System Integration of the LabPET Small Animal PET Scanner

Abstract

To address modern molecular imaging requirements, a digital positron emission tomography scanner for small animals has been developed at Université 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 mm 2 (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 LabPET TM 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.

Roadmap to fully-digital PET/CT scanners

“…The signal issued from the APD are amplified with a charge sensitive preamplifier (CSP), designed in CMOS P18 technology [8], and digitized with offthe-shelf 45 MSPS 8-bit ADC (Fig 1). An FPGA harvests samples and performs relevant feature extraction like energy, timestamping and crystal identification (CI) prior to sending such information to scanner subsections coping with events sorting and coincidence extraction [3] [7]. This electronic architecture has all the required flexibility to explore new algorithms and better understand the underlying physical phenomena.…”

“…Digital signal processing, coupled to early digitization of single-pixel detector readouts, is among solutions that can fit the electronically-fused PET/CT problematic, simultaneously tackling all aforementioned aspects and, moreover, enabling new processing techniques such as Compton-scatter LOR computation or alternate random estimation [5] [6]. This paper will discuss the pros and cons of past and present digital methods implemented in the LabPET™ electronics [3] [7], and those foreseen on the roadmap to a fullydigital PET/CT scanner that would be suited to molecular imaging, notwithstanding commercialization issues.…”

PETonCHIP: architecture of a on-chip high-resolution, fully digital positron emission tomography scanner for small Animal Imaging

“…Currently, the firmware for a full virtual oscilloscope acquisition mode is still under development. This mode, as described in [9], provides access to the ADC's raw outputs enabling in-depth analysis for debugging and calibration purposes The system runs with three clocks, one at the sampling frequency of 52 MHz, one at the PCI frequency and one at the operating frequency of the SDRAM. It must be pointed out that the selection of the sampling frequency has been based on the availability in the prototyping board of a low-jitter clock at 125 MHz or 52.5 MHz.…”

A handy time alignment probe for timing calibration of PET scanners

“…One of them is the time offsets between coincident detectors in the scanner. In the case of the LabPET TM scanner [1,2], these offsets are mainly due to the different decay times of LYSO and LGSO crystals [3], the variation in routing lengths between APD detectors and ADCs, and the ADC clock distribution network [4]. To ensure the best overall timing resolution, it is necessary to compensate for the time offset of individual detectors.…”