psy 8960, fall ‘06 introduction to mri1 introduction to mri: nmr mri - big picture –neuroimaging...
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Introduction to MRI 1Psy 8960, Fall ‘06
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 2Psy 8960, Fall ‘06
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 3Psy 8960, Fall ‘06
Neuroimaging
Localization TimingHuman studies
Interpretation
Electrophysiology
Optical imaging
EEGelectroencephalography
MEGmagnetoencephalography
PETPositron emission tomography
fMRIfunctional MRI
Introduction to MRI 6Psy 8960, Fall ‘06
Hydrogen spectrum: electron transitions
http://csep10.phys.utk.edu/astr162/lect/light/absorption.html
1 electron volt = 1.6 × 10-19 J
Introduction to MRI 7Psy 8960, Fall ‘06
MagnetsDipole-dipole interactionsDipole in a static field
BN
S
N
S
Lowest energy
Highest energy
N
S
Lowest energy
N
S
Highest energy
N
S
N
S
€
E = −r μ •
r B
r τ =
r μ ×
r B
Introduction to MRI 8Psy 8960, Fall ‘06
http://csep10.phys.utk.edu/astr162/lect/light/zeeman-split.html
The Zeeman effect
• The dependence of electronic transition energies on the presence of a magnetic field reveals electron spin (orbital angular momentum)
Introduction to MRI 9Psy 8960, Fall ‘06
Stern-Gerlach experiment
http://www.upscale.utoronto.ca/GeneralInterest/Harrison/SternGerlach/SternGerlach.html
• Discovery of magnetic moment on particles with spins
• Electron beam has (roughly) even mix of spin-up and spin-down electrons
Introduction to MRI 10Psy 8960, Fall ‘06
NMR - MRI - fMRI timeline
1922Stern-GerlachElectron spin
1936Linus PaulingDeoxyhemoglobin electronic structure
1937Isidor RabiNuclear magnetic resonance
1952 Nobel prizeFelix Bloch, Edward PurcellNMR in solids
1973Paul Lauterbur, Peter MansfieldNMR imaging
1993Seiji Ogawa, et al.BOLD effect
1902Pieter ZeemanRadiation in a magnetic field
Introduction to MRI 11Psy 8960, Fall ‘06
Nucleus in magnetic fieldNucleus in free space
€
L = I(I +1)h
μ = γL
€
Lz = mh
μ z = γLz
€
Lz = ±h
2μ z = γLz
B
E
€
m = +1
2
€
m = −1
2
Single spin-1/2 particle in an external magnetic field
All orientations possess the same potential energy
Spin-up and spin-down are different energy levels; difference depends linearly on static magnetic field
Introduction to MRI 12Psy 8960, Fall ‘06
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)
B
E
€
m = +1
2
€
m = −1
2
Transition from high to low energy state emits radiation with characteristic frequency:
€
ΔE = hω
Proton gyromagnetic ratio: = 42.58 MHz/T = 2 =267,000,000 rad/T
Introduction to MRI 14Psy 8960, Fall ‘06
BM: net (bulk) magnetization
M
M
M||
Many spin-1/2 particles in an external magnetic field
Equilibrium: ~ 1 ppm excess in spin-up state creates a net magnetization
Excitation affects phase and distribution betweenspin-up and spin-down, rotating bulk magnetization
Introduction to MRI 15Psy 8960, Fall ‘06
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
Relaxation
Excitation
Introduction to MRI 16Psy 8960, Fall ‘06
Proton NMR spectrum: ethanol
/grupper/KS-grp/microarray/slides/drablos/Structure_determination