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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• 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.

DWI

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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