mr imaging and spectroscopy of the heart at 3t:technical challenges
DESCRIPTION
MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges. MR Imaging at 3T 2x S/N of 1.5 T 1/2 voxel size or 1/4 the acquistion time. MR Imaging at 3T Technical Challenges:-Body RF Coil Tissue Challenges:-T1’s get longer Regulatory Challenges: SAR Bo^2. MR Imaging at 3T - PowerPoint PPT PresentationTRANSCRIPT
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
MR Imaging at 3T
2x S/N of 1.5 T
1/2 voxel size or
1/4 the acquistion time
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
MR Imaging at 3T
Technical Challenges:-Body RF Coil
Tissue Challenges:-T1’s get longer
Regulatory Challenges: SAR Bo^2
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
MR Imaging at 3T
Technical Challenges:-Body RF Coil
Why Have a body coil?
-critical for applications outside
the head
-homogeneous transmit coil for
Phased array studies
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Original ResearchSensitivity and Power Deposition in a High-Field Imaging ExperimentDavid I. Hoult, MA, D, Phil *Institute for Biodiagnostics, National Research Council of Canada, Winnipeg, Manitoba, R3B 1Y6, Canada
Presented at the 7th Scientific Meeting of the ISMRM, Philadelphia, 1999
JMRI, 12:46-67,2000.
“SINCE THE EARLY DAYS of human imaging, it has been known that the electrical characteristics of tissue could adversely affect the fidelity of its image. Thus, Bottomley and Andrew ([1]) surmised that B1 field penetration effects could set an effective limit to the Larmor frequency of roughly 20 MHz, while independently but for the same reasons, Hoult and Lauterbur ([2]), in their paper on the signal-to-noise ratio (S/N) of the imaging experiment, suggested 10 MHz (0.25 T for protons) as a limit. Mansfield and Morris ([3]) adopted the same stance.”
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Ultrahigh field (7T) magnetic resonance imaging and spectroscopy
Kâmil Uurbil, , a, Gregor Adrianya, Peter Andersena, Wei Chena, Michael Garwooda, Rolf Gruettera, Pierre-Gil Henrya, Seong-Gi Kima, Haiying Lieua, Ivan Tkaca, Tommy Vaughana, Pierre-Francoise Van De Moortelea, Essa Yacouba and Xiao-Hong Zhua
Magnetic Resonance Imaging21:1263-1281,2003
Better Spectra
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7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head imagesJ.T. Vaughan 1 *, M. Garwood 1, C.M. Collins 2, W. Liu 2, L. DelaBarre 1, G. Adriany 1, P. Andersen 1, H. Merkle 1, R. Goebel 3, M.B. Smith 2, K. Ugurbil 1
Magnetic Resonance in Medicine
Volume 46, Issue 1, Pages 24-30
4T 7T (7T/4T) 7T/4T(calc)
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Poster #
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
3 Tesla Body Coil B1 Mapping• Resistive and Dielectric Properties of the Body Perturb RF
Uniformity at High Field
• Mapping of B1 in the body requires a fast, breathhold sequence
• Single shot FSE with different amplitudes of the excitation pulse was used
• Signal vs. amplitude was fit to approximately sinusoidal signal curve observed in phantoms
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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dielectric padsat 3T
pads nearcoil
pads nearpatient
dielectricshading
arrows indicate magnitudeand phase of B1
+ fieldcolor shows B1
+ field magnitude
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GE Company ConfidentialTim Skloss and Armen Kocharian
Dielectric effects exist at all field strengths
These effects appear as non-uniformity in MR images
The effects are exacerbated at higher field strengths
The effects are exacerbated with multi-channel coils
Dielectric Effects – The Facts
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GE Company ConfidentialTim Skloss and Armen Kocharian
So What Can Be Done to Minimize These Effects?
8-Channel Torso Coil
without any pad
8-Channel Torso Coil
with low conductivity pad
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GE Company ConfidentialTim Skloss and Armen Kocharian
Low Conductivity Pad
20 millimolar solution of Manganese Chloride in distilled water.
(3.958 grams of Manganese Chloride (tetrahydrate) per liter of
solution)
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A spiral volume coil for improved RF field homogeneity at high static magnetic field strength.
Alsop DC, Connick TJ, Mizsei G.
Magn Reson Med. 1998 Jul;40(1):49-54.
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The Wave Equation Demands Spatial Variation of B Field
SpatialVariationOf RF
Short Wavelength Effect
Conductivity Effect
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Birdcage vs. Spiral Coil
0°45°
90°135°
180° 0°45°
90°135°
180°
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4 Tesla Spiral Head Coil Prototype
Designed for Whole Brain Imaging25 cm diameter, 30 cm length, Eight conductors
Distributed CapacitanceSeven 6.8 pf ceramic capacitors per conductor
Integrated RF ShieldMechanically connected, 32 cm diameter
Shield Current ReturnVaughan et al. MRM 32:206 (1994)
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Coil Performance
High Q and Q ratioUnloaded Q 288, loaded Q 64
No tuning for load necessaryFrequency shift with load < 0.5 MHz
Excellent quadrature operationPolarity reversal dramatically reduced signal
Power deposition similar to birdcage100 mG B1 required 240W (CW)
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Effect of Spiral on Uniformity
Spiral coil uniformity was clearly improvedCompares favorably with birdcage
Radial intensity variations consistent with theoryTheory assumes cylindrical symmetry
Low flip angle gradient echo imaging intensity=B2
Phase gradient less than expected66% of gradient expected for geometry
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Radial Intensity Variation in 100% Isopropanol Phantom
Birdcage
Spiral
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Human Head Imaging
Multi-slice low flip angle gradient echo imagingOxford Instruments 1 m 4 T magnetGE Horizon Echospeed HardwareTR/TE 500/3 , 10°, 32 kHz BW
Spiral coil reduces center brighteningIntensity more uniform than birdcage
Signal intensity drops off near top of headBoundary condition effect ?Independent of distance head is in coil
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4 Tesla Head Imaging
Spiral Coil
Birdcage Coil
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Summary
Spiral coil design improves RF homogeneity
No apparent penalty in power depositionFurther comparison studies required
Must compensate for dielectric boundariesVarying spiral pitch, radius with axial distanceExternal dielectric pads
Coil designs can overcome short RF wavelengths
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Effect of External Dielectric
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Increased FSE Slice Coverage
• Many multi-slice FSE protocols are limited by SAR even at 1.5 Tesla
• 3 Tesla multi-slice acquisitions take 4 times longer due to slice restrictions from the 4 x higher SAR
• Reduced flip angles can be used to make power identical to 1.5 T with only a small effect on sensitivity– D.C. Alsop, Magn Reson Med 37:176-184 (1997)
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Sensitivity with Reduced Flip Angles
• Sensitivity drops only slowly with flip angle when tailored RF pulse trains are used for echo stability.
• Stimulated echo terms increase the effective T2 of the tissue but the images remain dominated by T2 contrast.
• Longer effective TE’s are required for the same T2 weighting.
For 90° pulses, SAR is reduced 4-fold but signal drops by just 14%
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
3 Tesla Reduced SAR FSE
• 90° asymptotic flip angles• 47, 3 mm slices in 3 acqs.• 16 ETL• 32 kHz BW• Flow compensation• TR 4000• 2 echoes• 256x256, 24 cm FOV• TE 12.4/112• 4 min 45 s total scan time
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
• Peripheral Gated Fastcard - SPGR• 19 Phases per 25 Second Breath Hold• 4 Element Cardiac Surface Coil Array
– GE R&D Center, Schenectady, NY
• Spatial Resolution: 1.3 x 1.5 x 8 mm
Cardiac Imaging Gradient Echo Imaging of the Heart
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
• End Diastole
Cardiac Imaging Gradient Echo Imaging of the Heart
• Mid Systole • End Systole
• Short Axis
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Cardiac Imaging Gradient Echo Imaging of the Heart
• End Diastole • Mid Systole • End Systole
• Long Axis
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Cardiac Imaging
2D FIESTA, Long and Short Axis
QuickTime™ and a decompressor
are needed to see this picture.
QuickTime™ and a decompressor
are needed to see this picture.
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BLACK-BLOODFSE CARDIAC
IMAGING: 1.5T VS 3.0T
Robert L. GreenmanJohn E. ShiroskyRobert V. Mulkern
Neil M. Rofsky
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• Published Studies– Gradient Echo - Signal ~ Sin– No Spin Echo (or FSE) Studies– Spin Echo - Signal ~Sin3
• B1 Heterogeneity– Conductive Effects (Signal Attenuation)– Dielectric Effects (Waveguide Effect) (or Resonant Cavity Effect)
FSE Black-Blood Imaging
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• Changes in T2 Relaxation Times:– Tumors– Infarction– Cardiac Transplant Rejection
• STIR – Sensitive to Both T1 and T2 Changes– Suppresses Fat
FSE Black-Blood Imaging
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• Blood Suppression– Minimizes Flow Artifacts
• Contrast– Vascular Walls– Endocardial Surfaces
• Double IR Pulse Sequence
FSE Black-Blood Imaging
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FSE Black Blood Imaging
2 x RR
RectangularNon-Selective180 Pulse
Slice-SelectiveAdiabatic 180Pulse
Aquisition
RR
Double Inversion Black Blood Sequence
TI
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1.5T Null Point = 625 ms
3.0T Null Point = 706 ms
3.0T Signal at Calculated 1.5T IR
Time = -0.07 M0
Black Blood Imaging
1.5T Null Point = 456 ms
3.0T Null Point = 490 ms
3T Signal at Calculated 1.5T IR Time = -0.03 M0
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• Double-IR FSE• Single Breathold• Matrix: 256 x 192• FOV: 40 cm• Slice Thick: 5 mm• Echo train Length (ETL): 24• Heart Rates: 45 - 75 BPM
Black Blood FSE Imaging 1.5T vs 3.0TMETHODS
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• T2-Weighted
– Effective TE: 42ms (6th echo)
– TR variable 1.5 - 2.5 Sec
• STIR
– IR time variable for best fat suppression
– TR variable 1.5 - 2.5 Sec
• Cycled IR Pulses On and Off
• B1 Field Maps
Black Blood FSE Imaging 1.5T vs 3.0TMETHODS
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• Body Coil Only• High-Pass Birdcage
• 1.5T Dimensions– 60 cm Diameter; 64 cm Long
• 3.0T Dimensions– 55 cm Diameter; 53 cm Long
Black Blood FSE Imaging 1.5T vs 3.0TMETHODS
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B1 (RF) Field Maps
1.5 Tesla 3.0 Tesla
Black Blood FSE Imaging 1.5T vs 3.0T
Results
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Black Blood FSE Imaging 1.5T vs 3.0T
Results
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T2-Weighted FSE Images
1.5T 3.0T
Black Blood FSE Imaging 1.5T vs 3.0T
Results
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Black Blood FSE Imaging 1.5T vs 3.0T
Results - T2 W SNR
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STIR FSE Images
1.5T 3.0T
Black Blood FSE Imaging 1.5T vs 3.0T
Results
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Black Blood FSE Imaging 1.5T vs 3.0T
Results - STIR SNR
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ROI MEASUREMENTS
Black Blood FSE Imaging 1.5T vs 3.0TMETHODS
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BLOOD SUPPRESSION PERFORMANCE
Black Blood FSE Imaging 1.5T vs 3.0T
Results
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Black Blood FSE Imaging 1.5T vs 3.0T
Results
BLOOD SUPPRESSION PERFORMANCE
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Black Blood FSE Imaging 1.5T vs 3.0T
Results
SNR (SIGNAL) UNIFORMITY
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Black Blood FSE Imaging 1.5T vs 3.0T
Results
SNR (SIGNAL) UNIFORMITY -P/A
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Black Blood FSE Imaging 1.5T vs 3.0T
Results
SNR (SIGNAL) UNIFORMITY -P/A
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Correlation of 23Na MR Imaging Findings with Cine, Late-Enhancement, and T2-weighted Findings
Note.—NA = not applicable.
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
RESULTS: All patients after subacute infarction and 12 of 15 patients with chronic infarction had an area of elevated 23Na signal intensity that significantly correlated with wall motion
abnormalities (subacute; r = 0.96, P < .001, and chronic; r = 0.9, P < .001); three patients had no wall motion abnormalities or elevated 23Na signal intensity. Only 10 patients in the subacute and nine in the chronic group revealed late enhancement; significant correlation with 23Na MR imaging occurred only in subacute group (r = 0.68, P < .05). Myocardial edema in subacute infarction correlated (r = 0.71, P < .05) with areas of elevated 23Na signal intensity but was extensively larger.
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Sodium Imaging of the Heart
5 inch circular coil
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3T MRI BIDMC DEPARTMENT OF RADIOLOGY
Sodium Imaging of the Heart
8 inch circular coil
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CHEMICALLY SELECTIVE PHOSPHORUS RARE
(FSE) IMAGING
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CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
PHANTOM RESULTS
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in vivo 31P RARE IMAGING PARAMETERS
• Modified FSE Sequence w/Chemical Selective Excitation
• Spatial Resolution: 4.7 X 4.7 X 25 mm (0.55 cm3)
• Scan Time: 4 Minutes/Metabolite Image (PCR or Pi)
CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
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Rest
Exercise
1H PCr Pi
FOREARM EXERCISE STUDY
CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
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CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
FOREARM EXERCISE STUDY
1H/CONTOUROVERLAY
Pi/PCr Ratio
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• Chronic High Glucose Levels Result in Functional Impairment of Circulation In Lower Extremities
• Ischemia and in 31P Metabolite Levels
• Neuropathy• Ulceration• Amputation
• Foot Muscle: Surrogate for Whole System
CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
ISCHEMIA IN DIABETIC FOOT
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CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
1H PCr Pi
ISCHEMIA IN DIABETIC FOOT
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CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
1H CONTOUROVERLAY
Pi/PCr Ratio
ISCHEMIA IN DIABETIC FOOT
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CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
Alternative Non-Invasive Method for Assessment of Ischemia:MRS Chemical Shift Imaging
Scan Time: 34 MinutesResolution: 10 X 10 X 25 mm
(2.5 cm3)
Scan Time: 4 Minutes/ImageResolution: 0.47 X 0.47 X 25 mm
(0.55 cm3)
PCr Pi
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3D RARE Pulse Sequence• Single Excitation
• Multiple Spin Echoes
• Readout Gradients Replace One CSI Phase Encode
31P Myocardial ImagingMethods - Pulse Sequence
Time
Readout
Phase Enc.
Slice Sel.
RF
ESP ESP ESP
ESP/ 2ESP ESP ESP
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31P Myocardial Imaging in vivo Results
• 3D Acquisition - 2 adjacent slices
• 12.5 mm x 12.5 mm x 25 mm Voxels (4 cc)
• Scan time: 9 Minutes 40 Seconds
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