First results in the application of Silicon Photomultiplier matrices to ...

First results in the application of Silicon 

Photomultiplier matrices to small 

animal PET 

G. Llosá, N. Belcari, M. G. Bisogni, G. Collazuol, A. del Guerra, S. 

Marcatili 

University of Pisa -INFN Pisa, Italy. 

M. Boscardin, M. Melchiorri, A. Tarolli, C. Piemonte, N. Zorzi 

FBK-irst, Trento, Italy. 

P. Barrillon, S. Bondil-Blin, V. Chaumat, C. de la Taille, N. Dinu, 

V. Puill, J-F Vagnucci 

Laboratoire de l'Accélérateur Linéaire, IN2P3-CNRS, Orsay, France. 

This work is partially supported by the European Commision's Sixth 

Framework Program through a Marie Curie Intra-European Fellowship 

gabriela.llosa@pi.infn.it NDIP08 Aix les Bains 15-20 June 2008 1

High resolution small animal PET 

! A high resolution small animal PET scanner with Silicon 

Photomultipliers as photodetectors is under development at the 

University of Pisa. 

! Silicon photomultiplier matrices from the Center from Scientific 

and technological research (FBK-irst , Trento, Italy) will be 

employed. 

! The use of SiPM matrices has significant advantages in the 

design of a detector head for a small animal PET tomograph wrt 

arrays of single SiPMs: 

High photodetector sensitivity: SiPM matrices leave low dead area 

wrt arrays of single SiPMs. 

Small pitch: high spatial resolution. 

Main drawback: high number of output channels 


PET detector head 

! Use of continuous scintillator slabs + finely pixellated SiPM 

matrix can provide better performance than pixellated scintillator 

blocks: 

- Very good spatial resolution with maximum likelihood 

algorithms. 

- high efficiency : no light losses due to small pixel size. 

- lower cost. 

• Stack of several detector layers: 

– Discrete DOI information that reduces 

parallax error => higher spatial 

resolution. 

– High efficiency: increased scintillator 

thickness. 


Detector head performance 

• Optimized geometry and performance estimated with GEANT4 

simulations. 

• Head geometry: stack of three detector layers (4 cm x 4 cm x 5 mm). 

– Continuous slab of LYSO, 5 mm thick=>15 mm total thickness. 

– SiPM matrix with 1.5 mm pitch elements as photodetector. 

• Head performance: 

– About 70% efficiency for 511 keV photons. 

– Intrinsic spatial resolution – 0.3 mm in the center of the crystal; 

< 1mm in the edges. 

• Center-of-gravity position determination algorithms worsen resolution and 

displacement errors towards the edges. 

• ML methods (skeweness and barycenter based) reduce error towards the 

edges. 

S. Moehrs et al. A detector head design for small-animal PET with silicon photomultipliers (SiPM). 

Phys. Med. Biol. 51 (2006) 1113-1127. 


PET applications: rotating tomograph 

4-head tomograph (same concept as YAP(S)-PET): 

– 2(4) rotating detector heads at 10-15 cm distance. 

– FOV 4 cm axial, 4 cm transaxial. 

– efficiency around 4% 

– Spatial resolution better than 1 mm 3 for a point source in the CFOV. 

– low cost. 


MR compatible ring tomograph 

• Ring tomograph to be inserted in a 

magnet bore of an MR system. 

• 16 detector heads, 7 cm x 2.4 cm; 

• FOV axial 7 cm, transaxial FOV ~6 cm. 

• Spatial resolution: 0.76 mm 3 for a 18 F point source 

in the CFOV with FBP. 

• efficiency around 11% for 250 keV energy 

threshold. 

Tests with SiPMs in MR system show no degradation of the data 

gradients off 

gradients on 

gradients off 

gradients on 


SiPM matrices from FBK-irst 

Composed of 16 pixel elements in a common substrate 

1 mm pixels in 1.06 mm pitch 

• Structure: n + -p-! -p + optimized for blue light: 

Shallow n + layer + specific antireflective coating. 

• 625 (25 x 25) microcells, 40 "m x 40 "m size. 

• Polysilicon quenching resistance. 

• Fill factor (GF) 30 – 35%. 

• PDE 8 % at #V=3.5V for $ = 420 nm. 

4 mm 

1 mm 


First results 

• Data acquisition system 

• Characterization 

• Measurements with pixellated crystals. 

• Measurements with continuous crystals. 


Readout: MAROC2 (LAL Orsay) 

• 64 channels (only 16 used) 

• Variable gain (6 bits), low noise preamplifier 

• Slow shaper (~20-150 ns, adjustable) 

• Fast shaper (15 ns) + 3 discriminators=> trigger signal 

• Designed for MAPMT – not optimized for SiPMs, but 

allows us to make the first tests satisfactorily. 


Readout: Test board 

• Altera FPGA 

• USB Port 

• ADC 

• ASIC calibration input. 

• LabVIEW software for 

data acquisition 


MAROC2 calibration 

• Calibration at minimum preamplifier gain: two linear ranges 

before saturation. 

• Good uniformity: % = 0.9 % at 7 pC; % = 1.5 % at 94 pC; 

• SiPM signals with pixellated crystals at high overvoltage (>2V) 

saturate the amplifier. 


Characterization 

• Very good uniformity 

• Breakdown voltage variations %= 0.3% 


Results with pixellated crystals 

Holder for precise positioning: 

Error < 50 "m. 

LYSO crystal array: 16 (4x4) 

crystals of 0.96 mm x 0.96 mm 

x 10 mm separated by 100 "m 

of white epoxy resin. 

Perfect match to the SiPM matrix 



NO COINCIDENCE 

BACKGROUND 

DUE TO 176 Lu 

– One SiPM pixel in the matrix 

– Na-22 energy spectra 

acquired with the 

oscilloscope (no amplifier) 

– 3V overvoltage. 

COINCIDENCE WITH 

PMT + YAP crystal 



• Na-22 spectra from all channels with MAROC2 board 

• Coincidence with 1x1x10 mm LSO crystal + single SiPM 

• Peak position variation %=4.4% 

2 V overvoltage 

peak position variation Na spectrum all channels 


Results with continuous crystals 

• Crystal 4 mm x 4 mm x 5 mm covering 

the whole matrix. 

• Na-22 spectrum summing signals from 

all channels. 

4 V overvoltage 

gabriela.llosa@pi.infn.it NDIP08 Aix les Bains 15-20 June 2008 16 

4 mm 

4 mm

Position determination 

~35 mm 5 mm 

• Coincidence with a 2 nd detector: 1 mm x 1 mm x 

1 mm crystal coupled to a SiPM 

• Source close to the matrix, far from 2 nd detector 

• Move together source and 2 nd detector in 0.5 mm 

steps. 


Position determination 

Hitmap for different source position with crystal array 

+ 1 mm + 2 mm 

+ 0.5 mm + 1.5 mm 


Future work 

• Position determination with continuous 

crystal 

• Timing measurements 

• First test prototype 

– 2 rotating heads 

– Pixellated crystals 

– New readout system 

• Matrices 64 pixels will soon be fabricated. 


Conclusions 

• FBK-irst has developed SiPM matrices with 16 SiPM pixels 

in a common substrate. The first results obtained are very 

good and lead to the fabrication of SiPM matrices with 64 

pixels. 

•A very high resolution PET tomograph for small animals 

employing SiPMs as photodetectors is under development 

at the University of Pisa. A spatial resolution of 0.76 mm 

FWHM is expected for a 18 F point source in water in the 

centre of the FOV, employing FBP for image reconstruction, 

according to GEANT4 simulations. 

• The first tests towards the PET scanner construction are 

ongoing with good results. Tests with both pixellated and 

continuous crystals are being carried on. 

• A first prototype with two rotating heads is being 

assembled, employing pixellated crystals. 


Silicon Photomultipliers 

SiPM : Multicell Avalanche Photodiode working in limited Geiger mode 

- 2D array of microcells: structures in a common bulk. 

-Vbias > Vbreakdown: high field in mult. region. 

- Microcells work in Geiger mode: the signal is independent of the particle energy 

- The SiPM output is the sum of the signals produced in all microcells fired. 

! Significant advantages wrt other photodetectors: 

• High Gain 

• Fast timing 

• High PDE 

• High S/N ratio 

• Insensitive to magnetic fields 

• Low operating voltage 

• Potential low cost 

n + cathode +VGM h& 

p high-electric field 

multiplication region 

! epilayer 

p + substrate 

gabriela.llosa@pi.infn.it NDIP08 Aix les Bains 15-20 June 2008 21 

4 µm 

oxide 

e - 

hole

New detectors 

• Different geometry, size, microcell size and GF. 

40x40!m => GF 44% 

50x50!m => GF 50% 

100x100!m => GF 76% 

• Matrices 16 elements (4x4) 

4 mm 

4 mm 

IV CURVES OF 9 

MATRICES. VERY 

UNIFORM 

BREAKDOWN 

POINT 

N41-2: C. Piemonte. Recent Progress in the 

Performance of Silicon Photomultipliers 

produced at FBK-irst. 

circular (1mm diam) 1x1mm 2x2mm 3x3mm (3600 cells) 4x4mm (6400 cells)

First results in the application of Silicon Photomultiplier matrices to ...

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