Introduction to MRI: NMR

1. Introduction to MRI: NMR • Physics reminders • Nuclei and atoms • Electromagnetic spectrum and Radio Frequency • Magnets • Vectors • NMR phenomena • nuclei, atoms and electron clouds (molecular environment) • excitation and energy states, Zeeman diagram • precession and resonance quantum vs. classical pictures of proton(s) Introduction to MRI

2. Electromagnetic spectrum http://www.nps.gov Introduction to MRI

3. Electromagnetic spectrum c =   = 3 x 108 m/s / www.yorku.ca/eye/spectru.htm Introduction to MRI

4. RF Antennae vs. RF coils Antennae disperse energy Coils focus energy www.yorku.ca/eye/spectru.htm Introduction to MRI

5. Nuclei and subatomic particles Introduction to MRI

6. Stern-Gerlach experiment: discovery of spin • Discovery of magnetic moment on particles with spins • Electron beam has (roughly) even mix of spin-up and spin-down electrons • Beam should be bent to the side because a force is exerted on moving charge in a magnetic field • Beam was also split vertically, because electrons posses inherent magnetic moment http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SternGerlach/SternGerlach.html Introduction to MRI

7. Sub-atomic particles have intrinsic angular momentum (spin), L Aligned with L is , a magnetic moment The quantum number I determines how many spin states a particle might be found in For a nucleus, the number of protons and neutrons determines I L and  are related by , the gyromagnetic ratio Spin and magnetic moment Introduction to MRI

8. Periodic table: some nuclei are magnetic Introduction to MRI

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

10. Magnets Dipole in a static field Units of magnetic field: 1 Tesla = 104 Gauss 0.5 G = earth’s magnetic field ~50 G = refrigerator magnet Lowest energy Highest energy B N S N S Introduction to MRI

11. Magnets Dipole in a static field Proton in a static magnetic field Lowest energy Highest energy : magnetic dipole B N S N S Introduction to MRI

12. 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

13. E B Resonant frequency Transition emits energy Excitation promotes transition • Resonant frequency is determined by gyromagnetic ratio, a property of the nucleus • At 3T, protons resonate at ~128 MHz • At 7T, protons resonate at ~300 MHz Introduction to MRI

14. Electromagnetic spectrum c =   = 3 x 108 m/s / www.yorku.ca/eye/spectru.htm Introduction to MRI

15. Hydrogen spectrum: electron transitions 1 electron volt = 1.6 × 10-19 J Fixed energy transitions result in discrete absorption lines http://csep10.phys.utk.edu/astr162/lect/light/absorption.html Introduction to MRI

16. Precession and resonant frequency E B Spin-up and spin-down are different energy levels; difference depends linearly on static magnetic field Torque exerted by magnetic force on dipole creates precession. Introduction to MRI

17. Gyromagnetic (magnetogyric) ratio Introduction to MRI

18. From spin-1/2 particles to bulk magnetization B M: net (bulk) magnetization isochromat Excitation affects phase and distribution between spin-up and spin-down, rotating bulk magnetization M|| M Equilibrium: ~ 1 ppm excess in spin-up (low energy) state creates a net magnetization M Introduction to MRI

19. 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

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