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A r i B o r t h a k u r, P h D

A s s i s t a n t P r o f e s s o r , D e p a r t m e n t o f R a d i o l o g y A s s o c i a t e D i r e c t o r , C e n t e r f o r M a g n e t i c R e s o n a n c e & O p t i c a l I m a g i n g

U n i v e r s i t y o f P e n n s y l v a n i a S c h o o l o f M e d i c i n e

BMB 601 MRI

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A brief history…

  Isidor Rabi wins Nobel prize in Physics (1944) for for his resonance method for recording the magnetic properties of atomic nuclei.

  Felix Bloch and Edward Purcell share Nobel Prize in Physics (1952) for discovery of magnetic resonance phenomenon.

  Richard Ernst wins Nobel Prize in Chemistry (1991) for 2D Fourier Transform NMR.

  His post-doc, Kurt Wuthrich awarded a Nobel Prize in Chemistry (2002) for 3D structure of macromolecules.

  Nobel Prize in Physiology & Medicine awarded to Sir Peter Mansfield and Paul Lauterbur (2003) for MRI.

  Who’s next? Seiji Ogawa for BOLD fMRI?

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Outline Slide

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  Hardware:   MRI scanner   RF coil   Gradients

  Image contrast   Software:

  Pulse sequences   Fourier Transform

slide 4

How does it work?

http://www.med.harvard.edu/AANLIB/cases/case17/mra.mpg

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slide 5

Original Concept*

1.  Rotating Gradient 2.  Filtered Back-projection

*Lauterbur, Nature (1973)

slide 6

Fourier Imaging*

1.  Three orthogonal gradients:   Slice Selection   Phase Encoding   Frequency Encoding

2.  k-space   Representation of encoded signal

*Kumar, et al. J. Magn. Reson. (1975)

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slide 7

MRI scanners

19 Tesla 4.7 Tesla 1.5 Tesla

To generate B0 field and create polarization (M0)

slide 8

MRI coils

Head coil

Mouse MRI platform

Torso coil To transmit RF field (B1) and receive signal

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slide 9

Gradients

To spatially encode the MRI signal

slide 10

Image Contrast

Contrast in MRI based on differences in:   Relaxation times (T1, T2, T2

*, T1ρ)   Local environment

  Magnetization = spin density   Concentration of water, sodium, phosphorous etc.

  Macromolecular interactions   Chemical-exchange, dipole-dipole, quadrupolar interactions

  Contrast agents   Gadolium-based, iron-oxide, manganese

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slide 11

Magnetization

Bitar et al. RadioGraphics (2006)

“spin” “spin ensemble” spin-up or

spin-down =

Boltzmann distribution

Net Magnetic Moment

slide 12

Energy levels

Larmor Equation

Energy gap

N+/N- = exp (-ΔE/kBT) Boltzmann distribution

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slide 13

MR experiment

M0 aligned along B0 Mz=M0

RF pulse applied in x-y “transverse”

plane

M “nutates” down to x-y plane

Mz=M0cosα and

Mxy=M0sinα

RF off

Detect Mxy signal

in the same RF coil

slide 14

Equation of motion

Bloch Equation (simple form)

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slide 15

Effect of RF pulse

The 2nd B field

Choosing a frame of reference rotating at ωrf, makes B1 appear static:

slide 16

Nutation

Lab frame of reference e.g. B1 oscillating in x-y plane

Rotating frame of reference e.g. B1 along y-axis

http://www-mrsrl.stanford.edu/ ~brian/mri-movies/

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slide 17

After RF pulse-Relaxation

Spin-Lattice relaxation time=return to thermal equilibrium M0

Spin-spin relaxation with reversible dephasing

Spin-spin relaxation without “reversible” dephasing

T1>T2>T2*

slide 18

Signal detection

http://www-mrsrl.stanford.edu/ ~brian/mri-movies/

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slide 19

How does MRI work?

Water Fat 4.7 ppm 1.2 ppm

NMR signal

? =

MR Image

1) Spatial encoding gradients 2) Fourier transform

slide 20

Pulse Sequence (timing diagram)

Less MRI time/low image quality

RF

phase

slice

freq

90°

TE

Echo planar imaging

Less

MRI

tim

e/lo

w im

age

qual

ity

RF

phase slice

freq

90°

TE

Spin-echo TR

180°

RF

phase slice

freq

α (<90°)

TE

Gradient-echo TR

RF

phase slice

freq

90°

Fast spin-echo TR

180° 180° 180°

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MRI

B0 M0 B1

Gz

Gx

Gy

slide 22

RF

phase

slice

freq

α

TE

Gradient-echo

TR

Slice Selection*

*Mansfield, et al. J. Magn. Reson. (1976) *Hoult, J. Magn. Reson. (1977)

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Mz

Slice Selection

B0

Mz

Mz

Mxy γGz•z RF

BWRF= γGz•Δz

Δz{

slide 24

Frequency Encoding*

RF

phase

slice

freq

α

TE

Gradient-echo

TR

*Kumar, et al. J. Magn. Reson. (1975)

a.k.a “readout”

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Frequency Encoding

Mz

γGx•x

BWread= γGx•FOVx

FOVx

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RF

phase

slice

freq

α

TE

Gradient-echo

TR

Phase Encoding*

*Kumar, et al. J. Magn. Reson. (1975) *Edelstein, et al. Phys. Med. Biol. (1980)

τpe

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Phase Encoding

γGy2•y

1/τpe= γΔGy•FOVy

FOVy

γGy1•y

γGy0•y

Each PE step imparts a different phase twist to the magnetization along y

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Traversing k-space

Spin-warp imaging*

RF

phase

slice

freq

α

TE

Gradient-echo

TR

ky

kx

*Edelstein, et al. Phys. Med. Biol. (1980)

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slide 29

FT

k-space

freq phase

image

Fourier Transform

slide 30

Spiral Radial Echo-planar

phase

freq

Echo planar freq

freq

Radial

freq

freq Spiral

Traversing k-space

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slide 31

Fourier Transform

slide 32

Typical FT pairs

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K-space

The signal a coil receives is from the whole object:

€

S tx ,ty( ) = ρ x,y( )eiγ Gxtx ⋅x+Gyty ⋅y( )dxdy∫∫

€

kx, y = γGx, ytx, y

€

S kx ,ky( ) = ρ x,y( )eiγ kx ⋅x+ ky ⋅y( )dxdy∫∫

The image is a 2D FT of the k-space signal*:

€

ρ x,y( ) = S kx ,ky( )e−iγ kx ⋅x+ ky ⋅y( )dkxdky∫∫

*Kumar, et al. J. Magn. Reson. (1975)

replace:

K-space signal:

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K-space weighting

High-frequency data=edges

Low-frequency data=contrast/signal

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slide 35

What is wrong?

Original

Freq. de-phaser too large!

*Pauly, http://www.stanford.edu/class/ee369b/tests/mt_sol.pdf

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What is wrong?

Original

ky step was too large or FOVy was too small!

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slide 37

What is wrong?

Original

Freq. gradient amplitude was too low!

slide 38

MRI Examples

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slide 39

3D Neuro MRI

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Susceptibility-Weighted Imaging (SWI)

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BOLD fMRI

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Phase Contrast MR Angiography

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TSE  Fat  Saturated  

TSE  TE  =  25  ms  

Resolution:200x200µm2

slide 44

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Rationale for Sodium MRI

[PG] decreases with Degeneration

Urban et al Spine 1998

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Rationale for Sodium MRI

Sodium is a natural biomarker for PG

Urban and Maroudas Bioc. Biop. Acta 1979 Urban and Winlove, J. Magn. Reson. Imag. 2007

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How is [Na] related to [PG]?

 Fixed Charge Density (FCD)  Side chains of PG   [Na] correlated with [PG] through [FCD]

Maroudas et al., in Adult Articular Cartilage (1979) Lesperance et al., J. Orthop. Res. (1992)

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Sodium MRI

What do you need?

1.  RF coil 2.  Transmit/receive switch 3.  Broadband capable scanner (all vendors) 4.  A short echo-time MRI pulse sequence

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slide 49

Sodium MRI Ex Vivo

Wang et al., Spine (2010)

•  Sodium maps in bovine discs

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Sodium MRI Ex Vivo

Wang et al., Spine (2010)

Sodium content vs. PG content

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Sodium MRI In Vivo

Wang et al., Spine (2010)

•  Sodium mapping vs. T2 MRI

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New 7T MRI!

  ultra-short echo pulse sequence

  TE/TR=200µs/25ms   2mm isotropic   15 minute acquisition

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slide 53

Sodium MRI Movies

Axial Coronal Sagittal

slide 54

Sodium MRI Montage

  SNR ~26:1 in cartilage

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slide 55

Take home… Slide # 55

  Magnet   Create B0   Produce M0

  RF coil   Transmit B1 field   Detect Signal

  Image contrast   Relaxation, concentration, interactions etc.

  Gradients   Spatial encoding   Signal in k-space

  Fourier Transform   From k-space to image space

  Pulse sequences   Traverse k-space   Image Artifacts