MRI Principles

8/6/2019 MRI Principles

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Yves De Deene

MRI PRINCIPLES

NMR imaging is non-invasive

Strong static magnetic field

Radiofrequent electromagnetic waves

Space and time dependent magnetic fields

MRIMRS MRRMM agnetic

RR esonantceSS pectroscopy

MM agneticRR esonance

ImagingImaging

MM agneticRR esonance

RR elaxometry

Spectra Relaxation timesDiffusion coefficients

Images

tt

R1,2

M M

g(R 1,2)

NN .. MM .. RRNN .. MM .. RR


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NN .. MM .. RRNN .. MM .. RR

Water

H2O

m p = 1,67.10 -24 g

mp = 0,00000000000000000000000167 g

+ ++++

++++

+q p = 1,6.10 -19 C

q p = 0,00000000000000000016 C

Properties of the atomic nucleus

.( 1) L I I = +. z I L m=

.( / 2 ) p L p pg q m L =

, .( / 2 ) p z L p p zg q m L =

N

Z

Otto Stern

Walther Gerlach Nobel price for physics in 1943

Discovery: Nuclear spin momentum of the proton19221922Frankfurt

Discovery of Nuclear Magnetic Resonance19381938

Nobel price physics in 1944

Isidor Isaac Rabi

From left to right: N. Bohr,J. Franck, A. Einstein, I. Rabi

Z

N

Z

Z

N

N

New York

( ) z

H F z

=

H z

H z


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( )dM

M Bdt

=

B

M

The Bloch equation

dM dt

Nuclear Magnetic Resonance:Nuclear spin precession

Cross-section of an NMR scanner

Water molecule

Cryogenic magnet

Radiofrequency coil

Gradient coil

Hydrogenproton transmitsaradiofrequent electromagneticwave(yellow) afterexcitationbyan RFpulse(red)

B0

B0 B0 B0

2 RF f B

=

EXCITATION PULSE

COIL

ImageProcessing(2D-FFT)

Nuclear Magnetic Resonance Imaging:Basic principle

Topview of precession(transverse plane)

= . B

EXCITATION

COIL OBJECT:spin-system

SIGNAL RECEPTION

COIL OBJECT:spin-system

Nuclear Magnetic Resonance Imaging:Excitation and signal reception


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= . BNuclear Magnetic Resonance Imaging:Dephasing

IN PHASE OUT OF PHASE

Illinois

First discovery of a spin-echo

- E. Hahn (1950) -

19491949

Erwin Hahn

Pulsed field NMRThe spin-echo experiment

Excitationpulse

Refocusingpulse

TE

Excitationpulse

Refocusingpulse

TR

Erwin Hahn1949

TE/2 TE/2

REFOCUSSINGPULSE

EXCITATIONPULSE

N

ZZ

N



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Excitationpulse

Refocusing-pulse

TE/2 TE/2

MAGNETIC DIPOLE MOMENT PHASORS


Erwin Hahn1949

NicolaasBloembergen

RobertPound

EdwardPurcell

EXCITATIONPULSE

REFOCUSINGPULSE

EXCITATIONPULSE

REFOCUSINGPULSE

Nuclear Magnetic Resonance Imaging:Relaxation mechanisms

B 0

BL

tOnewater molecule

BL

tManywater molecules

c

T1, T2

T1(64 MHz)

T2(64 & 128 MHz)

01

liquids solids

1210

410

110

110

910 610

T1(128 MHz)

c

T

Nuclear Magnetic Resonance Imaging:Relaxation mechanisms


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INTERMEDIATELAYER

FREEWATER

Hydrogen bridges

C

C

OO

C N BOUNDLAYER

+

+ +

+

-

-- +

+

- - +

-

- +

+++

Relaxation theory in tissue(Daskiewicz, Zimmerman, Brittin, Edzes en Samulski)

J

0

BL

t

F

Bound layer

Intermediatelayer

Free water

R2mosteffective

R1mosteffective

0 = 1/ c

0 > 1/ c

0 < 1/ c

Pound PurcellBloembergen

INTERMEDIATELAYER

FREEWATER

Hydrogen bridges

C

C

OO

C N BOUNDLAYER

+

+ +

+

-

-- +

+

- - +

-

- +

+++

Protein, polymer, cell membrane

M

M

M

t

t

tLow mobility

High mobility

Contrast in MR images is determinedby the interaction of spin systems

Nuclear Magnetic Resonance Imaging:Two important relaxation mechanisms

Spin-lattice relaxation Spin-spin relaxationT1 T2

Energy transfered to lattice (phonons)

Entropy increases

Repopulation of spins between spin energy levels

Restoration of longitudinal magnetization

Energy transferred between spins

No entropy change of total spin system

No repopulation of spins between spin energy levels

Dephasing of transverse magnetization

time

B0

time

B0

Interactions with magnetic field fluctuations at Larmor frequency Interactions with magnetic field fluctuations at low frequency


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From nuclear magnetic resonance signal to Magnetic Resonance ImagingAn analogon in acoustics

1 2

1

2

3

From nuclear magnetic resonance signal to Magnetic Resonance ImagingAn analogon in acoustics:

Localisation of glasses close together

Frequency encoding

Spatial encoding of the NMR signal is based onfrequency changes in the precession

B

M

B

M

B

M

RF COIL

Fouriertransform

/ f =

4.7 m =

64 f MHz=


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Nature 242, (1973), 190-191

Illinois / Nottingham

19731973

Nobel price for physiology and medicine (Lauterbur & Mansfield) in 2003

Paul Lauterbur Peter Mansfield

NMR scanner with backprojection

Zurich

19741974

Richard Ernst

NMR scanner with 2D Fourier transformation

Nobel price for chemistry in 1991

Slice selection

1.52 T

B

z

64.8 MHz

f = 64.8 MHz

= . B

Spatial encoding of the NMR signal

GRADIENT COILS


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Spatial encoding of the NMR signal

Frequency encoding

MHz64 6860

T1.5 1.61.4 = . B

BG

x

SLICE SELECTION

B

z

EXCITATIONPULSE

R = 64.8 MHz B = 1.52 T

f = 64.8 MHz

1.52 T

= . B

1.52 T

EXCITATIONPULSE

PHASE ENCODING

B

y


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EXCITATIONPULSE

REFOCUSINGPULSE

1.4

FREQUENCY ENCODING

B

y

T1.5 1.6

SLICE SELECTION

2D-FOURIERTRANSFORM

FREQUENCY AND PHASE ENCODING

FREQUENCY

PHASE

AMPLITUDEIMAGE

(f , )

The first MRI scanners ...

and recent ones

Mobile MRI unitOpen MRI unitInterventional MRI unit


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Anatomical imagingBone and soft tissue

T2 weighted(torn ligaments)

rheumatoid arthritiswhrist

rheumatoid arthritisknee

T2 weighted(hernia)

Osteoporosis (femur)

But there is more to MRIthan anatomical imaging ...

First NMR images

19721972State of the art

3D images dynamic images

sharp image resolution

20082008

In research phase quantitative imaging

celspecific contrastagents hyperpolarized MRI in vivo spectroscopy functional imaging

multimodality imaging

I visited Copenhagen frequently after the war. At one point, I gave a talk in Copenhagen, and then afterwards we met with Bjerrum. Bjerrum was a chemist and a great friend of Niels Bohr Bohr said to him: You know, what these people do is really very clever. They put little spies into the molecules and send radio signals to them, and they have to radio back what they are seeing. I thought that was a very nice way of formulating it. That was exactly how they were used. It was not anymore the protons as such. But from the way they reacted,you wanted to know in what kind of environment they are, just

like spiesthat you send out. That was a nice formulation.- Felix Bloch -

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MRI Principles