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Introduction to MRI: NMR

Introduction to MRI: NMR. MRI - big picture Neuroimaging alternatives Goal: understanding neurall coding Electromagnetic spectrum and Radio Frequency X-ray, gamma ray, RF NMR phenomena History (NMR, imaging, BOLD) Physics nuclei, molecular environment

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Introduction to MRI: NMR

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  1. Introduction to MRI: NMR • MRI - big picture • Neuroimaging alternatives • Goal: understanding neurall coding • Electromagnetic spectrum and Radio Frequency • X-ray, gamma ray, RF • NMR phenomena • History (NMR, imaging, BOLD) • Physics • nuclei, molecular environment • excitation and energy states, Zeeman diagram • precession and resonance quantum vs. classical pictures of proton(s) Introduction to MRI

  2. Related readings • Huettel, Chapter 1 • History, resonance phenomena described (pp. 11 - 22) • Definitions of contrast and resolution (pp. 6 - 11) • Example (of what I don’t like … pp. 12, 13) • Buxton, pgs. 64 - 72, 124 - 131 • Haacke, Ch. 1, 2 & 25 Introduction to MRI

  3. Neuroimaging Introduction to MRI

  4. Nuclei Introduction to MRI

  5. Periodic table Introduction to MRI

  6. Hydrogen spectrum: electron transitions 1 electron volt = 1.6 × 10-19 J http://csep10.phys.utk.edu/astr162/lect/light/absorption.html Introduction to MRI

  7. Magnets Dipole in a static field Dipole-dipole interactions Lowest energy Highest energy Lowest energy Highest energy B N S N S N S N S N S N S Introduction to MRI

  8. The Zeeman effect • The dependence of electronic transition energies on the presence of a magnetic field reveals electron spin (orbital angular momentum) http://csep10.phys.utk.edu/astr162/lect/light/zeeman-split.html Introduction to MRI

  9. Stern-Gerlach experiment • Discovery of magnetic moment on particles with spins • Electron beam has (roughly) even mix of spin-up and spin-down electrons http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SternGerlach/SternGerlach.html Introduction to MRI

  10. NMR - MRI - fMRI timeline 1922 Stern-Gerlach Electron spin 1952 Nobel prize Felix Bloch, Edward Purcell NMR in solids 1993 Seiji Ogawa, et al. BOLD effect 1902 Pieter Zeeman Radiation in a magnetic field 1937 Isidor Rabi Nuclear magnetic resonance 1973 Paul Lauterbur, Peter Mansfield NMR imaging 1936 Linus Pauling Deoxyhemoglobin electronic structure Introduction to MRI

  11. Single spin-1/2 particle in an external magnetic field E B Nucleus in free space Nucleus in magnetic field Spin-up and spin-down are different energy levels; difference depends linearly on static magnetic field All orientations possess the same potential energy Introduction to MRI

  12. E B Resonant frequency, two ways Spins in static magnetic field precess, with  = B or  = B where ,  = precession frequency (radians, Hz) ,  = gyromagnetic ratio (in rad/T or Hz/T) B = static (external) magnetic field (Tesla) Transition from high to low energy state emits radiation with characteristic frequency: Proton gyromagnetic ratio:  = 42.58 MHz/T  = 2 =267,000,000 rad/T Introduction to MRI

  13. Gyromagnetic ratio Introduction to MRI

  14. Many spin-1/2 particles in an external magnetic field B M: net (bulk) magnetization Excitation affects phase and distribution between spin-up and spin-down, rotating bulk magnetization M|| M Equilibrium: ~ 1 ppm excess in spin-up state creates a net magnetization M Introduction to MRI

  15. Information in proton NMR signal • Resonant frequency depends on • Static magnetic field • Molecule • Relaxation rate depends on physical environment • Microscopic field perturbations • Tissue interfaces • Deoxygenated blood • Molecular environment • Gray matter • White matter • CSF Excitation Relaxation Introduction to MRI

  16. Proton NMR spectrum: ethanol /grupper/KS-grp/microarray/slides/drablos/Structure_determination Introduction to MRI

  17. Water www.lsbu.ac.uk/water/ Introduction to MRI

  18. Magnetic Resonance Imaging • An MR image is (usually) a map of water protons, with intensity determined by local physical environment • Contrast and image quality are determined by • Pulse sequence • Field strength • Shim quality • Acquisition time Introduction to MRI

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