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

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1 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 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 16T, 32mm vertical bore Berry MV & Geim AK, "Of Flying Frogs and Levitrons", Euro J Phys 18: : (1997). 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 1

2 The move to higher fields 3.T whole body scanners FDA approved for commercial use in 22 Accounted for 8.5% of high-field revenue in 23 Signa Excite Intera Achieva MAGNETOM Trio General Electric Philips Siemens Whole body 4-9.4T scanners: in evaluation 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, 2cm FOV, 512x512 (.4mm x.4mm), 3mm thick slices Images courtesy of SIEMENS Medical Systems The future? 9.4 T whole body imaging Early test image of a kiwi GE Medical Systems 9.4 Univ. Illinois Chicago 2

3 The static magnetic field (B ) Modern superconducting magnet design Type II superconductors Niobium titanium (NbTi) windings Critical field limits upper field (< 1 T) Bypass by cooling < 4.2 K Niobium tin (Nb 3 Sn) for higher fields Brittle and difficult to wind Expensive to use Fields above 1 T likely to interleave both windings High-field siting challenges To keep constant homogeneity, as B 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 : 2 tons with cryogens + 1 tons shielding 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 3

4 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 <.2 ppm 3T 1.5T.5ppm -.5ppm Magnetic 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: <.5 ppm at 35 cm DSV Inhomogeneities/susceptibility errors Off-resonance displacements are in the frequency encode direction δ ppm γ B FOV x = BW 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) 4

5 Metal implants at 3 Tesla metal prosthetic 3T 1.5T BW=125 khz BW=5kHz Magnet Shielding Reduces problems of siting MRI in a confined space 5 G line reduced from 1-13 m => 2-4 m Passive Shielding high permeability material, such as iron, provides return path for stray field lines of B decreasing the flux away from the magnet. can be quite heavy 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 Isogauss plot of 1.5T actively shielded magnet Fringe Fields: 1.5T Fault condition: 5m radial x 7m axial for t<2s 5

6 Isogauss plot of 3.T actively shielded magnet Fringe Fields: 3.T Fault condition: 6 m x 7.5 m for t<1s FDA B Field Safety limits Guidance for Industry and FDA Staff: Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices, July 14 th, B field safety concerns Ferromagnetic projectiles Medical devices Translation Torque Interference Magnetohydrodynamic effects 6

7 Main field Safety: Torques and Force Torque (L) on an object in magnetic field L m B sin θ B sin θ Translational force on object in magnetic field Torque and translational force also proportional to susceptibility and volume of material 2 r r F ( m B ) B B 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 T.4 1.5T: 5G Fringe Field Force: 1.5T versus 3.T 3.T: 5G.35 Field Strength T T 3.T 1.5T: 5G 3.T: 5G Relative Force T 1.5T: 5G 3.T 3.T: 5G T 1.5T: 5G 3.T 3.T: 5G Axial Distance from Isocenter (m)

8 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 ~ B Force resisting flow ~ B 2 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 Effects Increased blood pressure due to additional work needed to overcome magnetohydrodynamic force has a negligible effect on blood pressure <.2% at 1 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 reduced or avoided by moving slowly in the main field 8

9 Radiofrequency at high-field B 1 field sensitivity goes as B 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 inhomogeneity Hyperintensity in middle of imaged volume Dielectric effects become more significant as B Oil filled phantoms more homogeneous Challenges for brain and body homogeneity 3T Profile 1.5T profile 1.5T 3.T B1 inhomogeneity 3T 1.5T T1-W Spin Echo Images using torso array coil 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. 9

10 RF Safety: Specific Absorption Rate (SAR) Conductivity (σ) in body gives rise to E field from RF SAR ~ σε 2 /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! SAR B (flip angle) (RF duty cycle) (Patient Size) 2 2 FDA SAR limits Guidance for Industry and FDA Staff: Criteria for Significant Risk Investigations of Magnetic Resonance Diagnostic Devices, July 14 th, 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 <.5 C) First Level controlled Mode Requires medical supervision SAR < 4 W/kg over 6 minutes, ( T < 1. C) Second level controlled mode IRB is needed to scan humans 1

11 SAR limits on imaging SAR B (flip angle) (RF duty cycle) (Patient Size) 2 2 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 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) 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): (1999). SiMultaneous Acquistion of Spatial Harmonics (SMASH) Sodickson DK, et al, Magn Reson Med. 38(4): (1997). 11

12 4 3 2 Aliased Images 1 Low Resolution Fully encoded image unfold Un-aliased Image SENSE Undersampled k-space phase-encode direction unalias SMASH Corrected k-space phase-encode direction Advantages 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 SAR Reduces T2/T2* blurring for echo-train sequences Reduction of acoustic noise Gradients at higher-fields High performance gradients needed to take advantage of increased SNR for high resolution/speed Max amplitude ~ 2-5 mt/m Max slew rates ~ 12-2 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 12

13 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 Field Strength and Image Quality What happens to SNR and contrast with increased main field? Signal as a function of field strength Where does the increase in signal come from? Sample magnetization proportional to B hγ B M N N s kt Faraday s Law: Induced e.m.f. in coil proportional to time rate of change of transverse magnetization Larmor Precession Frequency = ω = γb 13

14 Higher fields how much SNR? Signal versus field strength Signal ω M B 2 Noise versus field strength Noise σ + σ ab + bb 2 2 1/2 2 coil+ system sample High-field signal-to-noise ratio SNR 2 B B 1/2 2 ab B + bb 7/4 (low field limit) (mid-field regime) At high-field, B 1 (B ) is no longer easily quantifiable SNR is still nearly linear with B in this regime T1 relaxation as a function of B b T( ω ) Aω 1 T 1 (ms) gray matter white matter adipose B (T) Bottomley PA, et al, Med Phys (1987) 14

15 T1 relaxation as a function of B 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.T T2 and T2* relaxation as a function of B 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) 15

16 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 T2-weighted brain using 8 channel array 3T 1.5T 72 TR BW ETL 8 2 N avg 1 24x18 FOV 2x2 512x x224 1:55 Time 1:38 x =.47mm x.94mm x =.78mm x.89mm 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 T => 44 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 16

17 Imaging applications What are some of the major applications that will receive the highest boost from higher field imaging? Spectroscopy Increased spectral resolution and SNR Brain, prostate, breast (+ other body applications) Multi-nuclear: 31 P, 19 F, 23 Na, 13 C 1.5T NAA 3.T SNR Cr increased Cr Cr by factor of ~2 at 3T Cho Cho NAA 2hz, 131 deg, 99% 4hz, 14 deg, 99% Prostate & breast spectroscopy 3T Prostate MRS Cunningham, CH, et al, Magn Reson Med 53: (25) 4T Breast MRS Bolan, PJ, et al, Magn Reson Med. 5(6): (23) 17

18 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 increases by factor of from 1.5T to 3.T These net effects, and reasons for them, are complicated Krasnow, B, et al, NeruoImage (23) Gradient- echo BOLD fmri: 1.5T vs 3.T 1.5T: p < 7.9 x1-5 3.T: p < 1x1-1 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.T 3.T Diffusion and Diffusion Tensor Imaging 18

19 Body Diffusion MRI at 3T: Breast Ax T2 ADC Anisotropic Isotropic 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 1.5T T2* contrast T2* effect increased by field 3.T 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.T Dynamic contrast enhanced imaging 19

20 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 Gibbs, GF, et al, AJNR (24) 3D TOF ( 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 (+ 3% ) => 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 SSFP CINE +SENSE Cardiac Tagging 1.5T 3T 1.5T systole 3T diastole Gutberlet, M, Eur Radiol 15: (25). 2