Magnetic Resonance Imaging– Basic Principles –

EVELYNE BALTEAU

e.balteau@ulg.ac.beCyclotron Research Centre

Overview

• Brief history of MRI• Magnetic properties of the nuclei

• Interaction with B0

• Interaction with B1

• Relaxation• Signal Localization• Contrast

Brief history of MRI

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1940 1950 1960 1970 1980 1990 2000

1946 – Bloch & Purcell independently describe the NMR phenomenon1952 – Bloch & Purcell Nobel Prize in Physics

NMR developed as analytical tool (no medical application)

1973 – Lauterbur : Back-projection MRImaging

1971 – Damadian : NMR used to distinguish healthy and malignant tissues medical application but imaging technique…

1975 – Ernst : Fourier Transform based MRI (demonstrated by Edelstein in 1980)

1977 – Mansfield : Echo-Planar Imaging

1991 – Ernst Nobel Prize in Chemistry

1990 – Ogawa : functional MRI (BOLD)

2003 – Lauterbur & Mansfield Nobel Prize in Medicine

MRI : magnetic stuff !!

Magnetic properties of the NUCLEI

External magnetic field

B0 = 3 T

Electromagnetic field B1 (Radio-

frequency or RF)

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60000 the earth’s magnetic field !!!!

FM radio-waves : 88.8 – 108.8 MHz !!

Magnetic properties of the nuclei

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Nuclear MRI no radioactivity !! nucleus is like a small magnet

The nuclear SPIN characterized by a spin number I quantum mechanics !! a nucleus with I 0 behaves like a

small magnet

The Hydrogen nucleus the most abundant (~⅔ of the atoms in living tissues)

Behaviour of the nuclei interacting with :

1.The external magnetic field B0

Equilibrium state

2.The electromagnetic field B1 (RF)

Disturbance

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Interaction with B0

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1. Orientation :

Interaction with B0

2. Energy states :

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E = ħBo = ħo

Interaction with B0

3. Precession :

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Rotation or precession about the axis of the magnetic field Bo with frequency :

o = Bo

o = Larmor frequency = gyromagnetic ratio

Interaction with B0

3. Precession :

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At the equilibrium state :

- rotation in phase

- no transverse magnetization Mxy

y

x

Interaction with B0

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4. Summary : at the equilibrium state :

1. spin orientation « up » > « down »

longitudinal magnetization Mz

2. precession

no transverse magnetization Mxy

Interaction with B1

Resonance phenomenon

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TRANSITIONS

Transitions E1 E2 Mz decreases

REPHASING

Phase coherence increases Mxy increases

!!! RF frequency = Larmor frequency = 0 !!!

Interaction with B1

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Two different processes :

1. Transitions E1 E2 Mz decreases

2. Rephasing Mxy increases

The macroscopic magnetization flips from the z-axis to the xy-plane and precesses

From the macroscopic point of view…

Relaxation back to the equilibrium state…

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DEPHASING

Dephasing Mxy decreases T2 relaxation

TRANSITIONS

Transitions E2 E1 Mz increases T1 relaxation

Relaxation back to the equilibrium state…

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Two different processes :

1. Transitions E2 E1 Mz increases T1 relaxation

2. Dephasing Mxy decreases T2 (exponential)

relaxation

Free Induction Decay : received signal !! informations from the

tissues of interest

Signal localization

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Up to now : the signal received contains information from the

entire body !!

Not interesting ! Use field gradients to spatially encode the signal

Three steps :1. Slice selection slice = matrix2. Frequency-encoding columns3. Phase-encoding lines

Signal localization

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1. Slice selection gradient Resonance Phenomenon : RF = o !!!

Before Gz is applied : all the spins precess with the same Larmor frequency o all could resonate !!

During application of Gz : the spins precess with only spins with frequency = RF resonate

Signal localization

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2. Frequency-encoding gradient

Slice selection : but still no spatial discrimination within the slice !

Before Gx is applied : all the spins precess with the same Larmor frequency o

During application of Gx : the spins precess with frequencies Fourier Transform of the signal allows discrimination between columns !

Signal localization

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3. Phase-encoding gradient

Before Gy is applied : all the spins precess with the same Larmor frequency o

During application of Gy : the spins precess with frequencies induces phase difference between the linesAfter application of Gx : all the spins precess again at the same Larmor frequency, but with different phase shifts from line to line…

Contrast in MRI

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Grey-level images :

the intensity of a voxel depends on the intensity of the corresponding signal.

Contrast in MRI

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T1 (ms) T2 (ms) proton density

WM 500 75 0.65

GM 750 90 0.8

CSF 3000 200 1.0

Contrast in MRI

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Contrast depends on :

1. tissue properties : T1, T2, user-independent

2. sequence parameters : TR, TE, …TR = repetition time = time interval between two RF pulsesTE = echo time = when the acquisition is performed user-dependent

Contrast in MRI

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Sequence parameters : TR and TE

Contrast in MRI

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T2-weighted image : long TR – long TE

Contrast in MRI

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T2-weighted image : long TR – long TE

CSF

GM

WM

TR = 3370 msTE = 112 ms

Contrast in MRI

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T1-weighted image : short TR – short TE

Contrast in MRI

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T1-weighted image : short TR – short TE

WM

GM

CSF

TR ~ 500 msTE ~ 10 ms

Contrast in MRI

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Illustration : une pomme dans un verre d’eau…Contraste en T1 – TE court et TR variableCas d’une impulsion RF initiale de 90°

Contrast in MRI

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Illustration : une pomme dans un verre d’eau…Contraste en T1 – TE court et TR variableCas d’une impulsion RF initiale de 180°

Contrast in MRI

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Illustration : une pomme dans un verre d’eau…Contraste en T2 – TR long et TE variable

(Impulsion RF initiale de 90°)

The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre

The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre

The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre

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