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LASER APPLICATIONS TO MEDICINE AND
BIOLOGY
Prof. Dr. Moustafa M. Mohamed
Biophysics Department,
Medical Research Institute,
Alexandria University
FIRST OFF WHAT DOES
LASER STAND FOR?
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LIGHT
AMPLIFICATION BY
STIMULATED
EMISSION OF
RADIATION
Basic Concepts:
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Laser is a narrow beam of light of a
single wavelength (monochromatic) in
which each wave is in phase (coherent)
with other near it.
Laser apparatus is a device that
produce an intense concentrated, and
highly parallel beam of coherent light.
Basic theory for laser
(Einstein 1917) :
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Atom composed of a nucleus and electron
cloud
If an incident photon is energetic enough, it
may be absorbed by an atom, raising the
latter to an excited state.
It was pointed out by Einstein in 1917 that
an excited atom can be revert to a lowest
state via two distinctive mechanisms:
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spontaneous emission and
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stimulated emission.
Spontaneous emission:
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Each electron can drop back
spontaneously to the ground state
emitting photons.
Emitted photons bear no incoherent.
It varies in phase from point to point
and from moment to moment.
e.g. emission from tungsten lamp.
Stimulated emission :
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Each electron is triggered into emission by
the presence of electromagnetic radiation of
the proper frequency. This is known as
stimulated emission and it is a key to the
operation of laser.
e.g. emission from Laser
Excited state
hν
Ground state
Absorption:
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Let us consider an atom that is initially
in level 1 and interacts with an
electromagnetic wave of frequency n.
The atom may now undergo a transition
to level 2, absorbing the required
energy from the incident radiation. This
is well-known phenomenon of
E
absorption.
2
hn=E2 – E1
E1
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According to Boltzmann's statistics, if a
sample has a large number of atoms, No,
at temperature T, then in thermal
equilibrium the number of atoms in energy
states E1 and E2 are:
N1 = No e-E1/kT
N2 = No e-E2/kT
If E1 < E2
Then
N1 > N2
If E1 < E2
and N1 < N2 This is called
"population inversion".
Population inversion:
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Generally electrons tends to (ground
state). What would happen if a
substantial percentage of atoms could
somehow be excited into an upper state
leaving the lower state all empty? This
is known as a population inversion. An
incident of photon of proper frequency
could then trigger an avalanche of
stimulated photon- all in phase (Laser).
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Consider a gas enclosed in a vessel
containing free atoms having a number
of energy levels, at least one of which is
Metastable.
By shining white light into this gas many
atoms can be raised, through resonance,
from the ground state to excited states.
Population Inversion
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E1 = Ground state,
E2 = Excited state (short life time ns),
E3 = Metastable state (long life time
from ms to s).
Life times
hn =5500 Ao
E3
10-9 sec
E2
10-3 -1 sec
Output
(amplification)
E1
Excitation
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To generate laser beam three
processes must be satisfied:
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Population inversion.
Stimulated emission.
Pumping source.
COLLIMATED
BEAM
MEDIUM
MIRROR
PUMP
Pumping sources:
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Optical pumping: suitable for liquid
and solid laser because they have
wide absorption bands.
Electric pumping: suitable for gas
laser because they have narrow
absorption band.
Chemical reaction.
Types of lasers:
Lasers are classified according to laser active
medium into:
 Solid: for example :
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Ruby (Cr:Al2O3)
Neodymium- glass (Nd-Glass).
Nd- YAG (Nd-Yttrium, Aluminum granite)
Liquid lasers: (Dyes).
Gas lasers: He-Ne, Ar, CO2, He-Cd, N2, Kr,
Excimer (ArF, XeF, HF, DF).
Laser Beam
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Coherent (in phase)
Monochromatic (single wavelength)
Collimated (highly parallel)
Intense (Concentrated)
USES OF LASER TECHNOLOGY
INCLUDING:
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SCIENCE
MACHINING
COMMUNICATIONS
SECURITY/MILITARY
MEDICINE
Historical introduction
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1946: A German physician, Gerd Meyer, used
the sun to treat detached retinas and destroy
tumors in some of his patients eyes.
1948: High intensity xenon lamp used for
photocoagulation
1961: one year after Maiman built the first laser,
Milton Zaret used laser to produce ocular lesions
in animals.
1963: Chris Zweng treated retinal disease in his
patients using laser beam
Heat By Laser
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Intense Heat
Destructive effects can be extremely
selective and precisely controlled
Reversible
effect
37 C
Protein
Denaturation
60 C
Coagulation
80 C
Vaporization
and ablation
100 C
Homeostasis Welding
Cutting
Laser Tissue Interaction:
REFLECTION
SCATTERING
b
LASER BEAM
a
TARGET
TISSUE
Transmitting
c
d
e
f
g
FLUORESCENCE
For diagnostic
PHOTOCHEMISTRY
Destroy the
target
ee-
HEAT
PHOTODISSOCIATION
(Break molecular
bond)
SHOCK
WAVE
(Breaks
mineralized
deposits)
TREATMENT &
DIAGNOSTIC BY LASER
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PHOTOCOAGULATION OF THE
RETINA
Heating a blood vessel to a point where
the blood coagulate and block the
vessel.
Photocoagulation can be done by:
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1- Xenon lamp
2- Laser
Photocoagulation
Xenon lamp:
Laser
Spot size 750 m m
High energy deposited in the eye:
20-50 times greater than deposited
treatment by laser beam
Spot size 50 mm
low energy deposited in the eye
Longer exposure (1 sec)
than laser, so local anesthesia
must be used
Short exposure (ms to ms)
So local anesthesia are not needed
Laser Treatment & Diagnostics
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Treatment cover everything from the
ablation of tissue using high power
lasers to photochemical reaction
obtained with a weak laser.
Diagnostics cover the recording of
fluorescence after excitation at a
suitable wavelength and measuring
optical parameters.
Diagnostic Laser System
Several factor have to be consider in designing
a diagnostic laser system:
1- A suitable excitation wavelength.
2- Knowledge about fluorescence properties of
different chromospheres in tissue is needed.
3-Origin of the fluorescence spectra must be
identified.
4- Tumor seeking drugs (e,g. hematoporphoryin) is used to enhance the optical
demarcation of malignant tumors.
Surgical Application of Laser
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Tissue heating (Skin rejuvenation & tissue
welding)
Coagulation
Vaporization
Fragmentation of tattoo pigment
Cold cutting
Photoacoustic (lithotripsy)
Photodissociation (non-thermal ablation of
the cornea in ophthalmology).
Retina Treatment
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The dark brown melanin pigment of the retina
absorb the green beam of the argon laser.
The argon laser can destroy specific regions of the
retina without harming the other area of the eye,
which absorb different wavelength of light.
Red birthmarks (Port wine
stains)
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Red birthmarks also
absorb the argon laser,
which could be blue or
green depending on its
wavelength.
The absorbed light
destroys hundreds of
the extra blood vessels
that beneath the skin’s
outer layer and discolor
it.
Disadvantages
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The heat generated by the beam can
sometimes spread to parts of the skin other
than the abnormal blood vessels and cause
scarring or loss of pigments.
R. Rox Andrson and A. Jhon (1983) (Harvard
University) suggested that short exposure
less than 1 ms – to intense light would
destroy the absorption site but produce little
or no damage to adjacent tissue.
Advantage of wide damage
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Wide damage caused by the longer slower
heating of tissue can be turned advantage.
Removing of a damaged portion of the liver
cause extensive bleeding.
The long exposure to a continuous wave laser
reduces bleeding because heat spreads to the
capillaries nearby.
A CO2 laser with a wavelength 10.6 microns
may be used because it is absorbed by the
compound most common to tissue: Water
Intraocular Nd: YAG Laser
Damage Mechanisms of intraocular
Nd:YAG Laser Surgery (Single laser
pulse:
 Plasma formation and expansion
 Emission of acoustic transient
 Cavitations with jet formation
Pulsed lasers can also remove
tissue
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Er-YAG Laser (Erbium Yttrium-Aluminum
Garnet) which has a wavelength of 2.9
micron and pulse duration of 200 us, can
cleanly ablate calcified bone.
Xenon chloride excimer laser (0.308 microns
and pulse duration of 10 ns) can vaporize
bone with little or no associated thermal
damage.
Laser and Fiber Optics
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Coupling lasers with other technologies
such as fiber optics, one can achieve
non- thermal, as well as thermal, results
in previously inaccessible parts of the
body.
Photodynamic Therapy of
Cancer
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A dye selectively
concentrates in cancerous
tissue 48 to 72 hours after it
is injected.
Blue-violet light from
krypton laser, administrated
through an optical fiber,
causing dye to fluorescence,
so it can easily be observed
and diagnosed.
The optical fiber then drives
laser light of another wave
length, which destroys the
tumor.
Laser Angioplasty
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The removal of plaque in obstructed vessel
by laser, administrated through a fiber
optics.
Fluorescence characterization of the vessel
wall could be performed via the same fiber
as that used for the delivery of high-power
pulses for plaque removal.
Ultrastructural changes of Staph.
aureus by laser irradiation:
There are many benefits of
laser dentistry. They include:
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Faster healing.
Reduced risk of infection
Decreased Sensitivity.
Less time in the dental chair.
Less bleeding.
Less post-treatment discomfort
laser Doppler velocimeter
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The laser Doppler velocimeter sends a
monochromatic laser beam toward the target
and collects the reflected radiation.
According to the Doppler effect, the change in
wavelength of the reflected radiation is a
function of the targeted object's relative
velocity.
Thus, the velocity of the object can be obtained
by measuring the change in wavelength of the
reflected laser light, which is done by forming
an interference fringe pattern.
Typical Laser Doppler
Velocity meter (Velocimeter)
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A laser power source is the essential part
a Helium-Neon (He-Ne) or Argon ion laser
with a power of 10 mW to 20 W is used.
Lasers have many advantages over other
radiation/wave sources, including excellent
frequency stability, small beam diameter
(high coherence), and highly-focused
energy.
Doppler Effect
Comparison Between the Single Beam
and Cross-Beam Systems
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single-beam system,
- employs a focused laser beam which is
scattered from particles.
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A portion of the scattered light is sampled and
mixed with a portion of the unscattered laser
beam and collected by an o p t i c a l - p h o t o m
u l t i p l i e r system
The two l i g h t beams heterodyne to yield
the difference frequency between the two
light beams.
This difference frequency, or Doppler frequency,
is related to the particle velocity in the flow.
DfD = (vs/l)(cos(a-q’)-cos a)
where DfD Doppler frequency, Hz
vs particle velocity, m/s
l wavelength of laser beam, m
a angle between particle velocity vector and laser
beam, deg
q angle between laser beam and scattered light,
deg
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Lasers are classified according to the
hazard
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Class 1 and 1M (magnifier) lasers are considered safe
Class 2 and 2M (magnifier)
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Class 3R (Restricted) Laser
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produce visible and invisible light that are hazardous under direct
viewing conditions;
Class 3B lasers
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emit visible light at higher levels than Class 1,
eye protection is provided
can be hazardous if the beam is viewed directly with optical
instruments;
produce visible or invisible light that is hazardous under direct viewing
conditions
they are powerful enough to cause eye damage in a time shorter
Laser products with power output near the upper range of Class 3B
may also cause skin burns;
Class 4 lasers
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high power devices capable of causing both eye and skin burns,
heir diffuse reflections may also be hazardous
the beam may constitute a fire hazard;