High Field MRI:
Technology, Applications,
Safety, and Limitations
R. Jason Stafford, Ph.D.
Department
Department of
of Imaging
Imaging Physics
Physics
The
The University
University of
of Texas
Texas M.
M. D.
D. Anderson
Anderson Cancer
Cancer Center
Center
Houston,
Houston, TX
TX
Brief overview of high-field MRI
• Introduction
• Technical/Safety Issues
16T, 32mm vertical bore
– Main field
– RF Field
– Gradients
• Contrast changes
–
–
–
–
Spin lattice relaxation (T1)
Spin-Spin relaxation (T2)
Transverse relaxation (T2*)
Spectral Resolution
• Major Applications
Berry MV & Geim AK, "Of Flying Frogs and
Levitrons",
Levitrons", Euro J Phys 18:
18: 307307-313 (1997).
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The promise of high-field MRI
• Trade SNR increase into higher resolution/speed
– Higher resolution imaging
• More detail & less partial volume averaging
– Faster Imaging
• Breath holds, minimize motion, kinetics
• Exploration of new/altered contrast mechanisms
• Potential to significantly advance anatomic,
functional, metabolic and molecular MR imaging
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The move to higher fields
• 3.0T whole body scanners
– FDA approved for commercial use in 2002
– Accounted for 8.5% of high-field revenue in 2003
Signa Excite
Intera Achieva
General Electric
Philips
MAGNETOM Trio
Siemens
• Whole body 4-9.4T scanners: in evaluation
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High-field SNR: 7T versus 3T in brain
7T
white matter SNR =65
Gray matter SNR = 76
3T
white matter SNR =26
Gray matter SNR = 34
TSE, 11 echoes, 7 min exam, 20cm FOV, 512x512 (0.4mm x 0.4mm), 3mm thick slices
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Images courtesy of SIEMENS Medical Systems
The future? … 9.4 T whole body imaging
Early test image of a kiwi
GE Medical Systems
9.4 T @ Univ. Illinois Chicago
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http://www.uic.edu/depts/paff/newsbureau/mriphotos.htm
http://www.uic.edu/depts/paff/newsbureau/mriphotos.htm
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The static magnetic field (B0)
• Modern superconducting magnet design
– Type II superconductors
– Niobium titanium (NbTi) windings
• Critical field limits upper field (< 10 T)
• Bypass by cooling < 4.2 K
– Niobium tin (Nb3Sn) for higher fields
• Brittle and difficult to wind
• Expensive to use
– Fields above 10 T likely to interleave both windings
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High-field siting challenges
• To keep constant homogeneity, as B0 ↑ magnet
– size increases
– weight increases
– cryogen volume and consumption increases
• energy stored in windings increases
– stray field lines are extended
• Costs and siting concerns can be significant
– Modern 3T scanners weigh 2x as much as 1.5T
– >3T : 20 tons with cryogens + 100 tons shielding
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High-field siting challenges
• Challenge: minimize these costs while maintaining
field homogeneity?
• Magnet winding circuits
• Tighter/more windings to reduce length
• Reduced length => reduced cryogen volume/use
•
•
•
•
Conductor formats and joining techniques
Filament alloys
Shimming
Shielding
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Magnet shimming
• Need higher performing, automated shims to
maintain homogeneity
• Several stages
– Magnet => δ < 125 ppm
– Superconducting shims: δ < 1.5 ppm
– Passive + Room Temperature: δ < 0.2 ppm
0.5 ppm
3T
1.5T
0
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-0.5 ppm
Magnetic field homogeneity
• Often stated as the δ (in Hz or ppm) across a given
diameter of spherical volume (DSV).
• Homogeneity desired is often application
dependent
• For 1.5T:
– Routine imaging:
– Fast imaging (EPI):
– Spectroscopy:
< 5 ppm at 35 cm DSV
< 1 ppm at 35 cm DSV
< 0.5 ppm at 35 cm DSV
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Inhomogeneities/susceptibility errors
• Off-resonance displacements are in the frequency
encode direction
δ ⋅ γ B ⋅ FOV
∆x =
ppm
0
BW
– Minimize errors by
• Increased bandwidths (BW)
• Decreased field of view (FOV) and/or slice thickness
• Increase encoding matrix
• Don’t forget slice select geometric distortions!
– Increase RF bandwidth (if possible)
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Metal implants at 3 Tesla
3T
1.5T
metal prosthetic
BW=125 kHz
BW=50kHz
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Magnet Shielding
• Reduces problems of siting MRI in a confined space
– 5 G line reduced from 10-13 m => 2-4 m
• Passive Shielding
– high permeability material, such as iron, provides return path for
stray field lines of B0 decreasing the flux away from the magnet.
– can be quite heavy and expensive
•
Active Shielding
– secondary shielding coils produce a field canceling fringe fields
generated by primary field coils
– typically coils reside inside the magnet cryostat
– Commercial 3T scanners rely on this to minimize weight
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Isogauss plot of 1.5T actively shielded magnet
Fringe Fields: 1.5T
Fault condition: 5m radial x 7m axial for t<2s
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Isogauss plot of 3.0T actively shielded magnet
Fringe Fields: 3.0T
Fault condition:
7.5 m for t<100s
6mx
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FDA B0 Field Safety limits
Guidance for Industry and FDA Staff: Criteria for Significant Risk Investigations of Magnetic Resonance
Diagnostic Devices, July 14th, 2003.
http://www.fda.gov/cdrh/ode/guidance/793.pdf
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B0 field safety concerns
• Ferromagnetic
projectiles
• Medical devices
– Translation
– Torque
– Interference
• Magnetohydrodynamic
effects
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Main field Safety: Torques and Force
• Torque (L) on an object in magnetic field
L ∝ m ⋅ B0 ⋅ sin θ ∝ B02 ⋅ sin θ
• Translational force on object in magnetic field
r r
F ∝ ∇(m × B0 ) ∝ B0∇B0
• Torque and translational force also proportional to
susceptibility and volume of material
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Magnetic Field safety: Torques and Force
• Equipment formally designated as “MR Safe” at
1.5T may not be at 3T
• Force on a paramagnetic object at 3T can be about
5x the force at 1.5T
• Force on a ferromagnetic object can be about 2.5x
the force at 1.5T
• Effects are worse in “short bore”
• The “list” of tested devices
– www.mrisafety.com
www.simplyphysics.com
www.simplyphysics.com
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3.0
0.050
0.045
1.5T
3.0T
1.5T: 5G
3.0T: 5G
2.5
1.5T
3.0T
1.5T: 5G
3.0T: 5G
Fringe Field Force: 1.5T versus 3.0T
Field Strength
2.0
0.040
0.035
0.030
0.025
1.5
0.020
1.0
0.015
0.010
0.5
0.005
0.0
0.000
350 0
2
4
6
250
4.0 2
3
4
5
6
3.5
1.5T
1.5T: 5G
3.0T
3.0T: 5G
300
Relative Force
8
7
8
1.5T
1.5T: 5G
3.0T
3.0T: 5G
3.0
2.5
200
2.0
150
1.5
100
1.0
50
0.5
0
0.0
0
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2
4
6
8
2
3
4
5
6
7
8
Axial Distance from Isocenter (m)
7
Magnetohydrodynamic Effects
• Electrically conductive fluid flow in magnetic field
induces current and a force opposing the fluid flow
• Effects greatest when flow perpendicular to field
– Potential across vessel ~ B0
– Force resisting flow ~ B02
• T-wave swelling
–
–
–
–
ECG distortions during highest aortic flow
Induced potentials ~ 5 mV/Tesla
Effect exacerbated at high-fields
Challenge: obtaining reliable ECG’s at higher fields
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Magnetohydrodynamic Effects
• Increased blood pressure due to additional work
needed to overcome magnetohydrodynamic force
has a negligible effect on blood pressure
– < 0.2% at 10 Tesla
• Hypothesized that field strengths ranging from 18
Tesla are needed before a significant risk is seen in
humans.
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Transient effects from the static field
• Phenomena reported in association with patients moving
in/out of high field magnets
–
–
–
–
–
–
Nausea (slight)
Vertigo
Headache
Tingling/numbness
Visual disturbances (phosphenes)
Pain associated with tooth fillings
• Effects transient and cease after leaving magnet
– actively shielded & short bore high-field magnets
• larger spatial gradient
– reduced or avoided by moving slowly in the main field
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Radiofrequency at high-field
• B1 field sensitivity goes as B0
• RF propagation becomes increasingly
inhomogeneous
–
–
–
–
Wavelength now on order of patient size
Permittivity, conductivity and patient conformation
Reduced penetration
Increased dielectric effects
• RF phase and magnitude function of position
• Significant imaging challenges lie ahead …
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B1 inhomogeneity
• Hyperintensity in middle of imaged volume
– Dielectric effects become more significant as B0↑
– Oil filled phantoms more homogeneous
– Challenges for brain and body homogeneity
3T Profile
1.5T profile
1.5T
3.0T
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B1 inhomogeneity
3T
1.5T
T1-W Spin Echo Images using torso array coil
Destructive interference in the pelvis can lead to persistent
“black hole” artifacts.
Use a dielectric pad to help correct.
New coil designs to help minimize effects.
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RF Safety: Specific Absorption Rate (SAR)
• Conductivity (σ) in body gives rise to E field from RF
– SAR ~ σΕ2/ρ
• SAR = RF Power Absorbed per unit mass (W/kg)
– 1 W/kg => 1°C/hr heating in an insulated tissue slab
• RF power deposition causes heating
– Primary concern: whole body and localized heating
– Significant concern at high-fields
– Don’t forget about local heating of medical devices!
SAR ∝ B02 ⋅ (flip angle) 2 ⋅ (RF duty cycle) ⋅ (Patient Size)
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FDA SAR limits
Guidance for Industry and FDA Staff: Criteria for Significant Risk Investigations of Magnetic Resonance
Diagnostic Devices, July 14th, 2003.
http://www.fda.gov/cdrh/ode/guidance/793.pdf
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International Electrotechnical Commission
• 3T MR unit operating modes are set by
recommended IEC guidelines
– Scanners report SAR in real-time
– notify users of operating thresholds
• Normal Mode
– SAR < 2 W/kg over 6 minutes, (∆T < 0.5 °C)
• First Level controlled Mode
– Requires medical supervision
– SAR < 4 W/kg over 6 minutes, (∆T < 1.0 °C)
• Second level controlled mode
– IRB is needed to scan humans
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SAR limits on imaging
SAR ∝ B02 ⋅ (flip angle) 2 ⋅ (RF duty cycle) ⋅ (Patient Size)
• Puts restrictions on
–
–
–
–
Pulse repetition time
Number of RF pulses in a multi-echo sequence (FSE)
Slice efficiency in multi-slice imaging
Ability to use high SAR pulses for contrast
• Fat saturation
• Magnetization transfer pulses
• Inversion pulses
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Ways to work around SAR limitations
• RF pulse design
– Reduced flip angle (particularly for fast spin echo)
• RF coil design
– Use of arrays
• Transmit-receive arrays to reduce power
• Parallel imaging techniques (SENSE, SMASH)
• Imaging parameters
– Rectangular field of view
– Increased TR
– Less slice in multi-slice imaging (lower efficiency)
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Partially Parallel Imaging (PPI)
• PPI will be increasingly important in the
development of high field imaging
• Uses apriori localized sensitivity information from
multiple receiver array coils to recover full image
from undersamped k-space acquisition
• Standard software on new generation scanners
– SENSitivity Encoding (SENSE)
• Pruessmann KP, et al, Magn Reson Med. 42(5):952-62 (1999).
– SiMultaneous Acquistion of Spatial Harmonics
(SMASH)
• Sodickson DK, et al, Magn Reson Med. 38(4):591-603 (1997).
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Low Resolution
Fully encoded image
Aliased Images
43
2
Un-aliased Image
1
unfold
SENSE
3
2
1
unalias
SMASH
Corrected k-space
phase-encode direction
Undersampled k-space
phase-encode direction
4
Advantages of parallel imaging
• Facilitates faster acquisition by collecting less lines
of k-space
– Doesn’t compromise resolution
– SNR reduced by AT LEAST a factor of √2
• Less # of echoes => Less SAR
• Reduces T2/T2* blurring for echoecho-train
sequences
• Reduction of acoustic noise
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Gradients at higher-fields
• High performance gradients needed to take
advantage of increased SNR for high
resolution/speed
• Max amplitude ~ 20-50 mT/m
• Max slew rates ~ 120-200 T/m/s
• Increased reactive (inductive and capacitive)
coupling to bore/shims/RF coils
– increased eddy currents and non-linearities
– self-inductance limits maximum amplitude and slew rate
– lower inductance designs the easiest fix
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Gradient safety at higher fields
• Physiological constraints on dB/dt to prevent
peripheral nerve stimulation limit gradient
performance
– One strategy for overcoming: shorten linearity volume
• Acoustic noise
– force on the coils scales with the main field
• NOTE: Higher performance gradients are
usually linear over a more restricted FOV
– Increased geometric distortions
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Field Strength and Image Quality
• What happens to SNR and contrast with increased
main field?
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Signal as a function of field strength
• Where does the increase in signal come from?
• Sample magnetization proportional to B0
M 0 ∝ ∆N ↑↓ ≈ N s
hγ B0
kT
• Faraday’s Law: Induced e.m.f. in coil proportional
to time rate of change of transverse magnetization
Larmor Precession Frequency = ω 0 = γ B0
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Higher fields … how much SNR?
• Signal versus field strength
Signal ∝ ω 0 M 0 ∝ B02
• Noise versus field strength
2
2
Noise ∝ σ coil
aB01/ 2 + bB02
+ system + σ sample ∝
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High-field signal-to-noise ratio
SNR ∝
⎧⎪ B07 / 4 (low field limit)
∝⎨
aB01/ 2 + bB02 ⎪⎩ B0 (mid-field regime)
B02
• At high-field, B1(B0) is no longer easily
quantifiable
• SNR is still “nearly” linear with B0 in this regime
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T1 relaxation as a function of B0
T1 (ms)
T1 (ω 0 ) ≅ Aω 0b
a
gr
ym
at
ite
wh
r
te
m
at
r
te
adipose
B0 (T)
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Bottomley PA, et al, Med Phys (1987)
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T1 relaxation as a function of B0
• Spin lattice relaxation both lengthens and converges for
most tissues with increased field strength
• Consequences
– Contrast and SNR reduction
• Need longer TR and/or prepatory pulses
– Longer inversion times needed
• STIR and FLAIR
– Tissue and blood more easily saturated
– Reduced Ernst angles in gradient echo imaging
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T1-weighted imaging
• Can use SNR boost for higher resolution
– Keep similar scan time
• Spin-echo T1-W imaging will be SAR limited
– Number of slices
– Fat saturation
• Solutions
–
–
–
–
–
Use an array head coil
Reduce number of slices
Rectangular field of view
Longer TR
Multiple acquisitions
1.5T
3.0T
T2 and T2* relaxation as a function of B0
• T2 can decrease slightly at fields > 3T
• T2* decreases significantly at higher fields
– Changes vary strongly with tissue environment
– Effects
• Increased T2* contrast from contrast agents or blood
• Decreased signal on gradient echo images due to
susceptibility effects
– Use of shorter TE
• T2* filtering of echo trains in EPI
– Use of shorter echo trains (multi-shot or PPI)
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T2-weighted imaging
• Benefits from higher SNR
– Higher bandwidth => longer acceptable echo-trains
– Potential for higher resolution in similar time
• Longer TR to compensate for T1 lengthening
– Overcome time penalty with longer echo-train and
rectangular field of view acquisitions
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T2-weighted brain using 8 channel array
3T
1.5T
7200 TR 3500
62.5 BW 31.25
24 ETL 8
2
Navg 1
24x18 FOV 20x20
512x256 256x224
1:55 Time 1:38
∆x = 0.47mm x 0.94mm
∆x = 0.78mm x 0.89mm
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Spectral resolution at higher fields
• Larger spectral separation between different
chemical species
– MR spectroscopy applications will obviously benefit
from this and SNR increase
• Chemical shift between fat/water increases
– 220 Hz @ 1.5T => 440 Hz @ 3T
– faster accrual of phase between water/fat for a given TE
• In-phase, out-of-phase TE timings change
– exasperates chemical shift artifacts
• Use higher bandwidths
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Imaging applications
• What are some of the major applications that will
receive the highest boost from higher field
imaging?
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Spectroscopy
• Increased spectral resolution and SNR
– Brain, prostate, breast (+ other body applications)
– Multi-nuclear: 31P, 19F, 23Na, 13C
1.5T
Cr
NAA
3.0T
NAA
SNRCr increased
Cr
by factor of ~2
at 3T
Cho
Cho
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2hz, 131 deg, 99%
4hz, 140 deg, 99%
Prostate & breast spectroscopy
3T
Prostate MRS
Cunningham, CH, et al, Magn Reson Med 53:1033–1039 (2005)
4T
Breast MRS
Bolan, PJ, et al, Magn Reson Med. 50(6):1134-43 (2003)
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Blood Oxygen Level Dependent imaging
• Functional MRI relies on the BOLD effect
• BOLD facilitates neuronal activation measurements
without using exogenous contrast agents
• Activation: Oxy-blood increases while Deoxy-blood
(paramagnetic) decreases
– T2* is lengthened => signal increase
– BOLD contrast increased due to smaller T2* values
– SNR increase also leads to higher sensitivity
• CNR increases by factor of 1.8-2.2 from 1.5T to 3.0T
– These net effects, and reasons for them, are complicated
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Krasnow, B, et al, NeruoImage (2003)
Gradient- echo BOLD fMRI: 1.5T vs 3.0T
1.5T: p < 7.9 x10-5
3.0T: p < 1x10-10
Right Hand Sensorimotor Task
Diffusion Weighted Imaging
• SNR is crucial
– Thinner slices
• Reduce partial volume
artifacts
– Higher b-values
1.5T
3.0T
• Diffusion Tensor Imaging
(DTI)
– Same benefits as DWI
– Faster acq.=> minimize motion
• Shortened T2*
– limits benefits
– Use parallel imaging
techniques
3.0T Diffusion and
Diffusion Tensor
Imaging
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Body Diffusion MRI at 3T: Breast
Ax T2
Anisotropic
ADC
Isotropic
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Perfusion imaging
• Arterial Spin Labeling (ASL)
– Uses and inversion pulse to “tag” blood
– Images acquired as tagged blood perfuses into tissue
– Long T1 results in better tagging
• Dynamic Susceptibility Contrast (DSC)
– Bolus of paramagnetic agent
• T2* contrast
– T2* effect increased by field
1.5T
3.0T
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Contrast Enhanced imaging
• Higher SNR
• Longer tissue T1 vs. little
change in contrast agent
T1
1.5T
3.0T
– Better contrast
– Use less contrast
• Perform higher resolution
dynamic imaging
• Applications: brain, breast
and body imaging
Dynamic contrast enhanced imaging
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MR Angiography at higher fields
• In general SNR => better spatiotemporal resolution
• Time of flight (TOF)
– Relies on saturated normal tissue and bright inflow
– Longer T1 time => better background tissue saturation
• Magnetization Transfer Contrast can further suppress
– Must be careful of SAR limits
– Higher-field => increased inflow signal
• Contrast bolus
– Better T1-contrast
• Phase contrast
– More sensitive to slow flow
3D TOF
Gibbs, GF, et al, AJNR (2004)
(www.medical.philips.com)
Cardiac imaging at higher fields
• Speed is king in cardiac imaging
– Use parallel imaging techniques to their fullest
• CINE imaging
– SAR => reduce flip angles for SSFP (trueFISP/FIESTA)
• T2 weighting and SNR loss
• Black blood imaging (double inversion recovery)
– Increased T1 of blood (+ 30% ) => longer inversion time needed
– More SNR and slow T1 relaxation
• Chance to increase the limited slice efficiency of method
• Cardiac Tagging methods
– Persistance of tagging
• Emerging techniques: Perfusion, Delayed Enhancement
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Cardiac imaging at 3T
SSFP CINE
Cardiac Tagging
1.5T
+SENSE
1.5T
3T
systole
diastole
3T
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Gutberlet, M, Eur Radiol 15: 1586–1597 (2005).
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