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 4Psy 8960, Fall ‘06

Nuclei

Introduction to MRI 5Psy 8960, Fall ‘06

Periodic table

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

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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:

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ΔE = hω

Proton gyromagnetic ratio: = 42.58 MHz/T = 2 =267,000,000 rad/T

Introduction to MRI 13Psy 8960, Fall ‘06

Gyromagnetic ratio

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

Introduction to MRI 17Psy 8960, Fall ‘06

Water

www.lsbu.ac.uk/water/

Introduction to MRI 18Psy 8960, Fall ‘06

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