Evaluation of a Modular PET System Architecture with Synchronization over Data Links

Abstract

Abstract-A DAQ architecture for a PET system is presented that focuses on modularity, scalability and reusability. The system defines two basic building blocks: data acquisitors and concentrators, which can be replicated in order to build a complete DAQ of variable size. Acquisition modules contain a scintillating crystal and either a position-sensitive photomultiplier (PSPMT) or an array of silicon photomultipliers (SiPM). The detector signals are processed by AMIC, an integrated analog front-end that generates programmable analog outputs which contain the first few statistical moments of the light distribution in the scintillator. These signals are digitized at 156.25 Msamples/s with free-running ADCs and sent to an FPGA which detects single gamma events, extracts position and time information online using digital algorithms, and submits these data to a concentrator module. Concentrator modules collect single events from acquisition modules and perform coincidence detection and data aggregation. A synchronization scheme over data links is implemented that calibrates each link's latency independently, ensuring that there are no limitations on module mobility, and that the architecture is arbitrarily scalable. Prototype boards with both acquisition and concentration functionality have been built for evaluation purposes. The performance of a small PET system with two detectors based on continuous scintillators is presented. A synchronization error below 50 ps rms is measured, and energy resolutions of 19% and 24% and timing resolutions of 2.0 ns and 4.7 ns FWHM are obtained for PMT and SiPM photodetectors, respectively.

Development of a data acquisition architecture with distributed synchronization for a Positron Emission Tomography system with integrated front-end.

“…Different parts of the work described in this thesis were presented at the Real Time Conference in 2010 [5] and the Nuclear Science Symposium in 2012 [6], and subsequently published as peer-reviewed journal papers in IEEE Transactions on Nuclear Science describing the generic synchronization scheme [7] and the DAQ circuit boards [8]; they correspond roughly to the first and second parts of this text, respectively.…”

“…Usually, the only interesting data to be extracted from the pulses are the magnitude Q and the timing information. 6 The signals generated by detectors are typically very weak and often masked by high levels of noise, and so they cannot be used directly, but need to be amplified instead. It is therefore desirable to preamplify and process them as soon as possible, hopefully before they traverse transmission lines that further degrade signal quality ( [20], pp.…”

“…In this section, the later stages of the data acquisition system that deal with digital data are considered. 8 …”

“…This is accomplished by assigning a time value, or timestamp, to each such occurrence and then comparing said values. In order for these timestamps to be comparable and yield a valid time difference in the metric of physical time, 6 a common time reference needs to be established, as well as a time synchronization mecha-nism that allows each node that assigns time values to access the common time reference with a certain resolution.…”

“…In usual settings, the time scales of continuoustime signals are not stretched or shrunk and hence turn out to be of the form T (t) = t + t d for some constant t d that represents the relative delay of signal x (t) from its source to the observation point. 8 It follows that the time reference of a continuous-time signal may be described by a single delay value t d , and that the misalignment between two continuous-time signals in a DAQ is given by a relative delay difference.…”