Frances W. Y. Lau

Analog signal multiplexing for PSAPD-based PET detectors: simulation and experimental validation.

A 1 mm(3) resolution clinical positron emission tomography (PET) system employing 4608 position-sensitive avalanche photodiodes (PSAPDs) is under development. This paper describes a detector multiplexing technique that simplifies the readout electronics and reduces the density of the circuit board design. The multiplexing scheme was validated using a simulation framework that models the PSAPDs and front-end multiplexing circuits to predict the signal-to-noise ratio and flood histogram performance. Two independent experimental setups measured the energy resolution, time resolution, crystal identification ability and count rate both with and without multiplexing. With multiplexing, there was no significant degradation in energy resolution, time resolution and count rate. There was a relative 6.9 ± 1.0% and 9.4 ± 1.0% degradation in the figure of merit that characterizes the crystal identification ability observed in the measured and simulated ceramic-mounted PSAPD module flood histograms, respectively.

Convex Optimization of Coincidence Time Resolution for a High-Resolution PET System

We are developing a dual panel breast-dedicated positron emission tomography (PET) system using LSO scintillators coupled to position sensitive avalanche photodiodes (PSAPD). The charge output is amplified and read using NOVA RENA-3 ASICs. This paper shows that the coincidence timing resolution of the RENA-3 ASIC can be improved using certain list-mode calibrations. We treat the calibration problem as a convex optimization problem and use the RENA-3s analog-based timing system to correct the measured data for time dispersion effects from correlated noise, PSAPD signal delays and varying signal amplitudes. The direct solution to the optimization problem involves a matrix inversion that grows order (n3) with the number of parameters. An iterative method using single-coordinate descent to approximate the inversion grows order (n). The inversion does not need to run to convergence, since any gains at high iteration number will be low compared to noise amplification. The system calibration method is demonstrated with measured pulser data as well as with two LSO-PSAPD detectors in electronic coincidence. After applying the algorithm, the 511 keV photopeak paired coincidence time resolution from the LSO-PSAPD detectors under study improved by 57%, from the raw value of 16.3 ± 0.07 ns full-width at half-maximum (FWHM) to 6.92 ±0.02 ns FWHM (11.52 ±0.05 ns to 4.89 ± 0.02 ns for unpaired photons).

Fortifying analog models with equivalence checking and coverage analysis

As analog and digital circuits have become more intertwined, we need to create a validation approach that handles both circuit types gracefully. This paper proposes a model-first approach, where one creates functional models of the analog blocks that will work in a HDL simulator, and then uses these models in the same way as HDL models are used for other standard cells: they are used in the full system validation, and the underlying implementations are validated to ensure they meet this specification. While creating functional models for the analog blocks might seem difficult, almost all analog blocks can be modeled as linear systems and we use this property to help create the required functional model.

1 mm 3 resolution breast-dedicated PET system

We are developing a 1 mm3 resolution breast-dedicated Positron Emission Tomography (PET) system in an effort to increase the role of PET in earlier stages of breast cancer management. The system consists of two 16 cm × 9 cm × 2 cm detector panels constructed using stacked layers of 8×8 arrays of 1 mm3 LSO scintillation crystals coupled to Position Sensitive Avalanche Photodiodes (PSAPDs). Preliminary detector characterization indicates that analog multiplexed PSAPD signals coupled to ASIC readout electronics are able to resolve the 8×8 arrays of LSO crystals with an average peak-to-valley ratio of about 14, an energy resolution of 14.4% ± 0.8% at FWHM for the 511 keV photo-peak, and a paired coincidence photon time resolution of 7.3 ± 0.2 ns FWHM using the ASIC (5.2 ± 0.1 ns FHWM unpaired photon time resolution). Each 1 cm2 area PSAPD chip under bias generates 2 to 4 mW of power, and thus thermal regulation is required. A finite volume simulation of the detectors with thermal regulation features incorporated in the panels indicates that the maximum temperature variation across the thermally regulated imaging head is 4 degrees Celsius.

Color-Based multiple agent tracking for wireless image sensor networks

This paper presents an implementation of a color-based multiple agent tracking algorithm targeted for wireless image sensor networks. The proposed technique is based on employing lightweight algorithms and low-bandwidth data communication between multiple network nodes to track the path of autonomous agents moving across the fields of view (FOV) of the sensors. Segmentation techniques are applied to find the agents within the FOV, and a color histogram is constructed using the hue values of the pixels corresponding to agents. This histogram is used as a means of identification within the network. As such, the algorithm is able to reliably track multi-colored agents of irregular shapes and sizes and can resolve identities after collisions. The proposed algorithm has low computational requirements and its complexity scales linearly with the size of the network, so it is feasible in low-power, large-scale wireless sensor networks.

Physical effects of mechanical design parameters on photon sensitivity and spatial resolution performance of a breast-dedicated PET system

PURPOSE This study aims to address design considerations of a high resolution, high sensitivity positron emission tomography scanner dedicated to breast imaging. METHODS The methodology uses a detailed Monte Carlo model of the system structures to obtain a quantitative evaluation of several performance parameters. Special focus was given to the effect of dense mechanical structures designed to provide mechanical robustness and thermal regulation to the minuscule and temperature sensitive detectors. RESULTS For the energies of interest around the photopeak (450-700 keV energy window), the simulation results predict a 6.5% reduction in the single photon detection efficiency and a 12.5% reduction in the coincidence photon detection efficiency in the case that the mechanical structures are interspersed between the detectors. However for lower energies, a substantial increase in the number of detected events (approximately 14% and 7% for singles at a 100-200 keV energy window and coincidences at a lower energy threshold of 100 keV, respectively) was observed with the presence of these structures due to backscatter. The number of photon events that involve multiple interactions in various crystal elements is also affected by the presence of the structures. For photon events involving multiple interactions among various crystal elements, the coincidence photon sensitivity is reduced by as much as 20% for a point source at the center of the field of view. There is no observable effect on the intrinsic and the reconstructed spatial resolution and spatial resolution uniformity. CONCLUSIONS Mechanical structures can have a considerable effect on system sensitivity, especially for systems processing multi-interaction photon events. This effect, however, does not impact the spatial resolution. Various mechanical structure designs are currently under evaluation in order to achieve optimum trade-off between temperature stability, accurate detector positioning, and minimum influence on system performance.

Data acquisition system design for a 1 mm 3 resolution PSAPD-based PET system

We are developing an application specific, high resolution 1 mm3 3-D PET system that comprises two detector panels that are 16 cm times 9.1 cm times 2 cm in dimension with arrays of sandwiched position sensitive avalanche photodiode (PSAPD) detectors. The system will comprise 2240 PSAPD devices per panel with approximately 11,200 channels per detector head. A data acquisition architecture is being designed around dedicated readout ASICs. The readout ASIC, the RENA-3, was developed by NOVA R&D for low noise charge sensitive readout of solid state detectors, with shaping, triggering, and timestamp generation circuitry. Using the RENA-3 evaluation system, two PSAPD devices operated in coincidence achieved a 17.2% global energy resolution, 1 mm2 spatial resolution, and 11 ns (15.5 ns paired) FWHM coincidence time resolution. A Monte-Carlo simulation of a PET mammography configuration was used to generate realistic count rates. The realistic count rates were then run through an event based simulation model of the PET mammography system and data acquisition to determine the count rate performance of the architecture. The data acquisition we propose with the RENA-3 ASIC readout should be able to handle a 5 mCi imaged whole body dose in a 160 cm tall woman receding 75% of the events with a 100 keV low energy threshold.

Convex optimization of coincidence time resolution for high resolution PET systems

We are developing a dual panel breast-dedicated PET system using NOVA RENA-3 ASICs. The coincidence timing resolution of the RENA-3 ASIC can be improved with list mode calibration. We treat the calibration problem as a convex optimization problem and use the RENA-3’s analog based timing system to correct the measured data for time dispersion effects from correlated noise, position sensitive avalanche photodiode (PSAPD) signal delays and varying signal amplitudes. The direct solution to the optimization problem involves a matrix inversion that grows order (n2) with the number of parameters. An iterative method using single-coordinate descent to approximate the inversion grows only order (n). The system calibration method is demonstrated with measured pulser data as well as with two LSO-PSAPD detectors in electronic coincidence. After applying the algorithm, the 511keV photopeak paired coincidence time resolution from the LSO-PSAPD detectors under study improved by 57%, from raw value of 16.3±0.07 ns FWHM to 6.92±0.02 ns FWHM (11.52±0.05 ns to 4.89±0.02 ns for unpaired photons).

Front-end electronics for a 1 mm 3 resolution avalanche photodiode-based PET system with analog signal multiplexing

We are developing a 1 mm3 resolution breast-dedicated Positron Emission Tomography (PET) system with 4608 Position Sensitive Avalanche Photodiode (PSAPD) detectors. This paper focuses on three aspects of the system’s front-end electronics: multiplexing of the PSAPD analog output signals, scaling the dynamic range of the PSAPD output signals using charge attenuation, and positioning of individual interaction energy depositions that are below the 511 keV photo-peak.

Thermal regulation of tightly packed solid‐state photodetectors in a 1 mm3 resolution clinical PET system

PURPOSE Silicon photodetectors are of significant interest for use in positron emission tomography (PET) systems due to their compact size, insensitivity to magnetic fields, and high quantum efficiency. However, one of their main disadvantages is fluctuations in temperature cause strong shifts in gain of the devices. PET system designs with high photodetector density suffer both increased thermal density and constrained options for thermally regulating the devices. This paper proposes a method of thermally regulating densely packed silicon photodetectors in the context of a 1 mm(3) resolution, high-sensitivity PET camera dedicated to breast imaging. METHODS The PET camera under construction consists of 2304 units, each containing two 8 × 8 arrays of 1 mm(3) LYSO crystals coupled to two position sensitive avalanche photodiodes (PSAPD). A subsection of the proposed camera with 512 PSAPDs has been constructed. The proposed thermal regulation design uses water-cooled heat sinks, thermoelectric elements, and thermistors to measure and regulate the temperature of the PSAPDs in a novel manner. Active cooling elements, placed at the edge of the detector stack due to limited access, are controlled based on collective leakage current and temperature measurements in order to keep all the PSAPDs at a consistent temperature. This thermal regulation design is characterized for the temperature profile across the camera and for the time required for cooling changes to propagate across the camera. These properties guide the implementation of a software-based, cascaded proportional-integral-derivative control loop that controls the current through the Peltier elements by monitoring thermistor temperature and leakage current. The stability of leakage current, temperature within the system using this control loop is tested over a period of 14 h. The energy resolution is then measured over a period of 8.66 h. Finally, the consistency of PSAPD gain between independent operations of the camera over 10 days is tested. RESULTS The PET camera maintains a temperature of 18.00 ± 0.05 °C over the course of 12 h while the ambient temperature varied 0.61 °C, from 22.83 to 23.44 °C. The 511 keV photopeak energy resolution over a period of 8.66 h is measured to be 11.3% FWHM with a maximum photopeak fluctuation of 4 keV. Between measurements of PSAPD gain separated by at least 2 day, the maximum photopeak shift was 6 keV. CONCLUSIONS The proposed thermal regulation scheme for tightly packed silicon photodetectors provides for stable operation of the constructed subsection of a PET camera over long durations of time. The energy resolution of the system is not degraded despite shifts in ambient temperature and photodetector heat generation. The thermal regulation scheme also provides a consistent operating environment between separate runs of the camera over different days. Inter-run consistency allows for reuse of system calibration parameters from study to study, reducing the time required to calibrate the system and hence to obtain a reconstructed image.