J Jakubek

Pixelman: a multi-platform data acquisition and processing software package for Medipix2, Timepix and Medipix3 detectors

The semiconductor pixel detectors Medipix2, Timepix and Medipix3 (256x256 square pixels, 55x55 μm each) are superior imaging devices in terms of spatial resolution, linearity and dynamic range. This makes them suitable for various applications such as radiography, neutronography, micro-tomography and X-ray dynamic defectoscopy. In order to control and manage such complex measurements a multi-platform software package for acquisition and data processing with a Java graphical user interface has been developed. The functionality of the original version of Pixelman package has been upgraded and extended to include the new medipix devices. The software package can be run on Microsoft Windows, Linux and Mac OS X operating systems. The architecture is very flexible and the functionality can be extended by plugins in C++, Java or combinations of both. The software package may be used as a distributed acquisition system using computers with different operating systems over a local network or the Internet.

Directional radiation detector

Many applications likehomeland security, radiation protection, control of fissile material proliferation and other require not only detection of radioactive materials, but also their localization. We are presenting a directional detector based on an array of semiconductor detectors capable to determine direction where the radioactive source is placed. Semiconductor single pad detectors are arranged into rows and separated by a shielding material. Selection of the detectors and shielding material depends on the type and energy of the radiation desired to monitor (i.e. X-rays, gammas or neutrons). Level of the signal, i.e. count rate, in each detector depends on the angle of the incoming radiation. Analysis of the count rate in each detector allows calculating angular position of the source. A series of simulations and evaluating measurements of the directional radiation detection principle is presented.

Large area pixel detector WIDEPIX with full area sensitivity composed of 100 Timepix assemblies with edgeless sensors

The superior properties of the single particle counting semiconductor pixel detectors in radiation imaging are well known. They are namely: very high dynamic range due to digital counting, absence of integration and read-out noise, high spatial resolution and energy sensitivity. The major disadvantage of current pixel devices preventing their broad exploitation has been their relatively small sensitive area of few cm2. This disadvantage is often solved using tiling method placing many detector units side by side forming a large matrix. The current tiling techniques require rather large gaps of few millimeters between tiles. These gaps stand as areas insensitive to radiation which is acceptable only in some applications such as diffraction imaging. However standard transmission radiography requires fully continuous area sensitivity. In this article we present the new large area device WIDEPIX composed of a matrix of 10 × 10 tiles of silicon pixel detectors Timepix (each of 256 × 256 pixels with pitch of 55 μm) having fully sensitive area of 14.3 × 14.3 cm2 without any gaps between the tiles. The device contains a total of 6.5 mega pixels. This achievement was reached thanks to new technology of edgeless semiconductor sensors together with precise alignment technique and multilevel architecture of readout electronics. The mechanical construction of the device is fully modular and scalable. This concept allows replacing any single detector tile which significantly improves production yield. The first results in the field of X-ray radiography and material sensitive X-ray radiography are presented in this article.

Hard x-ray phase contrast imaging using single absorption grating and hybrid semiconductor pixel detector.

A method for x-ray phase contrast imaging is introduced in which only one absorption grating and a microfocus x-ray source in a tabletop setup are used. The method is based on precise subpixel position determination of the x-ray pattern projected by the grating directly from the pattern image. For retrieval of the phase gradient and absorption image (both images obtained from one exposure), it is necessary to measure only one projection of the investigated object. Thus, our method is greatly simplified compared with the phase-stepping method and our method can significantly reduce the time-consuming scanning and possibly the unnecessary dose. Furthermore, the technique works with a fully polychromatic spectrum and gives ample variability in object magnification. Consequently, the approach can open the way to further widespread application of phase contrast imaging, e.g., into clinical practice. The experimental results on a simple testing object as well as on complex biological samples are presented.

Charge collection from proton and alpha particle tracks in silicon pixel detector devices

The lateral spread of charge carriers under the influence of the electric field in a pixellated silicon detector hit by a heavy charged particle, such as a proton or an alpha-particle, causes a sharing of the charge between the electrodes and many pixels have a signal. The results of the charge sharing effect measured in the Medipix2 and Timepix pixel detectors of 300 mum thicknesses is shown as a function of particle energy and applied bias voltage. A model describing the effects of funneling, plasma and diffusion on the charge collection and its sharing will be also presented. Using Timepix, it is possible to measure directly the quantity of charge deposited in each pixel within the cluster and to follow changes in charge collection as a function of collection time. This allows 3D-visualization of individual tracks of charged particles in silicon with Timepix.

BrachyView: Proof-of-principle of a novel in-body gamma camera for low dose-rate prostate brachytherapy

PURPOSEnThe conformity of the achieved dose distribution to the treatment plan strongly correlates with the accuracy of seed implantation in a prostate brachytherapy treatment procedure. Incorrect seed placement leads to both short and long term complications, including urethral and rectal toxicity. The authors present BrachyView, a novel concept of a fast intraoperative treatment planning system, to provide real-time seed placement information based on in-body gamma camera data. BrachyView combines the high spatial resolution of a pixellated silicon detector (Medipix2) with the volumetric information acquired by a transrectal ultrasound (TRUS). The two systems will be embedded in the same probe so as to provide anatomically correct seed positions for intraoperative planning and postimplant dosimetry. Dosimetric calculations are based on the TG-43 method using the real position of the seeds. The purpose of this paper is to demonstrate the feasibility of BrachyView using the Medipix2 pixel detector and a pinhole collimator to reconstruct the real-time 3D position of low dose-rate brachytherapy seeds in a phantom.nnnMETHODSnBrachyView incorporates three Medipix2 detectors coupled to a multipinhole collimator. Three-dimensionally triangulated seed positions from multiple planar images are used to determine the seed placement in a PMMA prostate phantom in real time. MATLAB codes were used to test the reconstruction method and to optimize the device geometry.nnnRESULTSnThe results presented in this paper show a 3D position reconstruction accuracy of the seed in the range of 0.5-3 mm for a 10-60 mm seed-to-detector distance interval (Z direction), respectively. The BrachyView system also demonstrates a spatial resolution of 0.25 mm in the XY plane for sources at 10 mm distance from Medipix2 detector plane, comparable to the theoretical value calculated for an equivalent gamma camera arrangement. The authors successfully demonstrated the capability of BrachyView for real-time imaging (using a 3 s data acquisition time) of different brachytherapy seed configurations (with an activity of 0.05 U) throughout a 60 × 60 × 60 mm(3) Perspex prostate phantom.nnnCONCLUSIONSnThe newly developed miniature gamma camera component of BrachyView, with its high spatial resolution and real time capability, allows accurate 3D localization of seeds in a prostate phantom. Combination of the gamma camera with TRUS in a single probe will complete the BrachyView system.

BrachyView, A novel inbody imaging system for HDR prostate brachytherapy: Design and Monte Carlo feasibility study

PURPOSEnHigh dose rate (HDR) brachytherapy is a form of radiation therapy for treating prostate cancer whereby a high activity radiation source is moved between predefined positions inside applicators inserted within the treatment volume. Accurate positioning of the source is essential in delivering the desired dose to the target area while avoiding radiation injury to the surrounding tissue. In this paper, HDR BrachyView, a novel inbody dosimetric imaging system for real time monitoring and verification of the radioactive seed position in HDR prostate brachytherapy treatment is introduced. The current prototype consists of a 15 × 60 mm(2) silicon pixel detector with a multipinhole tungsten collimator placed 6.5 mm above the detector. Seven identical pinholes allow full imaging coverage of the entire treatment volume. The combined pinhole and pixel sensor arrangement is geometrically designed to be able to resolve the three-dimensional location of the source. The probe may be rotated to keep the whole prostate within the transverse plane. The purpose of this paper is to demonstrate the efficacy of the design through computer simulation, and to estimate the accuracy in resolving the source position (in detector plane and in 3D space) as part of the feasibility study for the BrachyView project.nnnMETHODSnMonte Carlo simulations were performed using the GEANT4 radiation transport model, with a (192)Ir source placed in different locations within a prostate phantom. A geometrically accurate model of the detector and collimator were constructed. Simulations were conducted with a single pinhole to evaluate the pinhole design and the signal to background ratio obtained. Second, a pair of adjacent pinholes were simulated to evaluate the error in calculated source location.nnnRESULTSnSimulation results show that accurate determination of the true source position is easily obtainable within the typical one second source dwell time. The maximum error in the estimated projection position was found to be 0.95 mm in the imaging (detector) plane, resulting in a maximum source positioning estimation error of 1.48 mm.nnnCONCLUSIONSnHDR BrachyView is a feasible design for real-time source tracking in HDR prostate brachytherapy. It is capable of resolving the source position within a subsecond dwell time. In combination with anatomical information obtained from transrectal ultrasound imaging, HDR BrachyView adds a significant quality assurance capability to HDR brachytherapy treatment systems.

Single grating method for low dose 1-D and 2-D phase contrast X-ray imaging

X-ray phase contrast imaging (XPCI) using a single absorption grating and a hybrid semiconductor pixel detector is a newly introduced approach with great potential for application in medicine, biology and material research. In comparison with a conventional grating interferometer technique, which requires a multiple-exposure (phase-stepping) procedure, our method is greatly simplified, because both phase gradient and absorption images are obtained from just one exposure. Consequently, the approach can significantly reduce the time-consuming scanning and also possibly the unnecessary dose. Examples of application of the single-grating approach as an imaging tool for investigations in biology are presented. Particularly, we present the extension of our 1-D single grating method to a two-direction sensitive technique. The novel 2-D sensitive XPCI method is based on precise sub-pixel position determination of the X-ray pattern projected by the two-dimensional transmission grating directly from the pattern image. In a single exposure, phase gradient images in two perpendicular directions together with the conventional attenuation image are produced. Results of the proof-of-concept experiment are presented.

Small Dosimeter based on Timepix device for International Space Station

The radiation environment in space is different, more complex and more intense than on Earth. Conventional devices and detection methods used nowadays do not allow to discriminate single particle types and the energy of the single particles. The Timepix detector is a position sensitive pixelated detector developed at CERN in a frame of the Medipix collaboration that provides capability to visualize tracks and measure energy of single particles. This information can be used for sorting the particles into different categories. It is possible to distinguish light charged particles such as electrons or heavy charged particles such as ions. Moreover, the Linear Energy Transfer (LET) for charged particles can be determined. Each category is assigned a quality factor corresponding to the energy a particle would deposit in the human tissue. By summing the dose of all particles an estimate of the dose rate can be calculated. For space dosimetry purposes a miniature device with the Timepix detector and a custom made integrated USB based readout interface has been constructed. The entire device has dimensions of a USB flash memory stick. The whole compact device is connected to a control PC and is operated continuously. The PC runs a software that controls data acquisition, adjusts the acquisition time adaptively according to the particle rate, analyzes the particle tracks, evaluates the deposited energy and the LET and visualizes in a simple display the estimated dose rate. The performance of the device will be tested during a mission on International Space Station planned towards the beginning of year 2012.

Highly sensitive silicon detectors of thermal neutrons

Planar semiconductor diodes supplemented with a layer of an appropriate neutron converter such as 6LiF can be used for thermal neutron counting or imaging. Neutrons interacting in the converter generate alphas and tritons which enter the semiconductor and are detected there. However, simple planar devices suffer from limited detection efficiency which cannot reach more than about 5%. The limit in detection efficiency can be overcome by etching a 3D microstructure of trenches, pores or columns in the detector and filling it with the neutron converter. The overall neutron detection efficiency of such structure with pores was simulated. The results indicate an increase in the detection efficiency by factor of 6 in comparison with a standard planar neutron detector. Samples with different silicon column sizes were fabricated to study the electrical properties of 3D structures. The charge collection efficiency in silicon columns from 10 mum to 800 mum wide and 80 mum high was measured. Single pad detectors with pores were also fabricated and tested for thermal neutron detection. The samples have square pores of 20 mum wide, ~60 mum deep. The pore pitch is 70 mum. 6LiF was used as the neutron converter in all cases. Pulse height spectra of the filled samples irradiated by thermal neutrons were measured. The measurement proved functionality of such detectors and its usability for thermal neutron detection.