AVALANCHE PHOTODIODES FOR HIGH-RESOLUTION PET

IMAGING SYSTEMS

R. Bugalho, B. Carriço, C. S. Ferreira, M. Ferreira, R. Moura, C. Ortigão

J. Pinheiro, P. Rodrigues, J. C. Silva, A. Trindade and J. Varela

LIP – Lab. de Instrumentação e Física Experimental de Partículas, Avenida Elias Garcia 14, 1000-149 Lisboa, Portugal

also at IST- Instituto Superior Técnico, Av Rovisco Pais, 1049-001 Lisboa, Portugal

Keywords: Avalanche photodiode, Dark current, Gain, Positron emission mammography, Quality control.

Abstract: A high-resolution Positron Emission Tomography (PET) scanner prototype, named Clear-PEM, was

developed by the Portuguese PET Consortium in the framework of the Crystal Clear Collaboration (CCC).

This scanner is a PET prototype dedicated for breast cancer imaging mammography, based on a novel

readout scheme constituted by fine-pitch scintillator crystals, avalanche photodiodes (APD), low-noise high-

gain frontend amplifiers and a reconfigurable FPGA-based electronics readout system. The Clear-PEM

scanner is designed to exam both the breast and the auxiliary lymph node areas, aiming at the detection of

tumours down to 2 mm in diameter. The prototype has two planar detector heads, each composed of 96

detector modules. Each detector module is composed of a matrix of 32 identical 2x2x20 mm

LYSO:Ce

scintillator crystals, read at both ends by Hamamatsu S8550 APD arrays (4x8) for Depth-of-Interaction

(DOI) capability. The APD arrays were characterized through the measurement of gain and dark current as a

function of bias voltage, under controlled conditions. A set of 984 APD arrays followed a well defined

quality control (QC) protocol, aiming at the rejection of arrays not complying with the defined

specifications. From the total of 984, only 1 (0.1%) was rejected, reassuring the trust in these detectors for

prototype assembly and future applications.

1 INTRODUCTION

New methods for breast cancer diagnosis are object

of heavy research efforts. One such research line

relies on the use of Positron Emission based

technology applied to breast cancer detection. In

spite of initial very encouraging results in limited

clinical trials, whole body PET systems have

considerable operational costs, with a low patient

turnover incompatible with systematic screening and

a low spatial resolution which limits the minimum

lesion size that can be detected. This has led to the

development of dedicated PET scanners, targeting

breast cancer imaging applications (Thompson et al,

1995, Moses, 2004). The PEM units are designed to

explore localized regions of the body, usually the

breast, and adopt several design principles, like fine

pixelized crystals, to achieve a better spatial

resolution, as well as a large count–rate capability.

This set of requirements has lead to the development

of a series of proof–of–principle and a few full–

assembled Positron Emission Mammography (PEM)

scanner prototypes, several of which have been used

in preliminary clinical trials, based on high density

and high–Z inorganic scintillator crystals. Of this

list, almost all lack the ability to measure the depth–

of–interaction and thus reconstructed images may

show significant aberrations due to the parallax

effect. An emerging technique to reduce this

aberration effect consists on the readout of the

scintillation light, produced in the crystal elements,

by two opposing photosensors and extracts the

coordinate of interaction along the longitudinal axis

from the asymmetry of light collection (Shao et al.,

2000). This is particular important since in a planar

detector with the active media located close to the

object under examination, the parallax effect can be

an important source of blurring in the spatial

resolution.

Several technical challenges need to be

addressed namely on how to readout the light from

the crystals without putting unacceptable amounts of

Bugalho R., Carriço B., S. Ferreira C., Ferreira M., Moura R., Ortigão C., Pinheiro J., Rodrigues P., C. Silva J., Trindade A. and Varela J. (2009).

AVALANCHE PHOTODIODES FOR HIGH-RESOLUTION PET IMAGING SYSTEMS.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 31-38

DOI: 10.5220/0001432900310038

 SciTePress

non-active media (like conventional

photomuiltplers) between the patient port and the

crystals, which would lead to a degradation on the

final image quality and lesion detection sensitivity.

To address these issues, the Clear–PEM scanner

(Lecoq and Varela, 2002, Abreu et al, 2006), was

developed by the Portuguese PET Consortium,

composed by: Laboratório de Instrumentação e

Física Experimental de Partículas (LIP), Instituto de

Biofísica e Engenharia Biomédica da Faculdade de

Ciências de Universidade de Lisboa (IBEB),

Instituto de Biomédico de Investigação de Luz e

Imagem (IBILI), Instituto de Novas Tecnologias

(INOV), Instituto de Engenharia de Sistemas e

Computadores Investigação e Desenvolvimento

(INESC-ID), Instituto de Engenharia Mecânica e

Gestão Industrial (INEGI), Hospital Garcia da Orta

(HGO), Instituto Português de Oncologia (IPO) in

Porto and with the participation of CERN

(Organisation Européene pour la Recherche

Nucléaire) through the international collaboration

Crystal Clear Collaboration.

The detector is based on pixelized LYSO:Ce

crystals optically coupled on both extremities to

avalanche photodiodes (APD) and readout by a fast,

low-noise electronic system. The choice of

avalanche photodiodes was dictated by its

compatibility with the implementation of a double

readout technique. The APDs also demonstrate good

energy resolution for a direct detection of X-rays

and for LYSO:Ce scintillator light readout, with low

multiplication noise and acceptable gain uniformity.

The Hamamatsu S8550 APD was selected for the

Clear-PEM detector, which offers a stable working

regime up to gain 200 (Abreu et al., 2007), and has a

dark current noise below 27 electrons ENC at room

temperature (Kapusta et al., 2003).

The total number of APD arrays needed for a

single scanner is 384, and the S8550 arrays were

manufactured by Hamamatsu Photonics Inc. Japan.

A total of 984 photodetectors were acquired for

assessment.

A quality control protocol and methodology were

developed in order to assess and characterize the

acquired APDs. In this paper the adopted quality

check procedures for the APD arrays is described

and the controlled parameters are presented (gain

dependence on bias voltage, gain variation at various

voltages, dark current dependence on the bias

voltage).

2 CLEAR-PEM IMAGING

SYSTEM

2.1 Detector Heads

The Clear-PEM imaging system (Figure 1) is based

on a two parallel detector heads design. The detector

consists in two compact and planar detector heads

with adequate dimensions for breast and axilla

imaging. A dedicated gantry was built in order to

allow the rotation of the detector heads in breast

exams as well as to permit exams of the axilla

region. Each detector head holds the scintillator

material matrices and the frontend readout circuitry

composed of multi–pixel APD photosensors,

frontend ASIC chips, free–sampling ADCs and

Channel Link LVDS serializers. Auxiliary sub–

systems are also assembled in the detector heads,

providing environmental monitoring, cooling, power

supply and system clock distribution. A dedicated

digital trigger and data acquisition system is used for

online selection of coincidence events with high

efficiency, large bandwidth and negligible dead-time

(Albuquerque et al 2006, Albuquerque et al 2008a).

Figure 1: The Clear-PEM imaging system (CAD image).

The structure of a detector head starts in the

detector module. The detector module is composed

by the LYSO:Ce crystal matrix, optically coupled

(through an optical glue) to a S8550 APD array, on

each end, for DOI measurements. The APD array is

mounted in a small PCB with a low footprint SMT

connector on the back side. The components of the

detector module are housed and sealed in a

dedicated plastic assembly. The assembly has two

empty slots in which two detector modules can be

plug–in, defining a ”double module”. 12 detector

modules are mechanically fixed and electrically

connected to a front and back Frontend Boards

(FEBs) forming a Supermodule (Figure 2)

(Albuquerque et al, 2008b).

BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices

Figure 2: Supermodule structure assembling 12x32

LYSO:Ce crystals and 24 APDs in double readout. Each

Frontend board has two 192 input channels ASICs.

The assembled modules underwent a quality

control measurements carried out at LIP (Amaral et

al., 2006). The electronics chips were mounted on

the external faces of the two FEBs. Connectors for

cables linking to the data acquisition system were

also mounted in the PCBs. In a detector head, eight

Supermodules, each with 12 modules, are mounted

together.

The final prototype has two planar parallel

detector heads with 192 detector modules, 6144

crystals and 384 APD arrays (12 288 APD pixels),

covering a 17×15 cm

Field-of-View.

2.1.1 Crystals

The chosen scintillator for the Clear-PEM was a

inorganic crystal, LYSO:Ce. The LYSO:Ce is

cerium activated lutetium–yttrium–ortho–sylicate

(Lu

2(1−x)

SiO

:Ce) and has similar properties to

LSO:Ce (Lu

SiO

) (Melcher and Schweitzer, 1992).

The choice of this scintillator was dictated by its

high light output (27-30 photons/keV) compatible

with good energy measurements (9% at 662 keV

with a PMT readout), as well as a fast rise time and

decay (42 ns time constant) compatible with high–

quality timing measurements and low dead time. Its

peak emission is short–blue/long ultraviolet (UV)

wavelength (420 nm).

For the Clear-PEM scanner, crystals with

dimensions of 2×2×20 mm

were chosen. The 20

mm longitudinal size guarantees a large detection

efficiency for 511 keV photons and the

determination of the DOI coordinate with a

resolution of 2 mm (FWHM) (Abreu et al., 2006,

Amaral et al., 2006).

In total the scanner uses 6144 pixel LYSO:Ce

crystals (Figure 3a) divided in 192 matrices,

composed by a 4×8 LYSO:Ce crystal array (Figure

3b). In this configuration, the crystals are optically

isolated by 300 µm BaSO

reflector walls. The

BaSO

provide the crystal support and provides the

diffuse reflecting surfaces needed to optimize the

light collection and the DOI measurements.

Figure 3: Photograph of (a) a sample of 2×2×20 mm

LYSO:Ce crystals produced for the Clear-PEM scanner

and (b) an assembled BaSO

–type matrix.

2.1.2 Photosensors

The S8550 APD array from Hamamatsu Photonics

with a 32 pixels in an 8×4 configuration was the

selected photodetector for the Clear-PEM scanner

(Figure 4a and b). The S8550 array is assembled

from two distinct monolithic silicon wafer parts, of

2×8 ”reverse” type structure pixel elements, and

each 2x8 group is called an sub-array. The sub-array

that contains the APD pixels from A1 to H1, and

from A2 to H2, is called sub-array 1. The remaining

pixels are contained in the sub-array 2. Each sub-

array is biased independently. The pixels are

mounted on a 1 mm thick ceramic package with a

0.5 mm thick epoxy window (Kapusta et al., 2003).

Figure 4: (a) Schematic representation of the 32 pixel

Hamamatsu S8550 APD array (dimensions in millimetres)

and (b) photograph of a S8550 APD array.

Each Si pixel element has a 1.6×1.6 mm

compatible with an individual 1:1 readout of 2×2

cross–section LYSO:Ce crystals. The element

pitch is 2.3 mm and all the pixels placed in the same

sub–matrix share the same common bias.

(a)

(b)

(a)

(b)

AVALANCHE PHOTODIODES FOR HIGH-RESOLUTION PET IMAGING SYSTEMS

A compilation of the Hamamatsu S8550 main

electrical and optical characteristics are present in

Table 1. The effective APD gain (ratio of the total

number of secondary avalanche electrons produced

by the initial number of electron–hole pairs due to

the scintillation light) is between 70 and 350 with a

specified inter–pixel gain variation less than 5%

r.m.s. and a dark current of 2–4 nA per pixel. The

terminal capacitance is 10 pF per pixel (Abreu et al.,

2007).

Table 1: Hamamatsu S8550 APD 32 channel electrical and

optical parameters (Abreu et al, 2007, Mosset, 2006).

Parameter Hamamatsu S8550

Pixel Size 1.6 x 1.6 mm

Pixel Pitch 2.3 mm

Window Type 0.5 mm epoxy resin

Peak Wavelength 600 nm

QE @ 420 nm 72-76%

Gain (M)

Gain Gradient @ M=70

Dark Current

Capacitance

50 – 350

3.6%/V

2.4 nA (pixel @ M=70)

10 pF (pixel @ M=70)

The S8550 APD operates at a bias voltage of

360-500 V, depending on the required gain. The

gain shows a temperature gradient of –2.4%/◦C at

gain 70. This is due to lattice vibrations in the silicon

structure of the APD which are enhanced as

temperature increases making more probable to

interact with avalanche secondary electrons.

Temperature drifts, if not controlled, may thus

originate gain drifts contributing to a detioration of

the energy resolution (Crespo et al., 2004,

Spanoudaki et al., 2005). The temperature gradient

is also function of the bias voltage, which means that

at higher gains the APDs show an increased

susceptibility to temperature drifts. All this imply

that the system has to operate under stable thermal

conditions. The gain gradient as function of the

polarizing bias is observed to vary as function of the

gain. For the highest gain the bias polarization

supplies must be very stable with controlled amounts

of tension ripple (Abreu et al., 2007).

2.2 Electronic Systems

The frontend and data acquisition electronics

systems are key components in the developed PET

system, enabling a high detection sensitivity and low

random background noise, without compromising

the spatial resolution, allowed by fine segmented

crystals. The data acquisition and trigger electronics

of the Clear–PEM scanner is composed by three

main blocks (Albuquerque et al., 2006):

 The Frontend Electronics System, performs

signal amplification, channel selection and

analog multiplexing, analog to digital

conversion and parallel–to–serial translation;

 The Trigger and Data Acquisition System,

which implements the temporary data storage,

first–level trigger (L1) computation and data

transfer to the acquisition computer (DAE

server);

 The Software Trigger (L2), implemented in the

DAE server, in which the received data from

the DAE crate is un–packed, extraction

algorithms (time, energy, DOI) applied and

the trigger re–validated;

The on–detector electronics includes the

amplifier and analog multiplexing integrated circuits

(frontend ASIC). The front-end system is based in a

data-driven synchronous design that identifies and

multiplexes the analog signals of channels above

threshold, reducing the number of channels by a

factor of 96. To transmit the signals from the

detector heads, digital serializers are used to

minimize the number of lines connecting to the

trigger and data acquisition system (Rodrigues,

2007).

The off–detector system receives the serialized

digitized data streams, applies a coincidence trigger

based on the computation of the pulses amplitude

and timing, and pushes the data into the data

acquisition computer. The trigger and data

acquisition logic is implemented in large FPGAs

with 4 and 3 million gates respectively, with the

trigger algorithm decomposed in a sequence of

elementary operations executed in pipeline mode.

The system was designed to operate at a maximum

input clock frequency of 100 MHz and be able to

sustain a data acquisition rate of 1 MHz with

efficiency above 90–95%, under a maximum total

single photon background rate in the detector of 10

MHz. The communication between the readout

system and the data acquisition computer is

established through a serial high–speed link

allowing the off–detector crate to be located several

meters away from the DAE server data acquisition

computer.

2.3 Mechanical Systems

The Clear-PEM mechanical system was designed to

allow the exam of both the breast and the auxiliary

lymph nodes. The system is used in conjunction with

a shielded examination table that enables exams to

be performed with the patient kept in prone position.

Configurable openings in the examination table

BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices

allow the exam of both breasts with the detector

heads positioned in each side of the breast (Abreu et

al., 2006). During the exam the detector heads can

rotate around the detector main axis in order to

collect data at several angular orientations as

required for tomographic reconstruction.

The examinations of the breast region close to

the chest and of the axilla region are performed in a

front-back configuration. In these exams one

detector head is facing the breast (complementary

exam), or the shoulder (axilla exam), under the

scanner table and the other is positioned against the

patient back.

3 QUALITY CONTROL OF

AVALANCHE PHOTODIODES

The scanner construction starts with the quality

control (QC) and gain characterization of all APD

arrays acquired by the PET consortium, in order to

assess which ones are useable in the final prototype.

The project commercially acquired 984 APD arrays

which needed a fast evaluation. An automatic

measuring setup was developed, based on the direct

measurement of the APD-array common cathode

current when illuminated by a blue LED light (to

simulate the LYSO:Ce scintillation). This automatic

measuring setup was named “APD Tester”. The blue

light (420 nm) from the LED is homogeneously

distributed by a light guide through all APD pixels.

The characterization consists in the measurement

of the bias voltage needed to operate the APD arrays

at gains 70, 150 and 350, the variation of gain per

volt (dM/dV) at each of these gain regions, and also

the Dark Current (Id), for the same gain regions.

Each individual APD sub-array is characterized

independently (1968 sub-arrays, 9 characterization

parameters each, gives a total of 17712 measurement

values).

For the QC part, the chosen acceptance values at

gain 70 were:

 Bias Voltage ≤ 500V;

 Gain Variation ≤ 4.0%/V;

 Dark Current ≤ 160 nA (10 nA per pixel);

The parameters were selected accordingly to the

Hamamatsu specifications (and tolerances) and are

applied to each sub-array. If one of the parameters is

not approved, the APD array is marked as “bad” and

sent back to the manufacturer.

3.1 APD Tester

The APD Tester is a dedicated electronic setup for

the Hamamatsu APD array S8550 gain and dark

Current quality control and characterization (Figure

5). One of the main features of this dedicated

electronic system is the automatization of the

measurement procedure, in order to save time and

manpower. It measures 16 APD arrays (32 sub-

arrays) in a single run (or several), in about 4 hours.

It also controls the blue LED, used to simulate the

scintillation light.

Figure 5: APD Tester, an electronic setup designed to the

automatic characterization of the S8550 APD Array.

The electronic was completely designed and

developed at LIP. The Tester was assembled inside a

metal box (Faraday cage) due to the need of a

protection from any outside signal interference, and

to keep the APD arrays away from the outside light.

The LED is located inside the box and is coupled to

a light guide to provide an equal light distribution to

all APD sub-arrays (and pixels). The tester setup is

composed by two major parts: HV Control and

Digital Control.

The digital control is composed by HV relays to

select the APD sub-array to be measured, from the

possible 32. The relays are controlled through a

FPGA Cyclone II (ALTERA DE2 Board) via serial

port. The LED is also controlled by the digital part.

The HV Control receives the HV from a Keithley

6487 Picommeter/Voltage Source, controlled via

serial port, and delivers it to the enabled APD sub-

array. The automatic setup, and the Keithley, are

controlled via serial ports by a LabView program,

which selects the measurement type and collects the

data into an excel file.

AVALANCHE PHOTODIODES FOR HIGH-RESOLUTION PET IMAGING SYSTEMS

3.2 Experimental Setup

The experimental setup is composed by the APD

Tester (in the Faraday cage), a Keithley 6487

Picoammeter / Voltage Source and a PC. The gain

calculation assumes that when the APD sub-array is

biased at 30V (with the LED on), the gain is 1. From

that value the M=70, 150 and 350 are calculated. For

the dark current measurements the LED is turned off

and the residual current is measured. The schematic

of the experimental setup can be found in Figure 6.

Figure 6: Schematic representation of the experimental

setup for gain and dark current measurements.

3.3 Measurement Procedure

Each measurement run is accompanied by a stability

check procedure (performance on a reference APD

array) in order to evaluate the setup in terms of

significant variations on the studied parameters. The

setup operates at a stable temperature (through AC

control) of approximately 24ºC, in order to minimize

temperature dependence deviations. The complete

measurement took about 30 working days (2 run sets

per day approximately). The stability check consists

in the measurement of a predefined APD array, the

control APD. Each measurement run characterized

15 new APD plus the control APD.

3.3.1 Bias Voltage Quality Control and

Characterization

The 984 APD matrices (1968 sub-arrays) were

characterized in bias voltage terms and Table 2

summarizes the results.

All APD passed the HV QC at M=70 (the bias

voltage values were all below 500V) -

Figure

7. The HV values oscillated between 360

and 459 V - Figure 8.

Table 2: Bias Voltage average values for gains M=70, 150

and 350 from the 984 APD arrays (also the maximum and

minimum HV).

Bias Voltage (V) Sub-array 1 Sub-array 2

M=70

Average (r.m.s) 419 ± 22 418 ± 21

Minimum 360 364

Maximum 458 459

M=150

Average (r.m.s) 434 ± 22 434 ± 21

Minimum 375 379

Maximum 473 474

M=350

Average (r.m.s) 443 ± 22 442 ± 21

Minimum 384 389

Maximum 482 481

Figure 7: High Voltage per APD sub-array distribution at

M=70.

Figure 8: High Voltage histogram for M=70 values.

3.3.2 Gain Variation Quality Control and

Characterization

The gain variation of the 984 APD matrices was

characterized and Table 3 summarizes the results.

All but one APD sub-array had a gain variation

of less than 4%/V, passing the Gain variation quality

control at M=70 – Figure 9.

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The APD that didn’t pass the QC procedure had

a dM/dV of about 22.7 % on the second sub-array

(the other had a normal value, 3.2 %/V).

Table 3: Gain Variation average values for gains M=70,

150 and 350 from the 984 APD arrays (also the maximum

and minimum dM/dV).

Gain Variation

(%/V)

Sub-array 1 Sub-array 2

M=70

Average (r.m.s) 3.6 ± 0.1 3.6 ± 0.6

Minimum 3.2 3.3

Maximum 4.0 22.7

M=150

Average (r.m.s) 6.3 ± 1.8 6.0 ± 0.8

Minimum 5.4 5.4

Maximum 26.5 19.4

M=350

Average (r.m.s) 14.0 ± 4.7 13.5 ± 4.6

Minimum 9.2 6.0

Maximum 30.7 31.6

Figure 9: Gain Variation per APD sub-array distribution at

M=70.

3.3.3 Dark Current Quality Control and

Characterization

The Dark Current characterization of the 984 APD

matrices was completed and

Table 4 summarizes the

results.

All APD sub-array passed the Dark Current QC

at M=70 (all the values were below the stipulated

maximum limit of 160nA, less than 10 nA per APD

pixel according to Hamamatsu Photonics) – Figure

10.

4 CONCLUSIONS

The quality control and characterization procedure

for the acquired APD arrays for the Clear-PEM

prototype was defined and implemented. A

dedicated automatic APD characterization

electronics was developed and built. The 984

Hamamatsu S8550 APD arrays were submitted to

the QC and characterization procedure at gains 70,

150 and 350, in terms of bias voltage, dark current

and gain variation per volt.

Table 4: Dark Current average values for gains M=70, 150

and 350 from the 984 APD arrays (also the maximum and

minimum Id).

Dark Current

(nA)

Sub-array 1 Sub-array 2

M=70

Average (r.m.s) 27.1 ± 13.3 24.2 ± 10.1

Minimum 7.8 8.2

Maximum 88.9 67.6

M=150

Average (r.m.s) 43.1 ± 21.1 40.0 ± 19.2

Minimum 10.1 12.1

Maximum 138.4 300.0

M=350

Average (r.m.s) 107.1 ± 67.0 103.7 ± 65.9

Minimum 15.9 17.6

Maximum 334.2 700.0

Figure 10: Dark Current per APD sub-array distribution at

M=70.

The QC (M=70) had very good results: average

bias voltage of 419 and 418V for sub-array1 and 2

respectively, average dark current of 27.1 and

24.2nA, for sub-array1 and 2, and average gain

variation of 3.6%/V for both sub-arrays.

From a total of 984 tested APD arrays only 1

(0.1%) didn’t pass the QC procedure (due to a

22.7%/V gain variation in the second sub-array).

The small variance on the different electrical

characterization parameters points out that current

available APDs are suitable for high-integration PET

prototypes which could not be implemented using a

classical photomultiplier readout.

AVALANCHE PHOTODIODES FOR HIGH-RESOLUTION PET IMAGING SYSTEMS

ACKNOWLEDGEMENTS

The authors would like to thank colleagues from the

Portuguese PET Consortium and the Crystal Clear

Collaboration for their suggestions and contribution.

This project is financed by AdI (Agência de

Inovação) and POSI (Operational Program for

Information Society), Portugal. The work of P.

Rodrigues and A. Trindade was supported by FCT

under grants SFRH/BPD/37233/2007 and

SFRH/BPD/37226/2007. The work of R. Bugalho,

B. Carriço, C. S. Ferreira, R. Moura, C. Ortigão and

J. F. Pinheiro was supported by AdI.

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