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The level of oxygen in the blood can be determined by measuring the absorption spectrumof hemoglobin in the blood at several wavelengths. Its absorption spectra can be used to compare with those of oxyhemoglobin and deoxyhemoglobin can be determined and used to compute the oxygen level by a device called an oxumeter.
The ruby laser was the first laser invented in 1960. Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal. Chromium atoms absorb green and blue light and emit only red light. For a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity flash lamp is spiraled around the ruby cylinder to provide energy that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level.
Another advantage of the directionality of a laser is that laser light can be focused down to a extremely small spot with a lens, this is because that a perfect parallel beam can be focused into a single point at the focal point. So, extremely high intensity can be obtained with a laser beam. Then away from the focal point, the laser light diverges and spreads out and corresponding intensity decreases. This property can be used to vary the intensity of a laser by 100~10000 times.
In most cases, the photons energy of laser light is transferred to blood, tissues or bone in the form of heat. In other cases laser can also transfer photon energy to chemical bond energy of the molecules in human body, such as DNA. This risks the modification of genetic information. There are generally three interaction mechanisms involved.
Proteins are body’s most important structural and functional chemicals. They form the muscles, connecting tissues and blood vessels, they transport oxygen necessary for the metabolism. When temperature is much higher than the body temperature, proteins are destabilized: their complex structures begin to uncoil and losing their natural order and forming dense tangled network.
Laser and its medical applications
Laser and its medical
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.
What is a laser?
Laser = Light Amplification by Stimulated Emission of Radiation
•The physics of laser
•The interaction of laser light with human tissue
Outline of the unit 11
•Simple atomic structure
•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
Atom is the smallest
building block of matters,
including body tissue and
•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
•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.
•electron orbits displayed as an energy level diagram
• energy is plotted vertically with the lowest (n=1) , or
ground stat, and with exited states (n=2, 3,4,…) above.
• n is called orbit number and can be only positive
(a) 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.
Atom will absorb and emit light photons at particular
wavelength corresponding to the energy differences
between orbits. The wavelength λ of emitted or absorbed
photon can be obtained by the formula:
where ∆E is the change in energy between the initial and
A variety of biological molecules have notable absorption
spectra in the visible, IR, and UV. This has many clinical
application. e.g. Oximeter.
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 preexisting 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.
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
Example of Ruby laser.
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.
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:
Characteristics of laser
The power and intensity of a laser
The power P is a measure of energy transfer rate;
Total energy output (J)
exposurte time (s)
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
Intensity (W/cm ) =
spot area (cm 2 )
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.
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.
= 8 × 10−5W / cm2
= 3 × 10 − 2W / cm 2
π (0.1cm) 2
I laser 3 × 10−2W / cm 2
I bulb 8 × 10 W / cm
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.
continuous wave (CW) lasers versus pulsed lasers
• CW lasers has a constant power output during whole operation
• pulsed lasers emits light in strong bursts periodically with no
light between pulses
• The tw may vary from milliseconds (1ms=10-3 s) to
femtoseconds (1fs=10-15s), but typically at nanoseconds
• 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
= E pulse ⋅ R
Where R is the repetition rate
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 R=1/T).
1mlliJoule 10−3 J
= − 9 = 106W
= E pulse ( J ) × R( Hz ) = 10− 3 J × 5Hz = 5 × 10− 3W
4. Mechanisms of laser
interaction with human tissues
When a laser beam projected to tissue
Laser light interacts with
tissue and transfers
energy of photons to tissue
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,
• 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.
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
• cells in the photocoagulated region die and a region of
dead tissue called photocoagulation burn develops
• can be removed or pull out,
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 vessel’s wall. The coagulation
restriction helps seal off the flow, while damaged
cells initiate clotting.
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 :
• complete removal of
thin layer of tissue.
Conditions for photo-vaporization
1.the tissue must be heated quickly to above the
boiling point of the water, this require very high intensity
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
Moderate (10 – 100)
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.
5. Selective absorption of laser
light by human tissues
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
The main absorbing components of tissues are:
• Oxyhemoglobin (in blood): the blood’s 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
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
• pulsed lasers are used.
Laser skin rejuvenation
IR lasers are used to remove extremely thin layer of skin
(<0.1 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.
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.
Laser removal of port-wine stain
Laser removal of tattoo
tattoo can be removed with
variety of laser depending
on the presence of inks in
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
eye’s lens on the retina. The most popular visible laser is the green
• Treatment of glaucoma: Argon laser is
focused externally on iris to make
incision, creating drainage holes for
excess aqueous humors to release
• Retina tear: photocoagulation burn to
repair retina tears due to trauma to the
• 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
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
• Correction of myopia: over focusing of the lens.
Excimer laser removal of surface of cornea to make it
Hazards to the eye
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
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.
Protection to the eye
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
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.