14.11.2006 Frontiers in Imaging Science - Rome 2006 1 MRI-PET; A new approach for multi modality imaging system Karl Ziemons Member of Crystal Clear Collaboration

14.11.2006 Frontiers in Imaging Science - Rome 2006

1

MRI-PET; A new approach for multi modality imaging system

Karl ZiemonsMember of Crystal Clear CollaborationDr. Karl Ziemons

Forschungszentrum Jülich GmbH

Central Institute of Electronics

Tel.: +49-2461-615685

Fax: +49-2461-613990

E-Mail: [email protected]

14.11.2006Frontiers in Imaging Science - Rome 20062

Forschungszentrum JülichCentral Institute of Electronics

Motivations

PET & MRI are medical imaging techniques

that in widespread use both for patient diagnosis and management, and in clinical research

playing a key role in a wide range of fields from mapping of the human brain

to the development of new treatments for cancer



Motivations

PET in-vivo information about metabolism and functionality

(f)MRI Anatomical information with a better soft tissue contrast

as CT and does not apply additional radiation dose

in-vivo information about neuronal activities;the so-called blood oxygenation level dependent (BOLD) contrast has proved to be a very sensitive MRI marker



Complementary nature of MRI & PET

Parameter MRI PET

Anatomical Detail Excellent Poor

Spatial Resolution Excellent Compromised

Clinical Penetration Excellent Limited

Sensitivity Poor Excellent

Molecular imaging Limited Excellent

Hence: The Sum of PET and MRI should be excellent and even better MRI + PET << MRI-PET

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are needed to see this picture.





Good reasons for Hardware Fusion PET/CT

80 MBq, 4h pi, 6 min / bed position

68Ga-DOTATOC

(M Hofmann, MH Hannover / Bern)

CT

•Highly specific tracer•Focal uptake

BUT: PET/CT is not truly simultaneous

Danger of movement artefacts in quantitative PET images

Fusion



Possible Geometries:

axial offset

Spatial co-registration better than

temporal co-registration

M.Schwaiger, S.Ziegler, et al., 2005

PET

hybrid scanner

PET scanner

PET insert

MR & PET scanner solutions:

Simultaneous measurements will provide functional and anatomical information with

nearly perfect spatial co-registration!



Expectations from MRI-PET

Resolution enhancement for PET is negligible! reduction of positron range in a strong magnetic field

(Iida et al. (1986)) Positron range limits the effective spatial resolution in PET

imaging However, only effective in the plane transverse to the MRI field.

0 Tesla 10 Tesla

Simulation of positron end points:68Ga point source

surrounded by water,Raylman et al. (1996)B0



Expectations from MRI-PET (2)

PET reduction of positron range from no magnetic field up to 10 Tesla(Table modified from Raylman et al. (1996))

Radionuclid Max. Energy (MeV)

FWHM (mm), 0T

FWHM (mm)

0 T vs. 10 T

11C 0.96 4.24 0.3913N 1.19 4.44 0.5215O 1.70 5.28 1.4018F 0.64 3.85 0.0768Ga 1.89 5.46 1.5682Rb 3.15 8.03 3.90



Application of Simultaneous MRI-PET Scans

Receptor displacement studies– Fusion of PET studies and functional MRI

– Explore BOLD effect (blood oxygenation level dependence)

– Investigate interaction between receptor activation and regional perfusion

NOTE: In general need arterial input function for model calculation– Appropriate monitoring devices have to be

developed!!



PMT based PET/MR design

Typical PET detector with long (1 - 4m) optical fibers

scintillators

Fibers >1m long

PSPMT

MR scanner

Systemelectronics

PSPMT



Technical Developments

1. APD detector arrays– Insensitive to magnetic field

– Higher QE as PM but lower gain

– compact

2. miniaturization of electronics– Multichannel preamplifier chips

– Analog and digital preprocessing with FPGA’s (field programmable gate arrays) or ASIC’s (application specific integrated circuit)



APD based PET/MR

Typical PET detector with special MR shielding

scintillatorsAPDs

MR scanner

electronics

Systemelectronics

Cables >1m long



MRI-PET Design Considerations

PET ring inside gradient:

1. Easy removal of PET ring for maintenance and repair

2. Higher S/N for PET

3. Annihilation photons need only traverse RF coil --> minimal scatter

4. Gradients need more current

5. Stronger coupling of RF coil

M.Schwaiger, S.Ziegler, et al., 2005



Limited space for the PET detector Need minimum space for patient

PET detector must not use magnetic materials Could distort MR image

PET detector must not emit in MR frequency Could produce MR image artifacts

MR-compatible PET shielding materials Could distort MR image

PET/MR Design Challenges




PET detector must not use magnetic materials

Distortions and interference

lines are produced in a uniform phantom image

APD pin containing iron and nickel

(3-4 cm distortion)

APD pin with thin nickel coating

(1-2cm distortion)

Courtesy by R.Grazioso, Siemens




PET detector must not emit in MR frequency

RF artifacts can be seen in

the RF noise measurement due to improper shielding of the PET electronics.

RF artifacts

Artifact-free





MR-compatible PET shielding materials

Bulk lead

Hot-pressed lead monoxide

BGOPure lead powder

Red lead oxide powder

(Courtesy of D. Struhl, et al., TNS 2003)

Spin

-echo

Gradient

-echo

Various materials can produce an MR signature even if it is not conductive or magnetic.



MR gradient field-eddy currents Could produce noise in detector Could heat detector

MR RF transmit Could produce false PET events

MR materials Will produce more gamma attenuation





MR Radio frequency

degrading the energy and timing resolution of the PET system.

PET detectorinside copper shield

PET signal

(123 MHz RF

being amplified)

RF antenna

(123MHz RF)




PET Insert for a Clinical MRI Scanner

Simultaneous Measurement in 1.5 T

1.5mm

2.0mm

2.5mm

3.0mm

3.5mm

1.0mm

APD based detector Multi-slice PET images High quality MR images

Courtesy by R.Grazioso a. M.Schmand, Siemens



BrainPET Project

(G)-APD PET insert:

trapezoid monolithic block design:LSO block with two (G)-APD in double readout technology and MR compatible

System peak sensitivity: > 15%Spatial resolution: < 1.3mm over the whole FOV

Current CCC project

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Goal:



MRT

PET

Simultaneous Imaging of a [F-18]-FDG Mouse Head

Step and shoot PET acquisition (12 angles, each 6 min) while MRT images were taken.Filtered Back Projection (2.5 mm Gaussian image filtering post reconstruction)

70/30 Bruker Biospec system. B-GA20 gradient set. Micro Imaging Coil. FLASH MRT sequence. 1mm slice thickness.

with Two Coincident APD Based LSO Block-Detectors and a 7 Tesla Small Animal MRT System

Courtesy by B.Pichler, Tübingen



“New” MR-compatible solid-state PET detectors

Magnetically insensitive detectors for MR/PET Silicon photomultipliers (SiPM)

– an array of geiger-mode APDs showing good timing and energy resolution but not fully developed yet (Eres:12.5%, Tres: 560 psec). (Lorenz et al., “Some studies for a development of a small animal PET based on LYSO crystals and geigermode-APDs”)

Cadmium Zinc Telluride (CZT)– has been investigated for a long time but new for PET. Some

recent results show good timing (2.6ns vs BaF2). (Verger at al., “New trends in gamma-ray imaging with CdZnTe/CdTe at CEA-LETI”)

Hybrid Photodiode (HPD)– sensitive to magnetic fields but can operate normally if

positioned parallel to the magnetic field. (Igor Rubashov, “Apparatus for combined nuclear imaging and magnetic resonance imaging, and method thereof,”)



MRI-PET: Which Problems to solve?

MRI

to measure or calculate attenuation maps for quantitative PET image reconstruction– Need such a solution for Brain and WholeBody

Imaging!!

to assess motion in the FOV– Motion can deteriorate MR and PET images and

may cause serious artefacts! This is especially true for images derived from multiple acquisitions!



MRI based AC: Brain

A) PET-based Attenuation CorrectionB) Corresponding MR imagesC) MRI-based segmentation

(whole brain, bone, soft-tissue, sinus arca)

(IEEE, 2006: E. Rota Kops, et al.)

A) PET-based Attenuation CorrectionB) Corresponding MR imagesC) MRI-based segmentation

(whole brain, bone, soft-tissue, sinus arca)

(IEEE, 2006: E. Rota Kops, et al.)





CT

MRI

DifferentBone signal



Problem of Patient Motion (18F-Altanserin)

[Herzog et al., JNM, 2005]

Motion affected

Motion corrected Should be derived from MRI?!



Why MRI-PET Hybrid Imaging?

MRI

Want true simultaneous data acquisition in a single device

Want combined functional and morphological data acquisition at the same time

Want multi modal functional acquisitions at the same time (fMRI / MRS - PET)

Want to cross-validate activations measured with PET and fMRI under the same conditions, at the same time, in the same status

but still want quantitative PET image



Conclusion

PET/CT was a medical revolution and a technical evolution

MR/PET seems to be a technical revolution and a medical evolution



Acknowledgments

Thank‘s to Ralph Ladebeck and Ron Grazioso (Siemens)

for MR specifications and material

N.J.Shah and U.Pietrzyk (IME) ,T. Beyer (Essen)

And all the others for images and material

Documents

14.11.2006 Frontiers in Imaging Science - Rome 2006 1 MRI-PET; A new approach for multi modality imaging system Karl Ziemons Member of Crystal Clear Collaboration