L 36 Modern Physics [2]

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L 36 Modern Physics 2

• How lasers work
• Medical applications of lasers
• Applications of high power lasers
• Medical imaging techniques
• CAT scans
• MRIs

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How lasers work

• First we must understand the difference between
  incoherent and coherent radiation
• Ordinary light sources (light bulbs, fluorescent
  lights, etc) produce incoherent light
• lasers produce coherent light? all atoms
  radiate in the same manner

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Spontaneous vs Stimulated Emission

• Coherent radiation is produced when an atom
  undergoes stimulated emission.
• Spontaneous emission occurs when an electron
  makes an unprovoked transition to a lower energy
  level
• Stimulated emission occurs when an incoming
  photon induces the electron to change energy
  levels? amplification

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Simulated emission

• With stimulated emission, one photon comes in and
  its vibrations cause the electron to fall to the
  lower energy level, emitting another photon (the
  bridge)
• Thus, one photon goes in and 2 come out, moving
  in step and in the same direction
• To get a system of atoms to undergo lasing
  action, we must arrange to have many atoms in the
  excited state ? this is called population
  inversion.

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Population Inversion

• In a normal situation (a) more atoms are in the
  lower state than the upper state
• If an external energy source is provided to
  excite electrons into a higher energy state, a
  population inversion can be created as in (b)
• this is called pumping

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The laser medium

• The atoms are pumped into an excited state
• The excited electrons fall into an intermediate
  state (upper laser state) and stay there until a
  photon comes along and causes them to fall down
  to the lower laser state.

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He Ne Laser (633 nm red)

• A HeNe laser is a gas laser which uses a medium
  of 15 Helium and 85 Neon.
• A high voltage discharge is produced in this gas
  mixture and this produces the population
  inversion.
• The lasing action starts when one atom emits a
  photon which then induces another atom to emit
  and so on.
• The partially silvered mirror keeps most of the
  photons in.
• Photons which are not moving horizontally do no
  become part of the laser beam.

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A Helium-Neon (HeNe) Laser
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Medical Applications of Lasers

• Laser surgery to correct for
• nearsightedness, and
• farsightedness

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Applications of High Power Lasers
Using lasers to Cut metals
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Laser Fusion
Multiple beams of a powerful laser are focused on
a tiny pellet containing fusion fuel. The laser
energy compresses the pellet producing a
mini-hydrogen bomb that produces energy
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pellet
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Solid State Laser Diodes
small
Come in a variety of different colors
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Lasers Diodes

• Diode lasers use semiconductor materials (tiny
  chips of silicon) as the lasing media
• When current flows through the silicon chip it
  emits an intense beam of coherent light.
• Diode lasers are used to read the information
  embedded in the pits in CDs and DVDs, and also
  to read UPCs in bar code scanners and in laser
  pointers!

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Medical Imaging Techniques

• x-rays
• CT and CAT scans (Computerized Tomography)
• MRIs (Magnetic Resonance Imaging)

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X-rays

• very short wavelength (0.01 0.1 nm)
  electromagnetic waves
• produced when energetic electrons slam into a
  metal target
• able to penetrate soft tissue, but not bone
• produces a two dimensional shadow image

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A pineapple and a bananna

• A shadow image can be misleading
• two shadows taken from different angles provides
  a better picture
• shadows taken at multiple angles gives a more
  complete picture
• this is what a CT or CAT scan does

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CAT Scans
X ray images are taken at many different
angles passing through the patient. Some of the
cuts overlap. A full three dimensional image can
be reconstructed using computers. ? this
procedure is called tomography.
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Computerized Tomography

• A computerized tomography or CT scan image is
  formed by analyzing x-ray shadow images taken at
  many different angles and positions
• an x-ray source and an array of electronic
  detectors rotates around the patient as the
  patient slowly moves through the ring.

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Is there a better medical diagnostic?

• A CAT scan does a good job of imaging bones, but
  it does not provide as good an image of soft
  tissue
• Also, it requires that the patient receives a big
  dose of x-rays, which can be harmful in
  themselves ? it is an invasive diagnostic
• Magnetic resonance imaging (MRI) is a better
  method of imaging soft tissue

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MRI- how does it work?

• MRI works by locating the hydrogen atoms inside
  the body. Since the body is mostly water, there
  are lots of hydrogen atoms
• the nucleus of a hydrogen atom is a single
  protons. Protons behave like tiny bar magnets
  with a north pole at one end and a south pole at
  the other end.
• If you put a bar magnet in a magnetic field, it
  will try to align itself with the field.

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A bar magnet in a magnetic field
magnetic field
Solenoid for producing a strong magnetic field
by passing a large current through a set of coils
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Magnetic Resonance Imaging

• The rules of atomic physics (quantum mechanics)
  require that the atomic hydrogen bar magnets can
  only have 2 orientations when placed in a
  magnetic field ? either parallel or antiparallel
  to it, we call this spin-up or spin-down

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Magnetic Resonance

• Protons has a spin that can be either up or
  down relative to the direction of the magnetic
  field
• If radio waves (FM) hit the protons, it can
  cause it them to flip from one spin state to the
  other at a frequency that depends on the strength
  of the magnetic field
• These spin flips result in the absorption or
  release of radio wave energy that can be detected
  electronically

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Magnetic Resonance Imaging

• In effect, the magnetic field makes the protons
  act like tiny radio transmitters that only
  broadcast their signal when the value of the
  magnetic field is just right
• By varying the strength of the magnetic field as
  a function of position in the body, the spin
  flips can be detected in various parts of the
  body
• A computer is used to combine the signals from
  various parts of the body to generate detailed
  cross-sectional images

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MRI DEVICE
Homer
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MRI safety considerations

• The magnetic field used in MRI are very strong
  30,000 times the strength of the earths magnetic
  field.
• Because the magnet coils are cooled to liquid
  helium temperatures, they are usually kept on all
  the time
• Because the magnetic field is on, all iron and
  steel objects must not be allowed to enter the
  room. http//www.mercola.com/2001/aug/15/mri.htm