m agnetic r esonance i maging – basic principles – e velyne b alteau [email protected]...
TRANSCRIPT
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Overview
• Brief history of MRI• Magnetic properties of the nuclei
• Interaction with B0
• Interaction with B1
• Relaxation• Signal Localization• Contrast
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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
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MRI : magnetic stuff !!
Magnetic properties of the NUCLEI
External magnetic field
B0 = 3 T
Electromagnetic field B1 (Radio-
frequency or RF)
index
60000 the earth’s magnetic field !!!!
FM radio-waves : 88.8 – 108.8 MHz !!
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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)
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Behaviour of the nuclei interacting with :
1.The external magnetic field B0
Equilibrium state
2.The electromagnetic field B1 (RF)
Disturbance
index
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Interaction with B0
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1. Orientation :
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Interaction with B0
2. Energy states :
index
E = ħBo = ħo
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Interaction with B0
3. Precession :
index
Rotation or precession about the axis of the magnetic field Bo with frequency :
o = Bo
o = Larmor frequency = gyromagnetic ratio
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Interaction with B0
3. Precession :
index
At the equilibrium state :
- rotation in phase
- no transverse magnetization Mxy
y
x
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Interaction with B0
index
4. Summary : at the equilibrium state :
1. spin orientation « up » > « down »
longitudinal magnetization Mz
2. precession
no transverse magnetization Mxy
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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 !!!
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Interaction with B1
index
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…
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Relaxation back to the equilibrium state…
index
DEPHASING
Dephasing Mxy decreases T2 relaxation
TRANSITIONS
Transitions E2 E1 Mz increases T1 relaxation
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Relaxation back to the equilibrium state…
index
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
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Signal localization
index
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
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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
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Signal localization
index
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 !
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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…
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Contrast in MRI
index
Grey-level images :
the intensity of a voxel depends on the intensity of the corresponding signal.
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Contrast in MRI
index
T1 (ms) T2 (ms) proton density
WM 500 75 0.65
GM 750 90 0.8
CSF 3000 200 1.0
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Contrast in MRI
index
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
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Contrast in MRI
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Sequence parameters : TR and TE
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Contrast in MRI
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T2-weighted image : long TR – long TE
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Contrast in MRI
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T2-weighted image : long TR – long TE
CSF
GM
WM
TR = 3370 msTE = 112 ms
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Contrast in MRI
index
T1-weighted image : short TR – short TE
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Contrast in MRI
index
T1-weighted image : short TR – short TE
WM
GM
CSF
TR ~ 500 msTE ~ 10 ms
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Contrast in MRI
index
Illustration : une pomme dans un verre d’eau…Contraste en T1 – TE court et TR variableCas d’une impulsion RF initiale de 90°
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Contrast in MRI
index
Illustration : une pomme dans un verre d’eau…Contraste en T1 – TE court et TR variableCas d’une impulsion RF initiale de 180°
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Contrast in MRI
index
Illustration : une pomme dans un verre d’eau…Contraste en T2 – TR long et TE variable
(Impulsion RF initiale de 90°)
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The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre
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The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre
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The 3.0 Tesla Allegra MR scanner at the Cyclotron Research Centre
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index