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High Field MRI:Technology, Applications,
Safety, and Limitations
High Field MRI:High Field MRI:Technology, Applications, Technology, Applications,
Safety, and LimitationsSafety, and Limitations
R. Jason Stafford, Ph.D.Department of Imaging Physics
The University of Texas M. D. Anderson Cancer CenterHouston, TX
R. Jason Stafford, Ph.D.Department of Imaging Physics
The University of Texas M. D. Anderson Cancer CenterHouston, TX
AAPM 2005AAPM 2005AAPM 2005
Brief overview of high-field MRIBrief 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 ApplicationsBerry MV & Berry MV & GeimGeim AK, "Of Flying Frogs and AK, "Of Flying Frogs and LevitronsLevitrons", Euro J Phys ", Euro J Phys 1818: 307: 307--313 (1997). 313 (1997).
16T, 32mm vertical bore16T, 32mm vertical bore
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The promise of high-field MRIThe 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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• 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 fieldsThe move to higher fields
General ElectricGeneral Electric
MAGNETOM TrioMAGNETOM Trio
SiemensSiemens
InteraIntera AchievaAchieva
PhilipsPhilips
Signa ExciteSigna Excite
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High-field SNR: 7T versus 3T in brainHigh-field SNR: 7T versus 3T in brain
7T 3Twhite matter SNR =65 white matter SNR =26Gray matter SNR = 76 Gray matter SNR = 34TSE, 11 echoes, 7 min exam, 20cm FOV, 512x512 (0.4mm x 0.4mm), 3mm thick slices
Images courtesy of SIEMENS Medical Systems
AAPM 2005AAPM 2005AAPM 2005
The future? … 9.4 T whole body imagingThe future? … 9.4 T whole body imaging
http://http://www.uic.edu/depts/paff/newsbureau/mriphotos.htmwww.uic.edu/depts/paff/newsbureau/mriphotos.htm
GE Medical SystemsGE Medical Systems9.4 T @ Univ. Illinois Chicago9.4 T @ Univ. Illinois Chicago
Early test image of a kiwi
3
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The static magnetic field (B0)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 challengesHigh-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 challengesHigh-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
4
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Magnet shimmingMagnet 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
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Magnetic field homogeneityMagnetic 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: < 5 ppm at 35 cm DSV– Fast imaging (EPI): < 1 ppm at 35 cm DSV– Spectroscopy: < 0.5 ppm at 35 cm DSV
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Inhomogeneities/susceptibility errorsInhomogeneities/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)
0ppm B FOVx
BWδ γ⋅ ⋅
∆ =
5
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Metal implants at 3 TeslaMetal implants at 3 Tesla
3T 1.5T
BW=125 kHz BW=50kHz
metal prosthetic
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Magnet ShieldingMagnet 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 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: 1.5TFringe Fields: 1.5TIsogauss 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
6
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Fringe Fields: 3.0TFringe Fields: 3.0TIsogauss plot of 3.0T actively shielded magnet
Fault condition: Fault condition: 6 m x 7.5 m for t<100s6 m x 7.5 m for t<100s
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FDA B0 Field Safety limitsFDA 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 concernsB0 field safety concerns
• Ferromagnetic projectiles
• Medical devices– Translation– Torque– Interference
• Magnetohydrodynamiceffects
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Main field Safety: Torques and ForceMain 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
20 sin sinL m B Bθ θ∝ ⋅ ⋅ ∝ ⋅
0 0 0( )F m B B B∝ ∇ × ∝ ∇rr
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Magnetic Field safety: Torques and ForceMagnetic 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.comwww.simplyphysics.comwww.simplyphysics.comwww.simplyphysics.com
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Fringe Field Force: 1.5T versus 3.0TFringe 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
1.5T3.0T1.5T: 5G3.0T: 5G
0
50
100
150
200
250
300
350
0 2 4 6 8
1.5T1.5T: 5G3.0T3.0T: 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
1.5T3.0T1.5T: 5G3.0T: 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
1.5T1.5T: 5G3.0T3.0T: 5G
Fiel
d St
reng
thR
elat
ive
Forc
e
8
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Magnetohydrodynamic EffectsMagnetohydrodynamic 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 EffectsMagnetohydrodynamic 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 fieldTransient effects from the static field
• Phenomena reported in association with patients movingin/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-fieldRadiofrequency 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 inhomogeneityB1 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
1.5T1.5T 3.0T3.0T
3T Profile1.5T profile
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B1 inhomogeneityB1 inhomogeneity
3T 1.5T
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.
T1-W Spin Echo Images using torso array coil
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RF Safety: Specific Absorption Rate (SAR)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!
2 20SAR (flip angle) (RF duty cycle) (Patient Size)B∝ ⋅ ⋅ ⋅
AAPM 2005AAPM 2005AAPM 2005
FDA SAR limitsFDA 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
AAPM 2005AAPM 2005AAPM 2005
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 humans
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SAR limits on imagingSAR 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 20SAR (flip angle) (RF duty cycle) (Patient Size)B∝ ⋅ ⋅ ⋅
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Ways to work around SAR limitationsWays 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– 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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Aliased Images Un-aliased Image
Und
ersa
mpl
edk-
spac
eph
ase-
enco
de d
irect
ion
SENSE
SMASH
1234
unfold
unalias
12
34
Corrected k-space
phase-encode direction
Low ResolutionFully encoded image
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Advantages of parallel imagingAdvantages 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 SARLess # of echoes => Less SAR•• Reduces T2/T2* blurring for echoReduces T2/T2* blurring for echo--train train
sequencessequences•• Reduction of acoustic noiseReduction of acoustic noise
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Gradients at higher-fieldsGradients 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
13
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Gradient safety at higher fieldsGradient 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 QualityField Strength and Image Quality
• What happens to SNR and contrast with increased main field?
AAPM 2005AAPM 2005AAPM 2005
• Where does the increase in signal come from?• Sample magnetization proportional to B0
• Faraday’s Law: Induced e.m.f. in coil proportional to time rate of change of transverse magnetization
Signal as a function of field strengthSignal as a function of field strength
00 s
h BM N NkTγ
↑↓∝ ∆ ≈
0 0Larmor Precession Frequency Bω γ= =
14
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Higher fields … how much SNR?Higher fields … how much SNR?
• Signal versus field strength
• Noise versus field strength
0
20 0Signal M Bω∝ ∝
2 2 1/ 2 20 0coil system sampleNoise aB bBσ σ+∝ + ∝ +
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High-field signal-to-noise ratioHigh-field signal-to-noise ratio
• At high-field, B1(B0) is no longer easily quantifiable
• SNR is still “nearly” linear with B0 in this regime
00
0
7 / 42
1/ 2 20 0
(low field limit)
(mid-field regime)
BBSNR
BaB bB
⎧⎪∝ ∝ ⎨+ ⎪⎩
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T 1(m
s)
B0 (T)
gray matte
r
white matte
r
adipose
01 0( ) bT Aω ω≅
T1 relaxation as a function of B0T1 relaxation as a function of B0
Bottomley PA, et al, Med Phys (1987)
15
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T1 relaxation as a function of B0T1 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
T1-weighted imagingT1-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 B0T2 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 imagingT2-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 arrayT2-weighted brain using 8 channel array
3T3T 1.5T1.5T
∆x = 0.47mm x 0.94mm ∆x = 0.78mm x 0.89mm
7200 TR 350062.5 BW 31.2524 ETL 82 Navg 1
24x18 FOV 20x20512x256 256x224
1:55 Time 1:38
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Spectral resolution at higher fieldsSpectral 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
17
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Imaging applicationsImaging applications
• What are some of the major applications that will receive the highest boost from higher field imaging?
AAPM 2005AAPM 2005AAPM 2005
SpectroscopySpectroscopy
• Increased spectral resolution and SNR– Brain, prostate, breast (+ other body applications)– Multi-nuclear: 31P, 19F, 23Na, 13C
NAA
Cho
Cr
NAA
Cho
Cr
1.5T1.5T 3.0T3.0T
2hz, 131 deg, 99% 4hz, 140 deg, 99%
SNRCr increased by factor of ~2
at 3T
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Prostate & breast spectroscopyProstate & 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)
3TProstate MRS
4TBreast MRS
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Blood Oxygen Level Dependent imagingBlood 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
Krasnow, B, et al, NeruoImage (2003)
Gradient- echo BOLD fMRI: 1.5T vs 3.0TGradient- echo BOLD fMRI: 1.5T vs 3.0T
3.0T: p < 1x10-101.5T: p < 7.9 x10-5
Right Hand Sensorimotor Task
Diffusion Weighted ImagingDiffusion 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
Imaging
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Body Diffusion MRI at 3T: BreastBody Diffusion MRI at 3T: Breast
Ax T2 ADC
Anisotropic Isotropic
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Perfusion imagingPerfusion 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 imagingContrast Enhanced imaging
• Higher SNR• Longer tissue T1 vs. little
change in contrast agent T1– Better contrast– Use less contrast
• Perform higher resolutiondynamic imaging
• Applications: brain, breast and body imaging
1.5T 3.0T
Dynamic contrast enhanced imaging
20
MR Angiography at higher fieldsMR 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 fieldsCardiac 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 3TCardiac imaging at 3T
1.5T
3T
+SENSE
Gutberlet, M, Eur Radiol 15: 1586–1597 (2005).
1.5T 3T
systole
diastole
SSFP CINE Cardiac Tagging