Introduction to MRI- NMR.ppt

1、Introduction to MRI,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. cla

2、ssical pictures of proton(s),Introduction to MRI,2,Electromagnetic spectrum,http:/www.nps.gov,Introduction to MRI,3,Electromagnetic spectrum,www.yorku.ca/eye/spectru.htm,c = = 3 x 108 m/s /,Introduction to MRI,4,RF Antennae vs. RF coils,www.yorku.ca/eye/spectru.htm,Antennae disperse energy,Coils foc

3、us energy,Introduction to MRI,5,Nuclei and subatomic particles,Introduction to MRI,6,Stern-Gerlach experiment: discovery of spin,http:/www.upscale.utoronto.ca/GeneralInterest/Harrison/SternGerlach/SternGerlach.html,Discovery of magnetic moment on particles with spins Electron beam has (roughly) even

4、 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,Introduction to MRI,7,Spin and magnetic moment,Sub-atomic particles have intrinsic

5、 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,Introduction to MRI,8,Periodic table: some

6、nuclei are magnetic,Introduction to MRI,9,Water,www.lsbu.ac.uk/water/,Introduction to MRI,10,Magnets,Dipole in a static field,B,NS,NS,Lowest energy,Highest energy,Units of magnetic field:1 Tesla = 104 Gauss0.5 G = earths magnetic field50 G = refrigerator magnet,Introduction to MRI,11,Magnets,Proton

7、in a static magnetic field,Dipole in a static field,B,NS,NS,Lowest energy,Highest energy,: magnetic dipole,Introduction to MRI,12,Nucleus in magnetic field,Nucleus in free space,Single spin-1/2 particle in an external magnetic field,All orientations possess the same potential energy,Spin-up and spin

8、-down are different energy levels; difference depends linearly on static magnetic field,Introduction to MRI,13,Resonant frequency,Transition emits energy,Excitation promotes transition,Resonant frequency is determined by gyromagnetic ratio, a property of the nucleusAt 3T, protons resonate at 128 MHz

9、At 7T, protons resonate at 300 MHz,Introduction to MRI,14,Electromagnetic spectrum,www.yorku.ca/eye/spectru.htm,c = = 3 x 108 m/s /,Introduction to MRI,15,Hydrogen spectrum: electron transitions,http:/csep10.phys.utk.edu/astr162/lect/light/absorption.html,1 electron volt = 1.6 10-19 J,Fixed energy t

10、ransitions result in discrete absorption lines,Introduction to MRI,16,Precession and resonant frequency,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,Gyroma

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

12、bulk magnetization,isochromat,Introduction to MRI,19,Information in proton NMR signal,Resonant frequency depends on Static magnetic field MoleculeRelaxation rate depends on physical environment Microscopic field perturbations Tissue interfaces Deoxygenated blood Molecular environment Gray matter White matter CSF,Relaxation,Excitation,Introduction to MRI,20,Proton NMR spectrum: ethanol,/grupper/KS-grp/microarray/slides/drablos/Structure_determination,