L. A. Hamel’s research while affiliated with Université de Montréal and other places

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Publications (116)


Fig. 1 In this 3D rendering, the catheter 1 passes close to the gamma detection modules (2a, 2b shown without shielding) then through the peristaltic pump 3. A Y-connector allows to control the flow towards a waste container or a carousel holding evacuated tubes 4. Acquisition boards are also shown 5. For this work, a configuration with a single detector module 2a was evaluated, shown here with its transparent-rendered shielding 2a. The actual tungsten shielding is shown in Fig. 3 
Fig. 2 a An exploded view of the detector and preamplifier assembly. The CZT crystal is shown in green while the preamp board with the Mini DisplayPort connector is on top. The 3D-printed white plastic case is also shown and is used to isolate the high-voltage cathode and physically protect the CZT crystal. b A 3D rendering of the detection module showing one layer of the PCB and conductive tracks used to make a virtual coplanar detector from a pixelated detector 
Fig. 3 a Technical drawing in millimetre of the shielding container and b the actual shielding made of ABS plastic. The container was filled with 97% pure tungsten cubes. The detector assembly fits in the blue zone while the red zone shows the catheter space. The bottom of the blue zone (detector module enclosure) was shielded by 1 cm of tungsten (not shown) while the opening was not shielded to allow cables to exit (see Fig. 7) 
Fig. 4 Signal workflow and connection diagram. The FPGA listens to the USB port and executes commands sent by the GUI 
Fig. 5 Screenshot of the GUI, showing the various controls and visualisation elements and a typical 22 Na acquisition 

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A CZT-based blood counter for quantitative molecular imaging
  • Article
  • Full-text available

June 2017

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225 Reads

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4 Citations

EJNMMI Physics

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Andrea Frezza

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Background Robust quantitative analysis in positron emission tomography (PET) and in single-photon emission computed tomography (SPECT) typically requires the time-activity curve as an input function for the pharmacokinetic modeling of tracer uptake. For this purpose, a new automated tool for the determination of blood activity as a function of time is presented. The device, compact enough to be used on the patient bed, relies on a peristaltic pump for continuous blood withdrawal at user-defined rates. Gamma detection is based on a 20 × 20 × 15 mm³ cadmium zinc telluride (CZT) detector, read by custom-made electronics and a field-programmable gate array-based signal processing unit. A graphical user interface (GUI) allows users to select parameters and easily perform acquisitions. Results This paper presents the overall design of the device as well as the results related to the detector performance in terms of stability, sensitivity and energy resolution. Results from a patient study are also reported. The device achieved a sensitivity of 7.1 cps/(kBq/mL) and a minimum detectable activity of 2.5 kBq/ml for ¹⁸F. The gamma counter also demonstrated an excellent stability with a deviation in count rates inferior to 0.05% over 6 h. An energy resolution of 8% was achieved at 662 keV. Conclusions The patient study was conclusive and demonstrated that the compact gamma blood counter developed has the sensitivity and the stability required to conduct quantitative molecular imaging studies in PET and SPECT.

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Conception and characterization of a virtual coplanar grid for a 11×11 pixelated CZT detector

March 2017

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67 Reads

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5 Citations

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Due to the low mobility of holes in CZT, commercially available detectors with a relatively large volume typically use a pixelated anode structure. They are mostly used in imaging applications and often require a dense electronic readout scheme. These large volume detectors are also interesting for high-sensitivity applications and a CZT-based blood gamma counter was developed from a 20x20x15 mm³ crystal available commercially and having a 11x11 pixelated readout scheme. A method is proposed here to reduce the number of channels required to use the crystal in a high-sensitivity counting application, dedicated to pharmacokinetic modelling in PET and SPECT. Inspired by a classic coplanar anode, an implementation of a virtual coplanar grid was done by connecting the 121 pixels of the detector to form intercalated bands. The layout, the front-end electronics and the characterization of the detector in this 2-channel anode geometry is presented. The coefficients required to compensate for electron trapping in CZT were determined experimentally to improve the performance. The resulting virtual coplanar detector has an intrinsic efficiency of 34% and an energy resolution of 8% at 662 keV. The detector's response was linear between 80 keV and 1372 keV. This suggests that large CZT crystals offer an excellent alternative to scintillation detectors for some applications, especially those where high-sensitivity and compactness are required.


Table 1 Parameters 
Fig. 1 Typical use of the device in a PET study  
A CdZnTe-based automated Blood Counter for Quantitative Molecular Imaging

January 2015

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69 Reads

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2 Citations

IFMBE proceedings

Positron Emission Tomography (PET) and Single- Photon Emission Computed Tomography (SPECT) are well established molecular imaging modalities used in many fields of the biomedical sciences. They allow in vivo investigations of biological processes at the molecular level, and provide valuable information on the onset and progression of diseases. The images obtained are based on a measurable number of nuclear disintegrations and, as such, are inherently quantitative. However, the quantitative nature of these modalities is usually dismissed, largely because tools and methods dedicated to quantitative imaging are lacking. Accurate quantification in PET and SPECT typically requires frequent assessments of blood activity through indirect image-based estimations or direct sampling and measurements in a well-counter to obtain a time-activity curve that will feed pharmacokinetic models. These methods however can be inaccurate and error-prone. A new automated tool for the determination of blood activity as a function of time is presented. The device relies on a peristaltic pump for continuous blood withdrawal and can package blood samples in evacuated tubes at specific times for further analysis. The device uses a 20×20×15 mm3 Cadmium Zinc Telluride (CZT) detector, read by a custom-made preamplifier and a FPGA-based signal processing unit. A Graphical User Interface (GUI) offers a user-friendly interface with preselected parameters to perform an acquisition. This paper presents the overall design of the tool as well as preliminary results related to detector performance in terms of linearity, stability and sensitivity. The sensitivity is estimated at 0.9 cps/Bq/μL.



Calcul analytique du transport des charges dans le a-Si :H

February 2011

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16 Reads

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1 Citation

Multitrapping transport of carriers through exponential band tails is studied for carriers generated at an arbitrary position in the device in presence of a linearly varying electric field, thus generalizing the usual treatment of time of flight experiments. An analytical expression is found for the free carrier density distribution n(x,t) for values of the dispersion parameter 0 < α < 1. In the case α = 1/2, analytical solutions are given for the transient current I(t) and the induced charge Q(t). Comparison with previous calculations is made for the special case of uniform electric field and carriers initially generated at one interface.


Monte Carlo Simulation of Transient Currents in a-Si:H

January 2011

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13 Reads

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1 Citation

Materials Research Society symposia proceedings. Materials Research Society

Multi-trapping transient currents through pure exponential band tails are calculated by Monte Carlo simulations of unprecedented accuracy. The time and energy dependent carrier distributions, the current and the charge signals are obtained. The transit time tT and the dispersion parameters α1, and α2, before and after the transit time, are extracted from the simulated currents. Results are presented for the expected temperature dependence of α1 and α2 and of the drift mobility μD for electrons and holes in a-Si:H. At low temperature, α1 = α2 = T/To is found, but above T ≈ 0.6To, α1, bends downwards to saturate at 1 at high temperature while α2 bends upwards, clearly improving the agreement with the experimental data. The temperature dependence of the drift mobility μD is well reproduced for holes with a microscopic mobility of μo = 0.5 cm2V−1s−1. For electrons, the agreement is also excellent but the surprisingly low value of μo = 4 cm2V−1s−1 is found.


Modelling of Drift Mobility Experiments on a-Si:H

January 2011

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16 Reads

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1 Citation

Materials Research Society symposia proceedings. Materials Research Society

The authors present recent results of a study of the behavior of electronic carriers in a-Si:H, using the model of multiple trapping (MT) in an exponential density of states. in previous publications, using Monte Carlo simulations, they showed that the standard low field MT model gives reasonable agreement with experiment particularly if the Meyer-Neldel effect is included in the model. They report here on the results of including two other effects. First, they have included a simple model of field assisted detrapping, to take account of the effect of high fields. They obtain very good agreement with the results of measurements on both electrons and holes, from a number of laboratories. In addition, they show here that the validity of an effective temperature approach can be checked easily by comparison with experiment. Second, they have presented a simple model of rapid relaxation of trapped carriers. This model offers the possibility of removing the apparent inconsistency between these measurements, and other experiments.


Simulation of charge collection processes in semiconductor CdZnTe γ -ray detectors

July 2009

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201 Reads

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65 Citations

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

We present a Monte Carlo algorithm for the simulation of charge collection behavior of semiconductor γ-ray detectors. The model takes into account the electrical properties of the detectors, transport properties of the material, trapping induced by impurities, experimental setup characteristics and γ-ray–matter interaction processes. We demonstrate how to include electrostatic repulsion as a correction to diffusion coefficient for Gaussian-shaped charge distribution. Comparison of simulated and measured data for different γ-ray sources for a CdZnTe single-sided charge-sharing strip detector are shown to demonstrate the efficiency of the model even for small electrode-pitch detector simulation.


Generalized demonstration of Ramo's theorem with space charge and polarization effects

December 2008

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128 Reads

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45 Citations

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

Ramo's theorem provides a convenient way of simulating the currents induced through the electrodes of a radiation detector by the movement of charge carriers. The concept of weighting potential or of weighting field is the key ingredient in the calculation of the induced currents. A new demonstration of the theorem, based on energy balance, is provided that explicitly takes into account the effect of material polarization. It is shown that Ramo's theorem is valid in an arbitrary linear material and that polarization charges at interfaces between different materials or in the bulk of inhomogeneous materials must be included in calculating the weighting potentials. The use of our definition of weighting potential is also demonstrated for non-linear materials. The present proof is thus more generally valid than previous ones.


Simulation of picosecond domain time-of-flight experiments in aSi:H

December 2007

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18 Reads

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3 Citations

Journal of Non-Crystalline Solids

Using a model, developed earlier, of trap controlled conduction, which includes both the Meyer–Neldel rule and field assisted detrapping, we have simulated picosecond timescale drift mobility and drift velocity measurements in a-Si:H. This model is able to duplicate both the picosecond and nanosecond data, for all values of temperature and field, using one set of fixed parameters. We observe that under certain conditions, the drift mobility is field independent. Coherence between the picosecond and nanosecond results is shown.


Citations (46)


... Counter [18] β with argon methane gas 250 4 15.3 (rat) -No c Swisstrace [32] γ coinc. with glass fiber/PMT Non-dependent 3 6 -Yes MR-Compatible BS [15] γ coincidence with LSO/APD Non-dependent 23 6 -Yes ABSS [33] γ coincidence with LSO/APD Non-dependent 18.5 13.2 (human) -Yes Online blood sampler [17] γ coincidence with LYSO/PMT Non-dependent 7 33.4 (rat) -No c GSO detector assembly [13] γ coinc/single with GSO/PMT Non-dependent 6.2 5.3 (rat) 70 No c CZT-based blood counter [34] γ coinc/single CZT detector Non-dependent 12.5 56.9 (human) -No c PBS-101 [35] γ single with BGO/PMT Non-dependent 20/70 47.1 (human) -No c Pico-count flow-through d. [36] γ coincidence with BGO/PMT Non-dependent 6.9 110 (human) -No c a [37]. b The detection volume and catheter wall thickness are given for mice (mainly PE10 catheter) unless otherwise noted. ...

Reference:

The ultra high sensitivity blood counter: a compact, MRI-compatible, radioactivity counter for pharmacokinetic studies in µL volumes
A CZT-based blood counter for quantitative molecular imaging

EJNMMI Physics

... This method prohibits the processing of preamplifier pulses corresponding to holes by comparing pulse height to its rise time. Modern unipolar detectors, i.e., single charge carrier sensors (Frisch and coplanar grids, strip, pixel, hemispherical, and multiple electrodes detectors) [6,12], use dominant electron collection of the charge carriers as well. Previous studies [12] showed that pixelated (CdZn)Te detector can also benefit from ...

Conception and characterization of a virtual coplanar grid for a 11×11 pixelated CZT detector
  • Citing Article
  • March 2017

Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment

... For the weighting potential ( ϕ k ) a transition from unity to zero must occur in the gaps between the selected anode and its neighbours. A boundary condition involving the gradient and the surface normal is commonly applied [31,39,43,44], represented by alternative B1. The inverse-distance weighting [45] method (alternative B2) has not been used previously, and was introduced to obtain way to parametrise and adjust the steepness of the lateral sides of the weighting potential and CIE. ...

Weighting potentials in CdZnTe γ-ray detectors with segmented electrodes
  • Citing Article
  • January 1998

... Studies with the BDS also confirmed that by altering the droplet formulation, different thresholds were achievable (17,19) . A method for the fitting of the neutron response function is given in ref. (20). Additional ground-based accelerator studies were further carried out to better understand the high-energy response characteristics of the detector for space application [see text below and the section International Space Station (ISS) missions]. ...

Neutron response functions for superheated droplet detectors
  • Citing Article
  • January 1998

... In order to build a robust, sensitive and compact device, the cadmium zinc telluride (CZT) detector technology was used in this work [14,15]. CZT detectors are known for their high energy resolution and stable operation (no temperature dependence). ...

A CdZnTe-based automated Blood Counter for Quantitative Molecular Imaging

IFMBE proceedings

... Incident photons on detectors equipped with multiple electrodes yield signals of opposing polarities at adjacent electrodes. (13)(14)(15) This phenomenon is associated with situations where the incident location of the photon lies near the electrode boundary. The mobilities of the electrons and holes in the detector differ significantly. ...

Single-sided CZT strip detectors
  • Citing Conference Paper
  • September 2004

Proceedings of SPIE - The International Society for Optical Engineering

... This performance is similar to that recorded in the initial testing of this detector. 10 The response to 662 keV photons from a 137 Cs source of 10 mm thick detector UNH-EV-11 is shown in Figure 10. The effect of electron trapping across the 10 mm thickness is evident in the photopeak pulse height as measured for different interaction depths (Z). ...

Energy and position resolution of a CdZnTe gamma-ray detector with orthogonal coplanar anodes
  • Citing Article
  • October 1999

Proceedings of SPIE - The International Society for Optical Engineering

... The backscattered ions, in a direction of 165 • with respect to the angle of incidence, are collected with a detector (20 keV of resolution) fixed at 12 cm from the sample. Using a computer program, namely the SIMNRA software [19][20][21], we treated the recorded RBS spectra. ...

Pulse height defects for 16O, 35Cl and 81Br ions in silicon surface barrier detectors
  • Citing Article
  • January 1990

Nuclear Instruments and Methods in Physics Research Section B Beam Interactions with Materials and Atoms

... The quadratic sum of these two effects can be calculated to estimate an upper limit on the maximal spread L max of a charge cloud [18,19] assuming a maximal drift length of 15 mm for near-cathode events, ...

Further studies of single-sided charge-sharing CZT strip detectors

Proceedings of SPIE - The International Society for Optical Engineering