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We present the design for OpenPET, an electronics readout system designed for prototype radiotracer imaging instruments. The critical requirements are that it has sufficient performance, channel count, channel density, and power consumption to service a complete camera, and yet be simple, flexible, and customizable enough to be used with almost any detector or camera design. An important feature of this system is that each analog input is processed independently. Each input can be configured to accept signals of either polarity as well as either differential or ground referenced signals. Each signal is digitized by a continuously sampled ADC, which is processed by an FPGA to extract pulse height information. A leading edge discriminator creates a timing edge that is “time stamped” by a TDC implemented inside the FPGA. This digital information from each channel is sent to an FPGA that services 16 analog channels, and information from multiple channels is processed by this FPGA to perform logic for crystal lookup, DOI calculation, calibration, etc. As all of this processing is controlled by firmware and software, it can be modified/customized easily. The system is open source, meaning that all technical data (specifications, schematics and board layout files, source code, and instructions) will be publicly available. [ABSTRACT FROM PUBLISHER]

Design, implementation, analysis and measured performance results of a new high-speed low-noise 16-channel charge-sensitive preamplifier (CSA) prototype with automatic detector leakage compensation are presented. The prototype has been fabricated in TSMC 0.35-μm CMOS process and is designed for use with avalanche photodiode (APD) based PET detectors. The CSA is used to read out charge signals from a 4 × 4 APD array having 3 pF of capacitance and 75 nA of leakage current per pixel. A single channel CSA has 16 gain settings measured to range from 31.7 mV/fC to 4.5 mV/fC. The gain settings for all channels are set by a 64-bit on-chip shift register. The signal rise time at the CSA output was measured to be as fast as 2.9 ns (5%-55% rise time) and 4.8 ns (20%-80% rise time). A feedback MOS transistor biased in the triode region is used to reset the CSA output and a very slow Gm-feedback loop performs automatic leakage compensation up to 10 μA of leakage current per channel. Minimum input referred rms noise of 350 e- was measured with a pure capacitive input load and 1200 e- with an actual APD load biased at -1.7 kV connected to the CSA input, both at 0.1-μs peaking time. The prototype chip draws less than 50 mA of total current from a +3.3 V supply. [ABSTRACT FROM AUTHOR]

In a conventional PET system with block detectors, a timing estimator is created by generating the analog sum of the signals from the four photomultiplier tubes (PMT) in a module and discriminating the sum with a single constant fraction discriminator (CFD). The differences in the propagation time between the PMTs in the module can potentially degrade the timing resolution of the module. While this degradation is probably too small to affect performance in conventional PET imaging, it may impact the timing inaccuracy for time-of-flight PET systems (which have higher timing resolution requirements). Using a separate CFD for each PMT might allow for propagation time differences to be compensated through calibration and correction in software. In this paper we investigate and quantify the timing resolution achievable when the signal from each of the 4 PMTs is digitized by a separate CFD. Several methods are explored for both obtaining values for the propagation time differences between the PMTs and combining the four arrival times to form a single timing estimator. We find that the propagation time correction offsets are best derived through an exhaustive search, and that the ‘weighted average’ method provides the best timing estimator. Using these methods, the timing resolution achieved with 4 CFDs (1052 ± 82 ps) is equivalent to the timing resolution with the conventional single CFD setup (1064 ± 216 ps). [ABSTRACT FROM AUTHOR]

The CPS Accel is a commercial PET camera based on a block detector with 64 LSO scintillator crystals (each 6.75 × 6.75 × 25 mm) read out with 4 photomultiplier tubes. The excellent timing resolution of LSO suggests that this camera might be used for time-of-flight (TOF) PET, thereby reducing the statistical noise significantly. Although the Accel achieves 3 ns coincidence resolution (a factor of two better than BGO-based PET cameras), its timing resolution is nearly an order of magnitude worse than that demonstrated with individual LSO crystals. This paper quantifies the effect on the timing of each component in the Accel timing chain to identify which components most limit the camera's timing resolution. The components in the timing chain are: the scintillator crystal, the photomultiplier tube (PMT), the constant fraction discriminator (CFD), and the time to digital converter (TDC). To measure the contribution of each component, we construct a single crystal test system with high-performance versions of these components. This system achieves 221 ps FWHM coincidence timing resolution, which is used as a baseline measurement. One of the high-performance components is replaced by a production component, the coincidence timing resolution is re-measured, and the difference between measurements is the contribution of that (production) component. We find that the contributions of the TDC, CFD, PMT, and scintillator are 2000 ps, 1354 ps, 422 ps, and 326 ps FWHM, respectively, and that the overall timing resolution scales like the square root of the amount of scintillation light detected by the PMT. Based on these measurements we predict that the limit for the coincidence timing resolution in a practical, commercial, LSO-based PET camera is 528 ps FWHM. [ABSTRACT FROM AUTHOR]