new rf technologies for very high field and very low field
TRANSCRIPT
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New RF technologies for very high field and very low field magnetic resonance imaging
Andrew Webb
C.J. Gorter Center for High Field MRI
LEIDEN UNIVERSITY MEDICAL CENTER
19-Apr-181
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Outline
1. How does RF technology fit into MRI?
Similarities and differences to radar/antennas
2. Advantages but also RF challenges of very high frequency MRI
Interactions with the body
3. What do we use very high frequency MRI for in the hospital?
4. High frequency components, current and future….
Expert advise welcome!
5. And now for something completely different….
Ultra low field MRI, why and how
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Magnet (DC), gradient coils (100s kHz), RF coil (100s MHz)
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Superconducting wire (tens of km, cooled with liq He)
Earths magnetic field: 50 mTesla
Typical clinical scanner: 3 Tesla
Ultra high field human scanner: 10 Tesla
High field NMR spectrometer: 30 Tesla
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Shielded superconducting magnet
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Why do we need a very strong magnet?
B0=3 Tesla f=128 MHz
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dV -
dt
Faradays’law
Effects of a pulse of RF energy
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What frequencies are we dealing with?
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Similarities and differences with radar
scattering
FAR FIELD
z
yx
E
B
v
NEAR FIELD
dimensions <<l
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Multiple coil arrays
circularly polarized
transmit
multiple channel
receive
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Quadrature
mixer
I
Q
(A/D)1
(A/D)2
preamplifier
filters
variable-gain
amplifiers
s(t) demodulator
LPF
Mixer
sout(t)
Demodulator
sdemod(t)
Quadrature mixer
Oscillator LPF
LPF
90o
phase
shifter
IFcos t
sI(t)
sR(t)
Mixer
Mixer
s(t)
sdemod(t)
0 IFcos t
low-pass filter
Superheterodyne detector
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Analogue-to-digital converters
SAR
16
14
12
10
8
6
4
2
104 105 106 107 108 109 1010
DS
Pipeline
folding/
interpolating
flash
resolution
(bits)
samples per second
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High current (400 Amps) gradient power supplies
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x
y
z
Bz
z
Bz
y
Bz
x
(x,y,z) = gB(x,y,z)
Imposing spatial information on precessional frequency
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What are the main differences to radar and antennas?
Operate in the near field – so coils not antennas
Relatively low frequency so scattering is not relevant
Spatial resolution is not wavelength-related but depends upon
the gradient strength, since we impose a frequency/phase
difference onto the sample
Magnetic field produces the signal
Electric field produces heating!!
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Part 2
Advantages and disadvantages of higher fields
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The move to higher fields
Higher signal-to-noise
Higher spatial resolution
Higher spectral resolution
Increased image contrast
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LUMC
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LUMC
C.J.Gorter Center for High Field MRI (established 2008)
Mission: Develop new technology for clinical, medical research and
biomedical research in MRI
Technique – application in volunteers – application in patients
Rapid translation of hardware and software – weeks not years
Most rapid – eye coil construction to first patient ~2 weeks
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Examples of 7T MRI projects in Leiden
MR angiography
small vessel diseaseCartilage for early
stage osteoarthritisCoronary artery
early stenosis
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Does it work first time?
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22
Effects of finite RF wavelength in tissue
1tissue
rfl
r~60
MuscleWavelength (cm)
frequency (MHz)
10
20
30
40
50
60
1.5 Tesla
3 Tesla
7 Tesla
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RF inhomogeneityconstructive/destructive interference
23
l~12 cm
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Image non-uniformities at high field
24
3T 4T 5T 6T 7T 8T 10T 12T
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Increased SAR and heating at 7T
25
300 MHz128 MHz300 MHz128 MHz
SAR Temperature rise
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EM modelling
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EM modelling
T (C)Geometry B1+ (mT) SAR (W/kg)
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Uveal melanoma and treatment planning
LUMC is Netherlands Center for uveal melanoma
200-300 patients per year
ultrasound
19-Apr-1828
Part 3
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Treatment options
29
Enucleation
Radiotherapy
External beam
therapy
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Ruthenium plaque therapy
For a patient to be eligible for ruthenium plaque brachytherapy the
maximum tumour prominence is 7 mm.
For tumour prominences greater than 7 mm the eye is removed.
19-Apr-1830
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Ultrasound
31
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Ultrasound
32
• Only 2D cross-sections
• Oblique cross-sections typically overestimate tumour size
• Inadequate for large, complexly, shaped tumours
• Low contrast between tumour and sclera
3D imaging technique with better contrast needed
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High resolution imaging of the eye
19-Apr-1833
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Uveal melanoma patients
19-Apr-1834
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Two patients
19-Apr-1835
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Two patients
Patient 1 – MRI-estimated tumour prominence was 6.2 mm
Patient 2 - MRI-estimated tumour prominence was 9.2 mm
For patient 1, this resulted a substantial change in treatment plan
from eye removal (based on the UBM scan) to treatment with
ruthenium plaque therapy
1-2 patients per week are now being referred to the 7T scanner
for treatment planning
A combined evaluation of all the different sequences
showed no extra-scleral extension for either patient.
19-Apr-1836
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3T 7T
Imaging patient before cochlear implant
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RF and image inhomogeneities
Part 4
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B1 inhomogeneity – parallel transmit approach
19-Apr-1839
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Parallel transmit approach
19-Apr-1840
A1, 1A2, 2
A3, 3
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Multi-element dipole antennas
19-apr-1841
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First attempts in vivo on the 7 Tesla
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Parallel transmit approach
19-Apr-1843
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Improving the RF amplifiers?
Class A/B amplifiers
Broadband
Class D?
Class E?
Class H?
On coil amplifiers to
avoid enormous power
losses in cables
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Different types of amplifiers
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Passively vs. actively fed dipoles
19-apr-1846
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Passively vs. actively fed dipoles
19-apr-1847
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5. Ultra-low field sustainable MRI for developing countries
19-Apr-1848
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First prototype system 2018
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The system
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A Halbach magnet – NdBFe material
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First iteration modelled
Central frequency 2.5 MHz
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And built
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First software written – open source platform
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How to spatially encode i.e. get an image
Two ring magnet and rotate the outside one
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Collaboration with signal processing from TU Delft
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19-Apr-1857
Steps to improve performance in next 5 years
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19-Apr-1858
Steps to improve performance in next 5 years
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Acknowledgements
19-Apr-1859
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Acknowledgements
19-Apr-1860
NWO TOPGO
STW 13375
STW 13783
ERC Advanced 670629