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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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Presentation on theme: "3T MRI BIDMC DEPARTMENT OF RADIOLOGY MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges."— Presentation transcript:

1 3T MRI BIDMC DEPARTMENT OF RADIOLOGY MR Imaging and Spectroscopy of the Heart at 3T:Technical Challenges

2 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

3 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

4 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

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 B 1 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 Ultrahigh field (7T) magnetic resonance imaging and spectroscopy Kâmil Uurbil,, a, Gregor Adriany a, Peter Andersen a, Wei Chen a, Michael Garwood a, Rolf Gruetter a, Pierre-Gil Henry a, Seong-Gi Kim a, Haiying Lieu a, Ivan Tkac a, Tommy Vaughan a, Pierre-Francoise Van De Moortele a, Essa Yacoub a and Xiao-Hong Zhu a Magnetic Resonance Imaging 21: ,2003 Better Spectra

7 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 T 7T (7T/4T) 7T/4T(calc)

8 3T MRI BIDMC DEPARTMENT OF RADIOLOGY Poster #

9 3T MRI BIDMC DEPARTMENT OF RADIOLOGY

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 coil pads near patient dielectric shading arrows indicate magnitude and phase of B 1 + field color shows B 1 + field magnitude

15 GE Company Confidential Tim 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

16 GE Company Confidential Tim 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

17 GE Company Confidential Tim 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)

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 Spatial Variation Of RF Short Wavelength Effect Conductivity Effect

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

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 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=B 2 Phase gradient less than expected 66% of gradient expected for geometry

24 Radial Intensity Variation in 100% Isopropanol Phantom Birdcage Spiral

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 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: (1997)

30 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%

31 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 echoes 256x256, 24 cm FOV TE 12.4/112 4 min 45 s total scan time

32 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

33 3T MRI BIDMC DEPARTMENT OF RADIOLOGY End Diastole Cardiac Imaging Gradient Echo Imaging of the Heart Mid Systole End Systole Short Axis

34 3T MRI BIDMC DEPARTMENT OF RADIOLOGY Cardiac Imaging Gradient Echo Imaging of the Heart End Diastole Mid Systole End Systole Long Axis

35 3T MRI BIDMC DEPARTMENT OF RADIOLOGY Cardiac Imaging 2D FIESTA, Long and Short Axis

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

37 Published Studies –Gradient Echo - Signal ~ Sin  –No Spin Echo (or FSE) Studies –Spin Echo - Signal ~Sin 3  B1 Heterogeneity –Conductive Effects (Signal Attenuation) –Dielectric Effects (Waveguide Effect) (or Resonant Cavity Effect) FSE Black-Blood Imaging

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

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

40 FSE Black Blood Imaging

41 1.5T Null Point = 625 ms 3.0T Null Point = 706 ms 3.0T Signal at Calculated 1.5T IR Time = M 0 Black Blood Imaging 1.5T Null Point = 456 ms 3.0T Null Point = 490 ms 3T Signal at Calculated 1.5T IR Time = M 0

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

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

44 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 3.0T METHODS

45 B1 (RF) Field Maps 1.5 Tesla3.0 Tesla Black Blood FSE Imaging 1.5T vs 3.0T Results

46 Black Blood FSE Imaging 1.5T vs 3.0T Results

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

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

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

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

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

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

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

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

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

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

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59 Correlation of 23 Na 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 23 Na 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 23 Na signal intensity. Only 10 patients in the subacute and nine in the chronic group revealed late enhancement; significant correlation with 23 Na 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 23 Na signal intensity but was extensively larger.

62 3T MRI BIDMC DEPARTMENT OF RADIOLOGY Sodium Imaging of the Heart 5 inch circular coil

63 3T MRI BIDMC DEPARTMENT OF RADIOLOGY 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 in vivo 31 P RARE IMAGING PARAMETERS Modified FSE Sequence w/Chemical Selective Excitation Spatial Resolution: 4.7 X 4.7 X 25 mm (0.55 cm 3 ) Scan Time: 4 Minutes/Metabolite Image (PCR or Pi) CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING

67 Rest Exercise 1H1H PCrPi FOREARM EXERCISE STUDY CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING

68 FOREARM EXERCISE STUDY 1 H/CONTOUR OVERLAY Pi/PCr Ratio

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

70 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING 1H1H PCrPi ISCHEMIA IN DIABETIC FOOT

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

72 CHEMICALLY SELECTIVE PHOSPHORUS RARE IMAGING Alternative Non-Invasive Method for Assessment of Ischemia: MRS Chemical Shift Imaging Scan Time: 34 Minutes Resolution: 10 X 10 X 25 mm (2.5 cm 3 ) Scan Time: 4 Minutes/Image Resolution: 0.47 X 0.47 X 25 mm (0.55 cm 3 ) PCrPi

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

74 31 P 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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