IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 53, NO. 1, FEBRUARY 2006 297 A Sampling ADC Data Acquisition System for Positron Emission Tomography Alexander Mann, Boris Grube, Igor Konorov, Stephan Paul, Lars Schmitt, David P. McElroy, and Sibylle I. Ziegler Abstract—A data acquisition system for a positron emission to- mograph (PET) based on avalanche photodiode (APD) readout of lutetium oxyorthosilicate (LSO) scintillator crystals is presented. The analog data of each APD are read out by fast analog to digital converters (ADCs) and processed within field programmable gate arrays (FPGAs). The ADCs are continuously sampling with a 80 MHz, low jitter clock, which is synchronous for the whole detector. The main tasks of the FPGAs are pulse detection and extraction of signal timing information from the digitized data stream. The de- tected signal pulse data are therefore compared to a predefined set of pulse shapes with known phase shifts with respect to the sample clock. By searching for the best match within this calibration set, a precise start time information for the signal pulse can be deter- mined. The calculated time values are then transmitted from the ADC cards via fiber optic links to multiplexer modules which com- bine the different data streams and can also perform further pro- cessing like search for coincident events. Finally, the preprocessed detector data are transmitted from the multiplexers to PCI cards in the image reconstruction computers by gigabit optical links. Index Terms—Data acquisition, field programmable gate array (FPGA), positron emission tomography, signal processing. I. INTRODUCTION T HE PRESENTED application is a data acquisition system (DAQ) for a small animal positron emission tomograph (PET), based on avalanche photodiode (APD) readout of lutetium oxyorthosilicate (LSO) scintillator crystals. The Mu- nich Avalanche Photo-Diode Positron Emission Tomograph (MADPET) was developed at the “Nuklearmedizinische Klinik und Poliklinik” at “Klinikum Rechts der Isar” of the “Tech- nische Universität München” to prove the feasibility of a single channel readout of LSO crystals with APD matrices and to develop the reconstruction algorithms for such a system [1]. The first prototype system is now already used in the medical research with small animals. The second version of this system (MADPET-II) is currently under development and will provide a full two layered detector ring with an increased number of detector crystals [2]. The de- tector ring is built from 18 detector modules, each including Manuscript received November 15, 2004; revised November 7, 2005. This work was supported in part by the Maier-Leibnitz-Labor and the Dr.-Ing. Leon- hard-Lorenz-Stiftung. This research is part of the EU Integrated Infrastructure Initiative Hadronphysics Project I3HP under Contract Number RII3-CT-2004- 506078. A. Mann, B. Grube, I. Konorov, S. Paul, and L. Schmitt are with the Physik-Department, Technische Universität München, Garching 85748, Ger- many (e-mail: Alexander.Mann@ph.tum.de). D. P. McElroy and S. I. Ziegler are with the Nuklearmedizinische Klinik und Poliklinik, Klinikum Rechts der Isar, Technische Universität München, München 81675, Germany. Digital Object Identifier 10.1109/TNS.2006.869830 Fig. 1. MADPET-II detector module. two layers of APD matrices on which the LSO scintillators are directly mounted. Each APD matrix provides 4 8 channels, which are read by a charge sensitive preamplifier on the module. The preamplifier converts the analog APD signals to differential signals which are transfered via flat cable to the data acquisi- tion system for digitization. The complete detector consists of APD channels which have to be read out and processed independently by the DAQ. Fig. 1 shows one of the 18 detector modules which includes the LSO crystals, APD diode matrices and the preamplifiers. The presented data acquisition system is based on modules developed for the COMPASS experiment at CERN [4]. In con- trast to the readout system described in [3], which uses mixed- signal application specific integrated circuits (ASICs) for analog signal processing, this design concept minimizes analog pro- cessing of detector data by sampling each channel with a fast analog to digital converter (ADC) and performing then all pro- cessing steps on the digitized data stream. Thus, the system is less susceptible to component parameter variations and noise in- terference. The signal processing algorithms are implemented in field programmable gate arrays (FPGAs) and are loaded dy- namically prior to operation. This ensures high flexibility in the design of the algorithms, which can be changed and optimized also after the final setup of the PET system. II. DATA ACQUISITION SYSTEM The whole readout system is built from four different base modules: the sampling ADC module as analog interface, a multiplexer module for combining the data streams of several ADCs, a clock distribution network and a PCI card as interface to standard PCs. These building blocks are combined in a hier- archical tree network to serve the channel count and data rate requirements of the detector. The same base modules are also used with different firmware in several experimental setups in particle physics and are commercially available from i-trOnics GmbH, Riemerling [5]. 0018-9499/$20.00 © 2006 IEEE