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

1. MRI-PET; A new approach for multi modality imaging system Karl ZiemonsMember of Crystal Clear Collaboration Dr. Karl Ziemons Forschungszentrum Jülich GmbH Central Institute of Electronics Tel.: +49-2461-615685 Fax: +49-2461-613990 E-Mail: K.Ziemons@fz-juelich.de Frontiers in Imaging Science - Rome 2006

2. 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 brainto the development of new treatments for cancer Frontiers in Imaging Science - Rome 2006

3. 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 Frontiers in Imaging Science - Rome 2006

4. Complementary nature of MRI & PET Hence: The Sum of PET and MRI should be excellent and even betterMRI + PET << MRI-PET Frontiers in Imaging Science - Rome 2006

5. Good reasons for Hardware Fusion  PET/CT Fusion CT • Highly specific tracer • Focal uptake 80 MBq, 4h pi, 6 min / bed position (M Hofmann, MH Hannover / Bern) 68Ga-DOTATOC BUT: PET/CT is not truly simultaneous Danger of movement  artefacts in quantitative PET images Frontiers in Imaging Science - Rome 2006

6. hybrid scanner PET insert axial offset PET PET scanner Possible Geometries: MR & PET scanner solutions: M.Schwaiger, S.Ziegler, et al., 2005 Simultaneous measurements will provide functional and anatomical information with nearly perfect spatial co-registration! Spatial co-registration better than temporal co-registration Frontiers in Imaging Science - Rome 2006

7. B0 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. Simulation of positron end points: 68Ga point source surrounded by water, Raylman et al. (1996) 0 Tesla 10 Tesla Frontiers in Imaging Science - Rome 2006

8. 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)) Frontiers in Imaging Science - Rome 2006

9. 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!! Frontiers in Imaging Science - Rome 2006

10. MR scanner PSPMT System electronics Fibers >1m long scintillators PSPMT PMT based PET/MR design Typical PET detector with long (1 - 4m) optical fibers Frontiers in Imaging Science - Rome 2006

11. Technical Developments • APD detector arrays • Insensitive to magnetic field • Higher QE as PM but lower gain • compact • miniaturization of electronics • Multichannel preamplifier chips • Analog and digital preprocessing with FPGA’s (field programmable gate arrays) or ASIC’s (application specific integrated circuit) Frontiers in Imaging Science - Rome 2006

12. MR scanner System electronics scintillators APDs electronics Cables >1m long APD based PET/MR Typical PET detector with special MR shielding Frontiers in Imaging Science - Rome 2006

13. MRI-PET Design Considerations PET ring inside gradient: • Easy removal of PET ring for maintenance and repair • Higher S/N for PET • Annihilation photons need only traverse RF coil --> minimal scatter • Gradients need more current • Stronger coupling of RF coil M.Schwaiger, S.Ziegler, et al., 2005 Frontiers in Imaging Science - Rome 2006

14. PET/MR Design Challenges • 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 Frontiers in Imaging Science - Rome 2006

15. PET detector must not use magnetic materials  Distortions and interference lines are produced in a uniform phantom image APD pin with thin nickel coating (1-2cm distortion) APD pin containing iron and nickel (3-4 cm distortion) PET/MR Design Challenges Courtesy by R.Grazioso, Siemens Frontiers in Imaging Science - Rome 2006

16. 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. Artifact-free RF artifacts PET/MR Design Challenges Courtesy by R.Grazioso, Siemens Frontiers in Imaging Science - Rome 2006

17. MR-compatible PET shielding materials PET/MR Design Challenges Spin -echo Gradient -echo (Courtesy of D. Struhl, et al., TNS 2003) Hot-pressed lead monoxide Red lead oxide powder Pure lead powder Bulk lead BGO Various materials can produce an MR signature even if it is not conductive or magnetic. Frontiers in Imaging Science - Rome 2006

18. PET/MR Design Challenges • 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 Frontiers in Imaging Science - Rome 2006

19. MR Radio frequency degrading the energy and timing resolution of the PET system. PET detector inside copper shield PET signal (123 MHz RF being amplified) RF antenna (123MHz RF) PET/MR Design Challenges Courtesy by R.Grazioso, Siemens Frontiers in Imaging Science - Rome 2006

20. Simultaneous Measurement in 1.5 T 2.0mm 2.5mm 1.5mm 3.0mm 1.0mm 3.5mm PET Insert for a Clinical MRI Scanner • APD based detector • Multi-slice PET images • High quality MR images Courtesy by R.Grazioso a. M.Schmand, Siemens Frontiers in Imaging Science - Rome 2006

21. BrainPET Project (G)-APD PET insert: trapezoid monolithic block design:LSO block with two (G)-APD in double readout technology and MR compatible Goal: System peak sensitivity: > 15% Spatial resolution: < 1.3mm over the whole FOV Current CCC project Frontiers in Imaging Science - Rome 2006

22. Simultaneous Imaging of a [F-18]-FDG Mouse Head with Two Coincident APD Based LSO Block-Detectors and a 7Tesla Small Animal MRT System 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) PET MRT 70/30 Bruker Biospec system. B-GA20 gradient set. Micro Imaging Coil. FLASH MRT sequence. 1mm slice thickness. Courtesy by B.Pichler, Tübingen Frontiers in Imaging Science - Rome 2006

23. “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,”) Frontiers in Imaging Science - Rome 2006

24. MRI-PET: Which Problems to solve? • 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 Frontiers in Imaging Science - Rome 2006

25. MRI based AC: Brain CT Different Bone signal PET-based Attenuation Correction Corresponding MR images MRI-based segmentation (whole brain, bone, soft-tissue, sinus arca) (IEEE, 2006: E. Rota Kops, et al.) MRI Frontiers in Imaging Science - Rome 2006

26. Problem of Patient Motion (18F-Altanserin) Motion affected Motion corrected Should be derived from MRI?! Frontiers in Imaging Science - Rome 2006 [Herzog et al., JNM, 2005]

27. Why MRI-PET Hybrid Imaging? • 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 MRI Frontiers in Imaging Science - Rome 2006

28. Conclusion PET/CT was a medical revolution and a technical evolution MR/PET seems to be a technical revolution and a medical evolution Frontiers in Imaging Science - Rome 2006

29. 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 Frontiers in Imaging Science - Rome 2006