Magnetic Resonance Imaging

Spin

A fundamental property of matter.

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Deuterium = electron
= neutron
= proton

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Helium

= electron
= neutron
= proton

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Properties of Spin

When placed in a magnetic field, particles with spin absorb specific energies.

n = g B

B = Magnetic Field Strenght
n = frequency
g = gyromagnetic ratio (g_H = 42.58 MHz / T)

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Nuclei with Spin

Almost every element has an isotope with a nuclear spin.

Nuclei with spin & of biological importance.

Nuclei	Unpaired Protons	Unpaired Neutrons	Net Spin	g (MHz/T)
1H	1	0	1/2	42.58
2H	1	1	1	6.54
31P	0	1	1/2	17.25
23Na	1	2	3/2	11.27
14N	1	1	1	3.08
13C	0	1	1/2	10.71
19F	0	1	1/2	40.08

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Energy Levels

Think of a particle with spin as a small bar magnet.

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No external magnetic field.

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In an external magnetic field.

 
   
 

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Transitions Between Energy Levels

E = h n
 
         
 

E = energy
n = frequency (Larmor frequency)
h = Plank's constant (6.626x10^-34 J s)

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Energy Level Diagrams

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DE = h g B

n is RF
NMR: 60 < n < 800 MHz
clinical ¹H MRI: n = 60 MHz

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CW NMR Experiment

Constant Frequency

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Constant Field

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Copyright © 2000-4 J.P. Hornak.
All Rights Reserved.