chap3 2

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Published on October 16, 2007

Author: Jolene

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Magnetic Resonance Imaging – Basic Principles –:  Magnetic Resonance Imaging – Basic Principles – EVELYNE BALTEAU e.balteau@ulg.ac.be Cyclotron Research Centre Overview:  Overview Brief history of MRI Magnetic properties of the nuclei Interaction with B0 Interaction with B1 Relaxation Signal Localization Contrast Brief history of MRI:  Brief history of MRI index 1946 – Bloch & Purcell independently describe the NMR phenomenon 1952 – 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 !!:  MRI : magnetic stuff !! index 60000  the earth’s magnetic field !!!! FM radio-waves : 88.8 – 108.8 MHz !! Magnetic properties of the nuclei :  Magnetic properties of the nuclei index The Hydrogen nucleus  the most abundant (~⅔ of the atoms in living tissues) Behaviour of the nuclei interacting with ::  Behaviour of the nuclei interacting with : 1. The external magnetic field B0 Equilibrium state 2. The electromagnetic field B1 (RF) Disturbance index Interaction with B0:  Interaction with B0 index 1. Orientation : Interaction with B0:  Interaction with B0 2. Energy states : index DE = għBo = ħwo Interaction with B0:  Interaction with B0 3. Precession : index  Rotation or precession about the axis of the magnetic field Bo with frequency : wo = gBo wo = Larmor frequency g = gyromagnetic ratio Interaction with B0:  Interaction with B0 3. Precession : index At the equilibrium state : - rotation in phase - no transverse magnetization Mxy Interaction with B0:  Interaction with B0 index 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:  Interaction with B1 Resonance phenomenon index !!! RF frequency = Larmor frequency = w0 !!! Interaction with B1:  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… Relaxation  back to the equilibrium state…:  Relaxation  back to the equilibrium state… index Relaxation  back to the equilibrium state…:  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 Signal localization:  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 = matrix 2. Frequency-encoding  columns 3. Phase-encoding  lines Signal localization:  Signal localization index 1. Slice selection gradient Resonance Phenomenon : wRF = wo !!! Before Gz is applied : all the spins precess with the same Larmor frequency wo  all could resonate !! During application of Gz : the spins precess with  w  only spins with frequency = wRF resonate Signal localization:  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 wo During application of Gx : the spins precess with  frequencies  Fourier Transform of the signal allows discrimination between columns ! Signal localization:  Signal localization index 3. Phase-encoding gradient Before Gy is applied : all the spins precess with the same Larmor frequency wo During application of Gy : the spins precess with  frequencies  induces phase difference between the lines After 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:  Contrast in MRI index Grey-level images :  the intensity of a voxel depends on the intensity of the corresponding signal. Contrast in MRI:  Contrast in MRI index Contrast in MRI:  Contrast in MRI index  Contrast depends on : 1. tissue properties : T1, T2, r  user-independent 2. sequence parameters : TR, TE, … TR = repetition time = time interval between two RF pulses TE = echo time = when the acquisition is performed  user-dependent Contrast in MRI:  Contrast in MRI index Sequence parameters : TR and TE Contrast in MRI:  Contrast in MRI index T2-weighted image : long TR – long TE Contrast in MRI:  Contrast in MRI index T2-weighted image : long TR – long TE Contrast in MRI:  Contrast in MRI index T1-weighted image : short TR – short TE Contrast in MRI:  Contrast in MRI index T1-weighted image : short TR – short TE Contrast in MRI:  Contrast in MRI index Illustration : une pomme dans un verre d’eau… Contraste en T1 – TE court et TR variable Cas d’une impulsion RF initiale de 90° Contrast in MRI:  Contrast in MRI index Illustration : une pomme dans un verre d’eau… Contraste en T1 – TE court et TR variable Cas d’une impulsion RF initiale de 180° Contrast in MRI:  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°) 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:  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 Slide34:  index

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