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MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges

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Presentation on theme: "MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges"— Presentation transcript:

1 MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges

2 MR Imaging at 3T 2x S/N of 1.5 T 1/2 voxel size or
1/4 the acquistion time

3 MR Imaging at 3T Technical Challenges:-Body RF Coil
Tissue Challenges:-T1’s get longer Regulatory Challenges: SAR Bo^2

4 -critical for applications outside the head
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

5  Original Research Sensitivity and Power Deposition in a High-Field Imaging Experiment David 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.”

6 imaging and spectroscopy
Better Spectra 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 Imaging 21: ,2003

7 4T T (7T/4T) T/4T(calc) 7T vs. 4T: RF power, homogeneity, and signal-to-noise comparison in head images J.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

8 Poster #

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10 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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14 dielectric pads at 3T pads near patient pads near coil dielectric
arrows indicate magnitude and phase of B1+ field color shows B1+ field magnitude pads near coil dielectric shading

15 Dielectric Effects – The Facts
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

16 with low conductivity pad
So What Can Be Done to Minimize These Effects? 8-Channel Torso Coil with low conductivity pad 8-Channel Torso Coil without any pad

17 Low Conductivity Pad 20 millimolar solution of Manganese Chloride in distilled water. (3.958 grams of Manganese Chloride (tetrahydrate) per liter of solution)

18 A spiral volume coil for improved RF field homogeneity at high static magnetic
field strength. Alsop DC, Connick TJ, Mizsei G. Magn Reson Med Jul;40(1):49-54.

19 The Wave Equation Demands Spatial Variation of B Field
Conductivity Effect Spatial Variation Of RF Short Wavelength Effect

20 Birdcage vs. Spiral Coil
180° 180° 90° 90° 45° 135° 45° 135°

21 4 Tesla Spiral Head Coil Prototype
Designed for Whole Brain Imaging 25 cm diameter, 30 cm length, Eight conductors Distributed Capacitance Seven 6.8 pf ceramic capacitors per conductor Integrated RF Shield Mechanically connected, 32 cm diameter Shield Current Return Vaughan et al. MRM 32:206 (1994)

22 Coil Performance High Q and Q ratio No tuning for load necessary
Unloaded Q 288, loaded Q 64 No tuning for load necessary Frequency shift with load < 0.5 MHz Excellent quadrature operation Polarity reversal dramatically reduced signal Power deposition similar to birdcage 100 mG B1 required 240W (CW)

23 Effect of Spiral on Uniformity
Spiral coil uniformity was clearly improved Compares favorably with birdcage Radial intensity variations consistent with theory Theory assumes cylindrical symmetry Low flip angle gradient echo imaging intensity=B2 Phase gradient less than expected 66% of gradient expected for geometry

24 Radial Intensity Variation in 100% Isopropanol Phantom
Spiral Birdcage

25 Human Head Imaging Multi-slice low flip angle gradient echo imaging
Oxford Instruments 1 m 4 T magnet GE Horizon Echospeed Hardware TR/TE 500/3 , 10°, 32 kHz BW Spiral coil reduces center brightening Intensity more uniform than birdcage Signal intensity drops off near top of head Boundary condition effect ? Independent of distance head is in coil

26 4 Tesla Head Imaging Spiral Coil Birdcage Coil

27 Summary Spiral coil design improves RF homogeneity
No apparent penalty in power deposition Further comparison studies required Must compensate for dielectric boundaries Varying spiral pitch, radius with axial distance External dielectric pads Coil designs can overcome short RF wavelengths

28 Effect of External Dielectric

29 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: (1997)

30 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%

31 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

32 Cardiac Imaging Gradient Echo Imaging of the Heart
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

33 Cardiac Imaging Gradient Echo Imaging of the Heart
Short Axis End Diastole Mid Systole End Systole

34 Cardiac Imaging Gradient Echo Imaging of the Heart
Long Axis End Diastole Mid Systole End Systole

35 2D FIESTA, Long and Short Axis
Cardiac Imaging 2D FIESTA, Long and Short Axis

36 BLACK-BLOOD FSE CARDIAC IMAGING: 1. 5T VS 3. 0T Robert L
BLACK-BLOOD FSE CARDIAC IMAGING: 1.5T VS 3.0T Robert L. Greenman John E. Shirosky Robert V. Mulkern Neil M. Rofsky

37 FSE Black-Blood Imaging
Published Studies Gradient Echo - Signal ~ Sin(q) No Spin Echo (or FSE) Studies Spin Echo - Signal ~Sin3(q) B1 Heterogeneity Conductive Effects (Signal Attenuation) Dielectric Effects (Waveguide Effect) (or Resonant Cavity Effect)

38 FSE Black-Blood Imaging
Changes in T2 Relaxation Times: Tumors Infarction Cardiac Transplant Rejection STIR Sensitive to Both T1 and T2 Changes Suppresses Fat

39 FSE Black-Blood Imaging
Blood Suppression Minimizes Flow Artifacts Contrast Vascular Walls Endocardial Surfaces Double IR Pulse Sequence

40 FSE Black Blood Imaging

41 Black Blood Imaging 1.5T Null Point = 456 ms 3.0T Null Point = 490 ms
3.0T Signal at Calculated 1.5T IR Time = M0 1.5T Null Point = 456 ms 3.0T Null Point = 490 ms 3T Signal at Calculated 1.5T IR Time = M0

42 Black Blood FSE Imaging 1.5T vs 3.0T METHODS
Double-IR FSE Single Breathold Matrix: 256 x 192 FOV: 40 cm Slice Thick: 5 mm Echo train Length (ETL): 24 Heart Rates: BPM

43 Black Blood FSE Imaging 1.5T vs 3.0T METHODS
T2-Weighted Effective TE: 42ms (6th echo) TR variable Sec STIR IR time variable for best fat suppression Cycled IR Pulses On and Off B1 Field Maps

44 Black Blood FSE Imaging 1.5T vs 3.0T METHODS
Body Coil Only High-Pass Birdcage 1.5T Dimensions 60 cm Diameter; 64 cm Long 3.0T Dimensions 55 cm Diameter; 53 cm Long

45 Black Blood FSE Imaging 1.5T vs 3.0T
Results B1 (RF) Field Maps 1.5 Tesla 3.0 Tesla

46 Black Blood FSE Imaging 1.5T vs 3.0T
Results

47 Black Blood FSE Imaging 1.5T vs 3.0T
Results T2-Weighted FSE Images 1.5T 3.0T

48 Black Blood FSE Imaging 1.5T vs 3.0T
Results - T2 W SNR

49 Black Blood FSE Imaging 1.5T vs 3.0T
Results STIR FSE Images 1.5T 3.0T

50 Black Blood FSE Imaging 1.5T vs 3.0T
Results - STIR SNR

51 Black Blood FSE Imaging 1.5T vs 3.0T METHODS
ROI MEASUREMENTS

52 Black Blood FSE Imaging 1.5T vs 3.0T BLOOD SUPPRESSION PERFORMANCE
Results BLOOD SUPPRESSION PERFORMANCE

53 Black Blood FSE Imaging 1.5T vs 3.0T BLOOD SUPPRESSION PERFORMANCE
Results BLOOD SUPPRESSION PERFORMANCE

54 Black Blood FSE Imaging 1.5T vs 3.0T SNR (SIGNAL) UNIFORMITY
Results SNR (SIGNAL) UNIFORMITY

55 Black Blood FSE Imaging 1.5T vs 3.0T SNR (SIGNAL) UNIFORMITY -P/A
Results SNR (SIGNAL) UNIFORMITY -P/A

56 Black Blood FSE Imaging 1.5T vs 3.0T SNR (SIGNAL) UNIFORMITY -P/A
Results SNR (SIGNAL) UNIFORMITY -P/A

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59 Correlation of 23Na MR Imaging Findings with Cine, Late-Enhancement, and T2-weighted Findings
Note.—NA = not applicable.

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61 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.

62 Sodium Imaging of the Heart
5 inch circular coil

63 Sodium Imaging of the Heart
8 inch circular coil

64 CHEMICALLY SELECTIVE PHOSPHORUS RARE (FSE) IMAGING

65 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
PHANTOM RESULTS

66 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
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)

67 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
FOREARM EXERCISE STUDY 1H PCr Pi Rest Exercise

68 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
FOREARM EXERCISE STUDY 1H/CONTOUR OVERLAY Pi/PCr Ratio

69 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
ISCHEMIA IN DIABETIC FOOT Chronic High Glucose Levels Result in Functional Impairment of mCirculation In Lower Extremities Ischemia and D in 31P Metabolite Levels Neuropathy Ulceration Amputation Foot Muscle: Surrogate for Whole System

70 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
ISCHEMIA IN DIABETIC FOOT 1H PCr Pi

71 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
ISCHEMIA IN DIABETIC FOOT 1H CONTOUR OVERLAY Pi/PCr Ratio

72 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING
Alternative Non-Invasive Method for Assessment of Ischemia: MRS Chemical Shift Imaging PCr Pi Scan Time: 4 Minutes/Image Resolution: 0.47 X 0.47 X 25 mm (0.55 cm3) Scan Time: 34 Minutes Resolution: 10 X 10 X 25 mm (2.5 cm3)

73 31P Myocardial Imaging Methods - Pulse Sequence
3D RARE Pulse Sequence Single Excitation Multiple Spin Echoes Readout Gradients Replace One CSI Phase Encode

74 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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