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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.

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Presentation on theme: "Technical challenges and clinical research applications of ultrahigh field MRI A.G.Webb Professor, Director C.J.Gorter Center for High Field MRI Department."— Presentation transcript:

1 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

2 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

3 3 Philosophy Σα βγεις στον πηγαιμό για την Ιθάκη, να εύχεσαι νάναι μακρύς ο δρόμος, γεμάτος περιπέτειες, γεμάτος γνώσεις.

4 7T system ~50 worldwide

5 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

6 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

7 The first technical challenge – design of customized detectors

8 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

9 Image non-uniformities at high field 9 3T4T5T6T7T8T10T12T

10 10 General observations at high fields Overall, RF wavelength in tissue decreases with B0 field strength Dielectric constant Relative dielectric constant frequency (MHz) Muscle Wavelength (cm) frequency (MHz)

11 RF inhomogeneity constructive/destructive interference 11 ~12 cm

12 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

13 The alternative and slightly cheaper method New, high permittivity materials

14 How do dielectric materials work? 14 Displacement currents in the dielectric material produce a secondary local RF field which increases the total B1+

15 Dielectric pads in imaging FLAIR TSE normal with pads

16 Abdominal imaging at 3 Tesla (a)(b)(c) (d)(e)(f)

17 17 Image non-uniformities Potential for heating the patient Questions about safety/implants Motion sensitivity Difficulties in image segmentation Complexity of cardiac triggering

18 18 General observations at high fields Conductivity increases with frequency Conductivity Conductivity of gray matter (S/m) frequency (MHz) P=1/2  E 2

19 RF inhomogeneity constructive/destructive interference 19 ~12 cm

20 Increased SAR and heating at 7T 20 7T 300 MHz3T 128 MHz SAR (W/kg) 300 MHz128 MHz Temperature rise ( o C)

21 21 General observations at high fields How do you ensure safety? The RF engineer is the first person to be tested!

22 22 General observations at high fields Call upon the medical physics specialists Requires flexibility

23 23 General observations at high fields lack of self-awareness

24 24 General observations at high fields Attention to detail

25 25 General observations at high fields Rigorous safety testing procedures

26 Electromagnetic simulations Phantom heating tests SAR point (W/kg)

27 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

28 Reduction in image quality in patients High quality obtained in volunteers is typically not reproduced in AD patients 28 Healthy volunteerAD patient

29 Normal volunteer AD patient In vivo results of f0 fluctuations before correction 29

30 On-line monitoring of frequency variations

31 Some examples Image quality is significantly improved 31

32 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

33 Reduced contrast makes segmentation difficult T2*-magnitudeT2*- phase T1

34 Specialized image segmentation algorithm

35 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

36 Problems with cardiac triggering Overwhelming magnetohydrodynamic effect

37 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

38 Develop acoustic triggering

39 Technical “solutions” High permittivity materials Accurate SAR modelling On-line “motion” monitoring Acoustic cardiac triggering Phase/magnitude image segmentation

40 7T Cardiovascular MR Coronary MRA 7T Cardiovascular MR Coronary MRA

41 van Elderen et al., Radiology 2010;257: T 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, , 2010.

42 7T Cardiovascular MR Ischemic Cardiomyopathy, RCA 7T Cardiovascular MR Ischemic Cardiomyopathy, RCA

43 General observations at high fields Carotid artery vessel wall imaging T1 T2 TOF

44 Cochlear imaging MIP

45 Inner ear imaging – cochlear implants

46 3T 7T

47 Musculoskeletal applications of 7 Tesla

48 High resolution imaging of the human vertebra Inflammation in spine and sacroiliac joints Ankylosing Spondylitis

49 Water/fat images of sacroiliac (SI) joint

50 High resolution imaging of the eye

51

52 Uveal melanoma patients ultrasound

53 Proton beam therapy planning

54 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

55 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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