1 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 2005 AAPM 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 & Geim Geim AK, "Of Flying Frogs and AK, "Of Flying Frogs and Levitrons Levitrons", Euro J Phys ", Euro J Phys 18 18: 307 : 307-313 (1997). 313 (1997). 16T, 32mm vertical bore 16T, 32mm vertical bore AAPM 2005 AAPM 2005 AAPM 2005 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, functional, metabolic and molecular MR imaging
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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
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
• 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!
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 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
• 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-
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eph
ase-
enco
de d
irect
ion
SENSE
SMASH
1234
unfold
unalias
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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
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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?
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• 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= =
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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)
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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
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Imaging applicationsImaging applications
• What are some of the major applications that will receive the highest boost from higher field imaging?
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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)
• 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
(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