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12.0 Physics of Magnetic Resonance
MRI produces highresolution, highcontrast crosssectional images throughout the
head and body.
Like ultrasound, it is noninvasive, limited mainly by power
deposition.
MRI is also limited by the fact that the signal is generated by the
nuclei in the tissue and the only way to increase this signal is to increase the
magnetic field, which is very expensive. Beyond imaging, MRI has functional
aspects such as chemical species sensitivity,
microscopic blood flow sensitivity
that makes brain neuronal activity accessible and diffusional sensitivity to evaluate
tissue microstructure. This section will cover the basics, without delving into
imaging schemes. Later sections will cover imaging methods.
•Microscopic Magnetization
•Macroscopic Magnetization
•Precession and Larmor Frequency
•Transverse and Longitudinal Magnetization
•RF Excitation
•Relaxation
•Bloch Equations
•Spin Echoes
•Contrast Mechanisms
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View Full Document Important Points from MRI Lecture 1
•Electrons in atomic orbits and nucleons in nuclear “shells” have internal angular momentum, S, and
orbital angular momentum, L, giving total angular momentum J=L+S.
•Electrons, protons and neutrons are Fermions with S=1/2
•The total angular momentum, either electronic or nuclear is determined by the total angular
momentum of unpaired electrons, neutrons or protons.
•Nuclei with Even #pEven #n have J=0, Even #pOdd #n nuclei or Odd #pEven #n nuclei have
J=1/2, 3/2,… (halfinteger), while Odd #pOdd #n nuclei have J=1,2,…(integer).
•Angular momentum of charged particles or internal charges (neutron) gives rise to magnetic
properties such that electrons in atomic orbits or nuclei have magnetic moments (magnetization) m
(or μ)=γħJ for nuclei and m=m
B
J/ħ for electrons.
•γ is the gyromagnetic ratio in units of rad/secTesla. For H
1
, γ=42.57MHz/T
•In general, a particle has a magnetic moment given by m=gm
P
J where g is the g factor that
depends on the particle and situation, m
P
is the Bohr magneton for the electron (m
B
=eħ/2m
e
) and m
P
is the nuclear magneton for the nucleon (m
N
=eħ/2m
N
).
•When placed in a magnetic field, B, magnetic moments of a spin=1/2 system can have two
orientations (up or down) that have an energy difference of ΔE=γħB.
This is like ΔE=ħν where the
frequency, ν= γB
•In a magnetic field nuclear magnetic moments precess at a Larmor frequency f
L
= γB
•Since m
e
~m
N
/1000, the resonant frequency associated with electrons is nearly 1000 times higher
than for nuclei (ie, GHz vs MHz)
Important Points from MRI Lecture 1 (continued!)
•A collection of nuclear magnetic moments in a field B
0
are only weakly aligned with the
field because of constant exchange of thermal energy from the surroundings. About 1 in
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This note was uploaded on 10/30/2010 for the course MP 230 taught by Professor Macfall during the Fall '10 term at Duke.
 Fall '10
 MacFall

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