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Magnetic Resonance Imaging: Physical Principles

5/29/2012. 2. Physics of MRI, Lecture 1. Nuclear Magnetic ResonanceNuclear spinsSpin precession and the Larmor equationStatic B0RF excitationRF detectionSpatial EncodingSlice selective excitationFrequency encodingPhase encodingImage reconstruction. Fourier TransformsContinuous Fourier Tr

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Magnetic Resonance Imaging: Physical Principles

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    1. Magnetic Resonance Imaging: Physical Principles Richard Watts, D.Phil., Yi Wang, Ph.D. Weill Medical College of Cornell University, New York, USA

    2. 5/29/2012 2 Physics of MRI, Lecture 1 Nuclear Magnetic Resonance Nuclear spins Spin precession and the Larmor equation Static B0 RF excitation RF detection Spatial Encoding Slice selective excitation Frequency encoding Phase encoding Image reconstruction Fourier Transforms Continuous Fourier Transform Discrete Fourier Transform Fourier properties k-space representation in MRI

    3. 5/29/2012 3 Physics of MRI, Lecture 2 Echo formation Vector summation Phase dispersion Phase refocus 2D Pulse Sequences Spin echo Gradient echo Echo-Planar Imaging Medical Applications Contrast in MRI Bloch equation Tissue properties T1 weighted imaging T2 weighted imaging Spin density imaging Examples 3D Imaging Spectroscopy

    4. 5/29/2012 4 Many spins in a voxel: vector summation

    5. 5/29/2012 5 Phase dispersion due to perturbing B fields

    6. 5/29/2012 6 Refocus spin phase – echo formation

    7. 5/29/2012 7 Spin Echo Spins dephase with time Rephase spins with a 180° pulse Echo time, TE Repeat time, TR (Running analogy)

    8. 5/29/2012 8 Frequency encoding - 1D imaging

    9. 5/29/2012 9 Slice selection

    10. 5/29/2012 10 3rd dimension – phase encoding

    11. 5/29/2012 11 Gradient Echo FT imaging

    12. 5/29/2012 12 Pulse sequence design

    13. 5/29/2012 13 EPI (echo planar imaging)

    14. 5/29/2012 14 Spin Echo FT imaging

    15. 5/29/2012 15 Spin Relaxation Spins do not continue to precess forever Longitudinal magnetization returns to equilibrium due to spin-lattice interactions – T1 decay Transverse magnetization is reduced due to both spin-lattice energy loss and local, random, spin dephasing – T2 decay Additional dephasing is introduced by magnetic field inhomogeneities within a voxel – T2' decay. This can be reversible, unlike T2 decay

    16. 5/29/2012 16 Bloch Equation The equation of MR physics Summarizes the interaction of a nuclear spin with the external magnetic field B and its local environment (relaxation effects)

    17. 5/29/2012 17 Contrast - T1 Decay Longitudinal relaxation due to spin-lattice interaction Mz grows back towards its equilibrium value, M0 For short TR, equilibrium moment is reduced

    18. 5/29/2012 18 Contrast - T2 Decay Transverse relaxation due to spin dephasing T2 irreversible dephasing T2/ reversible dephasing Combined effect

    19. 5/29/2012 19 Free Induction Decay – Gradient echo (GRE) Excite spins, then measure decay Problems: Rapid signal decay Acquisition must be disabled during RF Don’t get central “echo” data

    20. 5/29/2012 20 Spin echo (SE)

    21. 5/29/2012 21 MR Parameters: TE and TR Echo time, TE is the time from the RF excitation to the center of the echo being received. Shorter echo times allow less T2 signal decay Repetition time, TR is the time between one acquisition and the next. Short TR values do not allow the spins to recover their longitudinal magnetization, so the net magnetization available is reduced, depending on the value of T1 Short TE and long TR give strong signals

    22. 5/29/2012 22 Contrast, Imaging Parameters

    23. 5/29/2012 23 Properties of Body Tissues

    24. 5/29/2012 24 MRI of the Brain - Sagittal

    25. 5/29/2012 25 MRI of the Brain - Axial

    26. 5/29/2012 26 Brain - Sagittal Multislice T1

    27. 5/29/2012 27 Brain - Axial Multislice T1

    28. 5/29/2012 28 Brain Tumor

    29. 5/29/2012 29 3D Imaging Instead of exciting a thin slice, excite a thick slab and phase encode along both ky and kz Greater signal because more spins contribute to each acquisition Easier to excite a uniform, thick slab than very thin slices No gaps between slices Motion during acquisition can be a problem

    30. 5/29/2012 30 2D Sequence (Gradient Echo)

    31. 5/29/2012 31 3D Sequence (Gradient Echo)

    32. 5/29/2012 32 3D Imaging - example

    33. 5/29/2012 33 Spectroscopy Precession frequency depends on the chemical environment (dBcs) e.g. Hydrogen in water and hydrogen in fat have a ?f = fwater – ffat = 220 Hz Single voxel spectroscopy excites a small (~cm3) volume and measures signal as f(t). Different frequencies (chemicals) can be separated using Fourier transforms Concentrations of chemicals other than water and fat tend to be very low, so signal strength is a problem Creatine, lactate and NAA are useful indicators of tumor types

    34. 5/29/2012 34 Spectroscopy - Example

    35. 5/29/2012 35 Future lectures Magnetization preparation (phase and magnitude, pelc) Fast imaging (fast sequences, epi, spiral…) Motion (artifacts, compensation, correction, navigator…) MR angiography (TOF, PC, CE) Perfusion and diffusion Functional imaging (fMRI) Cardiac imaging (coronary MRA)

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