INTRODUCTION
Light is the electromagnetic spectrum which covers an extremely broad range,
from radio waves with wavelengths of a meter or more, down to X-rays with wavelengths
of less than a billionth of a meter. Optical radiation lies between radio waves
and X-rays on the spectrum, exhibiting a unique mix of ray, wave and quantum
properties (Abdul Qader, 2006).
It is possible to say that a light signal may have dual behavior. At X-ray
and shorter wavelengths, electromagnetic radiation tends to be quite particle
like in its behavior, whereas toward the long wavelength end of the spectrum
the behavior is mostly wavelike. The visible light portion of electromagnetic
spectrum occupies an intermediate position, exhibiting both wave and particle
properties in varying degrees. Short wavelength UV light exhibits more quantum
properties than its visible and infrared counterparts (Ryer,
1998).
The name LASER is an acronym for Light Amplification by Stimulated Emission
of Radiation. Thus, the term reflects the crucial role of the process of stimulated
emission for the quantum generators and amplifiers of coherent light (Tarasov,
1986). Therefore, the history of laser development should be traced as far
back as 1917 when Albert Einstein showed that the process of stimulated emission
must exist (Elliott, 1995). This was the first step towards
the laser. In 1953, a group at Columbia University headed by Charles H.Townes
operated a microwave device that amplifier radiation by stimulated emission
process, it was termed MASER, an acronym for Microwave Amplification by Stimulated
Emission of Radiation (Elliott, 1995; Oshea
et al., 1978). Over next year's Schawlow and Townes made important
contributions that help to extend these ideas from the microwaves to the optical
wavelength region. These efforts culminated in July 1960 when T.H. Mainian announced
the generation of a pulse coherent red light by means of a ruby crystal-the
first laser (Mosaad, 1997). After that laser started to
be used by engineering sciences and industry. An example of its use in computer
science such as information security, thus making it possible to secure transmission
of information from the patient to data stored in the hospital (Alanazi
et al., 2010a, b) or from a hospital to another
(Nabi et al., 2010). Also in the field of electronic
engineering and medical engineering which made it posibel to develop laser devices
to be used in medicin and medical applications (Jawad, 2008).
There is no universal laser device or set of laser light Parameters for effective
treatment of all medical diseases just as there is no universal drug for all
human disorders (Sisecioglu et al., 2011; Khosravi
et al., 2008). Therefore, greater acceptance of lasers for medical
treatment will come with better understanding of the proper choice of laser
light energy value needed to perform a specific treatment, when this knowledge
is identified from careful scientific studies in the research laboratory a safety
packaged laser device and delivery system operated within appropriate range
of conditions will greatly increase the acceptance of laser for clinical applications
(Keye, 1990).
LASER LIGHT PROPERTIES
Coherence: Coherence means that the electromagnetic waves of light rays
are in phase with each other in both space and time. The coherent nature of
laser radiation is derived from its generation by stimulated emission that means
the emitted photon is exactly in phase with the stimulating photon (Moseley,
1988). There are two types of coherence spatial and temporal (Powell,
1992). Spatial coherence means that the crests and troughs of all the waves
coincide along lines perpendicular to the rays. Temporal coherence means that
the frequency, wavelength and speed of travel are all constant (Wright
and Fisherm, 1993). Coherence is the most fundamental property of laser
and distinguishes it from the light from other sources. Thus, a laser may be
defined as a source of coherent light.
Brightness or intensity: This property arises from the parallelism or
collimation of the laser light as it moves through space maintaining its concentration
and thus, the characteristic brightness. The brightness translates to high concentrations
of energy when the laser is focused on a small spot (Powell,
1992).
Monochromaticity:Monochrome means that all the photons have the same
wavelength. The light produced by a Particular laser will be of a characteristic
wavelength or wavelengths (Niemz, 1996). This contrasts
greatly with a typical incandescent light bulb which emits wavelengths of the
entire spectrum, usually wavelengths from ultraviolet through the entire visible
and then into the infrared range or more (Powell, 1992).
Directionality: There is little divergence of the laser beam as it exits
the laser device and the beam can travel a considerable distance with very little
movement away from parallelism. By not diverging over distance, laser light
maintains brightness (Powell, 1992; Niemz,
1996).
LASER PRINCIPLES
To describe how a laser works, some basic aspects of light/matter interaction
will be reviewed. The electrons in an atom or molecule exist in very specific
energy level called (states). Each atom possesses electrons that are characteristic
to the Specific element or combination of elements (molecules) (Elliott,
1995). The transition of an atom or molecule from one state to another occurs
and is called the quantum transition. Quantum transitions may be induced by
various causes; in particular they can occur when atoms interact with optical
radiation (Tarasov, 1986). If the atom is in the upper
state and makes a transition to the lower state then energy as a photon of electromagnetic
radiation can be emitted with a frequency given by:
V = E2-E1/h, E2-E1
= hV, E = hc/λ |
where, E2-E1 is the energy difference between the two levels E2 and E1 which is E, h is plank’s constant (6.625x10-34 J sec-1), c is the speed of light (3x108 m sec-1), v is the frequency and λ is the wavelength in (m).
On the other hand, if the atom is initially in the lower energy state (E1)
and makes a transition to the higher state (E2), then energy and
hence radiation of frequency given by the above equation must be absorbed (Beesly,
1978). By absorbing a photon that has energy equal to the difference
between the lower state and a higher one. Electron can move to more energetic
state (called an excited state). This excited state is less stable than the
lower or ground state, thus electrons tend to give up energy by radiating a
photon of an energy equal to the energy difference between the two states and
returning to some lower state. This emission occurs after a period of time called
(life time of spontaneous emission) (Elliott, 1995; Mosaad,
1997). The emission of the photon can occur by two ways (Tarasov,
1986; Mosaad, 1997):
Spontaneous emission: where the atoms at level E2 tend to decay to level E1 spontaneously without any stimulus. The decay of different atoms occurs at random, the instant of the transition, direction of the emitted photon and its polarization all are random quantities
Stimulated emission: where the atom lies in the upper energy level then the same incident photon may play the role of a trigger and induce the transition from E2 to E1, the transition causes an emission of a photon. Both the inducing and the induced photons have the energy equal E2-E1, same direction and polarization
Therefore, if a large number of matching-energy photons come past an excited
state, there is a high probability that this process occurs, so that its lifetime
for stimulated emission can be much less than its spontaneous lifetime. This
process is called amplification; it is the critical process that makes a laser
possible (Elliott, 1995) as shown in Fig.
1.
In order to obtain a laser action, it must be ensured that more atoms in the
lasing medium are in an excited state than in the lower-energy state. When this
condition is met, it is said that a population inversion takes place in the
medium. Pumping energy into the lasing medium can create this condition (Tarasov,
1986; Mosaad, 1997; Oshea et
al., 1978). Now a stray photon of the correct wavelength, produced by
spontaneous emission, is enough to set off a chain of stimulated emissions.
The lasing medium lies between two mirrors, one of them is totally reflecting
and the other is partially reflecting. Photons can bounce back and forth, stimulating
more and more atoms to emit photons, thus rapidly increasing the intensity,
as they leave through the partially reflecting mirror (Oshea
et al., 1978). If the pumping energy is applied continuously, population
inversion is maintained and new excited atoms will recoup the exhaustive atoms
and give rise to a continuous wave laser (Ashour, 2006).
While if the pumping energy is applied intermittently, as in pulsed laser, the
stimulated emission die down as the atoms that are in an excited state are developed
and lose its excessive energy, destroying the population inversion (Mosaad,
1997).
ELEMENTS OF LASER DEVICES
In order to operate most laser devices, three basic conditions must be satisfied:
Active medium: This is a collection of atoms, molecules, or ions that
can be solid, liquid, gas, or plasma states. The composition of the lasing medium
determines the wavelength output and name of a particular laser (Keye,
1990).
Pumping source: The source of energy to pump the laser medium. When
the laser medium in the optical cavity is pumped, a laser beam is generated
that leaves the cavity through the partially transmitive mirror by which the
population inversion is created inside the active medium (Cember,
1987).
Optical resonator: This consists of two mirrors. The laser medium is
placed in the optical cavity and its axis is made to coincide with the common
axis of the mirrors. One mirror is generally fully reflective for the wavelength
of operation of the laser and the other is partially transmitive, by which the
selection of some photon states and the suppression of other states can be realized
(Oshea et al., 1978) as shown in Fig.
2.
LASER MODES
The longitudinal (or axial) mode of a laser is a function of the distance between
the resonating mirrors and determines photon wavelength. The optical cavity
is structured to optimize the amplification of only one frequency in order to
maintain monochromatizing output (Cember, 1987). The Transverse
Electromagnetic Mode (TEM) defines the special energy distribution pattern across
the face of the beam. The smallest effective beam diameter and hence the highest
power intensity occurs with a TEM00 beam when we have a Gaussian
or normal distribution of energy across the beam (Keye, 1990).
Types of laser operation: Lasers can operate in the following modes:
CW or continuous mode: If the partially transmitting end of the optical
cavity allows a fraction of the light energy that strikes it to escape and if
energy can be pumped in to the lasing medium at such a rate that the laser output
can be maintained uninterruptedly, then we get a continuous laser beam. Most
CW lasers employ gas as the lasing medium (Seckel, 1996).
Chopped mode: A shutter may interrupt The output of a CW laser that chops the beam into trains of short pulses. The maximum power level of each pulse is the same as that obtained in the CW mode. The duration of the pulse when the shutter is open is limited by the speed of the shutter and is typically 100 to 500 m sec. The useful proportion of the laser beam during which the light is transmitted by the chopped laser is called Duly Cycle (DC):
The duty cycle is obtained by multiplying the pulse width by the frequency
(in hertz) (Al-Qalamjy, 2001).
Pulsed: Gas lasers such as the CO2 laser can be gated or
pulsed electronically. The gating permits the duration of the pulses to be compressed;
producing a corresponding increase in peak power that is much higher than it
is commonly available in the CV mode (Dederich, 1993).
Q-Switched: Short and more intense pulse can be obtained with the technique
of Q-switching. By introducing a shutter into the resonant cavity of the laser,
the energy in the active medium is raised to a level far above that is obtainable
without the shutter or obstruction in the system. If the shutter is then rapidly
opened or the obstruction is removed to permit light to traverse the resonant
cavity, all of the stored energy is discharged in an extremely short period.
The result is a short duration pulse (1 μ sec to 1 n sec) whose peak intensity
reaches to 107 W or more. This extremely high-powered flash is also
referred to as a giant pulse (Miserendino and Pick, 1995;
Powell, 1992).
LASER PARAMETERS
Wavelength: Wavelength is the most important determinant in how light
affects tissue. It is the distance between two successive crests of the wave.
Each type of laser has a certain wavelength (or wavelengths) according to the
nature of the active medium. Laser wavelengths are commonly measured in units
of length: nanometers (nm) or micrometers (μm), depending on whether they
are in UV, visible or IR range of the electromagnetic spectrum. Simply stated,
the wavelength determines the quality or type of interaction between the laser
and the tissue (Dederich, 1993).
Energy and energy density: Radiant energy is the total amount of energy
radiated by optical source (Hadley et al., 2000).
CW lasers generate laser outputs in certain ranges of energy according to its
application. Therefore, the output energy could be manipulated within the range
of the system. Pulse energy is the energy contained in a single pulse. This
term is used in pulsed laser. The operator cannot change pulse energy. The units
o f the energy are Joules (J). In pulsed laser, the energy density term is used
to determine the energy deposition in a certain area of the target. In a single
pulse the energy density equals the energy of a single pulse divided by the
irradiated area:
In the multipulsed irradiation the number of the pulses multiplies the last term:
The energy density is measured in Joules per square centimeter (J cm-2 or J cm-1).
Pulse duration: This term is used in pulsed lasers. It refers to the full width at half maximum of the peak of the pulse. Pulse duration is measured in units of time (milliseconds, microseconds, nanoseconds, picoseconds or femtoseconds). The operator cannot control the pulse duration.
Repetition rate: It is the number of pulses per one second. The operator controls it through the manipulation in the control panel of the laser unit in a certain range. It is measured in Hertz (Hz) or (S-1).
Duty cycle: It is the useful proportion of the laser beam during which the light is transmitted by the chopped laser. It is a unit less quantity. The range of the duty cycle enables the operator to reduce the unwanted thermal effect of the CW laser beam. Duty cycle can be calculated from:
Power and power density: Radiant power is the amount of radiant energy. The average power of the laser is equal to the output energy over the exposure time.
In pulsed laser the peak power is expressed as the following:
Average power of the pulsed laser or mean power of the chopped laser is equal to the energy of the pulse multiplied by the repetition rate.
The units of the power are Watts (W).
For a CW laser the power density is the average output power in watts divided by the irradiated area in square centimeters:
The units of the power density are (W cm-2).
Peak power of the pulsed laser divided by the irradiated area gives the power density of the pulsed laser:
The units of the power density are (W cm-2).
Spot diameter: It is the diameter of the irradiated area on the target.
Using focusing lenses could change this area. The spot diameter (2W2)
is directly proportional to the focal length of the lens (F) and the laser wavelength
(λ) and is inversely proportional to the beam diameter (2W1):
The spot diameter is considered to be equal to the beam diameter when the lenses are not be used. The units of the spot diameter are usually centimeters.
LASER-TISSUE INTERACTION
When laser light strikes a tissue surface, it can be reflected and refracted,
scattered, absorbed or transmitted (Das, 1991). The fractional
intensity that goes into these different processes depends on the optical properties
of the tissue like it’s reflectivity, scattering and absorption coefficients,
particle size (Chopra and Chawla, 1992), as well as the
laser parameters like wavelength, energy, pulse duration, operation mode and
output spectral profile (Bedrym, 1997; Ashour,
2006). In medical laser applications, refraction plays a significant role
when irradiating transparent media like corneal tissue. In opaque media, usually,
the effect of refraction is difficult to measure due to the absorption and scattering
(Niemz, 1996). Laser light passing through the tissue
undergoes multiple scattering processes and is transformed from a narrow collimated
beam into a broad diffuse beam (Mosaad, 1997). Scattering
coefficient increases with the increase of the wavelength, thus, UV light is
scattered more than IR light (Niemz, 1996). All
the effects of light begin with the absorption of electromagnetic radiation
(Elliott, 1995). During absorption, the intensity of an
incident light is attenuated by passing through a medium due to a partial conversion
of light energy into heat motion or certain vibrations of molecules of the absorbing
material. The ability of a medium to absorb electromagnetic radiation depends
on a number of factors, mainly the electronic constitution of its atoms and
molecules, the wavelength of radiation, the thickness of the absorbing layer
and internal parameters such as temperature or concentration; Fig.
3 shows these processes.
Two laws are frequently applied; they describe the effect of either the thickness
or concentration on absorption, respectively. They are commonly called Lambert’s
law and Beer’s law (Beer-Lambert’s law) and are expressed by:
I (z) =Io exp (-α z)
I (z) =Io exp (-k c z)
|
where, z is the optical axis, I(z) is the intensity at a distance
z, Io is the incident intensity, α is the absorption coefficient
of the medium, c is the concentration of the absorbing agent and k depends on
the internal parameter other than concentration (extinction coefficient) (Niemz,
1996).
The most important optical property that decides the suitability of a laser
for a surgical procedure is the penetration depth of its radiation in the tissue.
It is equal to the inverse of the absorption coefficient (α) of the laser
radiation in the tissue and is defined as the depth at which the intensity of
the laser radiation reduces to 37% (drops to l/e) of its maximum value at the
surface of the tissue. The penetration depth changes significantly with the
wavelength of the laser radiation (Julia et al.,
1998). In the red portion of the spectrum and in near infrared region, the
penetration depth can be considerably greater. In spectral regions where the
absorption coefficient is relatively high, such as at 10.6 nm, the radiation
is absorbed in a thin layer near the surface (Mosaad, 1997;
Julia et al., 1998). In biological tissue, either
water molecules or macromolecules such as proteins and pigments mainly cause
absorption. The absorption of infrared light can be attributed to water molecules,
whereas UV and visible light absorbs by proteins and pigments (Julia
et al., 1998).
|
| Fig. 3: |
Reflected, refracted, scattered, absorbed or transmitted when
laser light strikes a tissue surface (Chopra and Chawla,
1992) |
The absorbed portion of the laser radiation can produce photochcmical and/or
Photothermal effects depending on the wavelength of the laser radiation and
nature of the tissue. It can produce fluorescence and this is used in dentistry
in diagnosis of initial dental carries that based on spot emission fluorescence
(Julia et al., 1998).
LASER INTERACTION MECHANISMS
The variety of interaction mechanisms may occur when applying laser light to
biological tissue due to specific tissue characteristics as well as laser parameters
(Julia et al., 1998; Khosravi
et al., 2008).
Wavelength dependent mechanisms
Photothermal interaction mechanisms: The most frequently used mechanism
of photon energy conversion in laser medicine is heating. Heating of irradiated
sample occurs with all methods of tissue destruction (coagulation, vaporization,
cutting, etc.) (Karu, 1999). Photons absorbed by the
tissue are thought to cause biological effect via nonspecific Photothermal effects
caused by kinetic mechanism, the external energy from the laser photons is deposited
into the target materials via transitional, rotational and vibrational modes
of movements of the target molecules. The rotational and vibrational modes of
movement which are in fact criteria of the temperature or Kinetic Energy (KE)
of the target molecules. The extracted energy from the incident light most efficient
when the frequency of the incident photons is close to the characteristic frequencies
of these modes (resonance absorption) (Bedrym, 1997; Fitzpatrick
and Goldman, 2000). When laser energy is converted into heat in the
tissue, thermal diffusion begins. Diffusion of heat through the tissue depends
on the thermal properties of the irradiated material. The thermal relaxation
(cooling) phenomenon is influenced by the thermal coefficient of the tissue,
the properties of the surrounding tissue or fluids and the temperature differential
between the irradiated and non irradiated tissue (Litvack
et al., 1988). However, depending on the duration and peak
value of the tissue temperature achieved, different effects like coagulation,
carbonization, vaporization and melting may be distinguished. For thermal decomposition
of tissues, it is important to adjust the duration of the laser pulse in order
to minimize thermal damage to adjacent structures. For laser pulse durations
τ<τthermal relaxation time , heat does not even diffuse
to the distance given by the optical penetration depth L. for τ>τthermal
relaxation time heat can diffuse to a multiple of the optical penetration
depth, i.e., thermal damage of tissue adjacent to the decomposed volume is possible
(Niemz, 1996). The microscopical and biochemical analysis
showed that as the temperature is raised, the large, specially configured molecules
necessary for life are shaken open. Most proteins, DNA, RNA, membranes and their
integral structures start to unwind or melt at temperatures ranging from 40-100°C,
the result is denaturation or loss of function (Niemz, 1996;
Fitzpatrick and Goldman, 2000).
The most important and significant tissue alterations are dependent on the
temperature of the tissue after absorption of the laser radiation, as follows:
| • |
At 37°C; no measurable effects are observed for the next
5°C above this |
| • |
The first mechanism by which tissue is thermally affected can be attributed
to conformational changes of molecules. These effects, accompanied by bond
destruction and membrane alterations are summarized in the single term hyperthermia
ranging from approximately 42-50°C. If such a hyperthermia lasts for
several minutes, a significant percentage of the tissue will already undergo
necrosis |
| • |
At 60°C, denaturation of proteins and collagen occurs which leads
to coagulation of tissue and necrosis of cells. The corresponding macroscopic
response is the visible paling of the tissue. Several treatment techniques
such as LITT aim at temperatures just above 60°C |
| • |
At higher temperatures (>80°C), the cell membrane permeability
is drastically increased, thereby destroying the otherwise maintained equilibrium
of chemical concentrations |
| • |
At 100°C, water molecules contained in most tissues start to vaporize.
Due to the large increase in volume during this phase transition, gas bubbles
are formed inducing mechanical ruptures and thermal decomposition of tissue
fragments |
| • |
At temperatures exceeding 150°C, carbonization takes place which is
observable by the blackening of an adjacent tissue and the escape of smoke |
| • |
Finally, melting may occur. The temperature must have reached a few hundred
degrees Celsius to melt the tooth substance which mainly consists of hydroxyapatite
crystals (a chemical compound of calcium and phosphate) (Niemz,
2004) |
Thermal effects of laser radiation are listed in Table 1.
The group of photochemical interaction mechanisms stems from empirical observations
that light can induce chemical effects and reactions within macromolecules or
tissues. Photochemical effects occur as a result of direct excitation of electronic
bonds by the laser energy (Litvack et al., 1988).
In general most of the molecules of the tissue have their bonding in the ultraviolet
frequency region (Bedrym, 1997). At shorter wavelengths,
tissue components become electronically excited, thus, this (photo excitation)
leads to rupture of molecular bonds and formation of molecular fragments (Litvack
et al., 1988). Photochemical reactions generally do not result in
a significant rise in temperature. Photochemical effects involved either a change
in the course of biochemical reaction due to the presence of an electromagnetic
field or photodecomposition due to high energy photons that rupture molecular
bonds (Das, 1991; Monajembashi et
al., 1986). Photochemical interaction mechanisms take place at very
low power densities (typically 1 W cm-2) and long exposure times
ranging from seconds to CW lasers . In most cases, wavelengths in the visible
range are used because of their high optical penetration depths. Several applications
of the photochemical interaction mechanisms have been used as in the following
sections (Niemz, 2004).
Photodynamic Therapy: The photodynamic therapy reaction is mediated
by exogenous chromospheres. At low light intensities, laser energy is absorbed
by exogenous chromospheres molecules called photosensitizes (photo acceptors).
In this case, the light is used for activation of molecules or drugs by a specific
wavelength of the laser light. The absorbing molecule can transfer the energy
to another molecule and this activated molecule can then cause chemical reactions
in the surrounding tissue.
The molecule may be transformed into toxic compound, often involving oxygen-free
radical that can cause cellular death through destruction of the DNA molecule
(Niemz, 2004; Khosravi et al.,
2008). This type of reaction is successfully used in the Photodynamic Therapy
(PDT) of tumor; where the photoabsorbing molecules are artificially introduced
into a tissue before irradiation. Irradiation of cells at certain wavelength
can also activate some of the native components (Sisecioglu
et al., 2011). In this way specific biochemical reactions as well
as whole cellular metabolism can be altered. This type of reaction is believed
to form the basis for low power laser effect.
Biostimulation: This process is also known as low energy, low light,
soft, cold laser and low intensity, low power therapy is the application of
monochromatic red light energy close to infrared wavelengths to stimulate growth
factor cells and improve wound/soft tissue healing. Low Laser Level Therapy
(LLLT) was pioneered in Europe and Russia in the early 1960’s (Monajembashi
et al., 1986). Biostimulation using light energy which is
usually considered a photochcmical effect, is a procedure that has attracted
interest in both the clinical and research arenas in both human medicine and
veterinary. To many scientists and clinicians, the idea is that low intensity
light energy can promote and upgrade metabolic processes that result in tissue
repair and pain relief which is unbelievable. Also in the area of injuries,
conditions are usually created preventing proliferation such as low oxygen concentration
or pH. The exposure to red or near infrared light might thus serve as a stimulus
to increase cell proliferation (Niemz, 2004). Reports
from almost every region of the world indicate that low intensity lasers promote
the repair process of skin, tendons, ligaments, bone and cartilage in experimental
animals as well as wounds from various etiologies in humans (Weesner,
1995).
Photoablation therapy: Photoablation was first discovered by Srinivasan
and Mayne-Banton in the year 1982. They identified it as ablative photodecomposition,
meaning that material is decomposed when exposed to high intense laser irradiation.
It occurs when the energetic photons of the laser light decomposes the molecules
by breaking the chemical bonds. In this interaction, photoablation is due to
the "volume stress" as a result of bond breaking. The removal of tissue is performed
in a very clean and exact fashion without any appearance of thermal damage such
as coagulation or vaporization. Photoablation takes place in the intensity range
of 104-1010 W cm-2 and interaction time in
the range of 10-3-10-10 sec. But the typical threshold
values of this type of interaction are 107-108 W cm-2
at laser pulse durations in the nanosecond range. The main advantages of this
ablation technique lie in the precision of the dental enamel etching process
and the lack of thermal damage to adjacent tissues (Niemz,
2004). Currently, most of the ablation work is done with UV excimer laser
(Beesly, 1978; Karu, 1999). When
the energetic photons of the laser light decompose the molecules by breaking
the bonds at the impart excess energy for ejection (Weesner,
1998; Hemachandran and Arumugam, 1983). Interaction
of light with biological tissue is seen in Fig. 4.
Wavelength independent mechanisms: When using power densities exceeding
1011 W cm-2 in solids and fluids or 1014 W
cm-2 in air, where the pulse duration is in picosecond or femtosecond
range, multiphoton ionization of atoms and molecules may occur a phenomenon
called optical breakdown occurs. The physical effects associated with optical
breakdown are plasma formation and shock wave generation .If breakdown occurs
inside soft tissues or fluids, cavitations and jet formation may additionally
take place. By means of plasma-induced ablation, very clean and well defined
removal of tissue without evidence of thermal or mechanical damage can be achieved
when choosing appropriate laser parameters (Gordon, 1966)
as shown in Fig. 5. Uncontrolled, the effect of the plasma
on the tissue surface can cause tissue damage (Featherstone
and Neslon, 1987).
|
| Fig. 4: |
Interaction of light with biological tissue (Wavelength dependent
mechanism) (Aboud, 2005) |
|
| Fig. 5: |
Interaction of light with biological tissue (Wavelenght independent
mechanism) (Aboud, 2005) |
The ultra short laser pulses with pulse durations shorter than 100 ps-each
of them having no thermal effect-may add up to a measurable increase in temperature
if it applied at repetition rates higher than about 10-20 Hz, depending on the
laser. The most important parameter of plasma-induced ablation is the local
electric field E which determines when optical breakdown is achieved. If E exceeds
a certain threshold value as in mode locked lasers, where the pulse duration
is in picosecond or femtosecond range, multiphoton ionization of atoms and molecules
may occur, optical breakdown is achieved (Wintner, 2001;
Stern, 1969). The important feature of optical breakdown
is that it renders possible an energy deposition not only in pigmented tissue
but also in nominally weakly absorbing media. This means that the interaction
does not depend on the wavelength (Wintner, 2001). During
photo disruption, the tissue is split by mechanical forces. Whereas plasma-induced
ablation is spatially confined to the breakdown region. For nanosecond pulses
optical breakdown is always associated with shock wave formation even at the
very high threshold. Since adjacent tissue can be damaged by disruptive forces,
the presence of these effects is often an undesired but associated symptom (Wintner,
2001). Picosecond or femtosecond pulses permit the generation of high peak
intensities with considerably lower pulse energies. With these extremely short
pulse durations, optical breakdown may still be achieved while significantly
reducing plasma energy and, thus, disruptive effects (Wintner,
2001). The important feature of optical breakdown is that it renders possible
an energy deposition not only in pigmented tissue but also in nominally weakly
absorbing media, This means that the interaction does not depend on the wavelength
(Stern, 1969). During photo disruption, the tissue is
split by mechanical forces. Whereas plasma-induced ablation is spatially confined
to the breakdown region. For nanosecond pulses optical breakdown is always associated
with shock wave formation even at the very high threshold. Since adjacent tissue
can be damaged by disruptive forces, the presence of these effects is often
an undesired but associated symptom (Stern, 1969). Picosecond
or femtosecond pulses permit the generation of high peak intensities with considerably
lower pulse energies. With these extremely short pulse durations, optical breakdown
may still be achieved while significantly reducing plasma energy and, thus,
disruptive effects (Wintner, 2001).
LASER HAZARDS AND SAFETY
Laser hazards effects: Laser radiation hazards must be identified and
evaluated. Types of laser hazards:
| • |
Eye: Acute exposure of the eye to lasers of certain wavelength
and power can cause corneal and retinal burns (or both). Chronic exposure
to excessive levels may cause corneal or lenticular opacities (cataracts)
or retinal injury |
| • |
Skin: Acute exposure to high levels of optical radiation may cause skin
burn; while carcinogenesis may occur for ultraviolet and near ultraviolet
wavelengths |
| • |
Chemical: Some lasers require hazardous or toxic substance operates (i.e.,
chemical dye) |
| • |
Electric shock: most lasers produce high voltage that can be lethal |
| • |
Fire hazards: The solvents used in dye lasers are flammable. High voltage
pulse or flash lamps may cause ignition. Direct beams may ignite flammable
materials or a specular reflection from high power Continues Wave (CW) infrared
lasers. |
| • |
Another hazard involves the potential inhalation of airborne biohazardous
materials that may be released as a result of the surgical application of
laser |
Inhaled airborne contaminants can be emitted in the form of smoke or plume
that generated through thermal interaction of surgical lasers with tissue (Miserendino
and Pick, 1995; Sisecioglu et al., 2011).
Laser plume evacuated device is used, to eliminate laser plume or smoke which
is irritant to the pulmonary tree because it is carrying particles of tissue
and microorganism. In addition, masks are necessary to use by the medical staff.
They act as filters to protect the pulmonary system from the possibilities of
an infection by microorganism. (Muncheryan, 1975; Al-Alawi,
2005). Laser and laser systems are grouped according to their capacity to
produce injury and specific controls are then described for each group. Lasers
manufactured after august 1976, are classified and labeled by the manufacturer.
Information on the label must include class, the maximum output power, the pulsed
duration (if pulsed) and laser medium or emitted wavelength (Ashour,
2006). Maximum Permissible Exposure (MPE): the level of laser radiation
to which person may be exposed without hazardous effect. or adverse biological
changes in the eye or skin.
Laser safety standards and hazard classification: This standard was
developed by the American National Standard Institute (ANSI) in year 1993. The
classification is based upon the beam output power or energy from the laser.
Basically, the classification is used to describe the capability of the laser
to produce injury to personnel. The higher the classification number, the greater
is the potential hazard (Niemz, 2004):
| • |
Class 1: Low-power lasers and laser systems that cannot
emit laser radiation levels greater than Maximum Permissible Exposure (MPE).
Class I CW lasers emitting at 400-550 nm and should have output power no
larger than 0.39 mW. Class one laser and laser system are incapable of causing
eye damage and therefore except from any control measures and considered
safe. No requirement for special safety measures |
| • |
Class 2: Visible low power lasers or laser systems and may be continuous
or pulsed that are incapable of causing eye damage unless they are viewed
directly for an extended period (greater than 1000 sec). the power output
is 1 mW or less. Eye protection is only special safety measures |
| • |
Class 3: Medium power lasers and laser systems capable of causing
eye damage with short-duration (<0.25 sec) exposures to the direct or
secularly reflected beam. Includes class 3a and 3b Lasers |
| • |
Class 3a: Lasers or laser systems that are normally would not produce
a hazard if viewed for only momentary period with unaided eye. They may
present hazard if viewed using collecting optics. It have power output limited
to 1-5 mW at wavelengths of 400-700 nm. Adequate eye protection must be
done |
| • |
Class 3b: Lasers have out power limits 5-500 mW at CW modes and
have energy density less than 10 J cm-2 at visible or invisible
laser light direct beam impact to the eye is always hazardous. This includes
intra beam viewing or specular reflections |
| • |
Class 4: Lasers have high out power and exceed of 500 mW at CW
mode and more than 10 J cm-2 for pulsed mode, this laser systems
capable of causing severe eye damage with short-duration (<0.25 sec).
Class 4 laser and laser systems are also capable of causing severe skin
damage and igniting flammable and combustible materials, as seen in Table
2. |
Laser safety recommendations and requirements
Precautions against electrical shock: Almost all laser systems operate
at voltages of dangerous levels (in Kilovolts). The following should be observed
before anyone attempts to work on the circuit:
| • |
The main switch should be turned off before any one handles
the circuit connections |
| • |
A jumper with an insulated handle should be used to discharge the capacitors
before any work is done on the circuits |
| • |
Any one should not work on the system unless he is familiar with the circuitry
and the proper safety precautions |
| • |
For maximum safety, all laser systems should have grounded outlets |
| • |
The circuitry should be covered up when no one is dealing with it (Muncheryan,
1975) |
Precautions against laser radiations
Eye protection: The human eye is the most vulnerable tissue to all types
of laser radiation .The tissue in the retina (the screen at the back of the
eyeball that receives the light or image) is susceptible to damage because the
lens concentrates and focuses the laser beam on the retina. The retina is surrounded
by a thin, dark-brown membrane containing arteries , veins and pigment cells,
it would easily absorb radiation .The retina is sensitive to all color wavelength
, from the 380 to 900 nanometer spectral range and to some degree to infrared
wavelengths beyond 900 nanometer (Muncheryan, 1975). Within
the retinal area, the most critical area for vision is the fovea.
| Table 2: |
Principle laser radiation hazards according to the laser class
(Estephan, 2007) |
 |
This area is about 1 mm in diameter and contains the highest density of cone
cells, resulting in the highest image resolution of the eye. Minimal damage
in the peripheral field of the retina may go undetected since the brain compensates,
up to a certain point, for small-area vision losses. The fovea is much more
susceptible to damage than the para-macular region of the retina. It is necessary
for the therapist to use a protective eye filter because of the back-scatter
qualities of the beam. Therefore, the eyes of everyone in the operating room
must be protected by safety goggles which should be procured for the specific
energy and wavelength of the beam under consideration (Catone
and Alling, 1997). For beam control and to minimize direct eye exposure
observe these precaution:
| • |
Do not intentionally look directly into the laser beam or
at a specular reflection, regardless of its power |
| • |
Terminate the beam path at the end of its useful path |
| • |
Locate the beam path at a point other than eye level when standing or
when sitting at a desk |
| • |
Orient the laser so that the beam is not directed toward entry doors or
aisles |
| • |
Minimize specular reflections |
| • |
Securely mount the laser system on a stable flat room to maintain the
beam in a fixed position during operation and limit beam traverse during
adjustment. |
| • |
Confined primary beams and dangerous reflections to the optical table |
| • |
Clearly identify beam paths and ensure that they don’t cross populated
area of traffic path |
| • |
When the beam path is not totally enclosed, locate the laser system so
that the beam will be outside the normal eye-level range which is 1.2 to
2 m from the floor. A beam path that exits from controlled area must be
enclosed where the beam irradiants exceeds the MPE |
| • |
Warning signs should be placed on the doors at the entrance to the operating
room |
kin protection: High power laser can inflect skin burns and the effect
of laser radiation of the skin depends on both the wavelength and the pigmentation
of the skin. In visible spectrum range, the skin can reflect much of then while
in the infrared region the skin become highly absorbing. The laser injury to
the skin may be much less serious than to the eye (Moseley,
1988). Skin of the face is also considered in this precaution by placing
a towel over the patient face. The working in the oral cavity, the lips and
nose also be protected teeth are covered with saline-soaked sponges to avoid
enamel teeth burns (Nicholas et al., 1995).
CONCLUSION
From this study, it is concluded that laser is an useful tool in many medical applications if used properly, with taking in consideration the good understanding of laser principle and interaction between different laser parameters with biological tissues. This will enhance better operators performances with different laser treatments. It is suggested that more researches on laser applications in medical fields are needed because laser is still a promising tool.
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