Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 1/60 What is “High Field MRI”? •  Main magnetic 0ield strength ≥ 3 Tesla ω0 = γ B0
f0 = γ B0
2/60 Main magnetic 0ield Larmor frequency γ
γ =
= 42.56 MHz T-­‐1 2π
( γ = gyromagnetic ratio) B0 = 3T à f0 ≈ 128 MHz B0 = 4.7T à f0 = 200 MHz B0 = 7T à f0 ≈ 300 MHz B0 = 9.4T à f0 = 400 MHz Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI* 1990 4T whole body First whole body 4T magnet developed by Oxford Magnet Technology (now Siemens Magnet Technology) and installed as part of a Phillips medical system at the University of Alabama. University of Minnesota (Siemens) and NIH (General Electric) received one shortly after. •  multiple engineering problems •  signi0icant technological developments before obtaining useful images 3/60 * Adapted from book ‘Ultra High-­‐0ield Magnetic Resonance Imaging’ by Robitaille PM and Berliner LJ (2006) Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 4T whole body 3T clinical scanner First 3T “clinical” research system (80 cm bore) developed by Magnex Scienti0ic (now Agilent) and installed at the Henry Ford Hospital in Detroit. •  more compact (smaller bore) thanks actively shielded gradient coils •  signi0icantly less expensive •  easy to site •  engineering problems much more manageable 4/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 4T 8T whole body whole body 3T clinical scanner First whole body 8T magnet (80 cm bore) developed by Magnex Scienti0ic (now Agilent) and installed at Ohio State University. à today it is in the storage ~ 40 high 0ield whole-­‐body magnets installed worldwide (thirty 3T and ten 4T) and operated as pure clinical research systems 5/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 4T 8T whole body whole body 3T clinical 7T scanner whole body First whole body 7T magnet (90 cm bore) developed by Magnex Scienti0ic (now Agilent) and installed at the University of Minnesota. GE had begun to deliver a second-­‐generation 3T Signa™ system that was close to being a routine clinical product (94 cm bore, actively shielded 3T magnet and a fully featured Signa™ clinical console) 6/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 1999 4T 8T 3T FDA whole body whole body certi0ied 3T clinical 7T scanner whole body The 3T system was formally FDA certi0ied, eight years after the 0irst 3T magnet was installed in a clinical research site 7/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 1999 2000-­‐2003 4T 8T 3T FDA whole body whole body certi0ied 3T clinical 7T 3 more 7T scanner whole body installed Three more 7T systems were installed: •  Massachusetts General Hospital (MGH) in Boston (USA) •  National Institutes of Health (NIH) in Bethesda (USA) •  Niigata University (Japan) 8/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 1999 2000-­‐2003 2004 4T 8T 3T FDA 9.4T whole body whole body certi0ied whole body 3T clinical 7T 3 more 7T scanner whole body installed First whole body 9.4T magnet (80 cm bore) developed by General Electric and installed at the University of Illinois in Chicago for primary application of sodium imaging. 9/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 1999 2000-­‐2003 2004 2011 4T 8T 3T FDA 9.4T whole body whole body certi0ied whole body 11.74T 3T clinical 7T 3 more 7T head only scanner whole body installed 11.74T head only (68 cm bore) magnet (Agilent/Siemens) was installed October 2011 at NIH Magnet quenched from full 0ield December 2011 during NEMA acoustics tests at NIH Current status: Under repair 10/60 The Evolution of High Field MRI 1990 1991 1997 1998 1999 2000-­‐2003 2004 2011 20?? 11.74T 4T 8T 3T FDA 9.4T whole body whole body whole body certi0ied whole body 11.74T 3T clinical 7T 3 more 7T head only scanner whole body installed The French Atomic Energy Commission (CEA) decided to design an 11.74 Tesla (~ 234,000 times the Earth’s 0ield) MRI for human studies (90 cm bore) as part of the NeuroSpin project. 11/60 Slide courtesy of Riccardo Lattanzi The Evolution of High Field MRI 1990 1991 1997 1998 1999 2000-­‐2003 2004 2011 20?? 11.74T 4T 8T 3T FDA 9.4T whole body whole body whole body certi0ied whole body 11.74T 3T clinical 7T 3 more 7T head only scanner whole body installed Center for Biomedical Imaging (UHF): •  128-­‐channel 3T Siemens Trio •  64-­‐channel 3T Siemens Skyra with 2Ch pTx •  32-­‐channel 7T Siemens Magnetom with 8 & 32 Tx channels •  3T Siemens Biograph mMR MR/PET 12/60 Slide courtesy of Riccardo Lattanzi Ultra-­‐high field MRI: Benefits vs Challenges •  Expected Bene0its, increase in v
Signal-­‐to-­‐Noise Ratio (SNR) •  Increase image resolution & acquisition speed –  More details & minimize motion •  Non-­‐proton MRI: Sodium (23Na), Phosphorus(31P) v
Spectral resolution •  Identi0ication of glutamine (Gln)/glutamate(Glu) v
Susceptibilityà new contrast •  Technical Challenges v
B1+, transmit magnetic 0ield, inhomogeneity •  Image contrast and SNR inhomogeneity •  Diminishes the quality and diagnostic value v
Speci0ic Absorption Rate (SAR) •  Patient safety limits: dangerous local hot spots v
v
13/60 System: magnet, pTx hardware… Susceptibility à decrease in T2* Last week
MSK group
Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 14/60 Signal to Noise RaBo (SNR): brief revisit signal
SNR=
standard deviation of the noise
(r )
emf ≈ iω ∫ d r M (r, t )B
3
0
⊥
(receive)∗
⊥
B0: main magnetic 0ield strength 3T,7T… γ: gyromagnetic constant excitation
volume
signal ~ {ω0=γB0 & transverse magnetization(MT)}
2
2
noise~ σ coil
aB01/2 + bB02
+ system + σ sample ~
B02
~M0~ B0
SNR~ B0
std of noise ~ {ω0=γB0 assuming sample noise is dominant}
SNR increases ’nearly’ linearly with field strength
Increase the image resolution à more details, better localization
Increase acquisition speed à maximize patient comfort, minimizes motion
15/60 SNR: 7T vs 3T in hip* GRE Image Flip Angle Map Raw SNR Map 16/60 *Deniz CM. et al. (2012): MRM
Technical Challenge
B1+ inhomogeneity
SNR analysis between 3T & 7T RF shimming for 7T only SNR normalization compensates • Spatial 0lip angle variations High resolution GRE images (low 0lip angle) *Raw SNR maps Normalized SNR maps Noise image ( short TR, no RF excitation) sin(θ) Flip angle map of the GRE acquisition 17/60 *Kellmann P. et al. (2005): MRM 54: 1439-47
SNR Results •  SNR analysis indicated 2.33 times greater normalized SNR at 7T
compared to 3T
v
v
18/60 Shows the benefits of higher fields for hip imaging
The difference in receive coil structure may effect SNR comparison
SNR Benefit: ex-­‐vivo pathology or in-­‐vivo imaging? A
B
Susceptibility Weighted Imaging (SWI): exploits the susceptibility differences between tissues using phase imagesà sensitive to venous blood and iron storage Yulin Ge, MD
19/60 Slide courtesy of Riccardo Lattanzi & Daniel Sodickson
7T MRI: A Powerful View of The Brain Yulin Ge, MD
20/60 Slide courtesy of Riccardo Lattanzi & Daniel Sodickson
High-­‐ResoluBon Knee Imaging 55 year old male -­‐ Healthy 51 year old male -­‐ Osteoarthri4s Normal thickness Smooth surface Diffusely thinned Irregular surface •  Fat-suppressed 3D Flash, TR/TE = 26/5.1 ms, 0.23 x 0.23 x 1 mm3,
60 partitions, acquisition time 6:58, QED knee coil with 28 receive elements
Gregory Chang, MD
21/60 Slide courtesy of Riccardo Lattanzi & Daniel Sodickson
Imaging Osteoporosis 55 year old female -­‐ Healthy 76 year old female -­‐ Osteoporosis •  3D Flash TR/TE = 20/5.1 ms
•  0.23 x 0.23 x 1.0 mm3, 80 partitions, acquisition time 7:09
Gregory Chang, MD
22/60 Slide courtesy of Riccardo Lattanzi
Imaging the Human Hippocampus Control
Patient
Oded Gonen, PhD
23/60 Slide courtesy of Riccardo Lattanzi
3T vs. 7T 3T TSE, 0.7 x 0.7 x 5.0 mm3
7T TSE, 0.5 x 0.5 x 3.5 mm3
30 slices, acquisition time 3:16
40 slices, acquisition time 3:06
24/60 Slide courtesy of Riccardo Lattanzi
25/60 Slide courtesy of Riccardo Lattanzi
Hip imaging at 7T* A
B
C
D
Hip Cartilage Hip Bone Microarchitecture Bone microarchitecture images are shown for intermediate-­‐weighted FSE and T1-­‐
weighted FLASH sequences in A and B, respectively. Cartilage images are shown for fat suppressed FSE image in C and for water excitation FLASH image in D. FSE 26/60 FLASH *Deniz CM. et al (2013) ISMRM Ultra High Field Workshop
Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 27/60 Spectral ResoluBon •  At low 0ield, the concentrations of glutamine (Gln) and glutamate(Glu) are often combined as Glx = Glu + Gln. àThis could mask relative changes in Glu and Gln. •  There are techniques to overcome this problem v  often time consuming v  may result in the loss of other metabolite signals which may be of interest B0
28/60 Schematics: courtesy of Daniel Sodickson
Spectral ResoluBon (Glu & Gln) 29/60 http://www.utdallas.edu/nsm/research/airc/faculty_choi.htm#
Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 30/60 Technical Challenges – B1+ inhomogeneity •  RF EM Fields and Tissue: Electrical Properties of Muscle Tissue vs. Frequency σ increases and εr decreases with increasing frequency of the applied 0ield At 300MHz (7T), one wavelength in tissue is on the order of 0.1m vs ~1m in vacuum RF wavelength within tissue depends on tissue’s electric properties σ and εr 31/60 Courtesy of Chris Collins: from Lecture 1 Jan 15th 2013
Center-­‐bright ArBfact Due to Interference 35 cm sphere of saline
125 MHz (3T)
T/R Surface Coil(s)
Coaxial with Sphere
λ/2
32/60 Courtesy of Chris Collins: from Lecture 1 Jan 15th 2013
Head in a Birdcage Coil (ConstrucBve Interference) 64 MHz
175 MHz
260 MHz
B1+
(Scale Max.
=5µT)
SI
(Scale Max.
=1.0)
SAR
(Scale Max.
=3xAve.)
0
0.2 0.4 0.6 0.8 1
from Tesla to frequencyω0=γB0
33/60 Courtesy of Chris Collins: from Lecture 1 Jan 15th 2013
Value
Scale Max.
345 MHz
RF Shimming1 Parallel RF ExcitaBon2,3 •  Common RF waveform •  Distinct RF waveform •  Distinct and time-­‐varying amplitudes and phases for each element •  Higher degrees of freedom in RF design process •  Distinct but time-­‐ constant amplitudes and phases for each element No him No Sshim
Phase nly shim Phase Oo
nly RF Shim
* Next lecture by Martijn Cloos
Prostate region
a2, ϕ2
a1, ϕ1
RF
a8, ϕ8
a7, ϕ7
34/60 RF
a3, ϕ3
a4, ϕ4
a5, ϕ5
a6, ϕ6
RF
RF
RF
a1 (t)
ϕ1 (t)
(t)
a 5(t) , ϕ 5
a2(t), ϕ2(t)
a6(t), ϕ6(t)
a3(t), ϕ3(t)
a7(t), ϕ7(t)
,
(t)
ϕ
4
)
t
,
a 4(
1-Hoult D., (2000) JMRI: p46-67
2-Katscher U. et al., (2003) MRM: p144-50
3-Zhu Y. (2004) MRM: p775-84
Schematics: Courtesy of Riccardo Lattanzi
* Volunteer study at NYU using 8Ch Tx array at 7T
a8 (t) ϕ
, 8 (t)
RF
RF
RF
RF
RF shimming capability at NYU •  TimTx Trueform (2 Ch RF excitation from body coil) •  TimTx array step 2 (8 Ch RF excitation from local coils) Magnetom 7T
Both system are research only and has built-in RF shimming software
à go play with themJ
35/60 RF shimming example at 3T (Willinek WA et al. Radiology 2010) •  Two radiologists rated the images with 5 point scale for liver pelvis and 4 point scale for spine v
v
36/60 5(4): Excellent diagnostic quality 1: Non diagnostic image quality RF shimming example at 7T **
Phase Only RF Shim
No Shim
*
Prostate region
***
****
The 16-element optimization of gradient echo
signal intensity maps of single slices and whole
brain at 300 MHz using EM field simulations.
(search algorithm)
37/60 *-Mao W et al.,(2006) MRM 56: 918–922 ***-Metzger GJ et al. (2008) MRM
**-Volunteer study at NYU using 8Ch Tx array at 7T ****-Deniz CM et al (2012) MRM
RF Shim GUI (standalone program) Available with sample datasets
http://cai2r.net/resources/software/parallel-rf-transmission-rf-shimming-gui
38/60 1.5 1 0.5 0 0 2 4 6 8 Acceleration •  Increased baseline SNR SNR (Normalized) SNR [a.u.] Side Note: High field benefits for parallel MRI 1 0.5 0 0 2 4 6 8 Acceleration •  Slower decline with acceleration Ohliger M. et. al., (2003) MRM 50: 1018
39/60 Slide courtesy of Riccardo Lattanzi & Daniel Sodickson
High Field MRI and Parallel Imaging •  High 0ield strength improves parallel MRI v
Increased SNR v
Increased feasible accelerations (increase in coil sensitivity difference) •  Parallel imaging improves high 0ield MRI v
Reduced susceptibility artifact v
Improved RF homogeneity (via parallel transmission) v
Reduced SAR (via parallel transmission and reception) •  Practical developments in parallel reception and transmission are crucial for high 0ield MRI to deliver on its promise 40/60 Slide courtesy of Riccardo Lattanzi & Daniel Sodickson
Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 41/60 MagneBc (B1+) vs Electric (E) Field RF
Flip angle (degrees) Object B0: main magnetic 0ield (T) B1+ 0ield (T) "
û
MR signal In MRI Speci0ic Absorption Rate (SAR, W/kg) Tissue heating Concomitant E 0ield (V/m) SAR is the radiofrequency (RF) energy absorption rate by the body Limited by FDA 42/60 Field maps are courtesy of Bei Zhang
r: spatial location σ: conductivity ρ: sample density RF Energy Causes HeaBng in MRI Bio-Heat Equation:
Due to concomitant E 0ield Heat generated by metabolism ∂T
ρC
= ∇ • (k∇T ) + SARρ +[− ρblood wCblood (T − Tcore )] + Qm
∂t
Blood perfusion (cooling effect) T: temperature ρ: material density C: heat capacity k: thermal conductivity w: perfusion by blood 43/60 RF energy caused heaBng and its assesment Oh S. et al., 2012 ISMRM e-­‐poster 3863 Proton Resonance Frequency (PRF)*:
ΔT (r, t ) =
44/60 φ2 (r, t ) − φ1 (r, t )
αγ B0TE
*-Ishihara Y. et al (1995) MRM
Φ1, Φ2: pre and post heat GRE phase images TE: echo time γ: gyromagnetic ratio B0: main magnetic 0ield strength α: temperature dependency of the chemical shift (~ 0.01 PPM/°C) Technical Challenges – SAR: RF heaBng increases with B0 •  If B1 and tissue electrical properties are independent of frequency, we can expect SAR to increase with B02 •  Citing wavelength effects, sometimes it is stated that local SAR becomes a bigger issue as B0 increases (in ultra-­‐high 0ield), v
SAR distribution is strongly dependent on distribution of tissue properties v
Some (not all) numerical simulations show an increase in (max local SAR)/(whole body SAR) with B0 This metric is used to limit RF deposition via relatively easy power measurements
It is also known as k-factor in Siemens coil files
45/60 SAR DistribuBon with B0 in Simple and Complex Models 200MHz
300MHz
xy plane SAR(W/Kg)
340MHz
xy plane SAR(W/Kg)
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Axial
40
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140
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Sagittal
20
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•  SAR distribution depends largely on sample geometry, heterogeneity, and complexity. •  Maximum 1g SAR levels tend to be higher in models of human geometries than in homogeneous models for a given magnetic 0ield strength (factor of 2 to 3)* 46/60 *- Collins et al., MRM 40:847, 1998
Regulatory Limits on RF HeaBng in MRI 1) Temperature
Temperature increase is the primary and well known concern of safety in RF regime (3 kHz to 300 GHz) International Electrotechnical Commission IEC 60601-­‐2-­‐33, 3rd Edition, 2010, p. 34 47/60 RF Safety Assurance by PredicBng and/or Monitoring Temperature in MRI(?) •  Core body temperature can be monitored with thermometers or other probes •  Local temperature can be monitored with MR-­‐based methods •  Local temperature can be predicted with computer models (FDTD, FIT simulations) considering the Maxwell equations and bioheat equations •  Temperature monitoring and prediction is generally not performed in MRI due to challenges related to patient comfort, imaging time, and complexity. 48/60 Regulatory Limits on RF HeaBng in MRI 2) Specific energy Absorption Rate (SAR)
International Electrotechnical Commission IEC 60601-­‐2-­‐33, 3rd Edition, 2010, p. 34 49/60 Regulatory Limits on RF HeaBng in MRI 2) Specific energy Absorption Rate (SAR)
Local SAR limits the maximum RF power to any 10g of tissue !! Local coils are also subject to limits on whole-­‐body SAR International Electrotechnical Commission IEC 60601-­‐2-­‐33, 3rd Edition, 2010, p. 34 50/60 IEC CategorizaBon of RF Coils •  Volume Excitation Coils – designed to produce a homogeneous excitation within a volume v
Body coils, head coils, extremity coils, … •  Local Excitation Coils – designed to excite a localized area v
Single channel loop coil •  Multi-­‐channel Transmit Coils have attributes of local and volume coils – appropriate control of SAR depends on use 51/60 PredicBng and Monitoring SAR •  Whole-­‐body, Head, or Partial-­‐body average SAR can be predicted or measured as the RF power transmitted through the coil divided by the mass of the exposed region of the subject à relatively easyJ •  Local SAR can be calculated as σ|E|2/2ρ where v
v
v
52/60 σ is electric conductivity (S/m) ρ is material density (kg/m3) E is peak RF electric 0ield magnitude (V/m) Reducing SAR in Single-­‐Channel Coils •  SAR is proportional to the time-­‐average RF energy •  For a given pulse duration, SAR∝B12 v
Reducing 0lip angle will reduce SAR •  For a given Flip angle, SAR∝(pulse duration)-­‐1 •  For a given set of RF pulses, SAR∝(TR)-­‐1 53/60 PredicBng and Monitoring SAR for Parallel Transmit Coils •  Whole-­‐body, Head, or Partial-­‐body average SAR can be measured approximately by* 1) determining forward and re0lected power through each channel (with directional couplers on each channel), v  2) using this and drive con0iguration to determine the total power transmitted, and v  3) dividing by mass of subject or exposed portion thereof. v
•  Local SAR prediction for pTx is an area of rapid development: currently it is mainly based on EM Pield simulations Reducing SAR using pTx is shown to be feasible** Handling SAR in pTx setting à Next lecture by Martijn Cloos 54/60 * Zhu Y et al., Magn Reson Med 2012;67:1367-1378
** Zhu Y (2004) MRM 51
Single
RF shimming example for SAR reducBon (Willinek WA et al. Radiology 2010) •  Two radiologists rated the images with 5 point scale for liver pelvis and 4 No signi0icant No signi0icant difference difference in in point s
cale f
or s
pine image image quality qb
uality ut shorter but 33% shorter Dual
v imaging 5(4): Excellent imaging (50% for T2w, d1iagnostic 8% for quality T1w) v  1: Non diagnostic image quality 55/60 Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 56/60 Technical Challenges – System PerspecBve 7T (90 cm bore) magnet
system in test
57/60 Shielding installation for a 9.4T (65
cm bore) magnet
Slide courtesy of Riccardo Lattanzi
Magnet Design and ConstrucBon for B0 > 3T •  Geometrical issues v  High homogeneity over imaging volume (~45 cm diameter) v  several solenoid wounds for corrections •  Mechanical issues v  Large forces and stresses on the conductors and magnet formers (mostly between solenoidal windings) àavoid high stresses in compensation coils (FEM simulations) •  Conductors v  Niobium-­‐Titanium superconductors (NbTi) at 4.2 K for B0 ≤ 9.4T àneed liquid helium 58/60 Slide courtesy of Riccardo Lattanzi
SuperconducBng wire: CriBcal parameters •  Critical temperature •  Critical magnetic 0ield •  Critical current density 59/60 NbTi – based superconducBng wires •  NbTi conductors (many 0ine 0ilaments of a niobium-­‐titanium (NbTi) alloy embedded in a copper matrix) •  The most commonly used (developed since the sixties) •  Tc (0T) = 9.5K ; Tc (11T) = 4.1K •  Feasible industrially (>2000 tons/year) •  Competitive cost (around 300 $/km, depending on wire diameter) •  Easy to handle and no special precautions for use (like copper wire) -  Helium cryogenic: relatively high refrigeration cost Assuming 500A in wire and 1m magnet (solenoid) diameter, producing a 3T 0ield would require about 4775 turns, or about 15km of wire. (4775x500A=2.4MA) Cross-section of
superconducting wire
showing NbTi filaments in
Cu matrix
60/60 Magnet Design and ConstrucBon for B0 > 3T •  Energy management v  ~ 80 MJ for 7T whole body magnet systems àmust be dissipated safely in emergency situations •  Cryostats v  Magnet immersed in a reservoir of liquid helium at 4.2K à avoid leakage •  Gradient coils v  Acoustic noise from vibration caused by Lorentz forces à safety issues 61/60 Slide courtesy of Riccardo Lattanzi
Magnet Shielding at High Field •  Stray (fringe) 0ield of ~ 5 Gauss several meters from magnet center v  Hazard to people with pacemakers v  Affect instrumentation (PC, X-­‐Rays, etc.) •  Passive shielding à Actively shielded 7Ts are available now v  Extra cost, volume, weight are too high for active shielding v  ~ 200-­‐500 tons of steel needed to halve the 5 Gauss contour v  Same center for shield and magnet to balance forces (à shield will pass 0loor level) •  Effect on magnet homogeneity v  Need to shim with steel plates placed in slots outside the 62/60 bore Slide courtesy of Riccardo Lattanzi
SuperconducBng StaBc Magnets •
Several miles of superconducting wire wound in a pattern to produce a very homogeneous 0ield in a 40cm diameter spherical region •
Outer windings with current in the opposite direction provide “active shielding,” serving to help contain the strong magnetic 0ield •
Superconducting wire is kept below 4K with liquid Helium. An outer container of Liquid Nitrogen is used to slow the loss of liquid helium 63/60 Outline •  Introduction •  Expected Bene0its v  Signal-­‐to-­‐noise ratio (SNR) v  Spectral resolution •  Technical Challenges v  B1+ inhomogeneity v  RF heating & SAR v  System complexity •  Pros vs Cons 64/60 Pros and Cons of High Field MRI •  Pros v  ↑ SNR (~ B0, variable) à ↑ resolution, ↑ speed, … v  ↑ spectral resolution v  ↑ susceptibility •  Cons v  ↑ SAR (~ B02, variable) à limits on speed, 0lip angle, … v  ↑ RF variability à ↓ homogeneity (transmit & receive) v  ↑ B0 variability à ↑ shimming challenges, ↓ T2*, ↑ phase-­‐
related artifacts v  ↑ susceptibility v  ↑ bulk, cost, etc. 65/60 Thank you for your afenBon 