Multinuclear MRI at 7T

Ravinder ReddyRavinder Reddy

Department of RadiologyDepartment of Radiology

University of PennsylvaniaUniversity of Pennsylvania

Siemens 7 Tesla Magnet 7T Whole body magnet7T Whole body magnet

By MagnexBy Magnex UnshieldedUnshielded 2.4mx3.4 m longx3.8m2.4mx3.4 m longx3.8m 90cm bore90cm bore Weight 35000 kgWeight 35000 kg 5 ppm peak to peak field 5 ppm peak to peak field

variation over a 45 cm variation over a 45 cm diameterdiameter

Shield ~400 tonsShield ~400 tons

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Why?

Multinuclear MRMultinuclear MR High specificity, low sensitivity and resolutionHigh specificity, low sensitivity and resolution

11H MRH MR Low specificity, high sensitivity and resolutionLow specificity, high sensitivity and resolution

Outline Non-proton MRINon-proton MRI

Field dependent RF wave patternField dependent RF wave pattern SAR SAR SNR and relaxation effects SNR and relaxation effects

Potential Multinuclear ProjectsPotential Multinuclear Projects 2323Na, Na, 3131P MRP MR 1717O NMR, O NMR, 1313C NMRC NMR

Behavior of the RF filed in biological tissue

==rr is the electric permittivity, is the electric permittivity,

= magnetic permeability, = magnetic permeability, = conductivity in the media, = conductivity in the media, = resonance frequency= resonance frequency

(() attenuation and () attenuation and () phase constants) phase constants

€

γ = jω μ (ε − jσ

ω) = α + jβ ...(m−1)

€

=

21 + (

σ

ωε)2 −1

⎡

⎣ ⎢

⎤

⎦ ⎥

1/ 2

€

=

21 + (

σ

ωε)2 +1

⎡

⎣ ⎢

⎤

⎦ ⎥

1/ 2

Electromagnetic wave in a uniform conducting medium is given by

Yang et al., MRM

RF wave properties in tissues

In biological tissuesIn biological tissues RF wave lengthRF wave length = = Skin depth Skin depth = = AsAsincreasesincreases

Conductivity,Conductivity,increases increases dielectric constant, dielectric constant, , decreases., decreases.

Frequency dependency of the RF wave length in the brain

tissue:

Yang et al., MRM

Effect of RF field propagation in brain and a phantom at 7T

Yang et al., MRM

RF Wavelength and Field Strength

C

Pa

SAR limits on imaging

SAR ~ BSAR ~ Boo2 2 (Flip angle)2 (RF duty cycle). (Patient Size)

Flip angle=γB1tw

For low gama nuclei, high B1 is required to achieve the same flip angle Puts restrictions onPuts restrictions on

Pulse repetition timePulse repetition time Number of RF pulses in a multi-echo sequenceNumber of RF pulses in a multi-echo sequence Slice efficiency in multi-slice imagingSlice efficiency in multi-slice imaging

Signal as a function of field strength

Sample magnetization is proportional to BSample magnetization is proportional to Boo

MMoo ~ N ~ Nss h hγγBBoo/kT/kT

Induced emf in a coil is proportional to time rate of change of Induced emf in a coil is proportional to time rate of change of transverse magnetizationtransverse magnetization Larmor Precession Frequency= Larmor Precession Frequency= oo = = γγBBoo

€

Signal ∝ωoMo ∝ Bo2

€

Noise ∝ σ Coil +system2 + σ Sample

2 ∝ αB01/ 2 + bB0

2

SNR as a function of field strength

For low fields, sample contribution to noise is negligibleFor low fields, sample contribution to noise is negligible SNR~ BSNR~ B00

7/47/4

For mid field and high fields, sample noise dominates SNR~ BSNR~ B00

€

SNR ∝Bo

2

αB01/ 2 + bB0

2

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

0

10

20

30

40

50

60

1 2 3 4 5 6 7 8 9

Field Strength

SNR

Low Gamma

Protons

NucleusNucleus II Resonance Resonance Freq (@7T) Freq (@7T) in MHzin MHz

Natural Abundance Natural Abundance (%)(%)

Abs Sensitivity at constant Abs Sensitivity at constant field (Product of rel field (Product of rel sensitivity and Natural sensitivity and Natural abundance)abundance)

T1T1 T2T2

11HH 1/21/2 298298 99.9899.98 11 0.3-3s 1-600 1-600 msms

2323NaNa 3/23/2 78.978.9 100100 9.3x109.3x10-2-2 20-50ms

0.5-20 ms

3131PP 1/21/2 120.6120.6 100100 6.6x106.6x10-2-2 2-5s 50-1000

1313CC 1/21/2 74.874.8 1.11.1 1.76x101.76x10-4-4 3-15 s 100-100-1000 1000 msms

1717OO 5/25/2 40.3640.36 0.0370.037 1.08x101.08x10-5-5 4-6 ms4-6 ms 1-2 1-2 msms

1414NN 11 21.421.4 99.699.6 1.0x101.0x10-3-3

1515NN 1/21/2 30.030.0 0.370.37 3.85x103.85x10-6-6

1919FF 1/21/2 280280 100100 .83.83

Potential Multinuclear projects

2323NaNa Alzheimer’s DiseaseAlzheimer’s Disease Diffuse Axonal InjuryDiffuse Axonal Injury Arthritis, Disc DegenerationArthritis, Disc Degeneration

3131PP Head and Neck TumorsHead and Neck Tumors HIV infection HIV infection

1717OO Stroke, AD, CancerStroke, AD, Cancer

1313CC Lactate Mapping in CancerLactate Mapping in Cancer

Sodium MR

Sodium and AD Rationale:

1. Loss of neurons increase in the volume of extra-cellular space increase in MRI-detectable 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.2. AD-related pathology resulting in an increase in [Na]. MRM (submitted)

Detecting OA in human tissue

mM

NormalNormal

OA specimenOA specimen

Sodium MR of Cartilage Degeneration

Normal Symptomatic subject

IVD and Sodium MR

Tand 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

0.0

0.2

0.4

0.6

0.8 PME

Pi

PDE

PCr

γ-ATP-ATP

-ATP

0 -5 -10 -15 -20

0.0

0.1

0.2

0.3

Frequency (ppm)

Pre treatment

Post treatment

PME reduction is an indication of response to therapy

10 0 -10

10 0 -10 -20

Non-responder

Responder

31P MR Spectroscopy of head and neck tumors

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 HIV-1 infection alters membrane PDE and PME during the process of virus entry and/or replication.during the process of virus entry and/or replication.

Changes in PDE/PME ratio Changes in PDE/PME ratio diagnostic for the presence of HIV-1 in primary diagnostic for the presence of HIV-1 in primary

immune cellsimmune cells

17O MRI in Swine brain

Coronal

Sagittal

Transverse

18.0

18.5

19.0

19.5

20.0

20.5

21.0

21.5

22.0

0 100 200 300 400 500 600

Time (sec)

Computed [H217O]

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 7TMultinuclear MR at 7T RF wave propagation and associated inhomogeneities RF wave propagation and associated inhomogeneities

are negligibleare negligible SNR increases as BSNR increases as Bo o

7/47/4 T1, and T2* do not change significantlyT1, and T2* do not change significantly High B1 requirements for a typical flip angleHigh B1 requirements for a typical flip angle Efficient MR of 23 23Na, Na, 3131P, P, 1313C, and C, and 1717O at high fieldsO at high fields