Unit XI Laser and its medical applications

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Unit XI Laser and its medical applications
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A wide application of laser in medicine and
beauty therapy

• Surgical laser removing tumors, making
  incisions.
• Cosmetic treatments resurfacing, removal of
  birth mark, age spots, spider veins, hair,
  tattoos,
• Ophthalmology inner eye surgery in removing
  cataract, repairing retina, correct
  nearsightedness.

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Laser repairing retina
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Laser removal of port-wine stain
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Laser skin rejuvenation
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What is a laser?
Laser Light Amplification by Stimulated
Emission of Radiation

• The physics of laser
• The interaction of laser light with human tissue
  

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Outline of the unit 11

• Simple atomic structure
• Light emission
• Characteristic of laser
• Laser power and intensity
• Mechanisms of laser interaction with human
  tissues
• Selective absorption of laser light by human
  tissues
• Applications of lasers in healthcare and beauty
  therapy

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1. Summary about atom
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Atom is the smallest building block of matters,
including body tissue and fluids,

• it is electrically neutral, it contains three
  elementary particles the electron , the proton,
  and the neutron.
• electrons make circular motion around the nucleus
  (containing proton and neutron) in different
  level of orbits, called stationary orbits, they
  correspond to different energy levels.
• the number of electrons is equals to the number
  of protons in an atom,
• a nucleus counts for vast majority of the atomic
  mass.
• The so-called atomic number (Z number) is the
  proton number (also electron number) in an atom.
  Ions are formed when atoms obtained extra
  electrons or loose electrons.

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2. Light emission
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• electron orbits displayed as an energy level
  diagram
• energy is plotted vertically with the lowest
  (n1) , or ground stat, and with exited states
  (n2, 3,4,) above.
• n is called orbit number and can be only
  positive integer.

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• The electron can absorb energy and jump to a
  higher level, the process is called excitation.
• (b) A photon is emitted when an electron change
  from a higher orbit to a lower orbit with a
  characteristic emission spectrum. This process
  is called de-excitation.
• (c) If an atom absorbs a photon, an electron
  jumps from a lower orbit to a higher orbit with a
  characteristic absorption spectrum.

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Atom will absorb and emit light photons at
particular wavelength corresponding to the energy
differences between orbits. The wavelength l of
emitted or absorbed photon can be obtained by the
formula
where ?E is the change in energy between the
initial and final orbits. A variety of
biological molecules have notable absorption
spectra in the visible, IR, and UV. This has many
clinical application. e.g. Oximeter.
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3. How laser works
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Spontaneous emission and stimulated emission
An excited electron may gives off a photon and
decay to the ground state by two processes

• spontaneous emission neon light, light bulb

• stimulated emission the excited atoms interact
  with a pre-existing photon that passes by. If the
  incoming photon has the right energy, it induces
  the electron to decay and gives off a new photon.
  Ex. Laser.

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Optical pumping
many electrons must be previously excited and
held in an excited state without massive
spontaneous emission this is called population
inversion. The process is called optical pumping.
Example of Ruby laser.
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Optical pumping

• Only those perpendicular to the mirrors will be
  reflected back to the active medium, They travel
  together with incoming photons in the same
  direction, this is the directionality of the
  laser.

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Characteristics of laser

• The second photon has the same energy, i.e. the
  same wavelength and color as the first
• laser has a pure color
• It travels in the same direction and exactly in
  the same step with the first photon
• laser has temporal coherence

Comparing to the conventional light, a laser is
differentiated by three characteristics. They are

• Directionality,
• pure color,
• temporal coherence.

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Characteristics of laser
Pure color
Directionality Temporal coherence
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The power and intensity of a laser
The power P is a measure of energy transfer rate

where the unit of power is Joules/s or W.
The energy encountered by a particular spot area
in a unit time is measured by the intensity (or
power density)
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laser versus ordinary lights
The directionality of laser beam offers a great
advantage over ordinary lights since it can be
concentrate its energy onto a very small spot
area. This is because the laser rays can be
considered as almost parallel and confined to a
well-defined circular spot on a distant object.
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Sample problem we compare the intensity of the
light of a bulb of 10 W and that of a laser with
output power of 1mW (10-3 W). For calculation, we
consider an imagery sphere of radius R of 1m for
the light spreading of the bulb, laser beams
illuminate a spot of circular area with a radius
r 1mm.
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Fluence, F is defined as the total energy
delivered by a laser on an unit area during an
expose time TE, F(J/cm2)I(watts/cm2) x TE(s)
The advantage of directionality of a laser we
can focus or defocus a laser beam using a lens.
This can be used to vary the intensity of the
laser.
f
Incoming parallel ray
Focused spot
Diverged beam
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continuous wave (CW) lasers versus pulsed lasers

• CW lasers has a constant power output during
  whole operation time.
• pulsed lasers emits light in strong bursts
  periodically with no light between pulses

usually TgtgtTw
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• The tw may vary from milliseconds (1ms10-3 s) to
  femtoseconds (1fs10-15s), but typically at
  nanoseconds (1ns10-9s).
• energy is stored up and emitted during a brief
  time tw,
• this results in a very high instantaneous power
  Pi
• the average power Pave delivered by a pulsed
  laser is low.

Instantaneous power Pi
Average power Pave
Where R is the repetition rate
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Example A pulsed laser emits 1 milliJoule (mJ)
energy that lasts for 1 nanoseconds (ns), if the
repetition rate R is 5 Hz, comparing their
instantaneous power and average power. (The
repetition rate is the number of pulses per
second, so the repetition rate is related to the
time interval by R1/T).
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4. Mechanisms of laser interaction with human
tissues
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When a laser beam projected to tissue
Five phenomena

• reflection,
• transmission,
• scattering,
• re-emission,
• absorption.

Laser light interacts with tissue and transfers
energy of photons to tissue because absorption
occurs.
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Photocoagulation
What is a coagulation?

• A slow heating of muscle and other tissues is
  like a cooking of meat in everyday life.
• The heating induced the destabilization of the
  proteins, enzymes.
• This is also called coagulation.
• Like egg whites coagulate when cooked, red meat
  turns gray because coagulation during cooking.

A Laser heating of tissues above 50 oC but below
100oC induces disordering of proteins and other
bio-molecules, this process is called
photocoagulation.
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Consequence of photocoagulation
When lasers are used to photocoagulate tissues
during surgery, tissues essentially becomes
cooked

• they shrink in mass because water is expelled,
• the heated region change color and loses its
  mechanical integrity
• cells in the photocoagulated region die and a
  region of dead tissue called photocoagulation
  burn develops
• can be removed or pull out,

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Applications of photocoagulation

• destroy tumors
• treating various eye conditions like retinal
  disorders caused by diabetes
• hemostatic laser surgery - bloodless incision,
  excision

due to its ability to stop bleeding during
surgery. A blood vessel subjected to
photocoagulation develops a pinched point due to
shrinkage of proteins in the vessels wall. The
coagulation restriction helps seal off the flow,
while damaged cells initiate clotting.
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Photo-vaporization
With very high power densities, instead of
cooking, lasers will quickly heat the tissues to
above 100o C , water within the tissues boils and
evaporates. Since 70 of the body tissue is
water, the boiling change the tissue into a gas.
This phenomenon is called photo-vaporization.
Photo- vaporization results in complete removal
of the tissue, making possible for

• hemostatic incision,or excision.
• complete removal of thin layer of tissue. Skin
  rejuvenation, resurfacing

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Conditions for photo-vaporization

1. the tissue must be heated quickly to above the
   boiling point of the water, this require very
   high intensity lasers,
2. a very short exposure time TE, so no time for
   heat to flow away while delivering enough energy,

highly spatial coherence (directionality) of
lasers over other light sources is responsible
for providing higher intensities
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Intensity requirement
Intensity (W/cm2) Resulting processes Low
(lt10) General heating Moderate (10
100) Photocoagulation High (gt100) Photo-vaporiz
ation
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Photochemical ablation
When using high power lasers of ultraviolet
wavelength, some chemical bonds can be broken
without causing local heating this process is
called photo-chemical ablation. The
photo-chemical ablation results in clean-cut
incision. The thermal component is relatively
small and the zone of thermal interaction is
limited in the incision wall.
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5. Selective absorption of laser light by human
tissues
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Selective absorption
Selective absorption occurs when a given color of
light is strongly absorbed by one type of tissue,
while transmitted by another. Lasers pure color
is responsible for selective absorption.
The main absorbing components of tissues are

• Oxyhemoglobin (in blood) the bloods oxygen
  carrying protein, absorption of UV and blue and
  green light,
• Melanin (a pigment in skin, hair, moles, etc)
  absorption in visible and near IR light (400nm
  1000nm),
• Water (in tissues) transparent to visible light
  but strong absorption of UV light below 300nm and
  IR over 1300nm

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Selective absorption
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6. Applications of lasers
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Lasers in beauty therapy
Lasers application in beauty therapy are based on

• selective absorption of absorbing components.
• photo-vaporization process for removal of the
  treated components.
• pulsed lasers are used.

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Laser skin rejuvenation
IR lasers are used to remove extremely thin layer
of skin (lt0.1 mm). In the absence of pigment in
general, they take advantage of the presence of
water in the skin to provide an ability to remove
skin and body tissue.
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Laser hair removal
selective absorption absorbing component being
melanin pigment in hair and follicle, it is best
worked with a red light ruby laser. White hair
can not be treated with any laser due to the lack
of absorbing component.
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Laser removal of port-wine stain
Yellow laser is absorbed by the presence of
hemoglobin in blood vessels.
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Laser removal of tattoo
tattoo can be removed with variety of laser
depending on the presence of inks in the tattoo.
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Lasers in ophthalmology
For retina operation, visible laser can be used.
Visible light is transparent to the cornea and
crystalline lens, and can be focused with eyes
lens on the retina. The most popular visible
laser is the green argon laser.

• Treatment of glaucoma Argon laser is focused
  externally on iris to make incision, creating
  drainage holes for excess aqueous humors to
  release pressure,
• Retina tear photocoagulation burn to repair
  retina tears due to trauma to the head.
• Diabetic retinopathy inadequate blood supply to
  the retina due to diabetes. Small
  photocoagulation burn by green argon laser to
  repair the retina due to vessels leakage.

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Lasers in ophthalmology
For cornea and lens, UV light emitted by the
excimer laser is strongly absorbed by water and
proteins, so their energy can be absorbed by
transparent cornea and lens, permitting laser
surgery on these areas.

• Cataracts a milky structure in the lens of the
  eye. Photo-vaporization by using UV laser to
  remove the obaque regions.
• Correction of myopia over focusing of the lens.
  Excimer laser removal of surface of cornea to
  make it flatten.

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7. Laser hazards and protections
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Absorption of the eye
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Hazards to the eye
The retina The directionality of a laser beam
permits the ray to be focused to an extremely
small spot on the retina. A collimated laser will
be concentrated by a factor of 100,000 when
passing from cornea to retina.
Visible or near IR lasers (400 nm to 1400nm) are
particularly dangerous to the retina and always
requires eye-protection when working with these
kind of lasers.
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Hazards to the eye

• The cornea and lens
• Cornea is accessible to danger of UV and most of
  IR lasers,
• UV-A, UV-B (between 295nm and 320 nm) and IR-A
  (between 1 to 2 mm) are dangerous for lens,
• 308-nm (UV-B) excimer XeCl laser is particular
  dangerous because of it can simultaneously damage
  the lens, the cornea and the retina.

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Protection to the eye
Eye protection Eyewear (goggles) is the most
common laser protective measure, especially for
open laser beams. It should be good design with
all around shielding and adequate visible light
transmission. Identification of the eyewear
All laser protective eyewear shall be clearly
labelled with information adequate to ensure the
proper choice of eyewear with particular lasers.