Post on 15-Jan-2016
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
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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Philosophy
Σα βγεις στον πηγαιμό για την Ιθάκη,
να εύχεσαι νάναι μακρύς ο δρόμος,
γεμάτος περιπέτειες, γεμάτος γνώσεις.
7T system~50 worldwide
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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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
The first technical challenge – design of customized detectors
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Image non-uniformities
Potential for heating the patient
Questions about safety/implants/dental wires
Motion sensitivity
Difficulties in image segmentation
Complexity of cardiac triggering
Image non-uniformities at high field
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3T 4T 5T 6T 7T 8T 10T 12T
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General observations at high fields
1tissue
rf
• Overall, RF wavelength in tissue decreases with B0 field strength
Dielectric constant
Relative dielectric constant
100
150
200
250
300
350
400
450
500
45
50
55
60
65
70
75
frequency (MHz)
MuscleWavelength (cm)
frequency (MHz)
100
150
200
250
300
350
400
450
500
10
20
30
40
50
60
RF inhomogeneityconstructive/destructive interference
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l~12 cm
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General observations at high fields Solution 1 - multiple transmit channels
RF waveformgenerator 1
Power amplifier
Tx/Rx switch 1 Coil element 1
Digital receiver 1
RF waveformgenerator 2
Power amplifier
Tx/Rx switch 2 Coil element 2
Digital receiver 2
RF waveformgenerator 3
Power amplifier
Tx/Rx switch 3 Coil element 3
Digital receiver 3
RF waveformgenerator N
Power amplifier
Tx/Rx switch N Coil element N
Digital receiver N
The alternative and slightly cheaper method
New, high permittivity materials
How do dielectric materials work?
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Displacement currents in the dielectric material produce a secondary local RF field which increases the total B1+
Dielectric pads in imaging
FLAIR TSE
normal
with pads
Abdominal imaging at 3 Tesla
(a) (b) (c)
(d) (e) (f)
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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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General observations at high fields
Conductivity increases with frequency
0Imeff ion r
Conductivity
100 150 200 250 300 350 400
0.35
0.40
0.45
0.50
0.55
Conductivity of gray matter (S/m)
frequency (MHz)
P=1/2 sE2
RF inhomogeneityconstructive/destructive interference
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l~12 cm
Increased SAR and heating at 7T
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7T 300 MHz3T 128 MHz
SAR (W/kg)
300 MHz128 MHz
Temperature rise (oC)
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General observations at high fields How do you ensure safety?
The RF engineer is the first personto be tested!
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General observations at high fields Call upon the medical physics specialists
Requires flexibility
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General observations at high fields
lack of self-awareness
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General observations at high fields
Attention to detail
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General observations at high fields
Rigorous safety testing procedures
Electromagnetic simulationsPhantom heating tests
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1
0.1
SARpoint (W/kg)
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
Reduction in image quality in patients
High quality obtained in volunteers is typically not reproduced
in AD patients
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Healthy volunteer AD patient
50 100 150
-10
0
10
20
time (s)
freq
(H
z) /
A.U
.
resp. beltnav. phase
250 300 350
-10
0
10
20
time (s)
freq
uenc
y (H
z)
Normal volunteer
AD patient
0 50 150
0 50 150
In vivo results of f0 fluctuations
before correction
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On-line monitoring of frequency variations
Some examples
Image quality is significantlyimproved
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Image non-uniformities
Potential for heating the patient
Questions about safety/implants/dental wires
Motion sensitivity
Difficulties in image segmentation
Complexity of cardiac triggering
Reduced contrast makes segmentation difficult
T2*-magnitude T2*- phase T1
Specialized image segmentation algorithm
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
Problems with cardiac triggering
Overwhelming magnetohydrodynamic effect
Develop acoustic triggering
Principle developed by Niendorf group on Siemens 7T platformCommercially available for mildly ridiculous price
Develop an open-source Arduino-based system for continuousImprovement amongst users
Develop acoustic triggering
Technical “solutions”
High permittivity materials
Accurate SAR modelling
On-line “motion” monitoring
Acoustic cardiac triggering
Phase/magnitude image segmentation
7T Cardiovascular MRCoronary MRA
7T Cardiovascular MRCoronary MRA
van Elderen et al., Radiology 2010;257:254-259
7T Cardiovascular MRCoronary MRA, 7T versus 3T
7T Cardiovascular MRCoronary 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.
7T Cardiovascular MRIschemic Cardiomyopathy, RCA
7T Cardiovascular MRIschemic Cardiomyopathy, RCA
General observations at high fields Carotid artery vessel wall imaging
T1
T2
TOF
Cochlear imaging
MIP
Inner ear imaging – cochlear implants
3T 7T
Musculoskeletal applications of 7 Tesla
High resolution imaging of the human vertebra
• Inflammation in spine and sacroiliac joints
Ankylosing Spondylitis
Water/fat images of sacroiliac (SI) joint
High resolution imaging of the eye
High resolution imaging of the eye
Uveal melanoma patients
ultrasound
Proton beam therapy planning
Acknowledgements
Itamar RonenHermien KanMaarten VersluisThijs van OschSanneke van RoodenEce ArcanFrancesca BranzoliSebastian AussenhoferEidrees GhariqWouter TeeuwisseMark van BuchemWyger BrinkPaul de HeerJeroen van der Grond
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