Jan Visser

Construction and test of an X-ray CT setup for material resolved 3D imaging with Medipix based detectors

A prototype computerized tomography (CT) setup has been recently built at Nikhef in order to exploit the material resolved capabilities of Medipix based detectors in X-ray imaging. The CT scanner contains a Hamamatsu 90 kVp microfocus X-ray tube and an entirely remotely controllable sample alignment system. The complete setup is fully integrated with the detector operation software. Moreover the 120 frames/s RelaxD readout system [1] allows real time X-ray imaging of fast moving samples. In this work, the description of the setup is given and the first results obtained with Medipix2 [2] and Timepix [3] detectors are presented. They concern detector calibration with fluorescence lines, CT reconstruction of small biological and non-biological samples and material resolved 3D micro-imaging [4].

Proton Radiography With Timepix Based Time Projection Chambers

The development of a proton radiography system to improve the imaging of patients in proton beam therapy is described. The system comprises gridpix based time projection chambers, which are based on the Timepix chip designed by the Medipix collaboration, for tracking the protons. This type of detector was chosen to have minimal impact on the actual determination of the proton tracks by the tracking detectors. To determine the residual energy of the protons, a BaF 2 crystal with a photomultiplier tube is used. We present data taken in a feasibility experiment with phantoms that represent tissue equivalent materials found in the human body. The obtained experimental results show a good agreement with the performed simulations.

Coincidence velocity map imaging using Tpx3Cam, a time stamping optical camera with 1.5 ns timing resolution

We demonstrate a coincidence velocity map imaging apparatus equipped with a novel time-stamping fast optical camera, Tpx3Cam, whose high sensitivity and nanosecond timing resolution allow for simultaneous position and time-of-flight detection. This single detector design is simple, flexible, and capable of highly differential measurements. We show detailed characterization of the camera and its application in strong field ionization experiments.

Measurement of the energy response function of a silicon pixel detector readout by a Timepix chip using synchrotron radiation

In view of applications in X-ray spectral computed tomography, we determine the energy response function of a 300 μm thick silicon pixel detector, read out by a Timepix ASIC, for a wide range of energies, by using synchrotron radiation in the range 5-32.5 keV. We employ a simple analytical model of the charge transport in the sensor to fit the data and to parametrize the energy response function. This allows to interpolate the function between the energies at which the measurements were performed and to extrapolate it outside the experimentally accessible range. The response function thus parametrized is used to predict how an incoming X-ray spectrum will be distorted during the detection process in the sensor. The comparison of the calculation with measurements shows good agreement.

Combined X-ray CT and mass spectrometry for biomedical imaging applications

Imaging technologies play a key role in many branches of science, especially in biology and medicine. They provide an invaluable insight into both internal structure and processes within a broad range of samples. There are many techniques that allow one to obtain images of an object. Different techniques are based on the analysis of a particular sample property by means of a dedicated imaging system, and as such, each imaging modality provides the researcher with different information. The use of multimodal imaging (imaging with several different techniques) can provide additional and complementary information that is not possible when employing a single imaging technique alone. In this study, we present for the first time a multi-modal imaging technique where X-ray computerized tomography (CT) is combined with mass spectrometry imaging (MSI). While X-ray CT provides 3-dimensional information regarding the internal structure of the sample based on X-ray absorption coefficients, MSI of thin sections acquired from the same sample allows the spatial distribution of many elements/molecules, each distinguished by its unique mass-to-charge ratio (m/z), to be determined within a single measurement and with a spatial resolution as low as 1 μm or even less. The aim of the work is to demonstrate how molecular information from MSI can be spatially correlated with 3D structural information acquired from X-ray CT. In these experiments, frozen samples are imaged in an X-ray CT setup using Medipix based detectors equipped with a CO2 cooled sample holder. Single projections are pre-processed before tomographic reconstruction using a signal-to-thickness calibration. In the second step, the object is sliced into thin sections (circa 20 μm) that are then imaged using both matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) and secondary ion (SIMS) mass spectrometry, where the spatial distribution of specific molecules within the sample is determined. The combination of two vastly different imaging approaches provides complementary information (i.e., anatomical and molecular distributions) that allows the correlation of distinct structural features with specific molecules distributions leading to unique insights in disease development.

Active-Edge planar silicon sensors for large-area pixel detectors

We study the influence of active edges on the response of edge pixels by comparing simulations of the electrostatic-potential distribution to position-defined measurements on the energy deposition. A laser setup was used to measure the edge-pixel response function and shows the sensitive edge is only about 2 µm from the physical edge. 3D reconstruction of tracks from high-energy pions and muons, produced at the SPS H6 test beam facility at CERN, enabled to relate the energy deposition at edge pixels to the particles interaction depth. A clear correlation is observed between the simulated electric-field distortion and the reconstructed interaction-depth dependent effective size.

X-ray based methods for 3D characterization of charge collection and homogeneity of sensors with the use of Timepix chip

Timepix is a universal readout chip for pixel detectors which can be connected to various semiconductor sensors. The device has a 256×256 matrix of square pixels with a pitch of 55 µm. Every single pixel is able to measure the collected charge. The traditional material used for sensors is mono-crystalline silicon (Si). However, other materials such as gallium arsenide (GaAs) or cadmium telluride (CdTe) are applicable as well. To describe the properties of the sensors it is important to probe and evaluate the charge collection efficiency and its homogeneity across sensor area (or if possible even in its volume).

The influence of edge effects on the detection properties of cadmium telluride

Driven by the demand of various applications for a detection area that is larger than the active area of a single detector module, we explore the possibility to realise a large-area detector by a seamless tessellation of multiple detectors. This requires sensors with a minimum amount of dead area at the edge. In order to be able to reduce this area, edge effects must be understood and avoided or mitigated. In this paper, we report on first tests that are performed on diamond-blade diced slim-edge pieces of cadmium telluride with a last-pixel-to-edge distance of only 65 µm. The results indicate that the edge-pixel response is not significantly affected with respect to the leakage current and the charge collection efficiency. First measurements towards a quantification of the detective quantum efficiency have been made on edge pixels by determining the pixel response function and the noise power spectrum.

Proton energy and scattering angle radiographs to improve proton treatment planning: a Monte Carlo study

The novel proton radiography imaging technique has a large potential to be used in direct measurement of the proton energy loss (proton stopping power, PSP) in various tissues in the patient. The uncertainty of PSPs, currently obtained from translation of X-ray Computed Tomography (xCT) images, should be minimized from 3–5% or higher to less than 1%, to make the treatment plan with proton beams more accurate, and thereby better treatment for the patient. With Geant4 we simulated a proton radiography detection system with two position-sensitive and residual energy detectors. A complex phantom filled with various materials (including tissue surrogates), was placed between the position sensitive detectors. The phantom was irradiated with 150 MeV protons and the energy loss radiograph and scattering angles were studied. Protons passing through different materials in the phantom lose energy, which was used to create a radiography image of the phantom. The multiple Coulomb scattering of a proton traversing different materials causes blurring of the image. To improve image quality and material identification in the phantom, we selected protons with small scattering angles. A good quality proton radiography image, in which various materials can be recognized accurately, and in combination with xCT can lead to more accurate relative stopping powers predictions.

Proton Radiography to Improve Proton Radiotherapy: Simulation Study at Different Proton Beam Energies

To improve the quality of cancer treatment with protons, a translation of X-ray Computed Tomography (CT) images into a map of the proton stopping powers needs to be more accurate. Proton stopping powers determined from CT images have systematic uncertainties in the calculated proton range in a patient of typically 3–4% and even up to 10% in a region containing bone. As a consequence, part of a tumor may receive no dose, or a very high dose can be delivered in healthy tissues and organs at risks (e.g. brain stem). A transmission radiograph of high-energy protons measuring proton stopping powers directly will allow to reduce these uncertainties, and thus improve the quality of treatment. The best way to obtain a sufficiently accurate radiograph is by tracking individual protons traversing the phantom (patient). In our simulations, we have used an ideal position sensitive detectors measuring a single proton before and after a phantom, while the residual energy of a proton was detected by a BaF2 crystal. To obtain transmission radiographs, different phantom materials have been irradiated with a 3 × 3 cm2 scattered proton beam, with various beam energies. The simulations were done using the Geant4 simulation package. In this study, we focus on the simulations of the energy loss radiographs for various proton beam energies that are clinically available in proton radiotherapy.