Ari Borthakur, PhD Associate Director, Center for Magnetic Resonance & Optical Imaging Department of...

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

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 gD e p a r t m e n t o f R a d i o l o g y

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

Principles of MRI

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?

OutlineSlide

# 3

Hardware: MRI scanner RF coil Gradients

Image contrastSoftware:

Pulse sequences Fourier Transform

How does it work?

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

Original Concept*

1. Rotating Gradient2. Filtered Back-projection

*Lauterbur, Nature (1973)

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)

MRI scanners

19 Tesla 4.7 Tesla 1.5 Tesla

To generate B0 field and create polarization (M0)

MRI coils

Head coil

Mouse MRI platform

Torso coil

To transmit RF field (B1) and receive signal

Gradients

To spatially encode the MRI signal

Image Contrast

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

*, T1r) 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

Magnetization

Bitar et al. RadioGraphics (2006)

“spin” “spin ensemble” spin-upor

spin-down=

Boltzmanndistribution

Net MagneticMoment

Energy levels

Larmor Equation

Energy gap

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

MR experiment

M0 aligned along B0

Mz=M0

RF pulseapplied in x-y “transverse”

plane

M “nutates” down to x-y plane

Mz=M0cosand

Mxy=M0sin

RF off

Detect Mxy signal

in the same RF coil

Equation of motion

Bloch Equation (simple form)

Effect of RF pulse

The 2nd B field

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

Nutation

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

Rotating frame of referencee.g. B1 along y-axis

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

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*

Signal detection

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

How does MRI work?

Water Fat4.7 ppm 1.2 ppm

NMR signal

?=

MR Image

1) Spatial encoding gradients 2) Fourier transform

Pulse Sequence (timing diagram)

Less MRI time/low image quality

RF

phase

slice

freq

90

TE

Echo planar imaging

Les

s M

RI

time/

low

imag

e qu

ality RF

phase

slice

freq

90

TE

Spin-echoTR

180

RF

phase

slice

freq

(<90)

TE

Gradient-echoTR

RF

phase

slice

freq

90

Fast spin-echoTR

180 180 180

MRI

B0 M0B1

Gz

Gx

Gy

RF

phase

slice

freq

TE

Gradient-echo

TR

Slice Selection*

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

Mz

Slice Selection

B0

Mz

Mz

MxyGz•z RF

BWRF= Gz•z

z{

Frequency Encoding*

RF

phase

slice

freq

TE

Gradient-echo

TR


a.k.a “readout”

Frequency Encoding

Mz

Gx•x

BWread= Gx•FOVx

FOVx

RF

phase

slice

freq

TE

Gradient-echo

TR

Phase Encoding*

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

pe

Phase Encoding

Gy2•y

1/pe= Gy•FOVy

FOVy

Gy1•y

Gy0•y

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

Traversing k-space

Spin-warp imaging*

RF

phase

slice

freq

TE

Gradient-echo

TR

ky

kx

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

FT

k-space

freqphase

image

Fourier Transform

Spiral Radial Echo-planar

phase

freq

Echo planar freq

freq

Radial

freq

freq

Spiral

Traversing k-space

Fourier Transform

Typical FT pairs

K-space

The signal a coil receives is from the whole object:

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


replace:

K-space signal:

K-space weighting

High-frequency data=edges

Low-frequency data=contrast/signal

MRI Examples

3D Neuro MRI

Susceptibility-Weighted Imaging (SWI)

BOLD fMRI

Phase Contrast MR Angiography

Knee Imaging

TSEFat Saturated

TSETE = 25 ms

Resolution:200x200µm2

7T Project: Hyper-polarized 3He MRI

K Emami, et al., Mag Reson Med 2009

MRI

MRI “head coil”

Williams et al., PNAS 1994

7T Project: ASL-Perfusion MRI

Arterial Spin Labeling (ASL) technique

TAG

Ima

ge

Con

trol

ASL MRI & PET of Alzheimer’s Disease

ASL MRI Scan

18-FDG

PET Scan

CONTROL AD

Detre et al., Neuroimage 2009

Take home…Slide # 44

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

Ari Borthakur, PhD Associate Director, Center for Magnetic Resonance & Optical Imaging Department of...

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