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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
 MRI’s

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
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
Ei (larger energy)
photon
Ef (smaller energy)
Spontaneous emission
Incoming
photon
Stimulated emission
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.
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
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.
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.
A Helium-Neon (HeNe) Laser
Medical Applications of Lasers
Laser surgery to correct for
(a) nearsightedness, and
(b) farsightedness
Applications of High Power Lasers
Using lasers to
Cut metals
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
pellet
Solid State Laser Diodes
small
Come in a variety of
different colors
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 CD’s
and DVD’s, and also to read UPC’s in bar
code scanners and in laser pointers!
Medical Imaging Techniques
• x-rays
• CT and CAT scans (Computerized
Tomography)
• MRI’s (Magnetic Resonance Imaging)
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
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
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.
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.
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
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.
A bar magnet in a magnetic field
magnetic field
N
N
S
S
Solenoid for producing a strong
magnetic field by passing a large current
through a set of coils
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
magnetic
field
spin-down
spin-up
protons
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
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
MRI DEVICE
Homer
MRI safety considerations
• The magnetic field used in MRI are very
strong – 30,000 times the strength of the
earth’s 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