Physics Applied to Radiology RADI R250 -- Fall 2003

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Physics Applied to RadiologyRADI R250 -- Fall
2003

• Chapter 7

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Magnetism

• a fundamental property of matter
• enables matter to attract iron or steel
• charged particles in motion exhibit magnetism
• 1. spinning charged particle
• 2. directional motion of a charged particle
• creates a magnetic field in a plane to motion

1.
2.
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Magnetic Properties of e- p

• electrons always in motion
• e- spin on an axis (spin quantum number)
• e- move in orbit around a nucleus
• net magnetism (magnetic property displayed)
• paired e- (in outer shell) ordinarily do not
  exhibit net magnetism
• magnetic effect cancel each other out
• odd numbers of e- (in outer shell) have net
  magnetism

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Magnetism Terms

• magnetic domain
• net magnetism that occurs at the atomic level due
  to spinning e-
• property of atoms that exhibit magnetism
• results from more e- spinning in one direction
  than the other
• magnetic dipole
• result of a magnetic domain
• atom is viewed as a tiny two pole magnet

N
S
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Magnetism Terms (cont.)

• magnetic moment
• magnetic dipole of a spinning nucleus
• nuclear magnetism
• nucleus made up of particles
• each spins on an axis
• if odd of protons, net magnetism exists in the
  nucleus
• principle used in MRI

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Classification of Magnets

• 1. natural (found in nature)
• earth is a dipole
• spins on an axis
• rotates around sun
• magnetite lodestone or iron oxide (Fe3O4)
• natural ore that exhibits magnetism
• 2. artificial permanent (man made)
• hardened steel or alloy (alnico)
• ceramic
• 3. electromagnets
• created by a current of e-

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Laws of Magnetism

• 1. Every magnet has two poles
• designated N S
• Magnets will not exist without 2 poles
• Poles exhibit but opposite magnetic force
• Force is strongest at the poles

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Nature of Magnetism

• Magnetic materials usually in a non magnetized
  state
• domains in random arrangement
• net magnetism 0

• If domains align with same N to S orientation
• result net magnetism
• effectively one N pole and one S pole

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Laws (2)

• 2. Like poles repel unlike poles attract
• Analogous to like/unlike electric charges
• Results from a magnetic force field theory
• Magnetic Field
• area around a magnet where iron will react to the
  magnetic force of the magnet
• Symbol for a magnetic field B

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Magnetic Force Field

• lines of force (magnetic flux)
• vector
• NS outside the magnet
• SN inside the magnet
• form a closed loop
• each force field line is independent loop will
  not cross others

N
S
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Magnetic Field (cont.)

• Strength of field lines per unit area
• 1 gt 2
• parallel flux in same direction repel

N
S

• field lines pass through both magnetic non
  magnetic materials
• concentrated by magnetic materials
• unaffected by non magnetic materials

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Laws (3)

• Gauss's Law
• Analogous to Coulomb's laws
• The force between two magnetic poles varies
  directly as the strength of the poles and
  inversely as the square of the distance between
  poles.
• Formula

• Unit tesla T kg / (As2)
• gauss G 1x10-4T

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Magnetic Induction (magnetization)

• process of inducing a magnetic field in a piece
  of non magnetized iron (or similar material)
• principle temp. rearrangement of domains
• magnetic domains in a disordered array

• in the presence of a B the domains may realign to
  cause material to act temporarily as a magnet.

• lasts only as long as magnetic material is under
  influence of magnet

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More Magnetic Terms

• magnetic permeability (susceptibility)
• ease with which material can be magnetized
• measures materials ability to strengthens B
• magnetic retentivity
• ability of a material to hold magnetization
• Hard material which has retentivity
• used for permanent magnets
• Soft material which has retentivity
• used for temporary magnets
• permeability retentivity -- inversely related

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Magnetic Classification of Matter

• ferromagnetic
• exhibit strong magnetic properties
• e.g. iron, cobalt, nickel
• high permeability/low retentivity -- unless
  specially made as an alloy
• paramagnetic
• only slightly reactive to a magnetic field
• slight tendency to concentrate magnetic field
• very low permeability (feeble)

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Magnetic Classification (cont.)

• non magnetic (dimagnetic per Bushong)
• not reactive to a magnetic field
• no permeability or retentivity
• most materials fall in this category
• diamagnetic
• reduce the strength of a magnetic field
• spreads out the magnetic field lines
• no permeability