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Technical challenges and clinical research applications of ultrahigh field MRI A.G.Webb Professor, Director C.J.Gorter Center for High Field MRI Department of Radiology Leiden University Medical Center Leiden, The Netherlands
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2 Outline Why a very high field scanner? What can it do? Why doesn’t it work and give nice images automatically? How do we address the major challenges? Why do we need the input of medical physicists? Clinical applications and future input into radiotherapy
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3 Philosophy Σα βγεις στον πηγαιμό για την Ιθάκη, να εύχεσαι νάναι μακρύς ο δρόμος, γεμάτος περιπέτειες, γεμάτος γνώσεις.
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7T system ~50 worldwide
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Why ultra-high field MRI? Image quality is proportional to magnetic field strength Signal to noise at 7 tesla 2.3 times higher than 3 tesla Higher resolution and faster (for patients) MRI Improved sensitivity to diffuse iron deposition (neurodegeneration) Intrinsically better angiography to visualize small vessels Increased spectral resolution for metabolic studies
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6 Congratulations on purchasing your new Philips 7T - a bargain at €8,500,000 Compared to your old 3 Tesla Philips is delighted to offer significant increases in…….. Image non-uniformities Potential for heating the patient Questions about safety/implants/dental wires Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering But also significant decreases in Number of RF coils commercially available
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The first technical challenge – design of customized detectors
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8 Image non-uniformities Potential for heating the patient Questions about safety/implants/dental wires Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering
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Image non-uniformities at high field 9 3T4T5T6T7T8T10T12T
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10 General observations at high fields Overall, RF wavelength in tissue decreases with B0 field strength Dielectric constant Relative dielectric constant 100150200250 300 350400 450 500 45 50 55 60 65 70 75 frequency (MHz) Muscle Wavelength (cm) frequency (MHz) 100 150 200 250300350 400 450 500 10 20 30 40 50 60
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RF inhomogeneity constructive/destructive interference 11 ~12 cm
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12 General observations at high fields Solution 1 - multiple transmit channels RF waveform generator 1 Power amplifier Tx/Rx switch 1Coil element 1 Digital receiver 1 RF waveform generator 2 Power amplifier Tx/Rx switch 2Coil element 2 Digital receiver 2 RF waveform generator 3 Power amplifier Tx/Rx switch 3Coil element 3 Digital receiver 3 RF waveform generator N Power amplifier Tx/Rx switch NCoil element N Digital receiver N
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The alternative and slightly cheaper method New, high permittivity materials
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How do dielectric materials work? 14 Displacement currents in the dielectric material produce a secondary local RF field which increases the total B1+
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Dielectric pads in imaging FLAIR TSE normal with pads
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Abdominal imaging at 3 Tesla (a)(b)(c) (d)(e)(f)
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17 Image non-uniformities Potential for heating the patient Questions about safety/implants Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering
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18 General observations at high fields Conductivity increases with frequency Conductivity 100150200250300350400 0.35 0.40 0.45 0.50 0.55 Conductivity of gray matter (S/m) frequency (MHz) P=1/2 E 2
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RF inhomogeneity constructive/destructive interference 19 ~12 cm
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Increased SAR and heating at 7T 20 7T 300 MHz3T 128 MHz SAR (W/kg) 300 MHz128 MHz Temperature rise ( o C)
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21 General observations at high fields How do you ensure safety? The RF engineer is the first person to be tested!
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22 General observations at high fields Call upon the medical physics specialists Requires flexibility
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23 General observations at high fields lack of self-awareness
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24 General observations at high fields Attention to detail
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25 General observations at high fields Rigorous safety testing procedures
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Electromagnetic simulations Phantom heating tests 10 1 0.1 SAR point (W/kg)
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27 Image non-uniformities Potential for heating the patient Questions about safety/implants/dental wires Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering
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Reduction in image quality in patients High quality obtained in volunteers is typically not reproduced in AD patients 28 Healthy volunteerAD patient
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Normal volunteer AD patient 050150 050150 In vivo results of f0 fluctuations before correction 29
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On-line monitoring of frequency variations
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Some examples Image quality is significantly improved 31
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32 Image non-uniformities Potential for heating the patient Questions about safety/implants/dental wires Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering
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Reduced contrast makes segmentation difficult T2*-magnitudeT2*- phase T1
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Specialized image segmentation algorithm
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35 Image non-uniformities Potential for heating the patient Questions about safety/implants/dental wires Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering
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Problems with cardiac triggering Overwhelming magnetohydrodynamic effect
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Develop acoustic triggering Principle developed by Niendorf group on Siemens 7T platform Commercially available for mildly ridiculous price Develop an open-source Arduino-based system for continuous Improvement amongst users
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Develop acoustic triggering
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Technical “solutions” High permittivity materials Accurate SAR modelling On-line “motion” monitoring Acoustic cardiac triggering Phase/magnitude image segmentation
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7T Cardiovascular MR Coronary MRA 7T Cardiovascular MR Coronary MRA
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van Elderen et al., Radiology 2010;257:254-259 7T Cardiovascular MR Coronary MRA, 7T versus 3T 7T Cardiovascular MR Coronary MRA, 7T versus 3T S.G.C.van Elderen, M.J.Versluis, J.J.M.Westenberg, H.Agarwal, N.B.Smith, M.Stuber, A.de Roos and A.G.Webb, In vivo coronary magnetic resonance angiography at 7 Tesla: a direct quantitative comparison with 3 Tesla, Radiology, 257, 254-259, 2010.
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7T Cardiovascular MR Ischemic Cardiomyopathy, RCA 7T Cardiovascular MR Ischemic Cardiomyopathy, RCA
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General observations at high fields Carotid artery vessel wall imaging T1 T2 TOF
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Cochlear imaging MIP
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Inner ear imaging – cochlear implants
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3T 7T
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Musculoskeletal applications of 7 Tesla
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High resolution imaging of the human vertebra Inflammation in spine and sacroiliac joints Ankylosing Spondylitis
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Water/fat images of sacroiliac (SI) joint
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High resolution imaging of the eye
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Uveal melanoma patients ultrasound
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Proton beam therapy planning
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Acknowledgements Itamar Ronen Hermien Kan Maarten Versluis Thijs van Osch Sanneke van Rooden Ece Arcan Francesca Branzoli Sebastian Aussenhofer Eidrees Ghariq Wouter Teeuwisse Mark van Buchem Wyger Brink Paul de Heer Jeroen van der Grond
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Aλλά μη βιάζεις το ταξείδι διόλου. Καλλίτερα χρόνια πολλά να διαρκέσει· και γέρος πια ν’ αράξεις στο νησί, πλούσιος με όσα κέρδισες στον δρόμο, μη προσδοκώντας πλούτη να σε δώσει η Ιθάκη.
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