High Field MRI: Technology, Applications, Safety, and ... · Magnet Shielding • Reduces problems...

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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


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


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


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

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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


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


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

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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


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


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


Metal implants at 3 TeslaMetal implants at 3 Tesla

3T 1.5T

BW=125 kHz BW=50kHz

metal prosthetic


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


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

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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


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


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


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


Fringe Field Force: 1.5T versus 3.0TFringe Field Force: 1.5T versus 3.0T

Axial Distance from Isocenter (m)

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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


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.


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 …


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


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∝ ⋅ ⋅ ⋅


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


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∝ ⋅ ⋅ ⋅


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)


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

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Corrected k-space

phase-encode direction

Low ResolutionFully encoded image


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


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


Field Strength and Image QualityField Strength and Image Quality

• What happens to SNR and contrast with increased main field?


• 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+∝ + ∝ +


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

⎧⎪∝ ∝ ⎨+ ⎪⎩


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


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


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


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?


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


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


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


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

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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)


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


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

High Field MRI: Technology, Applications, Safety, and ... · Magnet Shielding • Reduces problems...

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