M R I Physics Course

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M R I Physics Course

• Jerry Allison Ph.D., Chris Wright B.S.,
• Tom Lavin B.S., Nathan Yanasak Ph.D.
• Department of Radiology
• Medical College of Georgia

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M R I Physics course Chapter 2 Basic Physical
Properties Magnetism Resonance
The M and R in MRI.
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Magnetism Magnetized objects exert forces of
repulsion or attraction on one another, resulting
from electric currents.
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Magnetic Properties

• All substances are magnetic (to various degrees).
• Magnetic susceptibility is the ability of a
  substance to become magnetized.

Q If all substances are magnetic, do all
substances have electric currents running
in them? A Well examine this question shortly
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Magnetic Properties (continued)

• External field is applied ? object becomes
  magnetized, according to susceptibility.

• Most standard materials fall into one of these
  classes (defined by the susceptibility)
• Diamagnetic
• Paramagnetic
• Super-paramagnetic
• Ferromagnetic

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Magnetic Properties (continued)

• Diamagnetic - develop a small magnetic field in
  opposition to an applied field (and have a small
  negative magnetic susceptibility).
• Non-Magnetic - very weakly diamagnetic

No field applied
Field applied
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Magnetic Properties (continued)

• Paramagnetic-develop a small magnetic field in
  alignment with an applied field (and have a small
  positive magnetic susceptibility).
• Super-paramagnetism is ascribed to small
  particles of iron oxide that can be used as MRI
  contrast agents.

No field applied
Field applied
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Magnetic Properties (continued)

• Ferromagnetic - strongly paramagnetic Fe, Co, Ni,
  Gd. Magnetism induced by an applied magnetic
  field may be retained. Ferromagnetic substances
  have a strong positive magnetic susceptibility.

No field applied
Field applied
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Magnetic Properties (continued)

• Stainless Steel can be ferromagnetic (exterior
  surface of the downtown VAMC) or non-magnetic
  (most surgical steel) depending upon the
  particular alloy (Fe, Cr, Ni, Mn). Fortunately,
  most (not all) surgical appliances (staples,
  clips, etc.) are an alloy that is non-magnetic.

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Induced Magnetism

• A ferromagnetic substance in an applied magnetic
  field will develop a magnetic field hundreds of
  times as strong as the applied field. A
  spherical iron ball at a distance of 1.6 m from
  a 1.0 T unshielded magnet will experience equal
  attractive forces from gravity and the magnet.
  This phenomenon is the basis for the projectile
  hazards in MRI.

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Induced Magnetism (continued)

• An electrical conductor wrapped around a
  ferromagnetic iron rod induces a very useful
  magnetic field when an electric current is
  flowing Electromagnets, Transformers, and
  Motors have iron cores. The induced field is
  much larger than the magnetic field created by
  the current flow in the conductor.

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Magnetism in Our World

• Magnetic fields are all around us
• Average field in Milky Way 5x10-6 Gauss
• Average field in Solar Wind 5x10-5 Gauss
• Average field at Moon 1x10-2 Gauss
• Average field at Jupiter 2x104 Gauss
• 1 Tesla 10,000 Gauss 1x104 Gauss
• 1 T 10,000 G

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Magnetism in Our World (continued)

• The Earths magnetic field is 0.5 - 1.0 Gauss at
  15 with axis of Earths rotation.
• A 1.5 Tesla (15,000 Gauss) field is 15,000 to
  30,000 times greater than the Earths magnetic
  field.

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Origins of Magnetism

• Stationary electrical charges have an electric
  field (E-field).
• Moving electrical charges develop a magnetic
  field (B-field).
• The basis for ALL MAGNETISM is the motion of
  electrical charges.

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Relationship of E- and B-fields
Stationary observer of stationary charge sees
E-field
B
E
E
Stationary observer of moving charge also sees
B-field
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Relationship of E- and B-fields

• Because of the inseparability of electric charges
  and magnetism, we refer to these phenomena in
  general as Electromagnetism.
• Phenomena of stationary charges ?
  Electrostatics

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Remember an accelerating or decelerating point
electric charge radiates electromagnetic
radiation.
Acceleration of electrons as they are deflected
by the nucleus of a tungsten atom in the target
produces Bremsstrahlung photons for diagnostic
x-ray applications.
tungsten target on X-ray tube anode
high speed electron
Bremsstrahlung photon
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Electric Fields (in this case, an electrostatic
field)
Stationary Point Electrical Charges
Electric Field Lines (arrows) represent
the direction of the fields
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Origins of Magnetism (continued)

• The right hand rule describes the direction of
  the magnetic field relative to the direction of
  movement of electric charges.

Thumb direction of positive charges
Fingers direction of magnetic field
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Magnetic Fields
A moving point electric charge develops a
magnetic field

Movement of a positive particle into the page
Movement of a positive particle out of the page
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Magnetic Fields
A moving point electric charge develops a
magnetic field
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Movement of an electron into the page
Movement of an electron out of the page
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Direction of magnetic field depends on 1)
particle charge, and 2) direction of motion.
Movement of a positive particle out of the page

Movement of a positive particle into the page
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-
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Movement of an electron out of the page
Movement of an electron into the page
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Topology of B-field
B
E
B
Charges moving in straight lines form circular
field.
Charges moving in a circle form a linear field in
the center of circle.
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Magnetic Properties
Q If all substances are magnetic, do all
substances have electric currents running in
them? Lets examine this question
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All Observable Matter is Composed of
Subatomic Particles
electrons mass 0.0005 amu charge -1
electrostatic unit(esu)
protons mass 1.0 amu charge 1 esu
neutrons mass 1.0 amu charge neutral
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Nucleons (or protons, neutrons) are composed of
charged quarks

Proton 2 Up quarks (2/3 esu) and 1 Down quark
(-1/3 esu) 1 esu net charge

-

Neutron 2 Down quarks (-1/3 esu) and 1 Up quark
(2/3 esu) no net charge
-
-

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Magnetic Properties
Subatomic particles have a property called
spin. They behave as if they are spinning on
their axis. So, lets think about little regions
on the surface of the particle, shown as boxes.
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Magnetic Properties
Each box contains charged particle
material. Therefore, as the particle rotates,
the boxes act like moving charges. So, our
particle behaves like a collection of currents,
and we generate a magnetic field.
B
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Spinning of the subatomic particles generates a
magnetic field, called a magnetic moment or
magnetic dipole.
Electron magnetic moment
Proton magnetic moment
Neutron magnetic moment

For both protons and neutrons, the spinning of
the charged quarks produces the magnetic moment.
So, although the neutron is electrically
neutral, its spinning quarks give it a magnetic
moment.
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Magnetic Properties
Q If all substances are magnetic, do all
substances have electric currents running in
them? A Yes, at the subatomic level. But the
currents are a result of particle spin.
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Resonance
Stimulated oscillation at the natural or normal
frequency
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A Classical demonstration using tuning forks
Tuning Fork 1
Tuning Fork 2
E
E
Sound waves from tuning fork 1 stimulate a
non-vibrating tuning fork 2, with same resonant
frequency, to RESONATE. It will absorb and give
off energy readily at this frequency.
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Tuned string
Tuning Fork 1
The same is true of a guitar string tuned to the
frequency of the tuning fork. But, if you detune
the string, it will (essentially) not resonate
anymore.
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Resonance (cont.)
In MRI, resonance relates to the stimulation of
proton magnetic moments (hydrogen nuclei) by RF
energy of the appropriate resonant
frequency. So, the protons will readily absorb
and release RF energy at this frequency. The
resonant frequency is tunable by the strength
of the magnetic field in which the protons are
spinning, as we shall see in a later lecture.
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Radio Frequency Energy

• (RF) - oscillating magnetic and electric fields
  (I.e., electromagnetic fields) having
  frequencies between 3 kilohertz (kHz) and 30
  Gigahertz (GHz).
• Examples
• Radio waves(AM 535-1605 kHz FM 88-108 MHz)
• MRI (21,43,64,128 MHz?protons in 0.5,1T,1.5T,3T
  lower for spectroscopy)
• Cellphones (824-848 MHz)
• TV transmission (50-900 MHz Ch.2-4 ? 54-72 MHz)
• Microwave Ovens (2.45 GHz)
• Radar (3-30 GHz)

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Radio Frequency Energy

• In MRI, magnetic fields oscillating at the
  appropriate resonant frequency are used to
  stimulate nuclei to either absorb energy or to
  release energy (spin flip transitions phase
  coherence).

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Summary

• All magnetism originates in the movement of
  electric charge.
• Magnetic susceptibility describes to what extent
  a material increases or decreases an applied
  magnetic field.
• Resonance periodic stimulation at the natural
  frequency can cause energy exchange.

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