Jonathan Bouchard

Entropy generation in a parallel-plate active magnetic regenerator with insulator layers

This paper proposes a feasible solution to diminish conduction losses in active magnetic regenerators. Higher performances of these machines are linked to a lower thermal conductivity of the Magneto-Caloric Material (MCM) in the streamwise direction. The concept presented here involves the insertion of insulator layers along the length of a parallel-plate magnetic regenerator in order to reduce the heat conduction within the MCM. This idea is investigated by means of a 1D numerical model. This model solves not only the energy equations for the fluid and solid domains but also the magnetic circuit that conforms the experimental setup of reference. In conclusion, the addition of insulator layers within the MCM increases the temperature span, cooling load, and coefficient of performance by a combination of lower heat conduction losses and an increment of the global Magneto-Caloric Effect. The generated entropy by solid conduction, fluid convection, and conduction and viscous losses are calculated to help und...

Exergy Analysis of a Parallel-Plate Active Magnetic Regenerator with Nanofluids

This paper analyzes the energetic and exergy performance of an active magnetic regenerative refrigerator using water-based Al2O3 nanofluids as heat transfer fluids. A 1D numerical model has been extensively used to quantify the exergy performance of a system composed of a parallel-plate regenerator, magnetic source, pump, heat exchangers and control valves. Al2O3-water based nanofluids are tested thanks to CoolProp library, accounting for temperature-dependent properties, and appropriate correlations. The results are discussed in terms of the coefficient of performance, the exergy efficiency, and the cooling power as a function of the nanoparticle volume fraction and blowing time for a given geometrical configuration. It is shown that while the heat transfer between the fluid and solid is enhanced, it is accompanied by smaller temperature gradients within the fluid and larger pressure drops when increasing the nanoparticle concentration. It leads in all configurations to lower performance compared to the base case with pure liquid water.

Performance characterization of a dual-threshold time-over-threshold APD-based detector front-end module for PET imaging

The LabPET II front-end module is an avalanche photodiode (APD) based pixelated detector designed to achieve submillimetric spatial resolution in pre-clinical Positron Emission Tomography (PET). This technology is also based on Time-over-Threshold (ToT) signal processing and designed to be used as a generic platform for ultra-high resolution PET imaging of small and medium-size animals. The basic building block uses a 4×8 array of 1.12×1.12×12 mm3 Lu1.9Y0.1SiO5:Ce (LYSO) scintillator pixels with one-to-one coupling to a 4×8 pixelated APD array mounted on a ceramic carrier. Four of these detectors are mounted on a PCB with two 64-channel ASICs interfacing to two detector modules each. Signals from each APD pixel can be individually processed by dedicated dual-threshold ToT channels providing timing and energy data. Energy calibration was performed using gamma ray sources in the range 300-1275 keV to correct the non-linearity of the ToT signal and obtain energy spectra. Energy and timing performance of the complete front-end module was evaluated. Results confirm the functionality of the dual threshold ToT circuit implemented in the 64-channel ASIC, as well as the physical performance of the most recent LabPET II version of APD-based detectors for applications in high-resolution PET imaging.

Preliminary results of a low-cost 4-channel time-correlated single photon counting system for time-domain diffuse optical tomography

Time-domain diffuse optical tomography (TD-DOT) provides information-rich data that have not yet been fully exploited for image reconstruction, notably to increase imaging spatial resolution. Current TD-DOT scanners suffer from a very low sensitivity owing to their small number of detection channels. This leads to excessively long acquisition times for in vivo imaging. To obtain a higher number of detection channels, thus increasing detection density, a low-cost time-correlated single photon counting (TCSPC) system dedicated to TD-DOT imaging was designed and developed, resorting solely to off-the-shelf electronic components to reduce costs, in distinction to custom application-specific integrated circuit (ASIC) solutions. It features 4 input channels with a 13.02 ps bin width and a 18.1 ps FWHM accuracy throughout a measurement dynamic range of 12.5 ns. Each channel includes a leading-edge discriminator, with a programmable threshold, for direct interfacing with off-the-shelf photodetector modules. A software-programmable delay line was added to the channel signal path to compensate for undesired propagation delays. The system also supports a virtually unlimited number of TCSPC channels using a daisy-chain configuration through an onboard Ethernet switch.

A Low-Cost Time-Correlated Single Photon Counting System for Multiview Time-Domain Diffuse Optical Tomography

Diffuse optical tomography resorting to time-domain measurements (TD-DOT) provides information-rich data that have not yet been fully exploited for image reconstruction, notably to increase imaging spatial resolution. Current TD-DOT scanners suffer from a very low sensitivity owing to their small number of detection channels. This leads to excessively long acquisition times for in vivo imaging. To obtain a higher number of detection channels, thus increasing detection density, a low-cost time-correlated single photon counting (TCSPC) system dedicated to TD-DOT was designed, resorting solely to off-the-shelf electronic components to reduce costs, in distinction to custom application-specific integrated circuit solutions. The system features four input channels, each of them including a leading-edge discriminator for direct interfacing with off-the-shelf photodetector modules and a programmable delay line to compensate for undesired propagation delays, thus also avoiding manual adjustment of cable lengths. The system supports a large number of TCSPC channels using a daisy-chain configuration through an onboard Ethernet switch. It also features a 13.02 ps bin width with a 12.5 ns dynamic range. Results show excellent linearity, with a

Preliminary results of an embedded timing probe for calibrating PET scanner

\sigma _{\mathrm{ DNL}}

Poster - 01: LabPET II Pixelated APD-Based PET Scanner for High-Resolution Preclinical Imaging

of 1.7% least significant bit (LSB), peak differential nonlinearity of 5% LSB, and peak integral nonlinearity of 10% LSB and an average accuracy of 19 ps full-width at half-maximum (FWHM) throughout two separate four-channel systems. The system was tested in a TD-DOT acquisition setup, and 37 ps FWHM instrument response functions were obtained using a single-photon avalanche diode (SPAD) detector. Diffuse intrinsic measurements were also acquired and compared with a reference TCSPC system showing similar results.

Model of a porous regenerator used for magnetic refrigeration at room temperature

With the aim of improving the CNR of the LabPET II scanner, a method to correct the channel-to-channel coincidence time difference has been developed. The LabPET II, an avalanche photodiode-based PET scanner, features up to 55 000 channels which benefit from precise timing alignment. The correction process is fully automated and embedded in the scanner hardware and firmware. It uses a timing probe designed to react with the excess kinetic energy of a positron, enabling an absolute time reference of the positrons emission inside the probe. The time measurement is performed by a 312.5 ps time-to-digital converter (TDC) implemented in the LabPET II coincidence unit FPGA featuring a differential nonlinearity of −0.23 LSB peak and an integral nonlinearity of 0.14 LSB peak. The system computes probe-to-channel coincidences in order to obtain the absolute time difference between each channel of the scanner. The time correction is then applied to each channel to align all coincidence spectra, enhancing time resolution, thus improving CNR. Finally, these corrections are applied in real-time to each event during a typical PET acquisition.

A fully automated and scalable timing probe-based method for time alignment of the LabPET II scanners

Purpose: LabPET II is a new generation APD-based PET scanner designed to achieve sub-mm spatial resolution using truly pixelated detectors and highly integrated parallel front-end processing electronics. Methods: The basic element uses a 4×8 array of 1.12×1.12 mm2 Lu1.9Y0.1SiO5:Ce (LYSO) scintillator pixels with one-to-one coupling to a 4×8 pixelated monolithic APD array mounted on a ceramic carrier. Four detector arrays are mounted on a daughter board carrying two flip-chip, 64-channel, mixed-signal, application-specific integrated circuits (ASIC) on the backside interfacing to two detector arrays each. Fully parallel signal processing was implemented in silico by encoding time and energy information using a dual-threshold Time-over-Threshold (ToT) scheme. The self-contained 128-channel detector module was designed as a generic component for ultra-high resolution PET imaging of small to medium-size animals. Results: Energy and timing performance were optimized by carefully setting ToT thresholds to minimize the noise/slope ratio. ToT spectra clearly show resolved 511 keV photopeak and Compton edge with ToT resolution well below 10%. After correction for nonlinear ToT response, energy resolution is typically 24±2% FWHM. Coincidence time resolution between opposing 128-channel modules is below 4 ns FWHM. Initial imaging results demonstrate that 0.8 mm hot spots of a Derenzo phantom can be resolved. Conclusion: A new generation PET scanner featuring truly pixelated detectors was developed and shown to achieve a spatial resolution approaching the physical limit of PET. Future plans are to integrate a small-bore dedicated mouse version of the scanner within a PET/CT platform.