High Field MRI:
Technology, Applications,
Safety, and Limitations
High Field MRI:
High Field MRI:
Technology, Applications,
Technology, Applications,
Safety, and Limitations
Safety, and Limitations
R. Jason Stafford, Ph.D. Department of Imaging Physics
The University of Texas M. D. Anderson Cancer Center Houston, TX
R. Jason Stafford, Ph.D.
Department of Imaging Physics
The University of Texas M. D. Anderson Cancer Center Houston, TX
AAPM 2005 AAPM 2005AAPM 2005
Brief overview of high-field MRI
Brief overview of high-field MRI
• Introduction
• Technical/Safety Issues
– Main field – RF Field – Gradients• Contrast changes
– Spin lattice relaxation (T1) – Spin-Spin relaxation (T2) – Transverse relaxation (T2*) – Spectral Resolution
• Major Applications
Berry MV & Berry MV & LevitronsLevitrons", Euro J Phys ", Euro J Phys GeimGeimAK, "Of Flying Frogs and AK, "Of Flying Frogs and 1818: 307: 307--313 (1997). 313 (1997). 16T, 32mm vertical bore16T, 32mm vertical bore
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The promise of high-field MRI
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,
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• 3.0T whole body scanners
– FDA approved for commercial use in 2002 – Accounted for 8.5% of high-field revenue in 2003
• Whole body 4-9.4T scanners: in evaluation
The move to higher fields
The move to higher fields
General Electric General Electric MAGNETOM Trio MAGNETOM Trio Siemens Siemens Intera
InteraAchievaAchieva
Philips Philips Signa Excite Signa Excite AAPM 2005 AAPM 2005AAPM 2005
High-field SNR: 7T versus 3T in brain
High-field SNR: 7T versus 3T in brain
7T 3T
white matter SNR =65 white matter SNR =26
Gray matter SNR = 76 Gray matter SNR = 34
TSE, 11 echoes, 7 min exam, 20cm FOV, 512x512 (0.4mm x 0.4mm), 3mm thick slices Images courtesy of SIEMENS Medical Systems
The future? … 9.4 T whole body imaging
The future? … 9.4 T whole body imaging
GE Medical Systems
GE Medical Systems
9.4 T @ Univ. Illinois Chicago
9.4 T @ Univ. Illinois Chicago
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The static magnetic field (B
0)
The static magnetic field (B
0)
• 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
High-field siting challenges
• To keep constant homogeneity, as B
0↑
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
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
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 0 0.5ppm ppm 3T 1.5T AAPM 2005 AAPM 2005AAPM 2005
Magnetic field homogeneity
Magnetic field homogeneity
• Often stated as the
δ
(in Hz or ppm) across a given
d
iameter of
s
pherical
v
olume (DSV).
• Homogeneity desired is often
application
dependent
• For 1.5T:
– Routine imaging: < 5 ppm at 35 cm DSV
– Fast imaging (EPI): < 1 ppm at 35 cm DSV
– Spectroscopy: < 0.5 ppm at 35 cm DSV
Inhomogeneities/susceptibility errors
Inhomogeneities/susceptibility errors
• Off-resonance displacements are in the
frequency
encode direction
– 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) 0 ppm B FOV x BW
δ
⋅γ
⋅ ∆ =AAPM 2005 AAPM 2005AAPM 2005
Metal implants at 3 Tesla
Metal implants at 3 Tesla
3T 1.5T BW=125 kHz BW=50kHz metal prosthetic AAPM 2005 AAPM 2005AAPM 2005
Magnet Shielding
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 B0decreasing the flux away from the magnet. – can be quite heavy and expensiveheavy 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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Fringe Fields: 1.5T
Fringe Fields: 1.5T
Isogauss plot of 1.5T actively shielded magnet
Fault condition: 5m radial x 7m axial for t<2s Fault condition: 5m radial x 7m axial for t<2s
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Fringe Fields: 3.0T
Fringe Fields: 3.0T
Isogauss plot of 3.0T actively shielded magnet
Fault condition: Fault condition: 6 m x 7.5 m for t<100s 6 m x 7.5 m for t<100s AAPM 2005 AAPM 2005AAPM 2005
FDA B
0Field Safety limits
FDA B
0Field 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
B
0field safety concerns
B
0field safety concerns
• Ferromagnetic
projectiles
• Medical devices
– Translation
– Torque
– Interference
• Magnetohydrodynamic
effects
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Main field Safety: Torques and Force
Main field Safety: Torques and Force
• Torque (L) on an object in magnetic field
• Translational force on object in magnetic field
• Torque and translational force also proportional to
susceptibility and volume of material
0 2 0
sin
sin
L
∝ ⋅
m B
⋅
θ
∝
B
⋅
θ
0 0 0(
)
F
∝ ∇ ×
m B
r
r
∝ ∇
B
B
AAPM 2005 AAPM 2005AAPM 2005Magnetic Field safety: Torques and Force
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 www.simplyphysics.com AAPM 2005 AAPM 2005AAPM 2005
Fringe Field Force: 1.5T versus 3.0T
Fringe Field Force: 1.5T versus 3.0T
Axial Distance from Isocenter (m) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 0 2 4 6 8 1.5T 3.0T 1.5T: 5G 3.0T: 5G 0 50 100 150 200 250 300 350 0 2 4 6 8 1.5T 1.5T: 5G 3.0T 3.0T: 5G 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050 2 3 4 5 6 7 8 1.5T 3.0T 1.5T: 5G 3.0T: 5G 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 2 3 4 5 6 7 8 1.5T 1.5T: 5G 3.0T 3.0T: 5G F ield Stren g th Relative F o rce
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Magnetohydrodynamic Effects
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
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.
Transient effects from the static field
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
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Radiofrequency at high-field
Radiofrequency at high-field
• B
1field sensitivity goes as B
0• 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
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
1.5T 1.5T 3.0T3.0T 3T Profile 1.5T profile AAPM 2005 AAPM 2005AAPM 2005
B1 inhomogeneity
B1 inhomogeneity
3T 1.5TDestructive 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)
RF Safety: Specific Absorption Rate (SAR)
• Conductivity (σ) in body gives rise to Efield 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!
2 2
0
SAR∝B ⋅(flip angle) ⋅(RF duty cycle) (Patient Size)⋅
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FDA SAR limits
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
International Electrotechnical Commission
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 humansAAPM 2005 AAPM 2005AAPM 2005
SAR limits on imaging
SAR limits on imaging
• 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
2 2
0
SAR∝B ⋅(flip angle) ⋅(RF duty cycle) (Patient Size)⋅
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Ways to work around SAR limitations
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)
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
–SENSitivityEncoding (SENSE)
• Pruessmann KP, et al, Magn Reson Med. 42(5):952-62 (1999).
–SiMultaneousAcquistion of Spatial Harmonics (SMASH)
Aliased Images Un-aliased Image Unders ampl ed k-s p ace phas e -e n code direct ion SENSE SMASH 1 2 3 4 unfold unalias 1 2 3 4 Cor rected k-spa ce phas e -e n code direct ion Low Resolution Fully encoded image
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Advantages of parallel imaging
Advantages of parallel imaging
• Facilitates faster acquisition by collecting less lines
of k-space
– Doesn’t compromise resolution
– SNR reduced by AT LEASTa factor of √2
•
•
Less # of echoes => Less SAR
Less # of echoes => Less SAR
•
•
Reduces T2/T2* blurring for echo
Reduces T2/T2* blurring for echo
-
-
train
train
sequences
sequences
•
•
Reduction of acoustic noise
Reduction of acoustic noise
Gradients at higher-fields
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
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
Field Strength and Image Quality
• What happens to SNR and contrast with increased
main field?
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• Where does the increase in signal come from?
• Sample magnetization proportional to B
0• Faraday’s Law: Induced e.m.f. in coil proportional
to time rate of change of transverse magnetization
Signal as a function of field strength
Signal as a function of field strength
0 0 s
h B
M
N
N
kT
γ
↑↓∝ ∆
≈
0 0AAPM 2005 AAPM 2005AAPM 2005
Higher fields … how much SNR?
Higher fields … how much SNR?
• Signal versus field strength
• Noise versus field strength
0 2 0 0Signal
∝
ω
M
∝
B
2 2 1/ 2 2 0 0coil system sample
Noise
∝
σ
++
σ
∝
aB
+
bB
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High-field signal-to-noise ratio
High-field signal-to-noise ratio
• At high-field,
B
1(
B
0) is no longer easily
quantifiable
• SNR is still “nearly” linear with B
0in this regime
0 0 0 7 / 4 2 1/ 2 2 0 0(low field limit)
(mid-field regime)
B
B
SNR
B
aB
bB
⎧⎪
∝
∝ ⎨
+
⎪⎩
T1 (m s ) B0(T) gray mat ter white mat ter adipose 0 1( 0) b Tω ≅AωT1 relaxation as a function of B
0T1 relaxation as a function of B
0AAPM 2005 AAPM 2005AAPM 2005
T1 relaxation as a function of B
0T1 relaxation as a function of B
0• 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
T1-weighted imaging
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
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T2 and T2* relaxation as a function of B
0T2 and T2* relaxation as a function of B
0• 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
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T2-weighted imaging
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
T2-weighted brain using 8 channel array
3T 3T 1.5T1.5T ∆x = 0.47mm x 0.94mm ∆x = 0.78mm x 0.89mm 7200 TR 3500 62.5 BW 31.25 24 ETL 8 2 Navg 1 24x18 FOV 20x20 512x256 256x224 1:55 Time 1:38
Spectral resolution at higher fields
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
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Imaging applications
Imaging applications
• What are some of the major applications that will
receive the highest boost from higher field
imaging?
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Spectroscopy
Spectroscopy
• Increased spectral resolution and SNR
– Brain, prostate, breast (+ other body applications) – Multi-nuclear: 31P, 19F, 23Na, 13C NAA Cho Cr NAA Cho Cr 1.5T 1.5T 3.0T3.0T 2hz, 131 deg, 99% 4hz, 140 deg, 99% SNRCrincreased by factor of ~2 at 3T AAPM 2005 AAPM 2005AAPM 2005
Prostate & breast spectroscopy
Prostate & breast spectroscopy
Cunningham, CH, et al, Magn Reson Med 53:1033–1039 (2005)
Bolan, PJ, et al, Magn Reson Med. 50(6):1134-43 (2003) 3T
Prostate MRS
4T Breast MRS
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Blood Oxygen Level Dependent imaging
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 increasesby factor of 1.8-2.2 from 1.5T to 3.0T
– These net effects, and reasons for them, are complicated
Krasnow, B, et al, NeruoImage(2003)
Gradient- echo BOLD fMRI: 1.5T vs 3.0T
Gradient- echo BOLD fMRI: 1.5T vs 3.0T
3.0T: p < 1x10-10 1.5T: p < 7.9 x10-5
Right Hand Sensorimotor Task
Diffusion Weighted Imaging
Diffusion Weighted Imaging
• SNR is crucial
– Thinner slices
• Reduce partial volume artifacts
– Higher b-values
• Diffusion Tensor Imaging (DTI)
– Same benefits as DWI – Faster acq.=> minimize motion
• Shortened T2*
– limits benefits – Use parallel imaging
techniques
1.5T 3.0T
3.0T Diffusion and Diffusion Tensor
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Body Diffusion MRI at 3T: Breast
Body Diffusion MRI at 3T: Breast
Ax T2 ADC Anisotropic Isotropic AAPM 2005 AAPM 2005AAPM 2005
Perfusion imaging
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
Contrast Enhanced imaging
• Higher SNR
• Longer tissue T1 vs. little change in contrast agent T1
– Better contrast – Use less contrast
• Perform higher resolution
dynamic imaging • Applications: brain, breast
and body imaging
1.5T 3.0T
MR Angiography at higher fields
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 (www.medical.philips.com) Gibbs, GF, et al, AJNR(2004)
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Cardiac imaging at higher fields
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