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Multinuclear MRI at 7T

Multinuclear MRI at 7T

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Multinuclear MRI at 7T

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  1. Multinuclear MRI at 7T Ravinder Reddy Department of Radiology University of Pennsylvania

  2. Siemens 7 Tesla Magnet • 7T Whole body magnet • By Magnex • Unshielded • 2.4mx3.4 m longx3.8m • 90cm bore • Weight 35000 kg • 5 ppm peak to peak field variation over a 45 cm diameter • Shield ~400 tons

  3. Why? • Multinuclear MR • High specificity, low sensitivity and resolution • 1H MR • Low specificity, high sensitivity and resolution

  4. Outline • Non-proton MRI • Field dependent RF wave pattern • SAR • SNR and relaxation effects • Potential Multinuclear Projects • 23Na, 31P MR • 17O NMR, 13C NMR

  5. Behavior of the RF filed in biological tissue Electromagnetic wave in a uniform conducting medium is given by • e=e0er is the electric permittivity, • m = magnetic permeability, • s = conductivity in the media, • w = resonance frequency • (a) attenuation and (b) phase constants Yang et al., MRM (2002)

  6. RF wave properties in tissues • In biological tissues • RF wave length l = 2p/b • Skin depth d = 1/a • As w increases • Conductivity, s increases • dielectric constant, e, decreases.

  7. Frequency dependency of the RF wave length in the brain tissue: Yang et al., MRM (2002)

  8. Effect of RF field propagation in brain and a phantom at 7T Yang et al., MRM (2002)

  9. 17O 13C 23Na 31P 1H RF Wavelength and Field Strength

  10. SAR limits on imaging • SAR ~ Bo2 (Flip angle)2 (RF duty cycle). (Patient Size) • Flip angle q=gB1tw • For low gama nuclei, high B1 is required to achieve the same flip angle • Puts restrictions on • Pulse repetition time • Number of RF pulses in a multi-echo sequence • Slice efficiency in multi-slice imaging

  11. Signal as a function of field strength • Sample magnetization is proportional to Bo • Mo ~ Ns hgBo/kT • Induced emf in a coil is proportional to time rate of change of transverse magnetization • Larmor Precession Frequency= wo = gBo

  12. SNR as a function of field strength • For low fields, sample contribution to noise is negligible • SNR~ B07/4 • For mid field and high fields, sample noise dominates • SNR~ B0

  13. Field dependence of SNR • At ultra-high magnetic fields (≥7T) • Proton SNR dependence on magnetic field is complicated due to RF inhomogeneities • SNR (proton)  B0 x T2*/T1 • T1 also increases • For other NMR nuclei (13C, 23Na, 17O or 31P) • SNR  B07/4 x T2*/T1 • for quadrupolar nuclei • T2* and T1 are not appreciably changed with the field

  14. Low Gamma SNR Protons Field Strength Field dependence

  15. Potential Multinuclear projects • 23Na • Alzheimer’s Disease • Diffuse Axonal Injury • Arthritis, Disc Degeneration • 31P • Head and Neck Tumors • HIV infection • 17O • Stroke, AD, Cancer • 13C • Lactate Mapping in Cancer

  16. Sodium MR

  17. Sodium and AD Rationale: Loss of neurons  increase in the volume of extra-cellular space  increase in MRI-detectable sodium (due to the longer T2). Positively charged sodium ions are attracted by the negatively charged side-chains on proteoglycan (PG) macromolecules resulting in an increase in sodium content.

  18. Sodium and AD When compared to the control brains [Na] increased in the AD brains, in the circular ROI located in the hippocampus. This 6% increase [Na] may be due to: An increase in extra-cellular volume of sodium. AD-related pathology resulting in an increase in [Na]. MRM (submitted)

  19. Detecting OA in human tissue mM Normal OA specimen

  20. Sodium MR of Cartilage Degeneration Normal Symptomatic subject

  21. IVD and Sodium MR

  22. T1r and Sodium image comparison between healthy 26yo male (left) and non healthy 24 yo male (right). Scale bar in milliseconds T1rho scale bar in ms Sodium scale bar in mM

  23. 31P MR

  24. 0.8 A PME PCr 0.6 a-ATP g-ATP Pi 0.4 b-ATP 0.2 0.0 PDE B 0.3 0.2 0.1 0.0 0 -5 -10 -15 -20 Frequency (ppm) Pre treatment Post treatment PME reduction is an indication of response to therapy

  25. 31P MR Spectroscopy of head and neck tumors Non-responder 10 -10 0 Responder -10 -20 10 0

  26. Proton decoupled 31P MRS of HN Cancer at 3T 3D CSI, TR=1000 ms, elliptical k-space sampling, voxel 2.3x2.3x2.3 cm3, 9 mins Kim et al, ISMRM2008 #3247 , E-poster computer #27 (May 8, 2:00 pm)

  27. HIV-1 infection and Phospholipid Ratio • HIV-1 infection alters membrane PDE and PME • during the process of virus entry and/or replication. • Changes in PDE/PME ratio • diagnostic for the presence of HIV-1 in primary immune cells

  28. Sagittal Coronal Transverse 17O MRI in Swine brain

  29. Cross polarization 13C Chemical Shift Imaging (CSI) During [1-13C] glucose infusion RIF-1; 16.7 mM constant serum conc.; 1x2x5 mm3 voxels; 50 min acq. Time Glucose Lactate

  30. Multinuclear MR at 7T • Multinuclear MR at 7T • RF wave propagation and associated inhomogeneities are negligible • SNR increases as Bo 7/4 • T1, and T2* do not change significantly • High B1 requirements for a typical flip angle • Efficient MR of 23Na, 31P, 13C, and 17O at high fields