Hybrid PET/MR Scanners:
A New Tool for
BIomedical Imaging?
Simon R. Cherry, Ph.D.
Center for Molecular and Genomic Imaging
Department of Biomedical Engineering
University of California, Davis
Image courtesy of Siemens
In Vivo Biomedical Imaging
Anatomic
Physiologic
Metabolic
Molecular
optical imaging
x-ray CT
PET/SPECT
MRI
ultrasound
PET/CT…a winning combination
Courtesy GE Medical Systems
Images courtesy of David Townsend, University of Tennessee
…a less successful combination
microwave/PC
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definition…
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recovering from a Windows
crash.
img.engadget.com
PET + MRI =
twice the power
or
double the trouble?
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Clinical
MRI
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Components of an MRI System
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Why PET/MR?
• Strengths
– “Near-perfect” registration of structural and molecular
imaging data
– Anatomically-guided interpretation of PET data
– Anatomic priors for PET reconstruction and data
modeling
– PET can be combined with advanced MRI techniques
such as DWI, DCE MR, MRS, cell tracking and MR
molecular imaging agents
• Weaknesses
– Technically difficult and likely expensive
– Uncertainty regarding throughput, cost effectiveness and
ultimate clinical role
Technical Challenges
in PET/MRI
• Interference on PET
– Static magnetic field
– Electromagnetic interference from RF and gradients
• Interference on MR
–
–
–
–
Electromagnetic radiation from PET electronics
Maintaining magnetic field homogeneity
Eddy currents
Susceptibility artifacts
• General Challenges
– Space
– Environmental factors (temperature, vibration…)
– Cost
Approaches to PET/MRI
“tandem” PET/MRI
PET
“integrated” PET/MRI
magnet
magnet
magnet
magnet
+ interference easier to avoid
+ largely use existing hardware
+ least expensive
+ simultaneous PET/MRI possible
+ higher throughput
+ best image registration
MR Compatible PET System
Animal MR System
PET Detectors
Magnet
Gradient Coils
RF coil
Concept
Detector Technology Used in PET
scintillator
scintillator
PSPMT
array of PMTs
position-sensitive PMT
Considerations: Photomultiplier tubes (PMTs) are fast,
have high gain and are very sensitive to magnetic fields
PMT Sensitivity to Magnetic Fields
7T = 70,000 gauss!
Position-Sensitive
Avalanche Photodiodes
Si avalanche photodiode with positionsensitive resistive anode
Active area: 8x8 to 28x28 mm
Gain ~ 1000 at 1750 V
Noise = 200 e– (FWHM)
Q. E. ≥ 60 % (400-700 nm)
Rise time ~ 1 ns
Capacitance 0.7 pf/mm2
MR-Compatible
PET Detector Module
Si avalanche photodiode with position-sensitive
resistive anode, field tolerant beyond 9.4 T
scintillator array
optical fiber bundle
PSAPD
preamplifiers
PET Insert
preamplifiers
PSAPDs
optical fibers
scintillator ring
PET-MRI Set Up
Gradient
set
RF coil PET insert
PET/MRI System
PET
Number of detector modules
16
Ring diameter
60 mm
Axial FOV
12 mm
Transaxial FOV
35 mm
Number of crystals
1024
Insert length
55 cm
Insert outer diameter
11.8 cm
MRI
Field Strength
7T
Gradients
40 G/cm, 0.2 G/cm/A
Clear Bore
12 cm
RF coil
Bruker 35 mm or custom
whole-body mouse
MR Effects on PET Data Acquisition
outside
magnet
inside 7T
magnet
inside magnet
spin echo
inside magnet
gradient echo
MR Effects on PET Data Acquisition
• Reconstructed Spatial Resolution
–
–
–
–
no sequences running
RARE sequence
Spin echo sequence
FLASH sequence
1.19 mm
1.18 mm
1.19 mm
1.19 mm
• Energy Resolution
–
–
–
–
no sequences running
RARE sequence
Spin echo sequence
FLASH sequence
22%
22%
22%
22%
single
single
pulse
pulse
100%
50%
baseline
baseline
100%
50%
3,000
2,900
2,800
2,700
2,700
2,600
2,600
2,500
2,400
2,300
2,200
2,200
2,100
2,100
2,000
2,000
100%
50%
Counts
Counts
MR Effects on PET Data Acquisition
X
X grad
grad
Y
Y grad
grad
Z
Z grad
grad
PET event rate measured in different conditions; baseline represents the events
recorded without running MR sequences; for the single pulse two different
repetition times were used; gradients were run at 100% and 50% power.
Positron Range
one factor limiting spatial resolution of PET
511 keV
positron
range
511 keV
Using High-Field Magnets
to Reduce Positron Range
Hammer BE, Christensen NL, Heil BG. Use Of A Magnetic Field To Increase The Spatial
Resolution Of PET. Medical Physics 21 (12): 1917-1920 Dec 1994.
Hammer BE, Christensen NL. Measurement Of Positron Range In Matter In Strong
Magnetic Fields. IEEE Transactions On Nuclear Science 42 (4): 1371-1376, 1995.
Positron energy 3 MeV
0 Tesla
7 Tesla
From Wirrwar et al, IEEE Trans Nucl Sci 44: 184-189, 1997
Positron Range Measurements
0.5 mm i.d. glass capillary tube in 5.6 cm diameter phantom
Profile through summed projection data
18F
86Y
76Br
Eavg= 250 keV
Eavg= 661 keV
Eavg= 1180 keV
Effect of PET Insert on MRI
Bruker 7T/30 BioSPEC system
Cylindrical phantom, Magnevist® in water (T1=250 ms)
Small animal RF coil (35 mm)
Spin Echo sequence (TR=1000 ms, TE=11.6 ms)
128 x 128 matrix size
PET insert
OUT
PET insert
IN
PET insert
IN + power
PET Effects on MR Data Acquisition
Signal-to-noise and uniformity
measured on homogeneous MRI phantom
g
w ith PET
w ithout PET
h
w ithout PET
100
Uniformity (%)
50
40
S/N
w ith PET
30
20
10
90
80
70
60
50
0
RARE
SE
MR sequences
FLASH
RARE
SE
MR sequences
FLASH
PET Effects on MR Data Acquisition
100
+1
_
0
PET on
=
-1
PET off
100
+1
_
0
PET off
=
PET off
-1
In Vivo Simultaneous PET/MRI
Mouse
FDG Tumor Imaging
–
–
–
–
–
–
PET
~200 µCi 18F-FDG
Voxel size: 0.35 x 0.35 x
1.5 mm3
MRI
RARE sequence
Whole body imaging RF
coil
FOV=4x4 cm2
Matrix size 256x256
Correlation
of ADC
and FDG
Signals
FDG-PET guided MRS
H20
Choline
Creatine
Mouse 1: Groin Tumor
H20
Creatine
Lipids
Creatine
H20
PRESS: TR/TE: 1685/10ms;
VAPOR H2O suppression
3mm3
voxel;
Lipids
Region
Cho/Cr
High FDG
Tumor
3.1
Low FDG
Tumor
1.7
High Choline may suggest high membrane Muscle
turnover rate = cell proliferation
Negligible
Human PET/MR
PET
•5.45 mCi FDG injected approx.
2.5 hours prior to acquisition
•OSEM 3D reconstruction
•Attenuation correction
performed based on MR data
MR
•T1 MP-RAGE, T2 SPACE
(shown), FLAIR, DTI, CSI, SVS
sequences run simultaneously
•CP coil
Catana/Benner/van der Kouwe/Andronesi/Jennings/Gerstner/Plotkin/Rosen/Sorensen (MGH)
Applications for PET/MRI
• Clinical applications in which MR is preferred
anatomic imaging modality to CT
• Applications requiring excellent spatial
registration between MR and PET images
• Correlation of functional or physiologic MR
measurements with PET
• Temporal correlation of signal from MR
contrast agents with radiotracers
• Temporal correlation of PET with MR
Spectroscopy (e.g. 19F/18F or 13C/11C)
Summary
• Prototype PET/MR systems have been
successfully developed for small animal and
human imaging
• Dual modality probes are being developed
for a range of applications
• PET/MR is a powerful multimodality platform
for biomedical research
• Translation for clinical applications is being
pursued
Acknowledgments
NIH R01 R01 EB000993
UC Davis: Ciprian Catana (MGH/Harvard), Yibao Wu,
Angelique Louie, Ben Jarrett, Jinyi Qi, Bo Peng, Jeff Walton
Caltech: Russell Jacobs, Daniel Procissi, Thomas Ng, Andrey
Demyanenko
RMD Inc.: Kanai Shah, Richard Farrell, Mickel McClish,
Purushottam Dokhale
University of Tübingen: Bernd Pichler, Martin Judenhofer
City of Hope: Andrew Raubitschek