High Field MRI: Technology, Applications, Safety, and Limitations. Brief overview of high-field MRI. The promise of high-field MRI

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High Field MRI:

Technology, Applications,

Safety, and Limitations

High Field MRI:

Technology, Applications,

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

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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 bore

16T, 32mm vertical bore

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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,

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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

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

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

GE Medical Systems

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 (Nb3_{Sn) 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 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

• 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 0 0.5ppm ppm 3T 1.5T AAPM 2005 AAPM 2005AAPM 2005

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

• 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

⋅ ∆ =

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Metal implants at 3 Tesla

3T 1.5T BW=125 kHz BW=50kHz metal prosthetic AAPM 2005 AAPM 2005AAPM 2005

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

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

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

₀

Field Safety limits

FDA B

₀

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

B

0

field safety concerns

B

0

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

• Translational force on object in magnetic field

• Torque and translational force also proportional to

susceptibility and volume of material

0 2 0

sin

L

∝ ⋅

m B

⋅

θ

∝

B

⋅

θ

0 0 0

(

)

F

∝ ∇ ×

m B

r

∝ ∇

B

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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 www.simplyphysics.com AAPM 2005 AAPM 2005AAPM 2005

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

• 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.

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

• B

₁

field 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

• 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

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.

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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

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

• 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

• 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

• 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

–SENSitivityEncoding (SENSE)

• Pruessmann KP, et al, Magn Reson Med. 42(5):952-62 (1999).

–SiMultaneousAcquistion of Spatial Harmonics (SMASH)

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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

• 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

sequences

•

• Reduction of acoustic noise

Reduction of acoustic noise

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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• 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

0 0 s

h B

M

N

kT

γ

↑↓

∝ ∆

≈

0 0

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Higher fields … how much SNR?

• Signal versus field strength

• Noise versus field strength

0 2 0 0

Signal

∝

ω

M

∝

B

2 2 1/ 2 2 0 0

coil system sample

Noise

∝

σ

+

σ

∝

aB

+

bB

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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

0

in this regime

0 0 0 7 / 4 2 1/ 2 2 0 0

(low field limit)

(mid-field regime)

B

SNR

B

aB

bB

⎧⎪

∝

_{∝ ⎨}

+

_⎪⎩

T1 (m s ) B0(T) gray mat ter white mat ter adipose 0 1( 0) b Tω ≅Aω

T1 relaxation as a function of B

0

T1 relaxation as a function of B

0

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T1 relaxation as a function of B

₀

T1 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

• 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

0

T2 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

• 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 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

• 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

• 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: 31_P,19_F,23_Na,13_C 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

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

• 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

3.0T: p < 1x10-10 1.5T: p < 7.9 x10-5

Right Hand Sensorimotor Task

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

Ax T2 ADC Anisotropic Isotropic AAPM 2005 AAPM 2005AAPM 2005

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

– Better contrast – Use less contrast

• Perform higher resolution

dynamic imaging • Applications: brain, breast

and body imaging

1.5T 3.0T

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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 (www.medical.philips.com) Gibbs, GF, et al, AJNR(2004)

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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

Cardiac imaging at 3T

1.5T 3T +SENSE 1.5T 3T systole diastole