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Multinuclear MRI at 7T Ravinder Reddy Department of Radiology University of Pennsylvania 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 QuickT i me™ and a T IFF (Uncompressed) decom pressor are needed to see this picture. Why? Multinuclear MR High specificity, low sensitivity and resolution 1H MR Low specificity, high sensitivity and resolution Outline Non-proton MRI Field dependent RF wave pattern SAR SNR and relaxation effects Potential Multinuclear Projects 23Na, 31P MR 17O NMR, 13C NMR Behavior of the RF filed in biological tissue Electromagnetic wave in a uniform conducting medium is given by jw m (e j w me a 1/ 2 s 2 1 ( ) 1 we 2 (a) attenuation and (b) phase constants b w me 1/ 2 s 2 1 ( ) 1 we 2 e=e0er is the electric permittivity, m = magnetic permeability, s = conductivity in the media, w = resonance frequency s ) a jb ...(m 1 ) w Yang et al., MRM (2002) 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. Frequency dependency of the RF wave length in the brain tissue: Yang et al., MRM (2002) Effect of RF field propagation in brain and a phantom at 7T Yang et al., MRM (2002) RF Wavelength and Field Strength 17O 13C 23Na 31P 1H SAR limits on imaging SAR ~ Bo2 (Flip angle)2 (RF duty cycle). (Patient Size) Flip angle q=B1tw 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 Signal as a function of field strength Sample magnetization is proportional to Bo Mo ~ Ns hBo/kT Induced emf in a coil is proportional to time rate of change of transverse magnetization Larmor Precession Frequency= wo = Bo Signal w o Mo Bo2 2 2 1/ 2 2 Noise s Coil s a B bB system Sample 0 0 SNR as a function of field strength SNR 2 o B aB1/0 2 bB02 For low fields, sample contribution to noise is negligible SNR~ B07/4 For mid field and high fields, sample noise dominates SNR~ B0 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 B0 7/4 x T2*/T1 • for quadrupolar nuclei •T2* and T1 are not appreciably changed with the field Field dependence 60 Low Gamma 50 40 SNR 30 20 Protons 10 0 1 2 3 4 5 Field Strength 6 7 8 9 Nucleus I Resonance Freq (@7T) in MHz Natural Abundance (%) Abs Sensitivity at constant field (Product of rel sensitivity and Natural abundance) T1 T2 1 1/2 298 99.98 1 0.3-3s 1-600 ms H 23 Na 3/2 78.9 100 9.3x10-2 2050ms 0.520 ms 31 P 1/2 120.6 100 6.6x10-2 2-5s 501000 13 C 1/2 74.8 1.1 1.76x10-4 3-15 s 1001000 ms 17 O 5/2 40.36 0.037 1.08x10-5 4-6 ms 1-2 ms 14 N 1 21.4 99.6 1.0x10-3 15 N 1/2 30.0 0.37 3.85x10-6 19 F 1/2 280 100 .83 Potential Multinuclear projects 23Na 31P Head and Neck Tumors HIV infection 17O Alzheimer’s Disease Diffuse Axonal Injury Arthritis, Disc Degeneration Stroke, AD, Cancer 13C Lactate Mapping in Cancer Sodium MR Sodium and AD Rationale: 1. Loss of neurons increase in the volume of extra-cellular space increase in MRIdetectable sodium (due to the longer T2). 2. Positively charged sodium ions are attracted by the negatively charged side-chains on proteoglycan (PG) macromolecules resulting in an increase in sodium content. 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: 1. An increase in extra-cellular volume of sodium. MRM (submitted) 2. AD-related pathology resulting in an increase in [Na]. Detecting OA in human tissue mM Normal OA specimen Sodium MR of Cartilage Degeneration Normal Symptomatic subject IVD and Sodium MR 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 31P MR 0.8 PCr 0.6 Pre treatment A PME Pi -ATP a-ATP 0.4 b-ATP 0.2 0.0 PDE B 0.3 Post treatment 0.2 0.1 PME reduction is an indication of response to therapy 0.0 0 -5 -10 (ppm)-15 Frequency -20 31P MR Spectroscopy of head and neck tumors Non-responder 10 -10 0 Responder 10 0 -10 -20 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) 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 MRI in Swine brain Sagittal Coronal Transverse 22.0 21.5 Computed [H217O] 17O 21.0 20.5 20.0 19.5 19.0 18.5 18.0 0 100 200 300 Time (sec) 400 500 600 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 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