I.F. Castro

X-ray imaging detector based on a position sensitive THCOBRA with resistive line

A new X-ray imaging detector based on a 2D-THCOBRA micropatterned structure using a simple position readout is proposed. It consists of a hybrid device that combines the prop- erties of a THGEM and a 2D-MHSP in a single structure, having two charge multiplication stages reaching the demanded gains for the use of charge division readout methods. For position determination, the new 10 10 cm 2 2D-THCOBRA structure uses two orthogonal resistive lines located at the end of the anode and top electrodes. The charge signal pulses collected at the end of each resistive line are digitized and processed in order to determine the center-of- gravity of the electron avalanche distribution. The detector uses a preamplification stage performed by a THGEM, followed by the 2D- THCOBRA, being operated in Ne/5%CH4 at atmospheric pressure. A characterization of the de- tector in terms of charge gain was made as a function of the voltage applied to the electrodes of the 2D-THCOBRA structure. The energy resolution of the system was also measured, envisaging Energy Weighting Technique (EWT) applications. The system characterization in terms of spatial resolution is presented together with imaging examples to evaluate its performance in X-ray imaging applications.

Impact of dark counts in low-light level silicon photomultiplier multi-readout applications

The Silicon Photomultiplier (SiPM) generates noise due to thermal excitation, producing dark counts. This may be a critical performance drawback of the SiPM for low light level applications, despite its otherwise promising features such as high internal gain and quantum efficiency, low power consumption and insensitivity to magnetic fields. The dark count rate is highly dependent on temperature and bias voltage, increasing with both. Dark count pulses are similar to single photon interactions and introduce complex challenges in low light level measurements. In applications where the pulse integration time is dependent on the decay time of a slow scintillation counter, the longer the signal integration the higher the probability of dark counts during the integration period, and this can be critical for detecting very low number of photons in that period. For higher light levels, the effects of dark counts can be reduced by setting an appropriate threshold. This work presents Monte Carlo simulation techniques to evaluate the impact of dark counts into position detection algorithms, in low light level imaging applications where multiple SiPMs are used to detect position of interaction.

Spectroscopic analysis of LYSO:Ce crystals

Rare earth orthosilicates are among the most widely used scintillator materials in the last decades. Particularly, lutetium-yttrium oxyorthosilicate (LYSO) is known to exhibit great potentialities in the field of radiation detectors for medical imaging. Consequently, an in-depth knowledge of the material properties is of utmost interest for the mentioned applications. In this work the spectroscopic properties of commercial cerium doped lutetium-yttrium oxyorthosilicate crystals (LYSO:Ce) were investigated by Raman spectroscopy, steady state photoluminescence, photoluminescence excitation and time resolved photoluminescence. Site selective excitation was used under steady state (325nm) and pulsed (266nm) conditions to separately investigate the temperature dependence of the 5d→4f Ce1 and Ce2 luminescence, allowing to establish the thermal quenching dependence of the Ce2 optical center. In the case of the Ce1 optical center, a luminescence quantum efficiency of 78% was obtained from 14K to room temperature with 266nm photon excitation.

Characterization of a small CsI(Na)-WSF-SiPM gamma camera prototype using 99mTc

A small field of view gamma camera is being developed, aiming for applications in scintimammography, sentinel lymph node detection or small animal imaging and research. The proposed wavelength-shifting fibre (WSF) gamma camera consists of two perpendicular sets of WSFs covering both sides of a CsI(Na) crystal, such that the fibres positioned at the bottom of the crystal provide the x coordinate and the ones on top the y coordinate of the gamma photon interaction point. The 2D position is given by highly sensitive photodetectors reading out each WSF and the energy information is provided by PMTs that cover the full detector area. This concept has the advantage of using N+N instead of N × N photodetectors to cover an identical imaging area, and is being applied using for the first time SiPMs. Previous studies carried out with 57Co have proved the feasibility of this concept using SiPM readout. In this work, we present experimental results from true 2D image acquisitions with a 10+10 SiPMs prototype, i.e. 10 × 10 mm2, using a parallel-hole collimator and different samples filled with 99mTc solution. The performance of the small prototype in these conditions is evaluated through the characterization of different gamma camera parameters, such as energy and spatial resolution. Ongoing advances towards a larger prototype of 100+100 SiPMs (10 × 10 cm2) are also presented.

Small prototype gamma camera based on wavelength-shifting fibres

We are studying and developing a small field of view gamma camera based on wavelength-shifting optical fibres coupled to both sides of an inorganic scintillation crystal and using for the light readout highly sensitive photon detectors, namely silicon photomultipliers (SiPMs) and high efficiency multi-anode photomultiplier tubes (MaPMTs). The coupling of the fibres in orthogonal directions allows obtaining 2D position information, while the energy signal is provided by a PMT. A first prototype laboratory system has been developed using a custom-made 50 × 50 × 3 mm3 CsI(Na) crystal with embedded 1 mm diameter fibres and reading out the light from several fibres in each direction, both with individual SiPMs and with a MaPMT. Proof-of-concept studies and results obtained with these systems using 57Co are presented and compared. The application of optical fibres combined with highly sensitive SiPMs or MaPMTs as light sensors in a compact gamma camera has the potential to improve the spatial resolution to the 1-2 mm FWHM level, thus improving the sensitivity of typical scintigraphy techniques and making such camera clinically useful. Results demonstrate the feasibility and imaging capability of the system using both types of photon detectors for imaging. In the case of SiPMs, a temperature cooling system is necessary to improve the SNR and consequently achieve a better imaging performance. The development of larger prototypes with 10 × 10 cm2 and 12 × 12 cm2 is under way, using 1 mm2 SiPMs and 64 anode PMTs, respectively.

Scintillating fiber optic dosimeters for breast and prostate brachytherapy

Brachytherapy is a radiotherapy modality where the radioactive material is placed close to the tumor, being a common treatment for skin, breast, gynecological and prostate cancers. These treatments can be of low-dose-rate, using isotopes with mean energy of 30 keV, or high-dose-rate, using isotopes such as 192Ir with a mean energy of 380 keV. Currently these treatments are performed in most cases without in-vivo dosimetry for quality control and quality assurance. We developed a dosimeter using small diameter probes that can be inserted into the patients body using standard brachytherapy needles. By performing real-time dosimetry in breast and prostate brachytherapy it will be possible to perform real-time dose correction when deviations from the treatment plan are observed. The dosimeter presented in this work was evaluated in-vitro. The studies consisted in the characterization of the dosimeter with 500 μm diameter sensitive probes (with a BCF-12 scintillating optical fiber) using an inhouse made gelatin breast phantom with a volume of 566 cm3. A breast brachytherapy treatment was simulated considering a tumor volume of 27 cm3 and a prescribed absolute dose of 5 Gy. The dose distribution was determined by the Inverse Planning Simulated Annealing (IPSA) optimization algorithm (ELEKTA). The dwell times estimated from the experimental measurements are in agreement with the prescribed dwell times, with relative error below 3%. The measured signal-to-noise ratio (SNR) including the stem-effect contribution is below 3%.

The EasyPET: A novel concept for an educational cost-effective positron emission 2D scanner

The easyPET concept proposed here, protected under a patent by the University of Aveiro, aims to realize a simple and affordable small dimension Positron Emission Tomography (PET) scanner. This innovative system is based on a single pair of detectors and a rotating mechanism with two degrees of freedom reproducing the functionalities of an entire PET ring. A 2D imaging prototype has been designed, commissioned and engineered, targeted to high level education for physics, engineering and nuclear medicine students. In this paper the performance of the prototype is reported, with a focus on the imaging capability and on the measurement of the uncertainty in the reconstruction of the source position. In addition, a detailed analysis is dedicated to the slice sensitivity and in particular to the effect of the energy threshold on the coincidence event selection.

Development of a micro PET system with improved spatial resolution through depth-of-interaction measurement

In small diameter positron emission tomography (PET) systems, the determination of the depth-of-interaction (DOI) of 511 keV gamma photons in scintillator crystals is of great importance, in order to achieve high DOI resolution with good uniformity within the entire field-of-view. In this work, we propose a new method for DOI determination, in which a single layer of LYSO crystals is read out on both ends through the use of silicon photomultipliers (SiPMs), but using wavelength-shifting fibers and a reduced number of SiPMs on one end. This design results in a simpler and less expensive readout when compared to the typical dual-ended readout method, which requires two photodetectors per crystal and corresponding readout electronics. GATE simulation of the system was carried out and experimental proof-of-concept studies were performed on a single detector cell (composed of two LYSO crystals operating in coincidence), to evaluate the amount of light detected on each side of the crystal and the achievable DOI resolution with this method, taking into account the attenuation of the light signal on the fiber side with crystal-SiPM distance. The feasibility of applying this new method in full detector rings for a small animal PET system is evaluated and discussed, considering different alternatives for position readout electronics.

Fiber optic dosimeter with silicon photomultipliers

A small dimension, real-time readout dosimeter is desirable for specific applications in medical physics as for example, dose measurement in prostate brachytherapy. This particular radiotherapy procedure consists in the permanent deposition of low energy, low-dose and low-dose rate small sized radioactive seeds. We developed a scintillating fiber optic based dosimeter suitable for in-vivo, real-time low dose and low dose rate measurements. Due to the low scintillation light produced in the scintillating fiber, a high sensitive and high gain light detector is required. The Silicon Photomultipliers are an interesting option that allowed us to obtain good results in our studies.