Els Koffeman

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.

Edgeless planar semiconductor sensors for a Medipix3-based radiography detector

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.

Study of Charge Diffusion in a Silicon Detector Using an Energy Sensitive Pixel Readout Chip

A 300 μm thick thin p-on-n silicon sensor was connected to an energy sensitive pixel readout ASIC and exposed to a beam of highly energetic charged particles. By exploiting the spectral information and the fine segmentation of the detector, we were able to measure the evolution of the transverse profile of the charge carriers cloud in the sensor as a function of the drift distance from the point of generation. The result does not rely on model assumptions or electric field calculations. The data are also used to validate numerical simulations and to predict the detector spectral response to an X-ray fluorescence spectrum for applications in X-ray imaging.

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.

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.

The optimal balance between quality and efficiency in proton radiography imaging technique at various proton beam energies: A Monte Carlo study

Proton radiography is a novel imaging modality that allows direct measurement of the proton energy loss in various tissues. Currently, due to the conversion of so-called Hounsfield units from X-ray Computed Tomography (CT) into relative proton stopping powers (RPSP), the uncertainties of RPSP are 3-5% or higher, which need to be minimized down to 1% to make the proton treatment plans more accurate. In this work, we simulated a proton radiography system, with position-sensitive detectors (PSDs) and a residual energy detector (RED). The simulations were built using Geant4, a Monte Carlo simulation toolkit. A phantom, consisting of several materials was placed between the PSDs of various Water Equivalent Thicknesses (WET), corresponding to an ideal detector, a gaseous detector, silicon and plastic scintillator detectors. The energy loss radiograph and the scattering angle distributions of the protons were studied for proton beam energies of 150MeV, 190MeV and 230MeV. To improve the image quality deteriorated by the multiple Coulomb scattering (MCS), protons with small angles were selected. Two ways of calculating a scattering angle were considered using the protons direction and position. A scattering angle cut of 8.7mrad was applied giving an optimal balance between quality and efficiency of the radiographic image. For the three proton beam energies, the number of protons used in image reconstruction with the direction method was half the number of protons kept using the position method.

Prospects for spectral CT with Medipix detectors

In the development of X-ray Computed Tomography (CT) in medical imaging, one is working to implement spectral information. While keeping the dose level the same, or even lower, than in conventional systems, spectral CT offers the possibility to measure energy dependent features of different tissues that will allow the extraction of additional information about the patient, eventually leading to real color CT. Spectral CT can be achieved through the application of energy sensitive pixel detectors, such as Medipix-based semiconductor devices and by the implementation of reconstruction algorithms where the energy information is taken into account. In this paper, we present the latest results of our work on spectral CT with Medipix detectors and specifically on detector characterization and the development of algorithms that include energy information.