P UBLISHED BY IOP P UBLISHING FOR SISSA
R ECEIVED: May 16, 2011
ACCEPTED: June 20, 2011
P UBLISHED: July 5, 2011
A dual-ended readout PET detector module based on
GAPDs with large-area microcells
a Department
of Electronic Engineering, Sogang University,
1 Shinsu-Dong, Mapo-Gu, Seoul 121-742, Republic of Korea
b Department of Nuclear Medicine, Samsung Medical Center,
Sungkyunkwan University School of Medicine,
50 Ilwon-Dong, Gangnam-Gu, Seoul 135-710, Republic of Korea
E-mail: ychoi.image@gmail.com
A BSTRACT: The use of a dual-ended readout PET detector module based on Geiger-mode avalanche
photodiodes (GAPDs) with large-area microcells was proposed to obtain high photon detection efficiency (PDE) and overcome energy non-linearity problems. A simulation study was performed
and experimental measurement were taken for the single- and dual-ended PET detector modules
consisting of the two types of GAPDs with 50×50 µm2 and 100×100 µm2 microcells. A Monte
Carlo simulation was conducted to predict the number of incident photons impinging on the GAPD
entrance surface to estimate the light collection efficiency (LCE) and energy linearity performance.
A depth of interaction (DOI) ratio histogram was also obtained. An experimental study was performed to acquire the spectra of different energy γ-rays, and the energy linearity was evaluated
by analyzing the photo-peak channels. The simulation results showed that the LCE and energy
linearity of the dual-ended PET detector modules were considerably improved compared to the
single-ended one, with 100×100 µm2 microcell GAPDs. We also estimated that the proposed
method can provide accurate (3–4 mm) and uniform DOI resolution. In the experimental measurement, the 511 keV photo-peak channels of the dual-ended PET detector modules were increased
26% and 71% compared to the single-ended one, with 50×50 µm2 and 100×100 µm2 microcell
GAPDs, respectively. The coefficients of determination (R2 ) were increased from 0.97 to 0.99 and
from 0.86 to 0.93 with 50×50 µm2 and 100×100 µm2 microcell GAPDs, respectively. The results
of this study demonstrate that the dual-ended readout scheme using GAPDs with large-area microcells provides high LCE and DOI information with minimized energy non-linearity. This will
enable investigators to configure PET detector modules with high sensitivity and resolution.
K EYWORDS : Electronic detector readout concepts (solid-state); Optical detector readout concepts
1 Corresponding
author.
c 2011 IOP Publishing Ltd and SISSA
doi:10.1088/1748-0221/6/07/P07003
2011 JINST 6 P07003
Jihoon Kang,a,b Yong Choi,a,b,1 Key Jo Hong,a,b Wei Hu,a,b Jin Ho Jung,a,b
Yoonsuk Huh,a,b Hyun Keong Lima,b and Byung-Tae Kimb
Contents
Introduction
1
2
Materials and methods
2.1 Materials
2.2 Simulation study
2.2.1 Light collection efficiency (LCE)
2.2.2 Energy linearity performance
2.2.3 DOI capability
2.3 Experimental measurements
2.3.1 Energy spectra of the 511 keV γ-rays
2.3.2 Energy linearity performance
2.3.3 DOI capability
3
3
4
4
4
5
5
5
5
5
3
Results
3.1 Simulation study
3.1.1 Light collection efficiency (LCE)
3.1.2 Energy linearity performance
3.1.3 DOI capability
3.2 Experimental measurements
3.2.1 Energy spectra of the 511 keV γ-rays
3.2.2 Energy linearity performance
3.2.3 DOI capability
5
5
5
6
6
6
6
6
8
4
Discussion
8
5
Conclusion
9
1
Introduction
Recently, a next generation photo-sensor, the Geiger-mode avalanche photodiode (GAPD), has
been developed and actively studied as a PET photosensor [1]–[9]. GAPDs consist of a densely
packed matrix with hundreds or thousands of microcells, and have a typical size range from 5×5
to 100×100 µm2 . Each microcell operates independently in a Geiger mode, and the total number
of fired microcells reflects the number of absorbed photons.
However, a finite number of microcells induces a significant deviation of energy linearity when
the number of converted photons is greater than the number of microcells. This results from an
inherent GAPD property of operating as on/off switch for the photons. Consequently, only a unit
pulse is generated from a single microcell for a period of 0.1 to 10 µsec even if two or more photons
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1
(b)
(c)
Figure 1. GAPD non-linearity problems (a) and practical approaches ((b) and (c)) for PET applications. (a)
A finite number of microcells produces significant energy linearity deviation when the number of entering
photons is larger than the number of microcells. (b) A common approach for improving the energy linearity
is the use of GAPDs with small-area microcells at the expense of PDE and gain. (c) A proposed approach
is the use of a dual-ended readout PET detector configuration consisting of two GAPDs with large-area
microcells which improves the energy linearity and LCE without considerable decrease of the PDE and
gain of the GAPD.
are absorbed within this time [10]. Light collection efficiency (LCE) might also be decreased due
to these saturation effects. These non-linearity properties may render implementation of GAPDs
with large-area microcells impractical for high photon flux detection and PET (figure 1(a)) [4]–[6].
A common approach for overcoming problems with non-linearity is the use of GAPD with smallarea microcells for developing a LSO-based PET detector (figure 1(b)) [11]–[14]. However, this
approach is associated with drawbacks including decreased photon detection efficiency (PDE) and
gain caused by the increase of dead space in the active GAPD area.
We therefore propose a new approach involving a dual-ended readout PET detector configuration in which generated photons in the LSO crystal are detected by two GAPDs (figure 1(c)).
The main advantage of this method is an increase in the number of microcells detecting the photons without modifying the microcell size. It is possible to improve the energy linearity and LCE,
while there is no need to decrease the PDE or gain of the GAPD. Additionally, this configuration
could provide information about the depth of γ-ray interaction that could be used to improve the
degradation of spatial resolution caused by parallax error.
The aim of this study was to investigate the advantages of the dual-ended PET detector modules based on GAPDs with large-area microcells, compared to a single-ended one. A simulation
study was performed and experimental measurements were taken to characterize light collection
efficiency (LCE), energy linearity and depth of interaction (DOI) capability of both detector configurations.
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(a)
(b)
(c)
(d)
Figure 2. Schematics of the four PET detector modules. Single-ended (top row; (a) and (b)) and dual-ended
(bottom row; (c) and (d)) readout configurations were composed of the two types of GAPDs containing
50×50 µm2 (left column; (a) and (c)) and 100×100 µm2 (right column; (b) and (d)) microcells.
2
2.1
Materials and methods
Materials
Four PET detector modules were evaluated: two single-ended readout PET detector modules consisting of the two types of GAPD with 50×50 µm2 and 100×100 µm2 microcells (figure 2(a) and
(b)), and two dual-ended readout PET detector modules consisting of two different microcell sizes
as described above (figure 2(c) and (d)).
Each of the two types of 3×3 mm2 GAPDs (Hamamatsu Photonics, Hamamatsu, Japan) was
coupled to a 3×3×20 mm3 LYSO pixel crystal (Sinocera, Shanghai, China) with optical grease
(Saint-Gobain Crystals, Hiram, USA). The scintillation crystals were polished on all faces and
covered with white epoxy except for the surface facing the photosensor. The primary GAPD specifications according to the manufacturer’s data and specification sheets are listed in table 1 [15].
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(a)
Table 1. GAPD specifications.
GAPD type 2
50×50
100×100
No. of microcells
3600
900
Fill factors (%)
61.5
78.5
Bias voltage (VBias ) (V)
70.3
69.9
PDE @ 420 nm, 0.5V above VBias (%)
30
45
Gain
7.5×105
24×105
Dark counts @ 0.5 threshold (Mcps)
6.5
7
microcell sizes
2.2
Simulation study
A Monte Carlo simulation using DETECT2000 [16] was conducted to investigate the light distribution after conversion of the γ-ray in LYSO crystal using a light yield of 27 photons/keV [17, 18].
The LYSOs, only bottom surface was polished in the single-ended readout configuration, and both
top and bottom surfaces were polished in the dual-ended one, were coupled to the GAPD with
optical grease. The other surfaces of the LYSOs were covered with white reflective paint with a
reflection coefficient (RC) of 0.98. A total of 10000 γ-ray interactions with the different random
numbers were generated to obtain sufficient statistical accuracy. The impinging photons on the
GAPD entrance surface were simulated and recorded for each interaction event. The number of
fired cells was calculated using eq. (2.1) [19].
Nfired microcells = Ntotal microcells × 1 − exp − ε ×
Nphotons
Ntotal microcells
(2.1)
where, ε, PDE = Quantum efficiency × Fill factor × Geiger-mode avalanche probability
2.2.1
Light collection efficiency (LCE)
In order to evaluate whether or not LCE is improved by the proposed dual-ended PET detector
module compared to a conventional single-ended PET detector module, the number of fired cells
and output electrons for 511 keV γ-rays were estimated. The LCE was calculated using eq. (2.2).
LCE =
2.2.2
Number of fired microcells
Number of entered photons
(2.2)
Energy linearity performance
Energy linearity performance was estimated by simulating four γ-rays with different energies (122
keV, 511 keV, 662 keV and 1275 keV). The number of output electrons as a function of γ-ray energy was estimated and the linear fit of these four data points was determined for each of the four
PET detector modules.
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GAPD type 1
(µm2 )
2.2.3
DOI capability
For the two dual-ended PET detector modules, event-by-event data were analyzed for seven different DOI locations, ranging from 1 mm to 19 mm along the LYSO crystal, in 3 mm increments. The
DOI ratio was calculated using eq. (2.3) [20].
DOI ratio =
S2
S1 + S2
(2.3)
where, S1 is the detected fired cell number from one end and S2 is the detected fired cell number
from the other end.
Experimental measurements
GAPD outputs were fed into custom-made charge sensitive preamplifiers and then into shaping
amplifiers for signal shaping and inversion. The amplified signals were digitized and recorded
by a commercially available DAQ unit (VHS-ADC-V4; Lyrtech Inc, Quebec, Canada) [21]. The
GAPDs with 100×100 µm2 and 50×50 µm2 microcells were operated at 70.4 and 70.7 V, respectively. The temperature was stabilized at 25 ◦ C with a cooling fan during the measurements.
2.3.1
Energy spectra of the 511 keV γ-rays
The energy spectra of 511 keV γ-rays were acquired for 5 min for each of the four PET detector
modules. A Na-22 point source with a 0.5 mm diameter and an activity of ∼200 kBq was placed
10 mm away from the crystal.
2.3.2
Energy linearity performance
Additional energy spectra were acquired using different point sources (Co-57 with ∼130 kBq of
activity and Cs-137 with ∼260 kBq of activity) and energy linearity performance was assessed by
photo-peak channels obtained from Gaussian fit. These experiments were repeatedly performed for
the four PET detector modules using the same electronic setup described above.
2.3.3
DOI capability
The dual-ended PET detector modules were irradiated by a collimated F-18 source at different DOI
positions. The data acquired from each GAPD were further processed to calculate the energy spectra (output1 + output2) and DOI ratio (output2/ (output1 + output2)) for each incident interaction
as a function of the DOI position. The DOI resolution was analyzed from the full width at half
maximum (FWHM) for each histogram.
3
Results
3.1
3.1.1
Simulation study
Light collection efficiency (LCE)
The number of simulated incident photons was 5855 ± 59 for the single-ended PET detector module and 6395 ± 58 for the dual-ended module. The calculated results are summarized in table 2.
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2.3
Table 2. Improved LCE of the dual-ended PET detector modules with 50×50 µm2 and 100×100
µm2 microcell GAPDs.
Microcell size
50×50 µm2
100×100 µm2
Detector types
Single-ended
Dual-ended
Single-ended
Dual-ended
No. of fired microcells
1390
1684
852
1436
No. of output electron
1.0×109
1.2×109
2.0×109
3.4×109
LCE (%)
23.7
26.3
14.5
22.5
3.1.2
Energy linearity performance
The energy linearity of output electrons as a function of γ-ray energy improved considerably with
the dual-ended PET detector module configuration (figure 3). The estimated coefficient of determination (R2 ) for the dual-ended configurations increased from 0.96 to 0.99 and from 0.78 to 0.87 for
the 50×50 µm2 and 100×100 µm2 microcell GAPDs, respectively, compared to the single-ended
one.
3.1.3
DOI capability
Figure 4 shows the profiles generated by the incident γ-rays for the two dual-ended PET detector
modules at seven different depths. The average DOI resolutions were 3.8 ± 0.2 mm and 4.1 ± 0.3
mm, with the 50×50 µm2 and 100×100 µm2 microcell GAPDs, respectively.
3.2
3.2.1
Experimental measurements
Energy spectra of the 511 keV γ-rays
No considerable improvement of energy resolution was observed with the dual-ended configuration compared to the single-ended one. However, the photo-peak channels of the dual-ended PET
detector modules were increased 26% and 71% for the 50×50 µm2 and 100×100 µm2 microcell
GAPDs, respectively (figure 5). These increments were similar to those estimated by the simulation
study shown in table 2.
3.2.2
Energy linearity performance
Energy linearity performance was considerably improved (figure 6) and was similar to the results
obtained from the simulation study (figure 3). Compared to the single-ended GAPD, the R2 values
of the 50×50 µm2 and 100×100 µm2 microcell GAPDs increased from 0.97 to 0.99 and from 0.86
to 0.93, respectively.
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Compared to the single ended module, the numbers of fired microcells in the dual-ended PET detector modules were increased 21% with 50×50 µm2 GAPD and 68% with 100×100 µm2 one.
Moreover, the LCE values of the dual-ended PET detector modules were increased 9% for the
50×50 µm2 GAPD and 55% for the 100×100 µm2 one compared to the single ended module.
Figure 4. DOI ratio histograms for the dual-ended PET detector modules with 50×50 (left) and 100×100
µm2 (right) microcell GAPDs.
Figure 5. Energy spectra of the four PET detector modules with 50×50 µm2 (left) and 100×100 µm2
(right) microcell GAPDs.
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Figure 3. Improved energy linearity performance was achieved using dual-end PET detector modules with
50×50 µm2 (left) and 100×100 µm2 (right) microcell GAPDs.
Figure 7. DOI ratio histograms for the dual-ended PET detector modules with 50×50 µm2 (left) and
100×100 µm2 (right) microcell GAPDs.
3.2.3
DOI capability
Figure 7 shows the depth profiles for two dual-ended PET detector modules at seven different
depths. The average DOI resolutions were 3.7 ± 0.2 mm and 3.8 ± 0.2 mm, for the 50×50 µm2
and 100×100 µm2 microcells GAPDs, respectively.
4
Discussion
A simulation study was performed and experimental measurement were taken for the single- and
dual-ended PET detector modules designed using two different GAPDs with 50×50 µm2 and
100×100 µm2 microcell sizes (figure 2). Light collection efficiency (LCE), energy linearity, and
DOI capability were considerably improved with the dual-ended PET detector modules based on
GAPDs compared to the single-ended one. This improvement was predicted and well-characterized
by the simulated estimations.
The proposed design could provide several advantages. Improved energy linearity allows the
use of GAPDs with large-area microcells for PET by providing high photon detection efficiency
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Figure 6. Energy linearity performance using the dual-end PET detector modules with 50×50 µm2 (left)
and 100×100 µm2 (right) microcell GAPDs.
(PDE). Increased LCE would improve the time resolution by reducing rise time and spatial resolution by increasing signal to noise ratios [13]. Furthermore, the proposed method could provide
continuous DOI capabilities (accurate and uniform resolution of 3–4 mm) which represent an additional and inherent benefit of dual-ended readout PET detector configurations [22]–[24].
A major drawback of the dual-ended PET detector module is that it requires twice the number
of GAPDs and readout electronic, compared to the single-ended module. Additionally, significant
GAPD-related bias voltage and temperature dependence require a careful tuning of the bias voltage
settings and temperature stabilization for the pair of GAPDs in the dual-ended configuration.
Conclusion
This study established and examined the dual-ended PET detector module configuration to overcome non-linearity problems associated with GAPDs containing large-area microcells. This configuration can considerably improve the energy linearity properties which may render implementation of these GAPDs practical for PET applications. Our results demonstrated that GAPDs with
large-area microcells could be widely used for many applications including high photon detection,
and may replace conventional photomultipliers, avalanche photodiodes, GAPDs with small-area
microcells. Moreover, our proposed configuration has a number of potential advantages such as
providing high LCE and DOI capabilities. These will allow the configuration of PET detector
modules with high resolution and sensitivity capabilities using GAPDs with large-area microcells.
Acknowledgments
This research was supported by the Converging Research Center Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology
(2010K001109), and by the Technology Innovation Program funded by the Ministry of Knowledge
Economy (10030029), Republic of Korea.
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