Applications of Laser in Medicine A thesis Submitted in partial Fulfillment for the requirements for the Degree of Master of Science in physics By: Nuha Hassan Kinash Supervisor By: Dr. Omer I. Eid
University of Khartoum College of Post Graduate Studies
2008
Acknowledgement First of all I should offer my thanks obedience and gratitude to Allah. Most Gracious, most merciful from whom I receive guidance and help. I would also like to extend my sincere thanks and appreciation to my great supervisor Dr. Omer Eid for his valuable guidance assistance and help that enable me to complete my research project. Very special thanks to the Red Sea University, to all my friends and to Sudan Institute for Natural Sciences family. Last but not least, special warm thanks to my parents, the rest of my family and my uncle Suleiman for their great encouragement and support.
i
Dedication
I dedicate this thesis to my parents, My family And Whom love me
ii
Table of Contents
Serial
Items
No. Acknowledgement
i
Dedication
ii
Table of Contents
iii-v
Abstract (English)
vi
Abstract (Arabic)
vii
Introduction
1
Chapter One
3-11
Basic Concepts 1.1.
Laser Power
3
1.1.1.
Power Density
3
1.1.2.
Fluence
4
Mode of Operation
5
1.2.1.
Continuous Wave Operation
5
1.2.2.
Pulse Operation
5
1.2.2.1.
Q- Switched
5
1.2.2.2.
Modelocking
6
Laser-Tissue Interaction
6
Reflection
6
1.2.
1.3. 1.3.1.
iii
1.3.2.
Scattering
6
1.3.3.
Transmission
7
1.3.4.
Absorption
7
Chapter Two
12-40
Medical Lasers with Applications 2.1.
Solid-state laser
12
2.1.1.
The Ruby laser
12
2.1.2.
The Neodymium: Yag Laser
13
Gas Laser
15
2.2.1.
The Helium-Neon Laser
16
2.2.2.
The Argon Ion Laser
18
2.2.3.
The Carbon Dioxide Laser
20
2.3.
Liquid Laser
23
2.4.
Properties of Medical Laser
26
2.4.1.
The Ruby Laser
26
2.4.2.
The Neodymium : Yag Laser
27
2.4.3.
The Helium-Neon Laser
28
2.4.4.
The Argon Ion Laser
28
2.4.5.
The Carbon Dioxide Laser
29
2.4.6.
The Dye Laser
30
Clinical Laser Applications
32
2.5.1.
Laser Surgery
32
2.5.2.
Gynecology
33
2.5.3.
Urology
34
2.5.4.
General Surgery
34
2.5.5.
Dermatology
36
2.5.6.
Cardiology
37
2.2.
2.5.
iv
2.5.7.
Ophthalmology
37
2.5.8..
Gastroenterology
39
Chapter Three
41-45
A field Study Chapter Four
46-47
Conclusion References
48-49
Appendix A
50
Appendix B
51
Appendix C
52-60
v
Abstract This work gives an idea about the originality of laser beams and how we benefit from them in the treatment and the recovery of patients. Some of the important medical lasers are Neodymium-YAG Laser (Nd: YAG) is used in laparoscopic surgery, the Carbon Dioxide Laser (CO2) is one of the most widely used in surgical operation specially in gynecologic surgery. Moreover it’s used as scalpel, the Argon Laser (Ar+) is used for ophthalmology and the Dye Laser is recently used in the treatment of cancer tumour. We know that Argon and Dye Lasers beams produce intense visible light that can be seen by the naked eye. For this reason they can be easily controlled and directed by the surgeons’ and physician. But beams of Carbon Dioxide and YAG Lasers are invisible and they are located in the Zone of the infrared region. For this reason we use the beams of HeliumNeon lasers that have red colour with energy very low. So we use these lasers simultaneously with the above mentioned lasers beams to allow the operator to adjust spot size and operation distances efficiently. This enables the surgeons and physicians to accomplish more serious operations and to obtain more satisfactory results. From the field study of the laser instruments used in ophthalmology hospitals in Khartoum State it’s clear that the laser instruments used there are, Argon Laser, Yag Laser and Excimer Laser Instruments. vi
ﺍﳋﻼﺻـــﺔ ﻫﺬﻩ ﺍﻻﻃﺮﻭﺣﺔ ﺗﻌﻄﻲ ﻓﻜﺮﺓ ﻋﻦ ﺃﺷﻌﺔ ﺍﻟﻠﻴﺰﺭ ﻭﻛﻴﻔﻴﺔ ﺍﻻﺳﺘﻔﺎﺩﺓ ﻣﻨﻬﺎ ﰲ ﻋﻼﺝ ﻭﺷﻔﺎﺀ ﺍﳌﺮﺿﻰ. ﻭﻣﻦ ﺃﻫﻢ ﺍﻟﻠﻴﺰﺭﺍﺕ ﺍﻟﻄﺒﻴﺔ ﺍﳌﺴﺘﺨﺪﻣﺔ ﻟﻴﺰﺭ ﺍﻟﻨﻴﻮﺩﻳﻮﻡ ﻳﺎﻙ) (Nd: YAGﺍﻟﺬﻱ ﻳﺴﺘﺨﺪﻡ ﰲ ﺟﺮﺍﺣﺔ ﺍﳌﻨﺎﻇﲑ .ﻭﻟﻴﺰﺭ ﺛﺎﱐ ﺍﻛﺴﻴﺪ ﺍﻟﻜﺮﺑﻮﻥ) (CO2ﺍﻟﺬﻱ ﻳﺴﺘﺨﺪﻡ ﺑﺼﻮﺭﺓ ﻭﺍﺳﻌﺔ ﰲ ﺍﻟﻌﻤﻠﻴﺎﺕ ﺍﳉﺮﺍﺣﻴﺔ ﺧﺎﺻﺔ ﰲ ﺟﺮﺍﺣﺔ ﺍﻻﻣﺮﺍﺽ ﺍﻟﻨﺴﺎﺋﻴﺔ .ﺑﺎﻻﺿﺎﻓﺔ ﺍﱃ ﺍﻧﻪ ﻳﺴﺘﺨﺪﻡ ﻛﻤﺸﺮﻁ .ﻭﻟﻴﺰﺭ ﺍﻻﺭﻛﻮﻥ) (Ar+ﺍﻟﺬﻱ ﻳﺴﺘﺨﺪﻡ ﰲ ﻃﺐ ﺍﻟﻌﻴﻮﻥ .ﻭﻟﻴﺰﺭ ﺍﻟﺼﺒﻐﺔ ) (Dye laserﺍﻟﺬﻱ ﰎ ﺍﺳﺘﺨﺪﺍﻣﻪ ﺣﺪﻳﺜﺎ ﰲ ﻋﻼﺝ ﺍﻻﻭﺭﺍﻡ ﺍﻟﺴﺮﻃﺎﻧﻴﺔ. ﻭﳒﺪ ﺃﻥ ﺃﺷﻌﺔ ﻟﻴﺰﺭﻱ ﺍﻻﺭﻛﻮﻥ ﻭﺍﻟﺼﺒﻐﺔ ،ﺃﺷﻌﺔ ﺿﻮﺋﻴﺔ ﻣﺮﺋﻴﺔ ﻭﳝﻜﻦ ﺭﺅﻳﺘﻬﻤﺎ ﺑﺎﻟﻌﲔ ﻭﻟﺬﻟﻚ ﳝﻜﻦ ﺗﻮﺟﻴﻬﻬﺎ ﻭﺍﻟﺴﻴﻄﺮﺓ ﻋﻠﻴﻬﺎ .ﺃﻣﺎ ﺃﺷﻌﺔ ﻟﻴﺰﺭﻱ ﺛﺎﱐ ﺃﻛﺴﻴﺪ ﺍﻟﻜﺮﺑﻮﻥ ﻭﺍﻟﻴﺎﻙ ،ﻓﺈﻤﺎ ﺃﺷﻌﺔ ﻏﲑ ﻣﺮﺋﻴﺔ ﺗﻘﻊ ﺿﻤﻦ ﻣﺪﻯ ﺍﻻﺷﻌﺔ ﲢﺖ ﺍﳊﻤﺮﺍﺀ .ﻟﺬﻟﻚ ﻳﺴﺘﺨﺪﻡ ﺷﻌﺎﻉ ﻟﻴﺰﺭ ﺍﳍﻠﻴﻮﻡ ﻧﻴﻮﻥ ﺫﻭ ﺍﻟﻠﻮﻥ ﺍﻻﲪﺮ ﻭﺍﻟﻄﺎﻗﺔ ﺍﻟﻘﻠﻴﻠﺔ ﺟﺪﺍﹰ ،ﺑﺼﻮﺭﺓ ﻣﺰﺩﻭﺟﺔ ،ﻣﻊ ﺷﻌﺎﻉ ﺍﻟﻠﻴﺰﺭﻳﻦ ﺍﻟﺴﺎﺑﻘﲔ ،ﻻﻋﻄﺎﺀ ﺍﳉﺮﺍﺡ ﺍﻻﻣﻜﺎﻧﻴﺔ ﺑﺘﻮﺟﻴﻪ ﺍﻟﺸﻌﺎﻉ ﻋﻠﻰ ﺍﳌﻜﺎﻥ ﺍﳌﻄﻠﻮﺏ ﻭﺑﺪﻗﺔ. ﻭﻫﺬﺍ ﻳﻤﻜﻦ ﺍﻻﻃﺒﺎﺀ ﻭﺍﳉﺮﺍﺣﲔ ﺃﻥ ﻳﻘﻮﻣﻮﺍ ﺑﺎﺩﺍﺀ ﻋﻤﻠﻴﺎﺕ ﺃﻛﺜﺮ ﺧﻄﻮﺭﺓ ﻭﻳﺘﺤﺼﻠﻮﺍ ﻋﻠﻰ ﻧﺘﺎﺋﺞ ﻣﺮﺿﻴﺔ. ﺇﺗﻀﺢ ﻣﻦ ﺧﻼﻝ ﺍﻟﺪﺭﺍﺳﻪ ﺍﳌﻴﺪﺍﻧﻴﺔ ﺍﻟﱵ ﺍﺟﺮﻳﺖ ﰲ ﻣﺴﺘﺸﻔﻴﺎﺕ ﺍﻟﻌﻴﻮﻥ ﺑﻮﻻﻳﺔ ﺍﳋﺮﻃﻮﻡ .ﺃﻥ ﺃﺟﻬﺰﺓ ﺍﻟﻠﻴﺰﺭ ﺍﳌﺴﺘﺨﺪﻣﺔ
ﻫﻲ
ﻟﻴﺰﺭ
ﺍﻻﺭﻛﻮﻥ
ﻭ
vii
ﻟﻴﺰﺭ
ﺍﻟﻴﺎﻙ
ﻭ
ﻟﻴﺰﺭ
ﺍﻻﻛﺴﺎﳝﺮ.
Introduction The word laser is an acronym for Light Amplification by the Stimulated Emission of Radiation. Laser light today covers a wide range of wavelengths, which includes the visible range of the electromagnetic spectrum (1). (See Fig.1) Light Amplification by the Stimulated Emission of Radiation was originally described as a theoretical concept by Albert Einstein in 1917, but it was not until 1954 that the first "stimulated" emissions of microwave radiation (Maser) were generated by J.P Gorden and C.H. Townes at Bell Laboratories. Theoretical calculations for the construction of a visible light Maser or Laser were published in 1958. The first Laser was built in 1960 by T.H. Maiman. His laser consisted of a pink ruby rod with silvered ends for mirrors inserted in a helical coil of a photographic flash lamp, which generated millisecond pulses of coherent 694nm Ruby Laser (red) light (2). Today lasers have entered almost all fields of science and have made a wide step of progress in many of them. Laser has become the modern technique of the 20th century.
In medicine, it became the "beam that
heals" and has been utilized in nearly every discipline of medicine, in diagnosis, therapy, surgery and medical instrumentation. In order to realize the full advantages of using a laser beam, one should understand the process of interaction between the beams one uses and the spot one needs to treat. One should know something about beam properties, its origin and the requirements needed to provide such a beam; also one should know how to handle the laser system and how to work safely, with an awareness or all possible hazards and, therefore, taking all necessary precautions to avoid harm to oneself and damage to treated area (1). 1
The purpose of this thesis is to provide such information in a simple form and also to describe several types of lasers that find interest in the different fields of medicine, together with some applications.
Fig.1: The Electromagnetic Spectrum.
2
Chapter One Basic Concepts In this chapter we give certain introductory concepts. 1.1. Laser Power 1.1.1. Power Density: The rate of laser energy delivery is called power and is measured in watts. The wattage is equal to the amount of energy, measured in joules, divided by the duration of exposure, measured in seconds. Watts (W) =
Joules(J) Seconds (s)
(1.1)
An important factor in the effective application of the laser is a concept of power density, or irradiance. Power density is defined as the amount of power that is concentrated into a spot, or watts/cm 2. Power density =
Watts Spot size (cm2 )
(1.2)
The spot size of a laser beam depends on several variables, including the focal length of the lens, the wavelength of the laser, and the transverse electromagnetic mode of the beam. • The focal length of the lens determines the size of the beam spot. A lens with a short focal length can provide a smaller spot size, thus increasing the intensity of the beam. A CO2 laser with a 50-mm lens can produce a spot size of 0. I mm, whereas a 400-mm lens can produce a spot size of 0.8 mm. Thus the shorter 50-mm lens can concentrate the power in a smaller spot area. • The wavelength of the laser also limits the spot size and beam focusing. When all other factors are equal, shorter wavelengths can 3
generate smaller spots. Thus, an argon laser at 488/514 nm can produce a much smaller spot than the CO2 laser can at 10,600 nm. The choice of laser should be based on the specific effects of the laser on the tissue rather than based on its spot-size capabilities. • The
transverse
electromagnetic
mode
(TEM)
determines
the precision of the spot by the power distribution over the spot area. A
TEM01
mode
is
an
example
of
a
common
multimode distribution, meaning that the spot has a cool area in the center of the beam. The most common and fundamental mode is the TEM00, which produces an even power distribution over the spot, with most of the power concentrated in the center and the rest decreasing in intensity towards the periphery of the beam. The spot size of a TEM00 beam is the region that has approximately 86% of the total beam power (3). 1.1.2. Fluence: Fluence is one of the most important and critical concepts that affect precision during laser surgery. It involves three properties: watts, time, and spot size (or area). The tissue effect will vary if any of these parameters are changed. Fluence =
Watts × Time Spot size (cm2 )
(1.3)
A laser beam can impact tissue at 100 W for I second, thereby delivering 100 J to tissue. Another beam can impact tissue at 1 W for 100 seconds, thereby also delivering 100 J to tissue. The difference in tissue effect is that more adjacent tissue damage will occur with the longer duration of impact because of the laser’s tissue-heating effects. Using the
4
highest appropriate wattage for the shortest time minimizes any damage to adjacent healthy tissue (3). 1.2. Mode of Operation The output of a laser may be a continuous constant-amplitude output (known as CW or continuous wave); or pulsed, by using the techniques of Q-switching, and Modelocking. 1.2.1. Continuous Wave Operation: In the continuous wave (CW) mode of operation, the output of a laser is relatively consistent with respect to time. The population inversion required for lasing is continually maintained by a steady pump source (4). 1.2.2. Pulsed Operation: In the pulsed mode of operation, the output of a laser varies with respect to time, typically taking the form of alternating 'on' and 'off' periods. In many applications one aims to deposit as much energy as possible at a given place in as short time as possible (4). 1.2.2.1. Q-switching: Extremely high power levels can be obtained by using a technique Known as q-switching that momentarily stores excess energy. A shutter is placed in the optical path to prevent laser emission until a very large population inversion has built up. When the shutter is opened the electrons is rapidly fall to the ground state releasing a tremendous pulse of energy that lasts only a few nanoseconds. Powers in the megawatt range can be produced by this technique. Q-switching is commonly used with ruby and neodymium solid laser (5).
5
1.2.2.2. Modelocking: A modelocked laser emits extremely short pulses on the order of tens of picoseconds down to less than 10 femtoseconds. These pulses are typically separated by the time that a pulse takes to complete one round trip in the resonator cavity (4). 1.3. Laser-Tissue Interaction When laser energy is delivered to tissue four specific interactions can occur: reflection, scattering, transmission or absorption. The extent of the interaction depends on the wavelength of the laser, fluence, and tissue types (3). 1.3.1. Reflection: The laser beam has no effect on the target when it is reflected off the impact site (Fig. 1.1), but it can cause harm where the beam eventually hits (3).
Fig. 1.1: Reflection 1.3.2. Scattering: The distribution of the laser light energy within the tissue can be altered when the beam is scattered through the tissue (Fig. 1.2). If the scattered energy is ultimately absorbed, then it will be converted to heat. The laser beam can also backscatter, causing potential hazard (3).
6
Fig.1.2: Scattering. 1.3.3. Transmission: Some laser wavelengths can be transmitted through certain tissue but have little or no thermal effect (Fig. 1.3) (3).
Fig.1.3: Transmission. 1.3.4. Absorption: As the tissue absorbs the laser energy, heat energy is produced and tissue damage occurs (Fig. 1.4) (3).
Fig.1.4: Absorption. Each tissue has specific absorption characteristics base on its composition and chromophore content. The principal chromophores present in mammalian tissue are: • Hemoglobin 7
• Melanin • Water • Protein Infrared light is absorbed primarily by water,
while visible and
ultraviolet light are primarily absorbed by hemoglobin and melanin, respectively. As wavelength decreases toward the violet and ultraviolet, scatter or absorption from covalent bands in protein limits penetration depth in the range (Fig.1.5) (6).
Fig. 1.5: Absorption of the Main Chromophore. When laser light strikes tissue and absorption takes place, the cellular water is super-heated to over 100° C. Intracellular protein is destroyed as the heat continues to build. The water inside the cell then turns to steam. Since 1g of steam occupies more space than does 1 g of water, the cellular membrane bursts under this extreme pressure. Debris and smoke (laser plume) is spewed from the tissue. Adjacent tissue is warmed by the intense heat produced it the point of incidence. The degree of thermal damage depends upon the temperature to which the laser energy heats the tissue. Table 1.1 notes the changes that occur as the laser beam is absorbed. The mechanism of thermal laser surgery is illustrated in Figure 1.6. At the immediate laser- tissue impact 8
site there is a zone of vaporization. Immediately adjacent to this site is a zone of necrosis caused by the thermal spread. Farther from the impact site is a zone of coagulation as thermal injury decreases (Fig. 1.7). The depth of penetration of the laser beam depends upon the laser wavelength, color and consistency of the tissue, power of the beam, duration of beam exposure, and beam spot size. As the laser beam cuts through tissue, it will continue to heat and destroy deeper tissues. If there is concern about accidental damage to adjacent tissues, a backstop may be used. Materials such as wet sponges, quartz rods, or titanium rods can be used as back-stops (3). Table 1.1: Tissue Changes with Temperature Increases Temperature
Visual change
Biological change
37-60° C
No visual change
Warming, welding
60-65° C
Blanching
Coagulation
65-90° C
White/grey
Protein denaturization
90-100° C
Puckering
Drying
100° C
Smoke plume
Vaporization, carbonization
9
Fig. 1.6: Absorption of Laser Energy in Tissue.
Fig. 1.7: Thermal Zones after Laser Impact. 10
Laser surgery can be divided into three broad categories regarding tissue
response:
Thermal,
mechanical,
and
chemical
effects.
Approximately 85% of lasers used today produce a thermal effect at the tissue level. These lasers cut, coagulate, vaporize, and ablate tissue from the interaction site where the thermal response originates. The mechanical effect on tissue is produced by some lasers as the laser beam generates sonic energy that mechanically disrupts tissue. Breaking apart kidney stones in the ureter or disrupting the posterior capsule within the eye are examples of this type of mechanical effect. The chemical effect of some lasers is produced as the laser energy activates light-sensitive drugs to disrupt and change tissue. This process is used in photodynamic therapy to selectively destroy malignant cells (3).
11
Chapter Two Medical Lasers with Applications The majority of lasers in use today fall into three categories. These are: -Solid state lasers. -Gas lasers (atomic, ionic, molecular lasers). -Liquid lasers (dye laser). Laser of popular use in medicine are the Neodymium: YAG (solid state) laser, the argan ion Ar+ (ionic gas) laser, the carbon dioxide CO2 (molecular gas) and recently the dye (liquid) laser. In this chapter, we will demonstrate, for each type, the system operation and the characteristics of its output. We will also give some medical applications for each & clinical laser applications. 2.1. Solid-State Laser 2.1.1. Ruby Laser: A ruby laser was the first type of laser invented and was first operated by T. H. Maiman (2). 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 or reflect 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 lamp is spiraled around the ruby cylinder to provide a flash of white light (Fig.2.1). The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. Upon returning to their normal 12
state, the electrons emit their characteristic ruby-red light. The mirrors reflect some of this light back and forth inside the ruby crystal, stimulating other excited chromium atoms to produce more red light, until the light pulse builds up to high power and drains the energy stored in the crystal. The optically pumped, solid-state laser uses sapphire as the host lattice and chromium as the active ion. Ruby laser is three level solid-state lasers. It produces pulses of visible light at wavelength of 694.3nm (7, 8).
Fig. 2.1: Schematic Diagram of Ruby Laser. 2.1.2. The Neodymium: YAG Laser: These are the most popular types of solid-state laser. The host medium is often acrystal of Y3AL5O12 (called Yag, an acronym for yttrium aluminum garnet) in which some of the Y3+ ions are replaced by Nd3+ ions (9). The Nd3+ doped YAG is usually in the shape of a rod roughly of the size of a short pencil with polished and gold-coated ends to serve as resonator mirrors. The medium is pumped optically by using a lamp wrapped helically around the rod (Fig.2.1). Pulsed operation is the modest pumping of this type of laser as it follows four-level pumping scheme. Continuous operation is also performed. Both types of operation are achieved by using the appropriate lamps. 13
A simplified energy level diagram of the Nd3+ ion is shown in Figure 2.2. The pump lamp energy, in bands near 530nm, 580nm and 750nm, is absorbed by Nd3+ ion and excites the ion to levels corresponding to level (3) in the Figure. These ions then decay fast through non-radiative process to the upper laser level (4 F3/2) which corresponds to level (2) in the pumping scheme and lase at 1.06 µm to the terminal laser level (I11/2) which correspond to level (1) which lies above the ground state of the ion ( level 0). Further non-radiative decay from the lower laser level (1) to the ground state (0) of the neodymium ion completes the lasing cycle. Laser output power obtained for CW operation ranges between 15150W and for pulsed type ranges between 400-104W for duration of 1 s to 5 ms. In Q-switched operation peak pulses of 104 kW are obtained; more output power is gained when the laser is mode-locked, a power in the range of gW with pulse duration of pS range can be delivered. Nd: YAG laser is regarded an efficient laser. Its output efficiency range is 1-3 %. Furthermore, frequency-doubled, Nd: YAG laser finds interesting applications in medicine. The standard 1064 nm laser will reduce to a laser beam of 523 nm after passing through a frequency-doubiasc crystal (KTP); such visible beams of power up to 10 W have been used in the treatment pigmented tissue (1).
14
Fig. 2.2: Simplified Energy Level Diagram for the Neodymium ion (Nd3+) in YAG Showing the Principle Laser Transitions. 2.2. Gas Laser There are basically three distinct families of gas laser, divided according to the nature of the lasing species. These include the neutral gas laser (typified by the helium-neon laser), the ionized gas laser (typified by organ), and the molecular gas laser (typified by carbon dioxide). The widely used positive argon ion laser Ar+ in medicine falls with this group. These three types of laser will be described in some details in the following section.
15
2.2.1. The
Helium-Neon Laser:
A helium-neon laser was the first gas laser to be invented by Ali Gavan (17). It usual operation wavelength is 632.8nm, in the red portion of the visible spectrum. The active medium is a mixture of helium and neon gases. The energy or pump source of the laser is provided by an electrical discharge of around 1000 through an anode and cathode at each end of the glass tube. A current of 5 to 100 mA is typical for CW operation. The optical cavity of the laser typically consists of a plane, high-reflecting mirror at one end of the laser tube, and a concave output coupler mirror of approximately 1% transmission at the other end (Fig.2.3).
Fig. 2.3: Schematic Diagram of a Helium-Neon Laser. A simplified energy level diagram of helium-neon atoms is shown Figure 2.4. The laser process in a helium-neon laser starts with collision of electrons from the electrical discharge with the helium atoms in the gas. This excites helium from the ground state to the 23S1 and 21S0 long-lived, metastable excited states. Collision of the excited helium atoms with the ground-state neon atoms results in transfer of energy to the neon atoms,
16
exciting neon electrons into the 3s2 level. This is due to a coincidence of energy levels between the helium and neon atoms. (Fig.2.4). This process is given by the reaction equation: He (21S)* + Ne + ∆E → He (11S) + Ne3s2* Where (*) represents an excited state, and ∆E is the small energy difference between the energy states of the two atoms. The number of neon atoms entering the excited states builds up as further collisions between helium and neon atoms occur, causing a population inversion. Spontaneous and stimulated emission between the 3s2 and 2p4 states results in emission of 632.82 nm wavelength light, the typical operating wavelength of a helium-neon laser. After this, fast radiative decay occurs from the 2p to the 1s ground state. With the correct selection of cavity mirrors, other wavelengths of laser emission of the helium-neon laser are possible. There are infrared transitions at 3.39 µm and 1.15 µm wavelengths, and a variety of visible transitions, including a green (543.5 nm), a yellow (594 nm) and an orange (612 nm) transition. The typical 633 nm wavelength red output of a helium-neon laser actually has a much lower gain compared to other wavelengths such as the 1.15 µm and 3.39 µm lines, but these can be suppressed by choosing cavity mirrors with optical coatings that reflect only the desired wavelengths (10,11)
. The laser output power, usually CW varies from a fraction of 1mW
and up to about 50 mW. The efficiency of this laser is typically low, usually less than 0.1% (1).
17
Fig. 2.4: Simplified energy Level Diagram of Helium and Neon Atoms. 2.2.2. The Argon Ion Laser: The argon laser was invented in 1964(1) by William Bridge and is one of the classes of noble-gas ion lasers that operate in the visible and ultraviolet spectral regions. Historical these lasers have been known as ion lasers (12). The discharge tube is designed to resist high heat although it is water-cooled. An axial magnetic field is applied to keep the discharge from the tube walls. The tube ends are sealed by quartz windows attached at Brewster angle. Suitable mirrors are used for the optical resonator (Figure 2.5). The dimension of the tube and the operating voltage depend on the aimed laser output power. For example, a 5 W output power laser system (the device usual used in ophthalmology) has a tube of about 0.8 m length 18
and few millimeters bore diameter and needs about 10 kW input power, also water cooling.
Fig. 2.5: Construction of A typical Argon Ion Laser. The energy level diagram of argon-ion laser is shown at Figure 2.6. Population of the 4p upper level of the laser transition occurs by three clear processes, they are: (a) Collision of argon electrons with argon ions in their ground state (indicated by the route 1). (b) Collision of argon electrons with ions in the metastable levels (indicated by the route 2). (c) Radiative cascade from higher levels (indicated by the route 3) (13). The laser output power, usually CW, can be as high as 200 W. Commercial Ar+-lasers of output power as high as 20 W are available with an efficiency about 0.1% (1).
19
Fig. 2.6:Energy Level Diagram for Argon Ion Laser 2.2.3. The Carbon Dioxide Lasers: The carbon dioxide laser is one of the most powerful and efficient lasers available. It operates in the middle infrared on rotational-vibrational transitions in the 10.6 µm and 9.4 µm wavelength regions. Both pulsed and CW laser output occur in several different types of gas discharge configurations in mixture of carbon dioxide, nitrogen, and helium gases, typically with a CO2: N2 ratio of about 0.8:1 and with somewhat more helium than N2 (12). The carbon dioxide molecule is a linear molecule, with the three atom arranged in a straight line with the carbon atom in the middle. There are three different types of vibration that can occur in the molecule. These vibrations are illustrated schematically in Figure 2.7. In the first mode of 20
vibration, the carbon atom remains stationary and the oxygen atoms move in opposite directions along the line of symmetry, as indicated by the arrows. In the second mode of vibration, which is a bending motion, all the atoms move in a plane perpendicular to the line of symmetry, the carbon atom move in one direction, while the oxygen atoms move in opposite direction. The third mode is an asymmetric mode in which all the atoms move on the same line. At any time, the carbon atom is moving in a direction opposite to the oxygen atoms (14).
Fig. 2.7: Schematic Diagram of the Vibrations of the Carbon Dioxide Molecule. Each possible quantum state is labeled as follows: for the symmetric mode by 100,200,300 etc, for the bending mode by 010,020,030 etc and for the asymmetric mode by 001,003,003 etc. combinations of the three modes e.g. 342 are also possible (17). Figure 2.8 presents a partial energy level diagram of CO2 molecule. It shows the vibrational levels, with their corresponding rotational sublevels. These belong to the ground electronic state. The three vibrational modes are present and take part in the lasing process. CO2laser is electrically pumped and the discharge electrons can accomplish the population of the laser upper level 001. This is done in a similar way to 21
that used in He: Ne laser. Nitrogen molecules are added to support the excitation process. They absorb energy by collision with discharge electrons and transfer it to the CO2 molecule by collision, just as the helium atom transfers its energy to the neon atom in He: Ne laser. Two laser transitions occur at 10.6 and 9.6 µm as the excited molecules decay to the lower levels 100 and 020 levels respectively. From these two lower levels the molecule decay spontaneously to the ground state 000. CO2 lasers have relatively high efficiency, 15-20%, and are regarded the most energy–efficient laser devices that exist so far. They can generate very high average power. Commercial CO2-laser with average power greater than 20 kW are available. Output power of 80 kW can be obtained. CO2 laser is important in medical research application (1).
Fig. 2.8: Partial Energy Level Diagram for the CO2 Molecule. Each Vibrational Level is Associated with its Rotational Levels.
22
2.3. Liquid Lasers 2.3.1. The Dye Laser: Dye lasers use an active medium consisting of a solution of an organic dye in a liquid solvent such at ethyl or methyl alcohol. Or water. It is pumped optically either by any other laser or by a flash lamp (9). One of the most important features that dye lasers offer is tunability. The monochromatic output of available dye lasers can be tuned over abroad range, from ultraviolet to the near infrared. To introduce a specific dye material, let us consider the dye rhodamine 6G, one of the important laser dye materials. It has several benzene rings and the chemical formula C26H27N2O3Cl, with a molecular weight around 450. It is soluble in methyl and ethyl alcohols and in other organic solvents. It can be used to color silk or paper pink. Its absorption and emission spectra are shown in Figure 2.9.
Fig. 2.9: Absorption and Emission Spectra of rhodamine 6G 23
Atypical energy level diagram for a dye material is shown in Figure 2.10. Initially, the entire population of molecules is concentrated at the bottom of the ground level S 0 . When the dye is irradiated with light of wavelength corresponding to the energy difference between S 0 and S1 , some of the ground state molecules are raised to S1 . These levels are shown as broad energy states containing many vibrational and rotational sublevels, so as to form a continuous band. Thus the absorption and emission spectra are broad. When a dye molecule is raised to a position in S1 by the pump light, it relaxes in a very short time to the lowest sublevels
of S1 . This relaxation occurs by nonradiative transitions, which produce heat. The upper levels of S 0 are initially empty. Thus, one obtains a population inversion between the lower sublevels of S1 and the upper sublevels of S 0 .These states are radiatively coupled; that is, the transition from S1 to S 0 occurs with emission of fluorescent light. Because of the population inversion, gain by stimulated emission is possible, and laser action can occur. Because the laser transition occurs between the bottom of S1 and sublevel nears the top of S 0 , the laser light is at a wavelength longer
than the pump light. Because many sublevels near the top of S 0 are empty, the laser light may be emitted over a range of wavelengths. This fact allows the possibility of tuning the output of dye lasers. There is a competing process that reduces the number of dye molecules available for laser operation. Additional states, called triplet states and denoted in the Fig. (2.10) as T1 and T2 , are present. The states S 0 and S1 , which provide laser operation, are part of a different manifold of
states, called singlet states. When dye molecules are excited to S1 some of 24
them make a transition to T1 via collisions. The molecules that make this transition tend to be trapped because they remain in T1 for a relatively long time. The decay from T1 to S 0 proceeds slowly. As the result, T1 acquires a large population. This reduces the number of molecules available for laser operation. The situation is made worse by the presence of T2 . Absorptive transitions at the laser wavelength can take place between T1 and T2 . It often happens that such triplet transitions occur at the same wavelength as the laser operation. Thus, as T1 becomes populated, the medium becomes absorbing. This effectively shuts off the laser operation. The populating of T1 occurs rapidly, within less than a few microseconds .This means that dye
lasers require the use of very fast pumping sources to provide a short pulse of laser light before the triplet states become populated (14).
25
T2
S 1
Absorption
Decay Collisions
Pump
Laser
T1 Slow Decay
S0 Fig. 2.10: Energy levels relevant to Operation of Dye Lasers. 2.4. Properties of Medical Lasers Brief properties of some medical lasers with several applications are given for lasers discussed in the previous sections of this chapter. 2.4.1. The Ruby Laser: The Ruby laser emits red light with a wavelength of 694 nm, early ruby laser systems were use retinal surgery, but weren’t used widely for dermatologic work until the development of Q-Switching technology in the mid 1980’s for tattoo treatments. Ruby laser light is strongly absorbed by blue and black pigment, and by melanin in skin and hair. Modern ruby laser system are available in Q-Switched mode, with an articulating arm, “free running” (millisecond range) mode with a fiber optic cable delivery, or as dual mode lasers. Current uses include: 26
• Treatment of tattoos (Q-switched mode). • Treatment of pigmented lesions including freckles, liver spots, Nevus of Ota (Q-switched mode). •
Laser Hair Removal (free-running mode) (6).
2.4.2. The Neodymium: YAG Laser: The laser output is in the near infrared spectrum at wavelength 1060 nm. It has a poor absorption by blood, i.e. absorption coefficient, a = 4cm-1 and is less absorbed by water (a~0.1 cm-1) .Hence it can penetrate rather deep into the tissue and is transmitted through clear liquids. This allows its use in the eye and other water-like filled cavities such as the bladder. The Nd: YAG beam has a high degree of scattering upon impact with tissue. The homogeneous zone of thermal coagulation and nicrosis from the impact site but precise control is not possible. The laser beam is also used to coagulate vessels up to about 4 mm in diameter. These characteristics make the Nd: YAG laser an excellent tool for tissue coagulation but very crude, unprecise tool for cutting due to tissue damage. For treatments sapphires tips, diamond or quartz are used to improve precision and to allow better control to avoid excessive tissue destruction. Major specialties of the laser use is in the treatment of menorrhagia, e.g. uncontrolled bleeding from uterus. A laser fiber is passed into the uterus via a hysteroscope to coagulate the endomatrium. The pulsive form of Nd: YAG output, i.e. the Q-switched and the mode-locked form, has found a growing use in ophthalmology both for eye diagnosis and treatment. Laser can differentiate between a faulty neural system and a disturbed optical system of the eye. Thus a predictive ability is provided when considering operations such as cataract removal, vitrectomy, corneal transplant and glaucoma surgery (1, 3). 27
2.4.3. The Helium: Neon Laser: Due to the low output power of the laser beam, the laser is mostly used in students’ laboratory for teaching. In medicine, it has been used normally as aiming beam. Since the beams of some powerful lasers, as CO2 and Nd: YAG lasers are invisible to the eye, the red beam of He: Ne laser allows the operator to adjust spot size and operating distance before the operating beam is switched (1). 2.4.4. The Argon - Laser: The laser beam is visible as a bright blue-green light of wavelength 488-515 nm. It is better absorbed by blood (a = 34 cm-1) than the Nd: YAG laser, but its absorption in water (a < 0.001cm-1) is less than of Nd: YAG laser. It is easily transmitted through clear aqueous tissues. Certain tissue pigments such as chromogens, melanin and haemoglobin will absorb argon laser light very effectively. This localization of heat generation will be a highly effective coagulater. The first significant medical application of this selective absorption characteristic was in the treatment of diabetic retinopathy in 1965. The argon laser light is absorbed by the retinal pigment epithelium and the generated heat is then used to photocoagulate the retina. Dermatology and plastic surgery are the other major areas of applications of argon laser therapy. The laser light is more heavily absorbed by the pigmented tissue or pigmented lesion than by the surrounding tissue. A valuable application is the treatment of port-wine stains. Like other power output lasers, the argon laser beam when focused to a very small spot (or when its power is increased sufficiently), its power density is high enough ‘to result in vaporization of the target. Medical argon ion lasers are of power as high as 15W. The beam is easily delivered 28
to the site through optical fibers which can be coupled to operating microscope or hand pieces or to a variety of endoscopes (1). 2.4.5. The Carbon - Dioxide Laser: The laser emission is in the mid infrared wavelength of 10.6 µ m. This range is heavely absorbed by water (a = 230 cm-1). The biological tissue is composed of 70% to 90% water and its absorption to CO2 radiation is independent on the tissue color unlike its absorption to argon ion laser light. Due to this high degree of absorption, most of the incident energy is sharply absorbed i.e. having a very short penetration depth (~0.01 mm), limiting the lateral damage of the tissue and leaving a very small zone of coagulation and nicrosis. This makes the CO2 laser a precise surgical instrument. The major applications of CO2 laser is in cutting, vaporization and coagulation. A focused spot of the laser beam will allow a precise cut (laser scalpel) leaving a dry bloodless areas of damage. The depth of the cut is determined by the power density and the time of irradiation. Dry incisions heal in much the same way as conventional wounds. Vaporization can be performed with focused or defocused beams. It helps in removing tissue of one cell layer at a time from its delicate structure. When cutting, vessels up to 0.5 mm in diameter can be coagulated instantly; larger vessels can be treated with a defocused beam. Precision welding of arteries for microsurgery and anastomosis of small vessels and nerves can be achieved. The CO2 laser has found widespread applications in
medicine,
including
most
of
its
branches
i.e.
gynecology,
otolaryngology, neurosurgery, plastic surgery, ophthalmology, and others. Surgical CO2 lasers typically of 30W output and can go as high as 100W. The beam is delivered with the aid of an articulating arm and hand 29
pieces, or coupled to an operating microscope. Recently, fiber optics has become available (1, 3). 2.4.6. The Dye Laser: The dye fluorescence covers a broad spectrum of colours. The dye laser is used where a selective absorption characteristics of tissue upon certain wavelength is required in therapy; for example, in the treatment of port-wine stains, tattoos and other pigmented tissue. Currently, dye laser is used in photoradiation therapy (PRT), a new possibility for cancer tumour detection and treatment. This involves the use of a photosensitizing agent (tumour seeking agent), such as haematoporphyrin derivative (HPD) in combination with laser radiation. As illustrated in Figure 2.11, diluted (HPD) is intravenously injected into biological system. It spreads, but is selectively retained in tumour tissue after two to three days. When activated by ultraviolet radiation its emission is characterized by dual peaked fluorescence light distributed in the red spectral region (about 630 nm) sperimposed on the natural fluorescence (autofluorescence) spectrum of the tissue (the broad band emission in Figure 2.11).
30
Fig. 2.11: Simplified Illustration of Photo-Radiation Therapy. (a) Tumour marking. (b) Tumour detection using laser-induced fluorescence. (c) Tumour destruction by the selective release of singlet oxygen.
31
The spectral red lines of HPD identify the tumour and allow standard biopsy specimens to be taken at the-correct location. When laser radiation of 630 nm is delivered to HPD molecule, it absorbs it and transfers it to an oxygen molecule which gets excited. This excited molecular oxygen (singlet) is known to be a strong toxic agent. It oxidizes the surrounding (tumour) tissue. The laser radiation needed for this induced chemical process is normally provided by using a dye laser pumped by an argon-ion laser. Another application of dye lasers involves the pulsed type. They have been used to fragment gall-stones and kidney stones. These are typically microsecond pulses, emitting energy 50-100mJ at wavelengths in the blue-green region. Laser radiation is delivered to the stone by an optical fiber passed through one of the channels of an endoscope The laser lithotripter (stone breaker) have several advantages over the acoustic shockwave lithotripter currently used for kidney stones (1). 2.5. Clinical Laser Applications In this section we will demonstrate the laser surgery, the advantage and disadvantage of the use of laser, and some of the medical fields where laser is currently used. 2.5.1. Laser Surgery: It is a type of surgery that uses the cutting power of a laser beam to make bloodless cuts in tissue or remove a surface lesion such as a skin tumor. There are a number of different types of lasers that differ in emitted light wavelengths and power ranges and in their ability to clot, cut, or vaporize tissue (15).
32
When using laser in medicine, potential advantages for both surgeons and patients are provided .These advantages may be summarized as follows: - No-touch technique. - Possibility of operating in inaccessible regions. - Dry surgical field with sterilization of the operative site. - Reduced blood loss. - Reduced oedema and pain yielding more rapid recovery of the patient. - Limited fibrosis and stenosis. - Limited damage of the adjacent tissue (for a few tens of micrometers) - Precision. - Reduced postoperative pain. -
No evidence of causing genetic damage or cancer (10).
There are also disadvantages, at the present state of the technique, in the application of the laser tool as: • High cost. • Complexity of the laser surgical unit. . • Reliability and safety problems (1). 2.5.2. Gynecology: Gynecologists were among the specialists who first truly appreciated the potential of the laser. One of the first lasers they used was the CO2 laser, which they found tremendously effective in treating patients with erosion of the cervix. New gynecological applications were introduced and with instrumentation refinements, the laser advanced laparoscopic surgery to a fine art. The laser was then found useful for cutting, coagulating and 33
vaporizing during intraabdominal procedures. Also, the laser was coupled with the hysteroscope to perform surgery within the uterus. Other clinical applications continue to be developed by innovative gynecologists who strive to provide less invasive less complicated procedures to benefit their patients. Many different wavelengths are being used effectively for a variety of gynecological applications, including lower tract, laparoscopic, hysteroscopic, and intraabdominal procedures (3). 2.5.3. Urology: Lasers were first experimentally used in urology within gas-filled bladders. The laser was inserted through conventional rigid cystoscopes with special deflecting prisms or shutoff windows to conduct the laser energy. Advances have led to the development of cystoscope accessories and quartz fibers to conduct the beam through a fluid-filled bladder. In the 1970s, Dr. A.Hofstetter successfully performed Nd: YAG laser cystoscopy to irradiate bladder tumors. This accomplishment led to tremendous advances in treating recurrent bladder tumors and initiated the development of other urological laser applications. A variety of laser wavelengths are used today to successfully treat many urological conditions. The CO2, Nd: YAG, frequency doubled YAG and argon lasers are commonly used for different procedures (3). 2.5.4. General Surgery: The laser has been slow to gain acceptance in the field of general surgery because conventional methods offer accessibility and visibility, and the physician feels comfortable with traditional surgical tools that are easy to control during procedure. Instrument a development has advanced to meet the needs of the physician to incise or excise diseased tissue 34
precisely and with fewer complications using minimally invasive techniques. Therefore, the general surgeon has not had a great desire to accept other tools, such as the laser. Because the laser has not been as quickly accepted by general surgeons as it has by other specialists, its fate is still being determined. Unfortunately for general surgeons, the laser has allowed endoscopists, cardiologists, radiologists and internal medicine specialists to perform the needed surgery from within organs or structures, thus eliminating the need for the general surgeon and incisional procedures. During the late 1980s and early 1990s, laparoscopy was aggressively introduced to the general surgeon. Laparoscopic cholecystectomy became the preferred procedure over the open upper-abdominal method. Laser technology was the tool of choice for these beginning procedures
since
the physicians who developed this laparoscopic technique were also skilled in using the laser (3). CO2 lasers are used in surgical operations, such as, otolaryngology. This is the branch of medical Science, which deals with problems of the ENT and the oral cavity. The output power of these lasers is of the order of 50W in the CW mode. Several problems in the neck and head can treat with laser radiation. By using conventional surgical method, there is a possibility of losing a large quantity of blood. However, while doing surgical operations with lasers, it is imperative that the eyes of the patients are well protected. Protective materials such as cotton, sponge swabs etc are usually used for protecting certain vital organs like nerves, veins and arteries from unduly getting heated while the near by tissue is being worked upon. 35
Lasers are also used in brain surgery for cutting tissues, boring holes in the skull and vaporising lesions and cauterizing blood vessels. The damage around the region, which is treated, is less than Imm. This is extremely important since the least amount of tissue should be removed or destroyed in the brain. Using CO2 lasers, neurosurgical coagulation of blood vessels up to l mm in external diameter can be done (13). 2.5.5. Dermatology: Laser is already being successfully used for cauterization and local treatment of skin growths and skin deformities. Laser treatment provides two major advantages over conventional treatment in case of burn injuries and skin grafting. They are: (a) The area, which has been treated remains sterile and hence provides an ideal bed for immediate skin grafting. (b) The blood loss is minimal. Laser finds wide application in dermatology especially in the following areas: (a) Homeostasis, which means stopping of bleeding. (b) Removal of hair, tattoos, warty keratoses, cell carcinomas, freckles, acne and various growths, both benign and malignant. CO2 lasers have been used for treating skin tumours. Another application of laser more specifically photomedicine, is in the surgical treatment of port wine stains (PWS). A port wine stain is a defect in the skin, which is manifested by discoloured or blotchy and darkened patches in the skin. The most commonly used laser in the treatment of PWS is the pulsed argon laser (13).
36
2.5.6. Cardiology: Another important area of medical application of lasers is the laser assisted balloon angioplasty, which has become very common in clearing blocked arteries. In balloon angioplasty, a tiny deflated balloon, housed in a thin catheter, is threaded through the artery in the blocked region. When it reaches the right spot the balloon is inflated to open up the vessel by compressing the obstructing plaque against the walls of the artery. This method is less expensive and less prone to complications, as compared to conventional bypass surgery. The disadvantage with balloon angioplasty is that the vessel can close again because there is no permanent removal of the clot. About an estimated 30% case need repetition of the process. This technique is of no use in case of totally blocked arteries. Lasers have helped in overcoming this problem. Lasers can pave the way for the balloon or clear the arteries on their own. In laser-assisted angioplasty, an Ar+ laser delivers the energy through the catheter and disposable balloon. The balloon helps in inflating the site for blood flow cessation and centering the optical fibre, which transmitting the laser and the opening the vessel. Taking turns, the balloon and the laser, clear the path for each other, through the block (13). 2.5.7. Ophthalmology: Laser has become a common tool now in photocoagulation such as the one used to reattach a detached retina. For example, the green beam of argon ion laser is focused on a certain point of the retina. The beam penetrates through the lens of the eye and the vitreous chamber without being absorbed. But the beam gets strongly absorbed by the red blood cells 37
of the retina and the resultant thermal effect leads to reattachment of retina. In the past, high-powered xenon lamps were also often used to concentrate enough heat in the area of the unattached retina. The problem faced here was that the optical power could not be focused sharply (13). Photocoagulation: The laser produces a pulse of light energy as directed by the surgeon, which is passed to the eye under treatment. The pulse of light is focused by the lens of the patients' eye to produce a minute lesion or coagulation of the tissue of the retina and the choroids (the vascular membrane of the eyeball between the sclera and retina). The retina is so welded to the choroids by a series of repeated pulses from the laser.
these lesions through several
The amount of energy required for
coagulation is different for different patients. The two lasers are commonly used in ophthalmology, are the ruby laser of 0.69 µ m and argon ion laser of 488 to 514 nm (13). Treatment of Corneal Ulcers: The other application of lasers in the treatment of eye disease is in corneal ulcers. Here, an argon-ion laser is used in combination with a fluorescein dye. The infected site is dyed with the fluorescein dye by using antibodies, specific for the infecting organism, labelled with the dye. Fluorescein absorbs argon laser strongly and raises the temperature of the organism, thereby destroying it, without damaging the surrounding tissues (13).
Laser in the Treatment of Glaucoma : Glaucoma is caused by an increase in pressure inside the eye, which destroys nerve cells and causes vision loss. Conventional surgery to treat this condition is to cut a channel in the eye to drain out the excess fluid. 38
However, this procedure is biset with a 5% risk of blindness. There are several drugs that could be administered to reduce the pressure. But they also cause serious side effects such as increase in blood pressure and bronchospasms. Laser surgery seems to give no side effect (13). 2.5.8. Gastroenterology: Gastrointestinal diseases primarily include ulcers and tumors of the esophagus, Stomach liver, gallbladder and intestine. The intestine further consists of the jejunum, ileum, colon, and rectum.
According to the
position of these organs, the gastrointestinal tract is subdivided into an upper and a lower tract. Most intestinal tumors are reported to occur inside the colon or the rectum. In general, any kind of ulcer or tumor can be treated with lasers if it is accessible with endoscopic surgery. Gastroenterology is one of the major domains of the CW Nd: Y AG laser only in photodynamic therapy are dye lasers applied. There exist mainly two indications for laser therapy: gastrointestinal hemorrhages and benign, malignant Since CW Nd: YAG laser is acting thermally; it can stop bleeding by means of coagulation. The CO2 laser is not suitable for clinical gastroenterology, since it is not transmitted through optical fibers which belong to the mandatory equipment of successful endoscopic surgery (16). By way of summary, performance data of the lasers described in this chapter are gathered in table 2.1.
39
Table 2.1: Characteristics of Medical Lasers Laser
Wavelength
Power range Mode
Efficiency
type
nm
W(J\S)
Ruby
694
(>30)
pulsed
0.1%
Nd: YAG
1060
5-120
CW and
1-3%
Qswitched Doubled
503
(>3)
Pulsed
0.1%
He:Ne
682.8
10-3 -10-2
CW
0.02%
Argon
458-515
0.001-25
CW and
0.1%
Nd: glass
ion CO2
pulsed 10600
0.1-100
CW and
10-20%
pulsed Dye
400-700
0.001-6
40
CW
0.5%
Chapter Three A field Study A field study of the laser instruments used in ophthalmology hospitals in Khartoum state. Table 3.1: Kind of the Laser Instruments. Laser Instrument Yag laser Argon laser
Purpose of Number of instrument treated sessions Capsuletomy 1 session Pan retinal More than 1 photocoagulation session Excimer laser Lasik 1 session Total
Frequency
Percent
4 6
33.3 50.0
2 12
16.7 100.0
From Table (3.1) it’s clear that the Argon laser instrument is the most widely used laser instrument. Its used at the rate of 50% compared with the Yag laser instrument which is used at the rate of 33.3% and the Excimer laser which is used at the rate of 16.7%.This is illustrated in Figure 3.1. 60
50
50
40
30
33
20
Percent
17 10
0
Yag laser
Argon laser
Excimer laser
kind of the laser instrument
Fig. 3.1: Kind of the Laser Instruments. 41
Table 3.2: Price of the Instruments Price
Frequency
Percent
Expensive
12
100.0
It is also clear from the above analysis that the price of the laser instruments is high and because they are very expensive they are found only in specialized centers of ophthalmology. Table 3.3: Kind of the Laser Instruments * the Instrument Firstly Used kind of the the instrument firstly used
laser instrument Yag laser Argon laser Excimer laser Total
1995 Count % of Total Count % of Total Count % of Total
1 8.3%
Count % of Total
1 8.3%
2000
1 8.3%
1 8.3%
2003
2004
1 8.3%
1 8.3% 1 8.3% 1 8.3%
1 8.3%
3 25.0%
1 8.3%
1 8.3%
2005
2007
1 8.3% 1 8.3%
1 8.3% 2 16.7%
4 33.3% 6 50.0% 2 16.7%
2 16.7%
3 25.0%
12 100.0%
From Table (3.3) we find that the first laser instrument used in the Khartoum state is the Yag laser instrument which was firstly used in 1995. No other laser instrument was used until the years 2000-2003 when the Argon laser instrument was firstly used. After 2003 another laser instrument (Excimer laser instrument) began to be used together with Argon laser and Yag laser instruments in specialized ophthalmology hospitals. This is illustrated in Figure 3.2.
42
Total
2006
2.2
the date 2.0
1995 1.8
2000
Count
1.6
2003
1.4
2004
1.2
2005
1.0
2006
.8
2007
Yag laser
Argon laser
Excimer laser
kind of the laser instrument Fig. 3.2: Kind of the Laser Instruments & the Instrument Firstly Used. Table 3.4: The Functioning of Instrument Work Answer Frequency Percent yes 10 83.3 No 2 16.7 Total 12 100.0 In the Table (3.4) we notice that the rate of the instruments functioning well is 83.3% and the rate of those don’t work due to the lack of the spare parts and experienced engineers and technicians is 16.7% (Fig.3.3).
43
100
80
83
60
40
Percent
20 17 0
yes
No
the functioning of instrument work
Fig. 3.3: The Functioning of Instrument Work. Table 3.5: The Availability of the Repairing Centers Answer Yes No Total
Frequency 3 9 12
Percent 25.0 75.0 100.0
Table 3.6: The Availability of the Spare Parts Answer Yes No Total
Frequency 3 9 12
Percent 25.0 75.0 100.0
From table 3.5 we notice that the laser instruments repairing centers are not available according to what is needed. There is a shortage at the rate of 75%. The rate of what is really available is 25%. That is because there are not enough experienced medical Engineers and technicians to
44
epair these instruments. The shortage of the spare parts is about 75% (Table 3.6).
45
Chapter Four Conclusion The aim of this work is to show the different methods of the applications of laser in medicine. Lasers are classified according to their active medium. Some of the important medical lasers are; Neodymium-YAG Laser (Nd-YAG), Argon Laser (Ar+), Carbon Dioxide Laser (CO2), and Dye Laser. Laser has special properties that makes it more important and useful than many other substances or instruments that are used in medicine. Some of its properties are, laser light has small divergence of beam & it has high energy. It has proved its great ability and benefits in the different fields of medicine specially surgery because it reduces blood loss due to the operation cutting, and including most of its branches i.e. gynecology, ophthalmology, dermatology, and others. Moreover laser has short pulses of light. This reduces pain and yields more rapid recovery to the patient. As the laser beam is very tiny the damage in adjacent tissues is very limited. In laser operations the surgeon uses few instruments. This enables the surgeon to have a clear vision of the spot of the operation. Laser operation causes no wounds so the patient can leave the hospital immediately after the operation. Laser operations have become more efficient, perfect and accurate due to the application of the computer control. The relationship between the different laser beams and the tissues depends on the properties of the laser beam according to its wavelengths, and its intensity.
46
The laser instruments were firstly used in the Khartoum state science 1995. Now the laser instruments which are used in the specialized ophthalmology hospitals are: Argon Laser, Yag Laser and Excimer Laser Instruments. The number of the Excimer laser instruments is few because it is not found in many hospitals because the cost of its treatment is too expensive for the patients. The difficulties that face the wide use of the laser instruments are due to: The shortage in the repairing centers of the medical engineers and technicians and the lack of spare parts. In general lasers have many hazards, some of the important hazards are: radiation, explosive, electrical, and toxic hazards. Due to these hazards it is very important that all the safety precautions requirements should be available in the hospital where the laser system is used. These safety precautions are: laser instrument should only be used by qualified and experienced technicians, surgeons and physicians. The laser instruments must be kept away from those who misuse them. These instruments must be occasionally checked, tested and maintained.
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References 1- Siham A. Kandela, “Laser Physics in Medicine”, ELHekima Publishing and Printing Establishment Bagdad, 1991. 2- Http:// www.arab-eng.org/vb/t55706.html. 3- Kaya Ball, “Lasers the Perioperative Challenge”, second Edition, Published in Nabcy L. Coon, USA. 1995. 4- Steen W.M, “Laser Materials Processing” Second Edition, 1998 5- Robert A. Mayers, “Enclopedia of Lasers & Optical Technology”, First Edition, Academic Press, 1991. 6- http://www.shorelaser.com/aboutlasermed.html. 7- Jeff. Hecht; “The Laser and Applications Layers “, Francis LTD London, 1971. 8- Jeff. Hecht; “Laser Guide Book”, contributing Editor printed bound by Donnelly and Son Company 1986. 9- Orazio Svelto, “Principles of Lasers”, fourth edition, London, Springer business Media, Inc, 1998. 10- Verdeyen, J.T,”Laser Electronics”, Third Edition, 2000. 11- Javan, A, Bennett, w. R and Herriot, D. R, ”Population Inversion and Continuous Optical Maser Oscillation in a Gas Discharge Containing a He-Ne Mixture”.phy. Rev.Lett. (1961). 12- William T. Silfast, “Laser Fundamentals” second edition, London, published by the press syndicate of the University of Cambridge, 2004. 13- K R. Nambiar, “Laser Principle’s Types and Applications”, New age international P limited publishers, 2004. 14- John. F. Ready, “Industrial Applications of Laser”, second Edition, Academic press, USA, 1997. 48
15- http://www.medterms.com/script/main/art:asp articleky=31889 16- Markolf H. Nienz, “Laser-Tissue Interaction’s Fundamental and Application”, second Revised Edition, Springer in London, 1998 . ﺟﺎﻣﻌﺔ ﺍ ﻟﺒﺼﺮﺓ,ﺗﺮﲨﺔ ﺣﺎﺳﺐ ﻋﺒﺪﺍﳊﺴﲔ. ﺎ ﺍ ﻟﻠﻴﺰﺭﺍﺕ ﻭﺗﻄﺒﻴﻘﺎ.ﺑﻴﺴﻠﻲ.ﺝ. ﻡ-17
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Appendix A University of Khartoum Faculty of Science Physics Department Questionnaire The purpose of knowing the use of laser instruments used in Ophthalmology 1- What is the kind of the laser instrument used? 2- What is the purpose of using this instrument? 3- The price of this instrument: (a) Cheap
(b) expensive
4- When is this instrument firstly used? 5- What is the number of patients treated with this instrument since it has been firstly used? 6-
Number of treated session needed for a patient? (a) One session
(b) More than one session
7- Is this instrument functioning well? (a) Yes
(b) No
8- What kind of difficulties that affect the use of this instrument? 1239- Are the repairing centers for this instrument available? (a) Yes
(b) No
10- Are the spare parts available? (a) Yes
(b) No 50
Appendix B Laser instruments used in ophthalmology centers & hospitals.
Name of Hospital
Laser Instruments Used
Alwalidain Eye Hospital-Omdurman
Argon Laser
Makkah Eye Center-Omdurman
Argon & Yag Lasers
Makkah Eye Center-Khartoum
Argon, Yag &Excimer Lasers
Sudan Eye Center-Khartoum
Argon & Yag Lasers
Nour Eluoon Military Hospital-Khartoum
Argon & Yag Lasers
Abdul Fadeel Almas Ophthalmology Nati- Argon Laser onal Technical center-Khartoum Barha Hospital-Khartoum Bahary
Excimer Laser
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Appendix C Laser Hazards and Laser Safety The increasingly wide spread use of lasers requires more people to become familiar with the potential hazards associated with the misuse of this valuable new product of modern science, applied in medicine. 1. Laser Hazard: The basic hazards from laser equipment can be categorized as follows: • Laser Radiation Hazards: Laser emit beam of optical radiation. Optical radiation (ultraviolet, visible, and infrared) is termed non ionizing radiation to distinguish it from ionizing radiation such as X-Rays and gamma rays which are known to cause different biological effects. Eye Hazards:The ocular hazards represent a potential for injury to several different structures of the eye. This is generally dependent on which structure absorbs the most radiant energy per volume of tissue. Retinal effects are possible when the laser emission wavelength occurs in the visible and near infrared spectral regions (0.4 µ m -1.4 µ m). Light directly from the laser or from a specular (mirror-like) reflection entering the eye at these wavelengths can be focused to an extremely small image on the retina. The incidental corneal irradiance and radiant exposure will be increased approximately 100000 times at the retina due to the focusing effect of the cornea and lens. Laser emissions in the ultraviolet and far infrared spectral regions (outside 0.4 µ m - 1.4 µ m) produce ocular effects primarily at the cornea. 52
However, laser radiation at certain wavelengths may reach the lens and cause damage to that structure (16). Optical Radiation Hazard:Effects of optical radiation at various wavelengths on various structures of the eye are shown in Figures. C. Actinic - ultraviolet, at wavelengths of 180nm to 315nm, is absorbed at cornea. These wavelengths are responsible for welder’s flash. Near ultraviolet radiation between 315nm and 400nm is absorbed in the lens and may contribute to certain forms of cataracts. Radiation at visible wavelengths, 400nm to 780nm, and near infrared wavelengths, 780 nm to 1400 nm, is transmitted through the ocular media with little loss of intensity and is focused to a spot on the retina 10 µ m to 20 µ m in diameter. Such focusing can cause intensity high enough to damage the retina. For this reason, laser radiation in the 400nm to 1400nm range is termed the retinal hazard region. Wavelengths between 400nm and 550nm are particularly hazardous. For long- term retinal exposure, or exposure lasing for minutes or hour. This is sometimes referred to as the blue light hazard. For infrared radiation with wavelengths of 3 µ m to 1mm is absorbed in the front surface of the eye. However, some middle infrared radiation between 1.4 µ m and 3 µ m penetrates deeper and may contribute to “glassblower’s cataract”. Extensive exposure to near infrared radiation may also contribute to such cataracts. The localization of injury is always the result of strong absorption in the specific tissue for the particular wavelength (16).
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Skin Hazard: From a safety standpoint, skin effects have been usually considered of secondary importance. However, with the more widespread use of lasers emitting in the ultraviolet spectral region as well as higher power lasers. Skin effects have assumed greater importance. Erythema, skin cancer, and accelerated skin aging are possible in the 230nm to 380nm, wavelength range (actinic ultraviolet). The most severe effects occur in the UV (280-315nm). Increased pigmentation can result following chronic exposures in the 280nm to 480nm wavelength range. At high irradiance, these wavelengths also produce “long-wave” erythema of the skin. In addition, photosensitive reactions are possible in the 310nm to 400nm (near ultraviolet) and 400nm to 600nm (visible) wavelength regions. The most significant effects in the 700nm to 1000nm range (infrared) will be skin burns and excessive dry skin (16).
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Fig. C: (a) Absorption sites of visible and near infrared radiation, (b) Absorption sites of middle infrared, far infrared and middle, ultraviolet radiation. (c) Absorption sites of near ultraviolet radiation. 55
• Chemical Hazards: Some materials used in lasers (i.e. excimer, dye, and chemical lasers) may be hazardous and /or contain toxic substances. In addition, laser- included reactions can release hazardous particulate and gaseous products (16). • Electrical Hazards: Lethal electrical hazards may be present in all lasers, particularly in high-power laser systems (16). • Expolsive Hazards: Explosions are most likely to occur in flash-lamp and capacitors, e.g. the flash-lamp used to pump solid state lasers. Chemicals and cryogenic solvents are often present in laser laboratories and they can cause numerous hazards (1). • Toxic Hazards: Toxic hazards result from rapid heating and vaporization of target, (substances, Food, Liquids, etc) as they are exposed to high- power laser beams. Depending on the target, toxins can vary from irritants to ocular and respiratory passages (such as sulfur oxides and nitrogen oxides) to protein poisons (such as lead and nickel compounds), to materials injuries to the hematopoietic system such as cyclohexene and toluene (1). • X-Ray Radiation Hazards: X-rays may be generated by electronic components of the laser system (e.g. high-voltage vacuum tubes and from laser-metal induced plasmas) (16).
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2. Classification of Laser Hazards: Laser can be categorized into four groups as per the American National standard and institute regulations. This will assist in choosing the required control measures for their safe use (3). Lasers are divided into class-1, class-2, class- 3A, class-3B and class-4. Class-1: Lasers are those lasers, which are not capable of emitting dangerous radiation under normal operating and viewing conditions. Class-2: Lasers are CW or pulsed lasers, in the visible range, of the order of 400-700nm. This can safely be viewed for reasonable duration only for the purpose of identification. Class-3A: Lasers, in the normal course, are not hazardous unless viewed through magnifying optical devices. They are used by surveyors. Class-3B: Lasers are dangerous if viewed with naked eyes. They, however, do not create hazardous diffuse reflections. Direct beam viewing has to be avoided. Their reflection are also to be avoided. Class-4: Lasers are those, which are capable of producing hazardous diffuse reflections. They are also fire hazards. Their average power output is above 500 milliwatts.(13) 3. Laser Safety: We have seen that the hazards of the laser originate from the supporting unit as well as from the laser beam itself. At the laser site, the following safety measures are to be observed. 57
a) Ensure proper grounding, insulation and shielding of all power supplies used at the laser site. b) Do not open any sub-unit or the laser scheme while the power is on. c) Provide circuit breakers; fuse etc at the junction boxes. Then, there is a requirement to guard against the chemicals and poisonous vapours that are required to be handled in the laser site. Many dyes and other solvents used in organic dye lasers are very poisonous and carcinogenic in nature. Adequate protection by way of wearing protective clothes, a gloves etc is essential. There has to be proper ventilation in the site to drive away these vapours and gases. The following measures may be adopted against the hazards posed by laser. a) Never ever look directly at the laser beam. Because the lens in the eye may focus the collimated laser beams into a tiny spot in the retina and ruptures it causing permanent blindness. b) Keep beam stops wherever possible and keep the laser beam at a level below the face. c) Use absorbing glass filters in goggles in tandem with clear plates behind. The colour, of the glass used is important. So, use green goggles while working with ruby-laser and red ones while working with frequency doubled YAG laser. The personnel working in the laser room or laser laboratory should wear protective clothing, gloves, special goggles etc. The laser room should be well ventilated and most have arrangement for darkening the room when required. All specification of “clear area” should be met such as dust. Free atmosphere and air conditioning. There should be aboard outside the room indicating a restricted entry by only qualified 58
personnel. It would also be ideal, if the laser room is away from the main building in a separate location (13). Below are some practical safety considerations in relation to the medical applications of Neodymium: YAG, Argon and Carbon Dioxide laser. Neodymiumd: YAG Laser: For surgical purposes, the laser is used through an endoscope. Special care of the endoscope and safety precaution of the eye must be taken into account. Filters and special safety goggles must be used by surgeons. The patient’s eyes should be protected. The Q-switched or mode-locked laser beam is used for ophthalmology. Reflection from corneal surface could be hazardous to assisting staff; therefore, safety glasses must be used in the operation theatre (1). Argon -Laser: For medical applications, this laser is used through handpiece or microscope or through a slit-lamp for ophthalmology. The eye protection is the central safety requirement. Special goggles for the organ laser wavelength should be used by the surgeons and their assistants. Patient’s eyes should be protected except for ophthalmic procedures. Argon ion laser is also used through an endoscope, and special filters or protective eye wear should be used (1). Carbon Dioxide - Laser: Although the laser wavelength is outside the retinal hazards region, it presents the potential for cornea or several burns when the beam directly hits the eye. Glass or plastic material absorbs the CO2 radiation. Thus one can wear his own corrective glasses with additional protective side shields, or wear clear glass goggles. Patient’s eyes must also be protected. The use of CO2 laser impose special safety precautions of fire hazards, dry sponges 59
and clothes will flame immediately upon impact with CO2 beam. Thus a container of water, wet sponges and moistened cloth drapes should always be kept available around the surgical field (1).
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