Q. Peng

High-performance electronics for time-of-flight PET systems

We have designed and built a high-performance readout electronics system for time-of-flight positron emission tomography (TOF PET) cameras. The electronics architecture is based on the electronics for a commercial whole-body PET camera (Siemens/CPS Cardinal electronics), modified to improve the timing performance. The fundamental contributions in the electronics that can limit the timing resolution include the constant fraction discriminator (CFD), which converts the analog electrical signal from the photo-detector to a digital signal whose leading edge is time-correlated with the input signal, and the time-to-digital converter (TDC), which provides a time stamp for the CFD output. Coincident events are identified by digitally comparing the values of the time stamps. In the Cardinal electronics, the front-end processing electronics are performed by an Analog subsection board, which has two application-specific integrated circuits (ASICs), each servicing a PET block detector module. The ASIC has a built-in CFD and TDC. We found that a significant degradation in the timing resolution comes from the ASICs CFD and TDC. Therefore, we have designed and built an improved Analog subsection board that replaces the ASICs CFD and TDC with a high-performance CFD (made with discrete components) and TDC (using the CERN high-performance TDC ASIC). The improved Analog subsection board is used in a custom single-ring LSO-based TOF PET camera. The electronics system achieves a timing resolution of 60 ps FWHM. Prototype TOF detector modules are read out with the electronics system and give coincidence timing resolutions of 259 ps FWHM and 156 ps FWHM for detector modules coupled to LSO and LaBr3 crystals respectively.

A front-end readout Detector Board for the OpenPET electronics system.

We present a 16-channel front-end readout board for the OpenPET electronics system. A major task in developing a nuclear medical imaging system, such as a positron emission computed tomograph (PET) or a single-photon emission computed tomograph (SPECT), is the electronics system. While there are a wide variety of detector and camera design concepts, the relatively simple nature of the acquired data allows for a common set of electronics requirements that can be met by a flexible, scalable, and high-performance OpenPET electronics system. The analog signals from the different types of detectors used in medical imaging share similar characteristics, which allows for a common analog signal processing. The OpenPET electronics processes the analog signals with Detector Boards. Here we report on the development of a 16-channel Detector Board. Each signal is digitized by a continuously sampled analog-to-digital converter (ADC), which is processed by a field programmable gate array (FPGA) to extract pulse height information. A leading edge discriminator creates a timing edge that is time stamped by a time-to-digital converter (TDC) implemented inside the FPGA. This digital information from each channel is sent to an FPGA that services 16 analog channels, and then information from multiple channels is processed by this FPGA to perform logic for crystal lookup, DOI calculation, calibration, etc.

Design of a 32-channel high-performance OpenPET Detector Board

A successful OpenPET electronics system needs to provide electronics, especially front-end electronics that are powerful enough to accommodate cutting-edge research. While there are a tremendous number of variations, the relatively simple nature of the data ultimately collected implies that there can be a common set of requirements. In this paper, we describe a 32-channel high-performance Detector Board (DB) designed for OpenPET to meet those requirements. The input stage of the 32-channel DB accepts differential voltages between -2 V and +2 V and has input diodes to protect against over and under voltage. As the input is differential, detector signals of either polarity can be accommodated by selecting which inputs (positive or negative) they are connected to. The input signals are then split and feed to the high-performance energy circuits and the high-performance timing circuits. The outputs of the two circuits are feed to a FPGA for TDC calculation, event data generation and transmission. We have designed, fabricated and tested a 16-channel prototype DB. It functions as we expected. The 16-channel prototype DB has a similar schematic structure and the same digital parts (ADC, FPGA and etc.) as the 32-channel DB described in this paper. Based on the 16-channel DB, we have completed the schematic design and functional simulation of the 32-channel DB.

A novel electronics for large scale SiPM array readout and advanced PET applications

The Silicon photomultiplier (SiPM) becomes a choice of photon sensors for advanced radiation detector development. However, reading out large-scale SiPM arrays is still a fundamental technical obstacle. We present a new method (named π-PET electronics) to address this issue. Very different from conventional front-end electronics design, the key innovation of the new electronics is to include almost all functions of front-end readout electronics inside a low-cost FPGA. That not only simplifies the analog components and reduce the cost (with only one linear amplifier) but also provides powerful and flexible signal processes to enable applying different algorithms to both enhance the performance and add new real-time dark current measurement and calibration features.

Progresses in designing a high-sensitivity dodecahedral PET for brain imaging

Dedicated brain PET has the potential to achieve better sensitivity, spatial resolution and image quality than conventional whole body PET cameras in brain tomography. Spherical PET (S-PET), that can achieve a higher geometrical sensitivity and a lower parallax error than conventional cylindrical ring PET scanners, is a good candidate for high performance dedicated brain imaging. Our simulation studies show that the S-PET has a local geometric efficiency 2.7 times higher than the 30cm cylinder PET, and 19 times higher than the 76cm cylinder PET in the zones around the cerebral cortex. However, it is very challenging to design detectors with curved surface for S-PET. Convex polyhedron PETs, such as dodecahedral PET, is easier to build and has the potential to achieve performances equivalent to those of S-PETs. We are building a high-sensitivity dodecahedron PET for brain imaging. In this paper, we report our progresses in: (1) sensitivity simulation studies, (2) design, simulation and fabrication of a pentagon-shaped detector module, (3) design and fabrication of the mechanic gantry, and (4) image reconstruction.

Performances of front-end electronics based on sigma-delta modulation — A simulation study

We present a novel front-end electronic design using sigma-delta (Σ-Δ) modulation. The new design needs only one analog amplifier, one differential digital input port and one digital output port from a FPGA. Both the energy and timing calculation are implemented in FPGA firmware. A Simulink model is established to simulate and evaluate the circuit design and the sample data processing. The energy is calculated directly by summing the sequences of 0 and 1 generated by the 1-bit Σ-Δ ADC module. The simulation disclosed the non-linear transfer function of the amplitudes of the input pulse and the calculated energy, which can be corrected using LUT(s). The timing is determined either directly by measuring the timing of first pulse generated by the 500MHz Σ-Δ ADC (timing resolution: 2ns), or by measuring the leading edge of the first pulse generated by FPGA differential input using a FPGA-based TDC.

Firmware and software framework of OpenPET electronics system for radiotracer imaging

The purpose of OpenPET project is to develop open source high-performance electronics for radiotracer imaging. We have developed OpenPET hardware electronics that mainly includes three custom electronics modules: the detector board (DB), the support board (SB), and the multiplexor board (MB). A reliable, scalable, flexible and user-friendly firmware and software frame need to be built to support the OpenPET hardware electronics. We adapted a firmware and software architecture that configures the OpenPET hardware electronics into a computer network with a tree topology. A data structure commonly used in reconfigurable computer networks are adapted and implemented to manage the hardware resources in the system, configure and calibrate the system, and control event data acquisition. The results of system test show that the firmware and software framework is a reliable, scalable and flexible platform for OpenPET system configuration, management, calibration, event data acquisition and processing.

Assessment of dedicated brain PET designs with different geometries

We compared the geometrical efficiencies and parallax errors of three different conceptual dedicated brain PETs, including a spherical cap design (R=18cm), a dodecahedron design (inscribed sphere radius = 15cm) and a cylindrical design (30cm in height and 30 cm in diameter) with their conventional whole body counterpart (a cylinder 20cm in height and 76 cm in diameter) by calculating the Solid Angle Fractions (SAF) and Average Gamma-ray Incident Angles (AGIA). The results show that spherical cap and dodecahedron have an identical SAF that is 58.4% higher than that of 30cm diameter cylinder, and 5.44 times higher than that of 76cm diameter cylinder. The 76cm diameter cylinder has the lowest AGIA. The spherical cap has an AGIA 16.2% lower than that of the 30cm cylinder, and 33.5% lower than that of dodecahedron in brain phantom simulation. In a region around the geometric center, the dodecahedron and the 30cm cylinder have similar parallax error. We conclude that the spherical cap is a better geometry for brain imaging compared to 30cm or 76cm cylinder. The dodecahedral PET consisting of 11 flat detector faces and an opening face, is a reasonable approximation of the spherical cap PET.

Imaging performance of the Tachyon Time-of-Flight PET camera

Tachyon, a single-ring “demonstration” Time-of-Flight (TOF) PET scanner has been developed to measure the improvement in image quality as a function of the timing resolution. The design of the detector module is optimized for timing by coupling the 6.1×25 mm2 of 6.1×6.1×25 mm3 LSO scintillator crystals onto a 1-inch diameter Hamamatsu R-9800 PMT with super-bialkali photocathode. We characterized the imaging performance of the system. The results show that Tachyon achieved a coincidence timing resolution of 314 ps +/-20 ps fwhm over all crystal-crystal combinations, better than any PET camera that has been reported on. Phantom experiments based on the NEMA NU 2-2007 standard were performed to evaluate the basic imaging performance of the scanner including noise equivalent count rates and image resolution.

PMT based pentagonal and hexagonal detector module designs for convex polyhedron PET systems

Our previous studies indicated that convex polyhedron PETs, such as dodecahedral PET, are able to achieve high performances similar to those of S-PETs. A convex polyhedron PET consisting of a couple of flat detectors is much easier to be constructed compared to an S-PET that needs detectors with curved surfaces. Typical convex polyhedrons suitable for PET include the dodecahedron (a regular convex polyhedron with 12 regular pentagonal faces) and the truncated icosahedron (an Archimedean solid with 12 regular pentagonal faces and 20 regular hexagonal faces). In both situations, pentagon-shaped and/or hexagon-shaped detector modules, instead of conventional square-shaped block detector module, are required. We presented two conceptual pentagonal detectors and two conceptual hexagonal detectors based on conventional lost-cost PMT-light-sharing scheme. Monte Carlo simulations were performed to investigate the crystal decoding performances of all four designs. The results show that high quality flood map can be archived by (1) increasing the areas of the PMT layer and (2) applying a light guide with optimized thickness in the detector modules. We conclude that that it is feasible to design and implement low-cost pentagonal or hexagonal detector modules for the convex polyhedron PETs.