Multinuclear MRI at 7T

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

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Why?

• Multinuclear MR
• High specificity, low sensitivity and resolution
• 1H MR
• Low specificity, high sensitivity and resolution

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Outline

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

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Behavior of the RF filed in biological tissue
Electromagnetic wave in a uniform conducting
medium is given by

• ee0er 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)
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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.

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Frequency dependency of the RF wave length in the
brain tissue
Yang et al., MRM (2002)
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Effect of RF field propagation in brain and a
phantom at 7T
Yang et al., MRM (2002)
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RF Wavelength and Field Strength
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SAR limits on imaging

• SAR Bo2 (Flip angle)2 (RF duty cycle). (Patient
  Size)
• Flip angle qgB1tw
• 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

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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

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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

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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

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Field dependence
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Nucleus I Resonance Freq (_at_7T) in MHz Natural Abundance () Abs Sensitivity at constant field (Product of rel sensitivity and Natural abundance) T1 T2
1H 1/2 298 99.98 1 0.3-3s 1-600 ms
23Na 3/2 78.9 100 9.3x10-2 20-50ms 0.5-20 ms
31P 1/2 120.6 100 6.6x10-2 2-5s 50-1000
13C 1/2 74.8 1.1 1.76x10-4 3-15 s 100-1000 ms
17O 5/2 40.36 0.037 1.08x10-5 4-6 ms 1-2 ms
14N 1 21.4 99.6 1.0x10-3
15N 1/2 30.0 0.37 3.85x10-6
19F 1/2 280 100 .83
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Potential Multinuclear projects

• 23Na
• Alzheimers Disease
• Diffuse Axonal Injury
• Arthritis, Disc Degeneration
• 31P
• Head and Neck Tumors
• HIV infection
• 17O
• Stroke, AD, Cancer
• 13C
• Lactate Mapping in Cancer

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Sodium MR
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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.

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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)
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Detecting OA in human tissue
mM
Normal
OA specimen
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Sodium MR of Cartilage Degeneration
Normal
Symptomatic subject
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IVD and Sodium MR
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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
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31P MR
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Pre treatment
Post treatment
PME reduction is an indication of response to
therapy
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31P MR Spectroscopy of head and neck tumors
Non-responder
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-10
0
Responder
10
0
-10
-20
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Proton decoupled 31P MRS of HN Cancer at 3T
3D CSI, TR1000 ms, elliptical k-space sampling,
voxel 2.3x2.3x2.3 cm3, 9 mins
Kim et al, ISMRM2008 3247 , E-poster computer
27 (May 8, 200 pm)
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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

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17O MRI in Swine brain
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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
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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