interface by large area flat-panel detector

The aim of the research was to investigate the cemented bone-implant interface be- havior (cement layer degradation and bone-cement interface debonding) with emphasis on imaging techniques suitable to detect the early defects in the cement layer. To simulate in vivo conditions a human pelvic bone was implanted with polyurethane acetabular cup using commercial acrylic bone cement. The implanted cup was then loaded in a custom hip simulator to initiate fatigue crack prop- agation in the bone cement. The pelvic bone was then repetitively scanned in a micro-tomography device. Reconstructed tomography images showed failure processes that occurred in the cement layer during the first 250 , 000 cycles. A failure in cemented acetabular implant — debonding, crumbling and smeared cracks — has been found to be at the bone-cement interface. Use of micro-focus source and high resolution flat panel detector of large physical dimensions allowed to r econstruct the micro-structural models suitable for investigation of migration, micro-motions and consecutive loosening of the implant. The large area flat panel detector with physical dimensions 1 20 × 120 mm with 50μm pixel size provided a superior image quality compared to clinical CT systems with 300 − 150 μm pixel size.

Measurement of energy levels in a silicon detector damaged by neutrons

The level of defects in a semiconductor silicon detector diode made of high resistivity N type material and exposed to neutrons in a research nuclear reactor was examined by measuring the thermally stimulated current (TSC). A modified TSC method was employed where the released charge was measured in the reverse direction on a diode with zero bias voltage. Electrons captured in cooled traps due to the photoelectric effect are released when the material is heated. The detector was irradiated with an integral neutron flux of 7.63 × 1015 n/cm2.

Real-time X-ray 2-D and 3-D micro-imaging of living animals with Medipix2 single photon counting detector

In this work we present the study of applicability of a desktop size radiographic/tomographic X-ray system for real-time microscopy and micro-tomography in the fields of biology, entomology, botanic and medical imaging. The apparatus is made up of the single photon counting pixel silicon detector, Medipix2 (matrix of 256×256 square pixels of 55 μm pitch) and a microfocus X-ray tube with a minimum spot size of 5 μm and a tungsten anode. The system has been used for observations of time-dependent processes inside living and still biological and organic samples. Excellent contrast and spatial resolution (micrometer scale) were obtained as a combination of a) low photon energy (40 kVp X-ray tube voltage), b) single photon counting operation, witch avoids integration of dark current c) energy discrimination in each pixel, allowing noise rejection and providing high SNR, d) high effective dynamic range for long exposures, which allows for high signal with high SNR, e) implementation of an original procedure for the energy response calibration of each pixel of the detector matrix, f) high speed read-out hardware and software, which opens the possibility to perform real-time studies of biological processes permitting, e.g., observation of morphological changes, mutations or metamorphosis of living animals and plants. Static and dynamic images of a parasite life cycle from the larva stage to pupa stage are presented here, as well as an in vivo computed tomography of the parasite living inside its host.

The use of contrast agent for imaging biological samples

The technique of X-ray transmission imaging has been available for over a century and is still among the fastest and easiest approaches to the studies of internal structure of biological samples. Recent advances in semiconductor technology have led to the development of new types of X-ray detectors with direct conversion of interacting X-ray photon to an electric signal. Semiconductor pixel detectors seem to be specially promising; compared to the film technique, they provide single-quantum and real-time digital information about the objects being studied. We describe the recently developed radiographic apparatus, equipped with Medipix2 semiconductor pixel detector. The detector is used as an imager that counts individual photons of ionizing radiation, emitted by an X-ray tube (micro- or nano-focus FeinFocus). Thanks to the wide dynamic range of the Medipix2 detector and its high spatial resolution better than 1μm, the setup is particularly suitable for radiographic imaging of small biological samples, including in-vivo observations with contrast agent (Optiray). Along with the description of the apparatus we provide examples of the use iodine contrast agent as a tracer in various insects as model organisms. The motivation of our work is to develop our imaging techniques as non-destructive and non-invasive. Microradiographic imaging helps detect organisms living in a not visible environment, visualize the internal biological processes and also to resolve the details of their body (morphology). Tiny live insects are an ideal object for our studies.

Enhancement of spatial resolution of roentgenographic methods using deconvolution

The spatial resolution of X-ray micro radiography (alike any other imaging method) is the crucial parameter, which determines the span of possible applications. Its improvement remains a major experimental and data processing challenge. From theoretical principles the lower limit of the spatial resolution of a radiographic method is given by the size of the so-called point spread function (PSF) of the imaging system. Hence, details in the radiograms smaller than the size of the PSF cannot be theoretically distinguished. In the case of high geometrical magnification, the X-ray spot size becomes the most important factor for PSF determination. We have successfully measured in 2-D the X-ray spot shape and using deconvolution the spatial resolution has been improved for high-contrast object up to three-fold. We have successfully illustrated the approach for real biological and material structure investigations. Another possibility for resolution enhancement is sub-pixel micro-shifting of the X-ray detector used. Although this approach is quite common for optical imaging, it has not been fully investigated for X-ray imaging. In our measurements we have used sub-pixel micro-shifts of the Medipix2 detector. The method is successfully presented on real objects.

High resolution radiography of ambers with pixel detectors

Radiography serves as a powerful non-destructive technique for studying inner structure of biological samples and materials. In the last years X-ray imaging has taken advantage of the developments in instrumentation such as table-top micro-focus X-ray tubes and quantum counting pixel detectors. The imaging setups used for our measurements allow for the observation of tiny samples including fossils in amber. The main goal of the study was to apply microradiography as representative of non-destructive and non-invasive methods for imaging fossils in amber. Those fossils are generally not easy to visualize, especially their internal structures. We investigated a combination of sources and detectors: (a) an X-ray unit for mammography with tungsten anode, emissive spot of 100 μm and an amorphous selenium imager; (b) a micro-focus X-ray tube with tungsten anode, emissive spot of 5 μm and a flat panel imager; (c) a nano-focus X-ray tube with tungsten anode, with gauge of emissive spot of 1 μm and as imager the pixel semiconductor detector Medipix2. The study of fossils in amber can be for example not well visible because of the presence of organic detritus from various sources. The amber preserves various ancient biological objects which are fully or partly saturated with amber resin. These samples attenuate X-rays similarly, but the use of pixel detectors enables capturing these differences, without permanent destruction of the samples (cracking, slicing, etc.). Microradiographic studies are completed by the observation of amber fossils in scanning and transmission electron microscopes.

Microradiography of biological samples with Timepix

Microradiography is an imaging technique using X-rays in the study of internal structures of objects. This rapid and convenient imaging tool is based on differential X-ray attenuation by various tissues and structures within the biological sample. The non-absorbed radiation is detected with a suitable detector and creates a radiographic image. In order to detect the differential properties of X-rays passing through structures sample with different compositions, an adequate high-quality imaging detector is needed. We describe the recently developed radiographic apparatus, equipped with Timepix semiconductor pixel detector. The detector is used as an imager that counts individual photons of ionizing radiation, emitted by an X-ray tube FeinFocus with tungsten, copper or molybdenum anode. Thanks to the wide dynamic range, time over threshold mode — counter is used as Wilkinson type ADC allowing direct energy measurement in each pixel of Timepix detector and its high spatial resolution better than 1μm, the setup is particularly suitable for radiographic imaging of small biological samples. We are able to visualize some internal biological processes and also to resolve the details of insects (morphology) using different anodes. These anodes generate different energy spectra. These spectra depend on the anode material. The resulting radiographic images varies according to the selected anode. Tiny live insects are an ideal object for our studies.

In‐Vivo Real‐Time X‐ray μ‐Imaging

The technique of X‐ray transmission imaging is available for more than 100 years and it is still one of the fastest and easiest ways how to study the internal structure of living biological samples. The advances in semiconductor technology in last years make possible to fabricate new types of X‐ray detectors with direct conversion of interacting X‐ray photon to an electric signal. Especially semiconductor pixel detectors seem to be very promising. Compared to the film technique they bring single‐quantum and real‐time digital information about the studied object with high resolution, high sensitivity and broad dynamic range. These pixel detector‐based imaging stand promising as a new tool in the field of small animal imaging, for cancer research and for observation of dynamic processes inside organisms. These detectors open up for instance new possibilities for researchers to perform non‐invasive studies of tissue for mutations or pathologies and to monitor disease progression or response to therapy.