Joel Riendeau

System Architecture of the LabPET Small Animal PET Scanner

To address modern molecular imaging requirements, a digital positron emission tomography (PET) scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to LYSO/LGSO phoswich detectors, the scanner supports up to 4608 channels in a 16.2 cm diameter, 11.25 cm axial field of view with an isotropic ~ 1.2 mm FWHM intrinsic spatial resolution at the center of the field of view. Custom data acquisition boards preprocess and sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at an average sustained rate of up to 1250 raw counts per second per mm2 (10 000 cps/channel). Real time digital signal analysis also filters out events outside the pre-selected energy window with crystal granularity to eliminate Compton events and electronic noise. Retained events are then merged into a single stream through a real-time sorting tree, at which end prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a host computer. The LabPET features four data recording modes, giving the user the choice to retain data for research or to minimize file size for high coincidence count rate and imaging purposes. The electronic system also supports time synchronized data insertion for flags such as vital signs used in gated image reconstruction. Aside from data acquisition, hardware can generate live energy and discrimination spectra suitable for fast, automatic channel calibration.

Real Time Implementation of a Wiener Filter Based Crystal Identification Algorithm

The recently launched LabPETtrade, a small animal Avalanche PhotoDiode (APD)-based PET scanner with quasi-individual readout and massively parallel processing, makes it possible to acquire real-time information necessary for Positron Emission Tomography (PET) image reconstruction. Since each APD is coupled to an LYSO/LGSO phoswich scintillator pair, an efficient crystal identification algorithm must be developed to sustain real-time crystal feature extraction in high PET count rate. Furthermore, a less application specific algorithm is needed to easily expand its use to a large range of crystal materials. For these reasons, a new ultra-fast crystal identification algorithm based on a Wiener filter is proposed. This optimum filter instantly recovers crystal parameters by minimizing a linear cost function. A one-dimension projection based discrimination is used to identify the scintillating crystal. The algorithm achieves a discrimination rate of for low-energy X-ray photons ( keV) and up to for high energy 511 keV photopeak photons, with a maximum throughput of 10 Mevents/sec when implemented in a field programmable gate array.

Timing improvement by low-pass filtering and linear interpolation for the LabPET TM scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (≪65 MHz) for reducing power consumption. Timestamps must then be interpolated between samples to achieve adequate time resolution for coincidence detection of annihilation radiation. A low-pass filter based interpolation algorithm adding up to 31 samples between original samples was designed to improve both the energy and timing resolution of the LabPETTM scanner. An energy resolution refinement of ˜2 bits can be achieved with such a technique. The better estimation of triggering threshold leads to a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (5-50% of maximum value). With the trigger threshold set at 20%, coincidence time resolution of ˜5.0 ns for LYSO-LYSO and ˜9.6 ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA.

Timing Improvement by Low-Pass Filtering and Linear Interpolation for the LabPET Scanner

Digital processing for positron emission tomography (PET) scanners commonly relies on low frequency sampling (MHz) to reduce power consumption. Timestamps must then be interpolated between samples to achieve adequate time resolution for coincidence detection of annihilation radiation. A low-pass filter based interpolation algorithm adding up to 31 samples between original samples was designed to improve timing resolution of the LabPET scanner. A 2-bit refinement in the determination of the pulse maximum amplitude leads to a better estimation of the triggering threshold, which in turn enables a more accurate timestamp generation. Timestamp accuracy was investigated as a function of trigger level (15%-50% of maximum value). With the trigger threshold set at 20%, coincidence time resolution of ns for LYSO-LYSO and ns for LGSO-LGSO are obtained. A real time implementation of the algorithm was achieved in a Xilinx FPGA.

System Integration of the LabPET Small Animal PET Scanner

To address modern molecular imaging requirements, a digital positron emission tomography scanner for small animals has been developed at Universite de Sherbrooke. Based on individual readout of avalanche photodiodes (APD) coupled to a LYSO/LGSO phoswich array, the scanner supports up to 3072 channels in a 16.2 cm diameter, 7.5 cm axial field of view with an isotropic 1.2 mm FWHM intrinsic spatial resolution at the center of the FOV. Custom data acquisition boards sample APD signals at 45 MHz and compute in real time crystal identification, energy and timing information of detected events at rates of up to 1250 raw counts per second per mm2 (10k cps/channel). Real time digital signal analysis also filters out events outside the photopeak with crystal granularity to eliminate Compton events and electronic noise. Retained events are then merged into a single stream through a real-time sorting tree, at which end the prompt and delayed coincidences are extracted. A single Firewire link handles both control and data transfers with a computer. The LabPETtrade features four data recording modes, giving the user the choice to retain data for research or to minimize file size for high coincidence count rate and imaging purposes. The electronic system also supports time synchronized data insertion for flags such as vital signs used in gated image reconstruction. Aside from data acquisition, hardware can generate live energy and discrimination histograms suitable for fast, automatic channel calibration.

High Rate Photon Counting CT Using Parallel Digital PET Electronics

Recent developments in detectors and electronics enable both positron emission tomography (PET) and X-ray computed tomography (CT) data to be acquired concurrently using the same detection front-end for dual-modality PET/CT imaging. Moreover, it would potentially allow substantial reduction of cost and housing size, in addition to facilitating image fusion. However, the lower energy signals (~60 keV vs. 511 keV) and higher photon flux per pixel (>1 Mcps vs. 10 kcps) in CT relative to PET cause significant pile-up and deadtime in CT data acquired in photon counting mode. A digital signal processing method was developed and implemented to improve processing of detector signals sampled at low frequency (~45 MHz) in presence of pile-up. The method consists in digitally subtracting the detector impulse response at the output of the preamplifier to restore the signal baseline for more accurate energy estimation. When compared to a fixed threshold counting technique, the proposed method features better noise immunity, higher energy resolution and 50% higher rates measured at an estimated true rate of 2.75 Mcps, making CT integration within modern digital PET hardware feasible.

Real Time Implementation of a Wiener Filter Based Crystal Identification Algorithm for Photon Counting CT Imaging

The recently launched LabPETtrade, a small animal avalanche photodiode (APD)-based PET scanner with quasi-individual readout and massively parallel processing, makes it possible to acquire both computed tomography (CT) and positron emission tomography (PET) images using the same detection system. However, since each APD is coupled to an LYSO/LGSO phoswich scintillator pair, an efficient crystal identification algorithm must be developed to meet the stringent requirements of CT data acquisition in single photon counting mode. We propose a new ultra-fast crystal identification algorithm based on a Wiener filter. This filter instantly recovers crystal parameters by minimizing a linear cost function. A simple one-dimension projection based discrimination is used to identify the scintillating crystal. The algorithm achieves a discrimination rate of 88% for low-energy X-ray photons (~60 keV) at a high count rate >1 M events/sec/channel when implemented in a field programmable gate array.

Roadmap to fully-digital PET/CT scanners

The ever-increasing needs of molecular imaging now require significant upgrade of conventional PET and CT scanners. Upcoming research protocols ask for low doses, submillimeter resolution, high sensitivity and multimodality. Current scanner technologies are mainly based on analog ASICs having a long design-cycle which hinders rapid scanner improvements and can hardly keep up with the new requirements of biomedical research. With new high-speed processors and configurable electronics, combined with early digitization of the signals from detectors, digital signal processing can flexibly and concurrently deal with many of those requirements. The present paper highlights past, present and foreseen developments in PET/CT signal processing. In particular, different model fits, filtered interpolation and neural networks are compared for timestamping and pulse shape discrimination. Recursive (ARMAX, AR...) and non recursive (Wiener, Fast Fourier transforms, Wavelets...) filtering are compared for crystal identification. Advanced pile-up correction, baseline restoration and energy measurement in photon-counting CT are also discussed. Finally, new techniques dealing with realtime event processing for Compton-scatter LOR computation and alternate random estimation will be briefly introduced. Pros and cons of each method are discussed and the best methods identified for a roadmap to fully digital PET/CT scanning is presented.

Novel CT detector based on an inorganic scintillator working in photon-counting mode

Detectors working in photon counting mode offer an interesting alternative to the common charge integrating detectors for computed tomography (CT), because they can potentially measure the energy of every detected X-ray photons and achieve better image contrast sensitivity for a given dose. Unfortunately, most current X-ray detectors suffer from limited count rate capability, due either to a long charge migration time in semiconductor and gas detectors, or to a slow decay time in ceramic scintillators. To overcome these difficulties, we propose to use pixel detectors based on fast light emitting inorganic scintillators individually coupled to avalanche photodiodes with parallel, low-noise, fast digital processing electronics to provide real time single event detection and recording. The proposed detector was investigated with 2 × 2 × 10 mm3 Lu1.9Y0.1SiO5 (LYSO), a fast decay time (40 ns), heavy (7.19 g/cc) scintillator that is also suitable for coincidence detection of annihilation radiation (511 keV) in positron emission tomography (PET). Therefore, the detector characteristics make it a good candidate for implementation in a combined PET/CT dual-modality scanner. Although only coarse spectral analysis is possible in the X-ray energy range, it is demonstrated that appropriate CT images for anatomical localization can be obtained at very low dose in counting mode using a PET/CT simulator set up for small animal imaging. Data are reported on CT image resolution, noise, contrast and dose.

Comparison of analytical and algebraic 2D tomographic reconstruction approaches for irregularly sampled microCT data

APD-based detectors with individual channel readout were developed for multi-crystal applications and have been implemented for the detection of annihilation radiation in the LabPETTM micro-scanner. The use of these APD-based detectors in X-ray imaging is currently being assessed with a microCT demonstrator in order to later combine PET and CT in one apparatus. This paper is focused on the tomographic reconstruction of the X-ray transmission data acquired with this demonstrator. Two aspects of the acquisition geometry need to be carefully considered: the radius of the detector arc and the irregular sampling of the detector bins. A specific shift- variant filtered backprojection formula derived to account for the detector curvature is applied to equiangularly resampled projection data while the simultaneous algebraic reconstruction technique is applied to both resampled and original projections. Images of physical phantoms reconstructed from measured projections using the different methods are presented and compared.