Abstract--The ISPA (Imaging Silicon Pixel Array)-tube is a
position sensitive photon detector based on the hybrid technology.
It detects light via a photocathode and an appropriate electric
field accelerates the emitted photoelectrons towards a silicon pixel
anode. This anode, finely segmented into pixel detectors provides
binary images and allows for the self-triggering of the tube.
Coupled to scintillating crystals, ISPA-tubes have been
successfully tested in the field of gamma ray imaging,
demonstrating real capabilities in both space and energy
resolution. Recently, we have concentrated our efforts on the
development of a compact and portable readout system based on
new electronics and able to provide full control and real-time
processing of ISPA-tubes. The system overview and the dedicated
interface are presented in this paper.
iagnostic imaging tools, namely gamma raw cameras
for nuclear medicine, have slowly improved since its
first invention by Anger in 1958 . A wide variety of
commercial gamma cameras systems are available in a wide
range of designs, but they still consist of upgrades of the
standard one, limited in spatial and energy resolution. Due to
their bulky size and significant dead space around the
periphery of the camera, those “all-purposes” systems are not
optimized to improve diagnostic accuracy in nuclear medicine
imaging for specific procedures and applications. The existing
alternatives are oriented on dedicated systems through compact
This work was supported in part by the National Institute for Nuclear
Physics (INFN) of Italy, the CERN, Switzerland, and the Portuguese
Foundation for the Science and the Technology (FCT) from the project
M. C. Abreu and P. Sousa are with the University of Algarve, FCT, 8000-
117 Faro, Portugal, and the Laboratory of Instrumentation and Experimental
Physics of Particles (LIP), 1000-149
F. De Notaristefani, V. Cencelli, E D’Abramo and G. Hull are with the
INFN Roma-III, I-00146 Rome, Italy.
C. D’Ambrosio H. Leutz and E. Rosso are with the CERN, CH-1211
F. Cindolo is with the INFN Bologna, I-40127 Bologna, Italy.
L. Peralta is with the LIP Lisbon, 1000-149 Lisbon, Portugal and the
Faculty of Science of the University of Lisbon, 1749-016 Lisbon, Portugal.
P. R. Mendes and C. Ortigão are with the LIP Lisbon, 1000-149 Lisbon,
1Manufacturers such as DIGIRAD, OY AJAT, ACRORAD, LABLOGIC,
INTRAMEDICAL, SIEMENS, IMARAD, Saint-GOBAIN, etc, provide new
competitive gamma camera imaging systems.
Lisbon, Portugal (e-mail:
gamma cameras based on new imaging semiconductors
technology. They are compact devices suited to high resolution
planar gamma imaging. Several prototypes of such imaging
probes are currently under development in laboratories [2-4],
and some of them are already in the market financed through a
In the framework of the ISPA Group at CERN, and taking
advantages on the existing technology for high particle
physics, the new gamma camera system we are developing
features a Hybrid Photon Detector (Fig. 1) intended for
biomedical applications which combines compactness and
portability with high spatial resolution. The concept of an
ISPA gamma camera, based on the HPD principle, has been
published since 1995 [5, 6]. Alternative studies using Monte
Carlo simulation were performed in order to optimize detector
arrangement, namely different crystal coatings and reflector
properties, several surface treatments and polish types. Some
collimator designs were also studied to evaluate the system
spatial resolution and sensitivity.
Fig. 1. Photograph of the cross-focusing ISPA-tube, inner tube dimensions
75 mm length x 50 mm diameter (ref. CERN-EX-0206003)
After the first encouraging results in the gamma ray imaging
field [7, 8], reasons to provide easier set-up and utility of the
ISPA-tube for clinical studies and basic research, lead us to re-
design new electronics. The new compact readout system
under development is able to control Omega3 and Alice1 
chips. The architecture is also readied to adapt to a new ASIC
chip under development. With this new concept, we intend to
substitute the present VME-NIM based system and provide a
Further Readout Electronics Development for
the ISPA gamma camera
Maria C. Abreu, Valentino Cencelli, Federico Cindolo, Enrico D’Abramo, Carmelo D´Ambrosio,
Francesco De Notaristefani, Giulia Hull, Heinrich Leutz, Catarina Ortigão, Luís Peralta,
Pedro Rato Mendes, Ettore Rosso and Patrick Sousa (Speaker)
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better portability of the equipment to satisfy the needs for
nuclear medicine applications, as for example inside clinical
II. SYSTEM DESCRIPTION
The previous readout architecture of the prototype high-
resolution gamma camera featured the Omega3/LHC1 pixel
detector readout chip  (Fig. 2), a tungsten collimator, the
RD19/Omega3 digital readout card, a discrete analog system
based on NIM modules and different power supplies to provide
threshold and bias voltages. PCI and VMEbus boards perform
intersystem communications between the detector and an
external PC running C programming language software for
Fig. 2. Photograph of the LHC1/Omega3 chip glued on a ceramic carrier
The new readout system (Fig. 3) consists of a compact
printed circuit board (PCB) to connect the ISPA-tube, and two
main electronic cards, one plugged onto the other.
Fig. 3. Block diagram of the new setup for gamma ray imaging
The first one was designed at INFN (Istituto Nazionale di
Fisica Nucleare) in collaboration with the firm CAEN
(Costruzioni Apparecchiature Elettroniche Nucleari) and is
based on the CAEN card S9007 , exploiting its abilities of
expansion and programmability. The second one, also
projected at INFN in collaboration with the firm CAEN, was
specially designed for phototube pixel readout and data
acquisition, and is called Piggy Back Board S9007 (PBBS).
A. The Silicon Sensor
The active sensor chip, originally developed at CERN for
high-energy physics  and later implemented on ISPA-tubes,
is a matrix of 128×16 C-type detector cells (conventional
rectangular ion implant diodes) of 50×500 µm2, manufactured
in a commercial 1 µm SACMOS technology process. The
readout chip is connected by solder bumps to a 300 µm thick
silicon sensor of the same geometry, allowing high gain, high
speed and low noise. This single-chip detector covers a
sensitive area of 8.0×6.4 mm2, and each cell comprises
preamplifier, comparator with externally adjustable threshold,
delay line, coincidence logic and memory (Fig. 4).
Fig. 4. Block diagram of the pixel cell
The vacuum sealed ISPA-tube is 4.0 cm long with 3.5 cm
diameter, featuring a YAP:Ce (Yttrium Aluminium Perovskite
Activated by Cerium) crystal of 2 mm thickness as gamma
converter. Photoelectrons from the photocathode (S20), which
is deposited on the inner side of the entrance window, are
accelerated to the pixel chip giving a spot that contains a
number of photoelectrons proportional to the incident photon’s
energy. Its centroid is used to determine with unprecedented
precision the incoming photon’s entrance position.
B. The S9007 electronic card
The S9007 card (Fig. 5) was first developed as an electronic
front-end for the AMS project (CERN experiment to search
antimatter in space), and is equipped with a FPGA (Field-
Programmable Gate Array) that has the functionalities of
buffer, memory and sequencer, and a fast Analog Devices DSP
(Digital Signal Processing) processor that performs the first
analysis of the data.
The FPGA is from ALTERA-APEX EP20K400 family and
the readout algorithms are realized in VHDL (Very high speed
integrated circuit Hardware Description Language) using
Altera’s Quartus development system compilation software,
directly programmed through Flash EPROM by means of
JTAG protocol. The DSP, from the ADSP218x family,
features two high-speed serial ports and a 16-bit internal DMA
port. It integrates 160K bytes of on-chip memory and operates
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with a 19 ns instruction cycle time. Besides a series of
feedings, this card is provided with an auxiliary connector that
allows the access to 100 I/O lines of the FPGA and two clock
signals, at 16 and 25 MHz respectively.
Fig. 5. Photograph of the S9007 electronic card
C. The PBBS electronic card
The Piggy Back Board S9007 (PBBS) (Fig. 6) is the main
interface of the system and is connected to the S9007 board to
substitute functionalities and missing features. Composed by
various subsystems, this card also has a FPGA ALTERA
EPM718S for the implementation of the logic interface with
the parallel port. The FPGA device is programmed with
Altera’s MAX+PLUS design software and a parallel port
protocol is implemented for external communication. These
allow the card to manage the interface with a standard personal
computer (PC) through which the user is able to control the
system and acquire data.
The ISPA-tube attached to the PCB with a pin grid array
(PGA) ZIP socket, is connected to the PPBS card through
standard flat connectors. The memorised data of the chip is
read through 32 differential lines in technology LVDS (Low
Voltage Differential Signaling), allowing high speed
communication, and 16 bi-directional lines are used for control
(strobe, read/write, reset, chip select, clock, etc).
To substitute the present analog front-end by integrated
components, part of the electronics is devoted to the
acquisition and consequent digitization of the back-pulse
signal used for triggering purposes and energy measurements.
An especially designed analog signal from the detector back-
pulse is generated on the ISPA PCB, shaped, sent to the PBBS
card, and sampled with a fast ADC. The analog circuit
comprises a EURORAD charge preamplifier module and two
INTERSIL semiconductor modules for shaping and amplifying
operations. The digital comparison of this ADC value with
pre-set window values will generate the trigger for the system.
The PPBS supplies also all the necessary feedback. There are
24 digital-to-analog converters (DAC) to provide voltage and
current references for the analog front-ends.
Fig. 6. Photograph of the PBBS electronic card
A new acquisition setup devoted to ISPA-tubes applications
was presented. The full electronic system is currently being
programmed and under test at CERN and INFN laboratories.
We also plan to implement the ALICE-chip (under-
development by the MIC group at CERN, Switzerland) in the
next ISPA-tube prototype.
At present, our efforts are concentrated in the electronics
and programming for full control of the ISPA-tube, but other
important issue is to apply Monte Carlo techniques on the
silicon pixel detector response. The next step of simulation
improvements will be the development of a GEANT4 
based simulation of the set-up. GEANT4 has a better visible
light tracking and surface description capabilities allowing for
a better reconstructed hitting point. Also the tracking of the
photoelectron in the tube electric field can be important
especially in non-homogeneous fields. These information are
essential for an accurately determination of the hit pixels and
thus for image formation.
In future, wireless communication between detector and
control electronics is foreseen,
implementation of a full computing unit based on PMC Power
PC Linux board.
together with the
We acknowledge the support of the TA2 group at CERN,
Geneva, Switzerland and thank the invaluable help of D.
Riondino and M. Pici. This work was partially supported by
FCT project POCTI/FNU/43672/2002.
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