2009 IEEE Nuclear Science Symposium Conference Record
M13-27
MR Compatible Brain PET Using
Tileable GAPD Arrays
Jin Ho Jung, Yong Choi, Member, IEEE, Key Jo Hong, Ji Hoon Kang, Wei Hu, Byung Jun Min, Yoon Suk Huh,
Seung Han Shin, Hyun Keong Lim, Dae Shik Kim, and Han Byul Jin
Abstract– The aim of this study is to develop a MR compatible
PET that is insertable to MRI and allows simultaneous PET and
MR imaging of human brain. The brain PET having 72 detector
modules arranged in a ring of 330 mm diameter was designed.
Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm ×
20 mm LYSO crystals coupled to a tileable 4 × 4 array Geigermode avalanche photodiode (GAPD) and designed to locate
between RF and gradient coils. Signals of the each module were
transferred to preamplifiers using flexible flat cable of 3 m long,
and then sent to a position decoder circuit (PDC), which outputs
digital address and an analog pulse of the one interacted channel
from preamplifier signals. The PDC outputs were fed into FPGAembedded DAQ boards. The analog signal was digitized, and
arrival time and energy of the signal were calculated and stored.
All electronics were located outside MR bore to minimize signal
interference between PET and MR. Basic performance of the
PET components and cross-compatibilities of the PET module
and MR were evaluated. Imaging performance of the designed
brain PET was investigated using Monte Carlo simulation and
experimental measurement. The degradation of PET
performance caused by the 3 m long cable and the PDC was
negligibly small. No obvious differences of the PET module
performance measured inside/outside MR bore were observed.
The SNR of various MR sequence phantom images acquired
with/without the PET module were also similar. Activity
distribution patterns of hot-rod phantoms were well imaged
without distortion, and rods down to a diameter of 3.5 mm were
resolved in both simulation and experiment. Gray and white
matter of the Hoffman brain phantom was also well imaged.
Preliminary experimental results demonstrate that MR
compatible high quality PET imaging is feasible using the GAPD
arrays, electronics, signal processing method and MR insertable
PET design schemes developed in this study.
development of combined PET/MR, which is a useful tool for
both functional and anatomic imaging [1],[2].
The purposed of this study is to design and fabricate a MR
compatible PET system that is insertable to MRI and allows
simultaneous PET and MR imaging of human brain. The
performance the designed PET was estimate using Monte
Carlo simulation method. The cross-compatibility of the PET
module based on Geiger-mode avalanche photodiode (GAPD)
arrays and MRI was evaluated. The performance of prototype
PET consisting of 72 PET modules and signal processing
components was also evaluated.
II. MATERIALS AND METHODS
A. MR Compatible Brain PET Design and Fabrication
1) PET detector module
PET detector module consisted of a LYSO (Sinoceramics,
Shanghai, China) scintillator blocks coupled to a 4 × 4 array
GAPDs (SensL, Cork, Ireland). The scintillator block
composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm crystals.
The individual crystal elements were mechanically polished
on all sides and optically isolated with a 0.3 mm white epoxy
resin. Each pixel of the GAPD array had a 2.85 mm × 2.85
mm sensitive area and a 3.3 mm pitch. The scintillator was
directly coupled to the GAPD without optical-coupling
material. Each PET detector module was encapsulated from
light.
I. INTRODUCTION
is a useful imaging modality to provide functional
Pinformation
about a specific organ or body system. PET,
ET
however, provides relatively poor spatial resolution and also
limited anatomic information. A combined PET/CT has been
utilized to overcome the limitation of PET using anatomic
information provided by CT. However, CT has low soft-tissue
contrast and lead to additional radiation exposure compared
with MRI. Recently, there has been great interest on the
This study was supported by a grant of the Mid-Term Industrial
Technology Development Program, the Ministry of Knowledge Economy
(10024198), by a grant of the Industrial Source Technology Development
Programs, the Ministry of Knowledge Economy (10030029), and by a grant of
the Radiation Technology Development Program through the Korea Science
and Engineering Foundation funded by the Ministry of Education, Science
and Technology (2007-00321), Republic of Korea.
The authors are with the Department of Nuclear Medicine, Samsung
Medical Center, Sungkyunkwan University School of Medicine, Seoul, 135710, Korea (e-mail: ychoi@skku.edu).
9781-4244-3962-1/09/$25.00 ©2009 IEEE
Fig. 1. 4 × 4 matrix LYSO crystals and a 4 × 4 array GAPD used to
construct PET detector module.
2) Analog and digital signal processing
The signals of the each module were fed into 16 channel
preamplifiers using flexible flat cable of 3 m long, as shown in
Fig. 2(b). Then, the preamplified signals were sent to a
position decoder circuit (PDC), which readout channel address
and analog pulse of the channel interacted with coincidence
event among 64 preamplified signals transmitted from 4
detector modules. Fig. 3 illustrates configuration of position
decoder circuit.
The output signals from the PDC were fed into VHS-ADC
Virtex-4 boards (Lyrtech, Quebec, Canada) with free-running
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analog to digital converters and field programmable gate array
(FPGA). Algorithm to calculate arrival time and energy of the
digitized signal was programmed in the FPGA. Processed
signals were stored in a list mode format.
Fig. 2. PET detector module and preamplifier connected using FFC of 300
cm long.
Fig. 4. Brain PET system consisting of PET detector, analog and digital
electronics.
Fig. 3. Configuration of position decoder circuit.
Fig. 5. Location of the PET detector and electronics inside MRI.
3) Brain PET system
The brain PET system consisted of 72 detector modules
arranged in a ring of 330 mm diameter, 72 preamplifiers, 18
PDCs and 3 DAQ boards, as shown in Fig. 4. PET detectors
were located between RF and gradient coils. All electronics
including preamplifiers were located outside MR bore to
minimize the signal interference between MR and PET. Fig. 5
illustrates the location of the PET detector and electronics
inside MRI.
B. Performance Estimation of the Brain PET Using Monte
Carlo Simulation
The spatial resolution over the FOV was estimated by
simulating the point source as a function of source location
using Geant4 application for tomographic emission (GATE).
Imaging performance of hot-rod phantom filled with 13 MBq
F-18 radioactive source and 3D digital Hoffman brain
phantom filled with 96 MBq were also estimated, as shown in
Fig. 6.
Fig. 6. Brain PET detector and 3D digital Hoffman brain phantom
simulated to estimate the PET imaging performance.
C. Basic Performance Evaluation of PET Components
1) Effect of cable length between detector module and
preamplifier on PET performance
Two PET detector modules were constructed and located at
the opposite side each other and separated by 10 mm. A 0.2
MBq Na-22 point source was placed at the center between
them.
The signals of the PET detector modules were transferred to
preamplifiers using a 10 cm or 300 cm FFC. Energy and
timing spectra were measured.
2) Performance evaluation of PET with and without
position decoder circuit
Two PET detector modules were constructed and located at
the opposite side each other and separated by 10 mm. A 0.2
MBq Na-22 point source was placed at the center between
them.
Energy and timing spectra were acquired with and without
position decoder circuit.
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D. Cross-compatibility of the PET module and MR
Two PET detector modules were located at the opposite
side each other, and a 0.2 MBq Na-22 point source was placed
at the center. Each PET module was encapsulated from light.
Energy and timing resolutions of a pair of PET detectors were
measured. Same experiments were performed with gradient
echo (GE) or spin echo (SE) sequences running after inserting
them into the bore of a 7T MRI (Bruker, Ettlingen, Germany).
MR cylinder phantom (10 mm diameter, 50 mm length) was
filled with CuSO4 and placed at the isocenter, as shown in Fig.
7. The phantom images were acquired with and without the
PET module while running GE or SE sequences.
phantom was well simulated. Fig. 10 shows tomographic
images of hot-rod and Hoffman brain phantoms.
Fig. 9. Spatial resolution as a function of source position.
Fig. 10. Tomographic images of Hot-rod phantom (left) and software
Hoffman brain phantom (right) acquired using GATE simulation.
Fig. 7. MR phantom (CuSO4) used to evaluate the cross-compatibility of
the PET module and MR.
E. Performance Measurement of the Prototype PET
The prototype brain PET consisting of 72 detector modules
was constructed. Hot-rod and Hoffman brain phantoms were
filled with 74 MBq and 55 MBq, respectively. Total
coincidence counts of hot-rod and Hoffman brain phantoms
acquired using developed electronics were 1.0 and 6.8 million,
respectively.
All phantom images were reconstructed using a 2D FBP.
Normalization and random correction were performed to
improve image quality.
B. Basic Performance Evaluation of PET Components
1) Effect of cable length between detector module and
preamplifier on PET performance
Average energy resolutions were 20.1±4.1% and 20.8±3.1%
by using 10 cm and 300 cm cable, respectively.
The timing resolutions were 1.8 ns and 1.9 ns by using 10
cm and 300 cm cable, respectively.
Fig. 11 illustrates energy and timing spectra acquired with a
10 cm and 300 cm FFC.
Fig. 11. Energy (left) and timing (right) spectra acquired with a 10 cm
(gray) and 300 cm (black) FFC.
Fig. 8. The prototype brain PET consisting of 72 detector modules. Each
detector module was independently encapsulated from light.
III. RESULTS
A. Performance Estimation of the Brain PET Using Monte
Carlo Simulation
Radial resolution of the brain PET was degraded from 3.3
mm to 6.1 mm, at a 100 mm off-center, as illustrated in Fig. 9.
The rods down to a diameter of 3.5 mm were clearly resolved
in hot-rod phantom image. Activity distribution pattern
between white and gray matter in the software Hoffman brain
2) Performance evaluation of PET with and without
position decoder circuit
The difference of energy resolutions measured with and
without the PDC was negligibly small.
As shown in Table I, the timing resolutions were 2.4 ns and
1.9 ns in the detector modules with and without the PDC,
respectively.
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Table I. Energy and timing resolution measured with PDC without PDC.
C. Cross-compatibility of PET Module and MR
As shown in Table II and III, no significant degradation of
PET performance caused by MR was observed. Distortion of
MR phantom image caused by PET detector module was not
found.
Table II. Energy and timing resolutions according to PET position and to
applied MR sequences.
timing resolution was decreased from 1.9 ns to 2.4 ns by using
the PDC because length of signal transmission line according
to the location of input channel on printed circuit board was
different. Nevertheless, the measured timing resolution was
comparable to that of commercial small animal PET (<2 ns)
[8,9] and human PET (<6 ns) [10].
The GATE simulation results on the spatial resolutions
across the FOV indicate that depth of interaction information
needs to be considered to improve the degradation of spatial
resolution at off-center of FOV.
Currently, timing improvement and scatter correction are
being implemented to improve the quality of the brain PET
image. Additionally, mechanics and magnetic shielding to
operate the prototype brain PET inside MR bore are being
designed.
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Table III. Effect of PET insert on MR phantom images.
D. Performance Measurement of the Prototype PET
Activity distribution patterns of hot-rod and Hoffman brain
phantoms were successfully acquired, as illustrated in Fig. 12.
The rods down to a diameter of 3.5 mm were resolved in hotrod phantom image. Activity distribution pattern between
white and gray matter in Hoffman Brain phantom was well
imaged.
Fig. 12. Tomographic images of Hot-rod phantom (left) and 3D Hoffman
brain phantom (right) acquired using the prototype PET.
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IV. DISCUSSION AND CONCLUSION
In this study, a MR insertable brain PET consisting of 72
detector modules based on GAPD array was designed and
constructed. PET images were successfully acquired using the
developed prototype PET. The results of this study
demonstrated that the designed PET detector module has good
MR compatibility. Minimal interference between PET and
MRI was observed. The cross compatibility was improved by
using the 300 cm FFC. The degradation of PET performance
by the 300 cm cable was negligibly small. The coincidence
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