Current Status of PET/MR The next disruptive technology? Barry A. Siegel, M.D. Mallinckrodt Institute of Radiology
Disclaimer/Disclosure • Simultaneous PET/MR scanner currently only available from one manufacturer, and this inherently biases presentation • Speaker is a consultant for Siemens Molecular Imaging
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PET/MR Options
PET/MR: Technical Challenges
• Sequential PET (or PET/CT) and MR with patient transport system to minimize motion • Simultaneous PET and MR in an integrated system
• MR gradients/B0/RF interferes with PMTs in PET
– Many engineering challenges, including PET effects on MR, MR effects on PET, space constraints, cost
– Solution: Avalanche photodiodes (APDs)
• PET electronics interfere with MR – Solution: RF shielding for PET components; eddy current suppression; initial validation shows no significant impairment
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PET/MR: Technical Challenges
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Biograph mMR
• Physical/mechanical constraints – Solution: Solid state APD's make low-profile PET detectors; 70 cm MRI frees enough space for solid-state detectors, associated electronics, and integrated cooling, leaving 60-cm gantry for patient (typical MR bore size)
• MR hardware causes attenuation/scatter of PET signal (annihilation photons) – Solution: PET-compatible RF coil design (Tim); hardware-based μ maps
Simultaneous acquisition of MR and PET data Fully integrated solid-state PET detector architecture PET-compatible Tim coil technology MR-based attenuation correction of PET data
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Biograph mMR Early adopters
MR-compatible, solid-state PET architecture
North America
Europe
MGH, Boston, USA
IMP Erlangen, Germany
NIH, Bethesda, USA
Klinikum rechts der Isar, Munich, Germany
Washington Univ., St. Louis, USA
Univ. Hospital Tübingen, Germany
Univ. of N. Carolina, Chapel Hill, USA
Univ. Hospital Leipzig, Germany
Mt. Sinai Medical Center, New York City, USA
CEMODI Bremen, Germany
Indiana University, Indianapolis, USA
Univ. Hospital Essen, Germany
Lawson Health Research Institute, London, Canada
University College London Hospitals, UK
Asia PLA 301, Beijing, China Parkway Novena Hosp., Singapore
SDN, Naples, Italy
UPMC, Pittsburgh, USA
Biopolis/CIRC, Singapore
DKFZ, Heidelberg, Germany
NYU, New York City, USA Cleveland Clinic Foundation, Cleveland. USA Stony Brook University, Stony Brook, USA New York-Presbyterian University Hospital of Columbia and Cornell, NY, USA
Youngnam University, Korea
Rigshospitalet, Copenhagen, Denmark
SNUH, Seoul, Korea
Trondheim, Norway
Apollo Hospitals, Delhi, India
DLRZ, Bonn, Germany
Fukushima Medical Univ, Fukushima, Japan
© Siemens AG 2011. All rights reserved.
© Siemens AG 2011. All rights reserved.
PET/CT vs. PET/MR PET/CT • Pros: – Validated for oncologic diagnosis, staging, and monitoring treatment – Fast acquisition (90 seconds/ station) – Established method for attenuation correction (AC) based on electron density/ x-ray transmission
• Cons: – Additional ionizing radiation – Low soft tissue contrast (especially if non-contrast CT)
PET/MR • Pros: – Simultaneous acquisition (better anatomic localization, motion correction of PET may be possible)
– High soft tissue contrast – Many research opportunities
• Cons: – AC still being refined – Long MR examination times compared to CT – Motion sensitive, requiring triggered, fast free breathing, or breath-held sequences
Photos courtesy Bob McKinstry
PET/MR: Attenuation Correction • No electron density information possible • Tissue segmentation approach (air, lung, soft tissue, fat) – 2-Point Dixon 3D-volumetric interpolated breath-held examination (VIBE) – Martinez-Moller A, et al., in a study of 35 patients showed minimal difference in SUV vs. PET/CT – Does not account for cortical bone – For brain, ultrashort TE (UTE) sequence and/or atlas/template-based approach may provide better performance
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NAC PET
AC PET
Fused
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PET/MR vs. PET/CT Comparison Aim: “…to evaluate comparability of the clinical performance between conventional PET/CT and PET/MR in patients with oncological diseases.“
T2 Haste
AC PET
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NAC PET
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• 32 patients with various cancers • Single‐injection exam • PET/CT → PET/MR • Measured/rated detectability, image quality, alignment, and SUV quantification Drzezga et al. Integrated simultaneous whole-body MR/PET: first comparison between MR/PET and PET/CT in patients. J Nucl Med, in press.
PET/MR vs. PET/CT Comparison • Anatomical localization comparable" using Dixon‐ MR versus low‐dose CT • Comparable PET/MR and PET/CT lesion detection • High correlation (p=0.93) between lesion SUVs measured by PET/MR and PET/CT • Concluded that PET/MR is "feasible in a clinical setting with high quality and in a short examination time”
PET/MR: When? • When repeated radiation exposure is a concern – Children, young adults
• Imaging of body regions where CT anatomy and contrast is suboptimal – Brain, skull base, head and neck, pelvis
• Capitalizing on improved tissue contrast of MRI and high sensitivity/metabolic specificity of PET – Breast, liver, prostate, cervix, rectum
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Drzezga et al. Integrated simultaneous whole-body MR/PET: first comparison between MR/PET and PET/CT in patients. J Nucl Med, in press.
Head and Neck Cancer Metabolism
• 66-year-old man with laryngeal cancer and suspicious level two lymphadenopathy
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PET/CT
Metabolism
PET images show an FDG-avid, right level 2 lymph node, which is more conspicuous compared with adjacent muscle on MRI image than on non-contrast CT.
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Cervical Cancer Staging • 31-year-old woman with cervical CA: initial staging evaluation
PET/MR and PET/CT images show uptake in primary cervical tumor, bilateral external iliac nodes and left para‐aortic nodes . Improved soft tissue contrast on the MR images allows more confident localization.
Diffusion-weighted MRI and FDG-PET: Complementary? ADC Cervical carcinoma. Note restricted diffusion in the cervical mass correlating with region of highest FDG uptake.
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T2 space
PET/MR: Opportunities AC PET
ADC
NAC PET
DWI
Cervical cancer with nodal metastasis. The external iliac lymph node is highly conspicuous on the high b value diffusion‐weighted image and has a low ADC indicating restricted diffusion.
Selected PET/MR Research Projects at Washington University
Courtesy of Ciprian Catana, MGH
PET/MR: Challenges
• Oncology – – – –
Optimization of body PET/MR protocols Tumor heterogeneity as a prognostic biomarker in cervical cancer Optimization of simultaneous PET/MR data acquisition in breast cancer PET/MR in patients undergoing therapeutic Y90 radioembolization
• Neurology – qBOLD MR vs. O-15 PET measurements of oxygen extraction fraction in patients with brain tumors – Functional neuroimaging in refractory major depression – PET of D2 receptors in schizophrenia with [11C]N-methylbenperidol – Amyloid PET, hippocampal volume, and fMRI assessment in dementia
• Cardiac – Development of a PET/MR method for characterization of myocardial infarction heterogeneity – Development of a streamlined PET/MR myocardial perfusion study
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• Optimal imaging protocols still being defined – Goal: to match timing and critical diagnostic information for both whole-body and focused exams – Most experience to date, Martinez-Moller A, et al. 2012: exam time 30 to 60+ minutes depending on body part – Need whole-body protocols that: » Are fast enough to compete with PET/CT » Provide added information value over PET/CT
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PET/MR: Challenges
Current Status of PET/MR?
• Need solutions to various technical problems – AC: cortical bone, susceptibility artifacts – Non-isotropic whole-body MR data – Study duration
• Training of NM physicians to interpret MRI • Showing that added value is worth the cost
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More questions than answers MIR
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