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

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

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

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  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. 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. 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 21:1263-1281,2003

  7. 4T 7T (7T/4T) 7T/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 #

  9. 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 3 Tesla Body Coil B1 Mapping

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

  11. 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

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

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

  14. 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.

  15. The Wave Equation Demands Spatial Variation of B Field ConductivityEffect Spatial Variation Of RF Short Wavelength Effect

  16. Birdcage vs. Spiral Coil 0° 180° 0° 180° 90° 90° 45° 135° 45° 135°

  17. 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)

  18. Coil Performance • High Q and Q ratio • 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)

  19. 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

  20. Radial Intensity Variation in 100% Isopropanol Phantom Spiral Birdcage

  21. 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

  22. 4 Tesla Head Imaging Spiral Coil Birdcage Coil

  23. 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

  24. Effect of External Dielectric

  25. 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) Increased FSE Slice Coverage

  26. 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. Sensitivity with Reduced Flip Angles For 90° pulses, SAR is reduced 4-fold but signal drops by just 14%

  27. 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 3 Tesla Reduced SAR FSE

  28. 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

  29. End Diastole Cardiac Imaging Gradient Echo Imaging of the Heart • Short Axis • Mid Systole • End Systole

  30. End Diastole Cardiac Imaging Gradient Echo Imaging of the Heart • Long Axis • Mid Systole • End Systole

  31. Cardiac Imaging 2D FIESTA, Long and Short Axis

  32. BLACK-BLOODFSE CARDIAC IMAGING: 1.5T VS 3.0TRobert L. GreenmanJohn E. ShiroskyRobert V. MulkernNeil M. Rofsky

  33. 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)

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

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

  36. FSE Black Blood Imaging

  37. Black Blood Imaging 1.5T Null Point = 625 ms 3.0T Null Point = 706 ms 3.0T Signal at Calculated 1.5T IR Time = -0.07 M0 1.5T Null Point = 456 ms 3.0T Null Point = 490 ms 3T Signal at Calculated 1.5T IR Time = -0.03 M0

  38. Black Blood FSE Imaging 1.5T vs 3.0TMETHODS • 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

  39. Black Blood FSE Imaging 1.5T vs 3.0TMETHODS • 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

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

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

  42. Black Blood FSE Imaging 1.5T vs 3.0T Results

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

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

  45. Black Blood FSE Imaging 1.5T vs 3.0T Results STIR FSE Images 1.5T 3.0T

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

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