A small Positron Emission Tomography demonstrator based on LYSO slabs and Silicon Photomultiplier matrices is under construction at the University and INFN of Pisa. In this paper we present the characterization results of the read-out electronics and of the detection system. Two SiPM matrices, composed by 8 × 8 SiPM pixels, 1.5 mm pitch, have been coupled one to one to a LYSO crystals array. Custom Front-End ASICs were used to read the 64 channels of each matrix. Data from each Front-End were multiplexed and sent to a DAQ board for the digital conversion; a motherboard collects the data and communicates with a host computer through a USB port. Specific tests were carried out on the system in order to assess its performance. Futhermore we have measured some of the most important parameters of the system for PET application

Ambrosi, Giovanni

Belcari, Nicola

Bisogni, Maria Giuseppina

Collazuol, Gianmaria

Corsi, Francesco

Foresta, Maurizio

Guerra Gutiérrez, Pedro

Marcatili, Sara

Marzocca, Cristoforo

Matarrese, Gianvito

Morrocchi, Matteo

Sportelli, Giancarlo

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

 Matteo Morrocchi, Sara Marcatili, Nicola Belcari, Maria Giuseppina Bisogni, Gianmaria Collazuol, Giovanni 
Ambrosi, Francesco Corsi, Maurizio Foresta, Cristoforo Marzocca, Gianvito Matarrese, Giancarlo Sportelli, Pedro 
Guerra Member IEEE, Andres Santos Senior Member IEEE, Alberto Del Guerra Senior Member IEEE 
 Abstract– A small Positron Emission Tomography 
demonstrator based on LYSO slabs and Silicon Photomultiplier 
matrices is under construction at the University and INFN of 
Pisa. 
In this paper we present the characterization results of the 
read-out electronics and of the detection system. Two SiPM 
matrices, composed by 8 x 8 SiPM pixels, 1.5 mm pitch, have 
been coupled one to one to a LYSO crystals array. 
Custom Front-End ASICs were used to read the 64 channels of 
each matrix. Data from each Front-End were multiplexed and 
sent to a DAQ board for the digital conversion; a motherboard 
collects the data and communicates with a host computer 
through a USB port. 
Specific tests were carried out on the system in order to assess 
its performance. 
Futhermore we have measured some of the most important 
parameters of the system for PET application. 
I. INTRODUCTION 
ILICON Photomultipliers (SiPM) represent an effective 
alternative to photomultiplier tubes used in Positron 
Emission Tomography[1]. 
The reduced dimension of the SiPM and the capability to 
assemble array of SiPMs with a millimetric pitch allow to 
overcome the limit in spatial resolution imposed by the  
current devices. 
For this reason a demonstrator of a PET module for preclinical 
applications is under development[2]; in small animal imaging 
systems, in fact, spatial resolution is a crucial parameter for 
the reduced anatomical dimensions of test rodens[3].  
SIPMs are also attractive for PET due their insensitivity to 
magnetic field; therefore a new integrated PET – MRI scanner 
can be developed combining the functional information of 
                                                           
Manuscript received November 14, 2011.  
Matteo Morrocchi is with University of Pisa and INFN sezione di Pisa, PI 
56127  Italy (telephone: 0039-329-1262837, e-mail: 
matteo.morrocchi@pi.infn.it).  
Sara Marcatili, Nicola Belcari, Maria Giuseppina Bisogni and Alberto Del 
Guerra are with University of Pisa and INFN sezione di Pisa, PI 56127 Italy. 
Gianmaria Collazuol is with INFN sezione di Pisa PI 56127 Italy. 
Giovanni Ambrosi is with INFN sezione di Perugia, PG 06100 Italy. 
Francesco Corsi, Maurizio Foresta, Cristoforo Marzocca and Gianvito 
Matarrese are with Politecnico di Bari and INFN sezione di Bari, BA 70100 
Italy. 
Pedro Guerra is with CIBER-BBN, CEEI-Modulo 3, Zaragoza 50018 
Spain. 
Andrres Santos and Pedro Guerra are with Universidad Politecnica de 
Madrid, Madrid 28040 Spian, and with CIBER-BBN, Spain. 
Positron Emission Tomography with the high anatomical 
resolution of a Magnetic Resonance Image  technique[4]. 
The demonstrator presented in this paper is composed by two 
8 x 8 SiPM matrices coupled to LYSO crystal matrices; a 
custom front – end electronics has been developed to read the 
devices and to communicate data to a PC. 
II. MATERIALS AND METHOD 
The detection system is composed of two SiPM matrices 
produced by FBK – Irst for the INFN DaSiPM2 collaboration, 
each SiPM is a 1,5 mm x 1,4 mm and  the matrix is composed  
by 64 (8 x 8) SiPMs built on a common silicon substrate, each 
SiPM of the matrix is composed of 840 microcelles [5]. A 
common bias voltage feeds the whole matrix. The two 
matrices are hosted on a custom board to enable the read-out 
of the channels. The matrix is shown in Fig. 1 (left) and the 
board is depicted in Fig 1 (right). 
 
 
Fig. 1.  SiPMs matrix used for the demonstration system: particular of the 
SiPMs array (left) and the custom board for the read – out (right). 
 
Each matrix is coupled to an 8 x 8 LYSO crystal array, 1,5 
mm pitch and 10 mm thick, so to have a one to one coupling 
with the SiPMs matrix; crystal and detector are coupled with 
optical grease in order to obtain a good coupling of the 
refractive indices. 
The spatial resolution of the matrix has been already 
measured [5], coupling the device with a LYSO crystal slab 
and obtaining a FWHM for the distribution of the centre of 
mass of the event of 0,9 mm. 
The two SiPM matrices are read by a DAQ system 
composed of two sets of DAQ boards connected to a central 
motherboard[6][7], as shown in Fig. 2. 
Each DAQ board hosts a custom front-end ASIC [8] 
developed by the INFN of Bari – Politecnico of Bari. The chip 
has 8 input channels, so 8 DAQ boards are needed to read the 
whole SiPMs matrix. Each channel of the ASIC is composed 
Characterization and test of a Data Acquisition 
System for PET 
S
2011 IEEE Nuclear Science Symposium Conference Record N15-4
978-1-4673-0119-0/11/$26.00 ©2011 IEEE 621
 of a Current Buffer with a 250 MHz bandwidth and input 
impedance lower than 20 Ohm. The signal coming from the 
current buffer is splitted in two paths. One is connected to a 
Current Discriminator; the trigger signal generated by the 
ASIC is obtained by a logic OR between the signal of the 
eight discriminators of the channels. The threshold level could 
be set at 12 different levels using four configuration bits. The 
other branch is connected to a CSA (Charge Sensitive 
Amplifier) to integrate the signal with three different gain 
levels and then to a Peak Detector to keep the signal fixed to a 
costant level during the reading process. The integration time 
of the CSA is set to 250 ns according to the decay time of the 
scintillator crystal. 
The DAQ hosts a 10 bits ADC and an FPGA Altera 
Cyclone II used to communicates with the motherboard and 
the ASIC.  
The central motherboard hosts a FPGA Altera Stratix III 
used to manage the I/O signals to/from the DAQ and to 
perform the time coincidence, data are stored in a FIFO and 
communicated to a host PC using a USB protocol. 
At the moment two DAQ boards are completed and 
available and others are under construction. 
In Fig. 3 the motherboard and the two DAQ boards used in 
the work are represented. 
 
 
    Fig. 2.  Scheme of the detection system. 
 
 
Fig. 3.  The acquisition system composed on a central motherboard 
and two DAQ boards. 
 
III. READ – OUT SYSTEM CHARACTERIZATION 
The first measurements have been executed to characterize 
the performance of the two Front – End ASICs. The linearity 
has been tested for each channel of both devices, injecting 
different values of charge to simulate the signal generated by 
the LYSO – SiPM module. 
A shaping circuit connected to a waveform generator has 
been used to reproduce the tipical signal shape obtained with 
crystal and SiPM. The mean value of the signal distribution 
has been considered for each amplitude of the charge 
injection. 
The plots of the ADC channel as a function of amplitude of 
the square wave for the two chips are shown in Fig. 4. 
The value of the pedestal for each channel has been 
obtained using a linear fit. 
The threshold values have been quantified by the same test 
signals. A defined number of pulses has been sent to the ASIC 
and the number of triggered events has been plotted as a 
function of the amplitude of the current peak of the signals. 
The error function so obtained has been derivated and the 
threshold level has been obtained with a gaussian fit. 
The Error Function plot of the lower threshold level is 
shown in Fig. 5, and the result of the first 8 levels is 
summarized in Fig. 6; as can be seen, the spacing between 
levels is constant.  
 
622
  
(a) 
 
(b) 
 
 Fig. 4.  Linearity test for the 8 channels of the first (a) and second (b) 
ASIC. The average ADC channel is plotted as a function of the amplitude of 
the signal of the pulse generator. 
 
 
Fig. 5.  Error function obtained with the lowest threshold level. 
 
Fig. 6.   The current peak injected in the ASIC is plotted as a function of 
the level of threshold. 
 
Timing perrformances has been finally assessed by sending 
test signals in time coincidence to the two ASICs.  
The two digital trigger signals generated by the Current 
Discriminator have been sent to an oscilloscope with passband 
of 1,5 GHz and sampling rate of 2 GS/s. 
The time jitter σt depends on the amplitude of the signals 
and on the noise level of the system according to the 
equation[9]: 
                         ( ) NS
t
dtdS
rn
t /
≈=
σ
σ                        (1) 
In which σn is the standard deviation of the noise, tr is the 
rise – time of the signal, S is its amplitude and N is the noise. 
The distribution of the time difference between the two 
signals is plotted and fitted obtaining as best result a FWHM 
of 160 ± 6 ps, as shown in Fig. 7. 
 
Fig. 7. Time delay distribution between the trigger signals of the two 
ASICs using test signals. 
IV. DETECTION SYSTEM CHARACTERIZATION 
In order to measure the performances of the whole detection 
system (comprehending crystal, SiPMs and read – out 
electronics) the two ASICs have been connected to a line of 
the SiPM’s matrix (8 pixels). 
Timing measurements have been performed using a 22Na 
source between the two lines of detectors and the lowest 
threshold. Due to the leading edge discrimination method used 
623
 in the chip, the system time resolution is limited by time walk 
effect of signals of same rise time and different amplitude. To 
avoid the saturation of the CSA an appropriate bias voltage for 
the two matrices has been chosen. 
The distribution of the time delay of the two triggers is 
shown in Fig. 8 and it shows a FWHM of 1,8 ± 0,2 ns. 
 
 
Fig. 8. Time delay distribution between the trigger signals of the two ASICs 
using . 22Na souces. 
 
Energy resolution has been estimated using both single line 
and two lines in time coincidence; the value of the pedestals 
previously founded have been used to calibrate the channels. 
Using a single line, the FWHM of the 511 keV peak of the 
22Na results in 15,3 ± 0,4 % and using the two devices in 
coincidence the result is 16,9 ± 0,4 %. 
The spectrum of single events is shown in Fig. 9. 
 
Fig. 9. 22Na  spectrum acquired with a single line of 8 SiPMs with a 1 to 1 
coupling to LYSO crystal of 10 mm width. 
 
To evaluate the imaging capabilities of the system, the two 
SiPM matrices has been set to 120 mm distance and the souce 
has been moved in 5 different positions in the transversal 
direction.  A coincidence window of 10 ns has been chosen. 
The set – up is shown in Fig. 10. Variating the distance of the 
source from the axis of the two matrices inside the Field of 
View (FOV) of the two lines, the position has been 
reconstructed taking into account the occurrence in each Line 
of Response (LOR). 
 
Fig. 10. Set –up for the test of position reconstruction with the matrices at a 
distance of 120 mm. 
 
As could be seen in Fig. 11 the position could be well 
reconstructed in all the situations analyzed; the error in the y  
axis is determined by the FWHM of the distribution of the 
LOR in the tansversal direction while the x error depends on 
the positioning of the source. 
 
Fig. 11. Reconstructed position as a function of the real position of the 
source with the two natrices at 120 mm distance. The centre of the FOV is at 
the midline. 
V. CONCLUSIONS 
A demonstrator for preclinical Positron Emission 
Tomography application composed of two SiPMs matrices 
and LYSO crystal is under development and the first results 
for the performances of the detection system are presented. 
The ASIC performances are complient with PET 
requirements; the electronic time jitter has been quantified to 
160 ± 6 ps (FWHM). 
Two DAQ boards have been used to read one line for each 
SiPM matrix. An energy resolution for the 511 keV peak of 
22Na of 15,3 ± 0,4 % has been obtained using a single line and 
a result of 16,9 ± 0,4 % has been obtained with the two lines 
in coincidence. 
A time resolution of 1,8 ± 0,2 ns (FWHM) has been 
measured using a single discrimination method. This result is 
affected by the time walk because photopeak events can not be 
selected with the present set-up; a better performance is 
expected with a double threshold composed of a low level for 
time information and a higher level at the peak energy. 
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[1] W. W. Moses, Photodetectors for nuclear medical imaging, Nuclear 
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[2] S. Marcatili et al., Development and characterization of a modular 
acquisition system for a 4D PET block detector, Nuclear Inst. and 
Methods in Physics Research, A 659 (2011), pp. 494-498 
[3] A. Del Guerra, N. Belcari , State-of-the-art of PET, SPECT and CT for 
small animal imaging, Nuclear Instruments and Methods in Physic A 
583 (2007) 119-124. 
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world's first whole body PET — MRI scanner, Nuclear Science 
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[5] Llosa et al., Characterization of a PET detector head based on 
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[6] G. Sportelli et al., Reprogrammable Acquisition Architecture for 
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