Transcript Laser - Pharos University in Alexandria

LASER APPLICATIONS TO
MEDICINE AND
Prof. Dr. Moustafa. M. Mohamed
Vice Dean
Faculty of Allied Medical Science
Pharos University
Alexandria
Dr. Mervat Mostafa
Department of Medical Biophysics
Pharos 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
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.
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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 absorption.
E2
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.
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Electric Pumping: Suitable For Gas Laser Because
They Have Narrow Absorption Band.
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Chemical Reaction.
Types of lasers
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According to the active material:
solid-state, liquid, gas, excimer or semiconductor
lasers.
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According to the wavelength:
Infra-red (IR), Visible, Ultra-violet (UV) or X-ray
Lasers.
Types of lasers
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Solid-state lasers have lasing material
distributed in a solid matrix (such as ruby or NdYAG). Flash lamps are the most common power
source. The Nd-YAG laser emits infrared light at
1.064 nm.
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Semiconductor lasers, sometimes called diode
lasers, are p-n junctions. Current is the pump
source. Applications: laser printers or CD players.
Types of lasers
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Dye lasers use complex organic dyes, such as
Rhodamine 6G, in liquid solution or suspension as
lasing media. They are tunable over a broad range
of wavelengths.
• Gas lasers are pumped by current. HeliumNeon (He-Ne) lasers in the visible and IR. Argon
lasers in the visible and UV. CO2 lasers emit light
in the far-infrared (10.6 mm), and are used for
cutting hard materials.
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Excimer lasers: (from the terms excited
and dimers) use reactive gases, such as
chlorine and fluorine, mixed with inert
gases such as argon, krypton, or xenon.
When electrically stimulated, a pseudo
molecule (dimer) is produced. Excimers
laser in the UV.
Solid-state Laser
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Example: Ruby Laser
Operation wavelength: 694.3 nm (IR)
3 level system: absorbs green/blue
Gain Medium: crystal of aluminum oxide (Al2O3)
with small part of atoms of aluminum is replaced
with Cr3+ ions.
Pump source: flash lamp
The ends of ruby rod serve as laser mirrors.
Ruby Laser
How Ruby laser works?
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1. High-voltage electricity causes the quartz
flash tube to emit an intense burst of light,
exciting some of Cr3+ in the ruby crystal to
higher energy levels.
How Ruby laser works?
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2. At a specific energy level, some Cr3+ emit
photons. At first the photons are emitted in all
directions. Photons from one Cr3+ stimulate
emission of photons from other Cr3+ and the light
intensity is rapidly amplified.
How Ruby laser works?
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3. Mirrors at each end reflect the photons back and
forth, continuing this process of stimulated
emission and amplification
How Ruby laser works?
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4. The photons leave through the partially silvered
mirror at one end. This is laser light.
High and Low Level Lasers
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High Level Lasers
–Surgical Lasers
–Hard Lasers
–Thermal
–Energy (3000-10000) mW
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Low Level Lasers
–Medical Lasers
–Soft Lasers
–Subthermal
–Energy (1-500) mW
–Therapeutic (Cold) lasers produce maximum
output of 90 mW or less (600-1000) nm light
Parameters
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Laser
–Wavelength
–Output power
– Average power
– Intensity
–Dosage
Wavelength
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Nanometers (nm)
Longer wavelength (lower frequency) = greater
penetration
Not fully determined
Wavelength is affected by power
Power
Output Power
–Watts or milliwatts (W or mW)
–Important in categorizing laser for safety
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Intensity
Power Density (intensity)
–W or mW/ cm2
– Takes into consideration – actual beam diameter
If light spread over lager area – lower power density
– Beam diameter determines power density
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Average Power
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Knowing average power is important in
determining dosage with pulsed laser
If laser is continuous – average power = peak
output power
If laser is pulsed, then average power is equal to
peak output power X duty cycle.
Energy Density
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Dosage (D)
Amount of energy applied per unit area
Measured in Joules/square cm (J/cm2)
– Joule – unit of energy
– 1 Joule = 1 W/sec
Dosage is dependent on:
–Output of laser in mW.
– Time of exposure in seconds.
– Beam surface area of laser in cm2
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.
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Diagnostics cover the recording of fluorescence
after excitation at a suitable wavelength and
measuring optical parameters.
Laser Tissue Interaction:
What Does Laser Do?
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Laser light waves penetrate the skin with no
heating effect, no damage to skin & no sideeffects.
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Laser light directs biostimulative light energy to
the body’s cells which convert into chemical
energy to promote natural healing & pain relief.
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Stimulation of wound healing
– Promotes faster wound healing/clotformation
–Helps generate new & healthy cells & tissue
What Does Laser Do?
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Increase collagen production
–Develops collagen & muscle tissue
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Increase macrophage activity
– Stimulates immune system
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Alter nerve conduction velocity
– Stimulates nerve function
What Does Laser Do?
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Improved blood circulation & vasodilation
– Increases blood supply
• Increases ATP production
• Analgesic effect
– Relieves acute/chronic pain
• Anti-inflammatory & anti-edematous effects
– Reduces inflammation
Tissue & Cellular Response
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Magnitude of tissue’s reaction are based on
physical characteristics of:
–Output wavelength/frequency
–Density of power
–Duration of treatment
– Vascularity of target tissues
Direct and indirect laser effects
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Direct effect - occurs from
absorption of photons
Indirect effect – produced by
chemical events caused by
interaction of photons emitted from
laser and the tissues
LASER Regulation
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Lasers are classified according to the hazard;
* Class 1 and 1M (magnifier) lasers are
considered safe
* Class 2 and 2M (magnifier)
- 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;
* Class 3R (Restricted) Laser
- produce visible and invisible light that are
hazardous under direct viewing conditions;
* Class 3B lasers
- 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
- 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;