Unit 11 Medical Laser

Unit 11 Laser and its medical applications

1. Summary about atoms • Atom is the smallest building block of matters, including body tissue and fluids, it is electrically neutral • it contains three elementary particles: the electron , the proton, and the neutron. The electron has a negative charge (e=-1.6x10-19 C), the proton has a positive charge (+e=-1.6x10-19 C), the neutron is neutral. • electrons make circular motion around the nuclei in different level of orbits, called stationary orbits, they correspond to different energy levels. the number of electrons is equals to the number of protons in an atom, a nucleus counts for vast majority of the atomic mass. The so-called atomic number (Z number) is the proton number (also electron number) in an atom. Ions are formed when atoms obtained extra electrons or loose electrons. •

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2. Light emission 2.1 absorption and emission of light From atomic model, we know that: • These orbits differentiate them each other by different level of energy. The energy change between orbits cannot be continued but rather quantized. • We can display the these electron orbits schematically using an energy level diagram: energy is plotted vertically with the lowest (n=1) , or ground stat, at the bottom and with exited states (n=2, 3,4,…) above. n is called orbit number and can be only positive integer.

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The electron can absorb energy and promoted to a higher level, the process is called excitation. A photon is emitted when an electron change from a higher orbit to a lower orbit with a characteristic emission spectrum. This process is called de-excitation. The “Neon light” is lights emitted in the glass tubes during an excitation and de-excitation processes: the gases are excited by external electric energy and different color of lights are emitted by de-excitation process. If an atom absorbs a photon, an electron jumps from a lower orbit to a higher orbit with a characteristic absorption spectrum.

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Atom will absorb and emit light photons at particular wavelength corresponding to the energy differences between orbits. The wavelength λ of emitted or absorbed photon can be obtained by the formula: hc ∆E = = hf λ where E is the change in energy between the initial and final orbits. Every element has its own characteristic pattern of frequencies, therefore atomic emission or absorption spectra (atomic spectroscopy) are the fingerprint of the chemical element. A variety of biological molecules have notable absorption spectra in the visible, IR, and UV. This has many clinical applications, e.g. Oximeter.

3. How lasers work 3.1 Stimulated emission The word "laser" stands for "light amplification by stimulated emission of radiation." Lasers are possible because of the way light interacts with electrons. Electrons can be bumped up to higher energy levels by the injection of energy-. The low orbit electrons can be excited to higher orbit levels by the injection of energy-for example, by a flash of light, flow of electric current. An excited electron may gives off a photon and decay to the ground state by two processes, one process called spontaneous emission. Examples: incandescent light bulb, they emit photons with different energies and in all directions.

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E2

E2

E2

E1

E1

E1

Absorption

Spontaneous emission

Incoming photon Emitted photon

Stimulated emission

A second process: the excited atoms can interact with a pre-existing photon that just happens to be passing by. If the incoming photon has the energy just equal to the energy difference between two states, it induces the electron to decay and gives off a new photon. This process is called stimulated emission. 3.2 Optical pumping In order the second process prevails the first in an atom; many electrons must be previously excited and held in an excited state without massive spontaneous emission: this is called population inversion. The process is called optical pumping. The process of optical pumping is can be illustrated by the example of a ruby laser. The ruby laser was the first laser invented in 1960. Ruby is an aluminum oxide crystal in which some of the aluminum atoms have been replaced with chromium atoms. Chromium gives ruby its characteristic red color and is responsible for the lasing behavior of the crystal. Chromium atoms absorb green and blue light and emit only red light.

In a ruby laser, a crystal of ruby is formed into a cylinder. A fully reflecting mirror is placed on one end and a partially reflecting mirror on the other. A high-intensity flash lamp is spiraled around the ruby cylinder to provide energy that triggers the laser action. The green and blue wavelengths in the flash excite electrons in the chromium atoms to a higher energy level. •

Only those perpendicular to the mirrors will be reflected back to the active medium, They travel together with incoming photons in the same direction, this is the directionality of the photon.

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The second photon has the same energy, i.e. the same wavelength and color as the first – laser has a pure color It travels in the same direction and exactly in the same step with the first photon – laser is in phase or temporal coherence

Comparing to the conventional light, a laser is differentiated by three characteristics. They are: • pure color, • temporal coherence, • directionality.

Directionality Keep pace: temporal coherencesolidincluding

There are many types of lasers, state, gas, semiconductor, or liquid. The ruby laser a solid-state laser. Solid-state lasers provide the highest output power of all laser types.

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3.3 Laser power and intensity The amount of energy output by laser depends up both the power and intensity (or power density) of a laser emission.

The power P is a measure of energy transfer rate; it depends on the energy carried by each individual photon (given by wavelength) and the amount of photons emitted in a unit time. Power(W)

=

Total energy output (J) exposurte time (s)

The total energy E during a exposure time TE can computed by E (J) = P(W) x TE(s), where the unit of power is Joules/s or W.

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Sample problem: A 5 mW He-Ne laser outputs red light with wavelength at 633 nm. Calculated the numbers of photons emitted per second. Solution: Power is just energy per second of the radiation, so if we get the energy of the photon from E=hc/λ , then we get the numbers of photons. The energy of the photon with 633 nm wavelength is: −9 E=hc/λ =(6.63x10-34 J-s x 3.0x108m/s)/λ =(6.63x10-34 J-s x 3.0x108m/s)/633x10 m=3.16x10-19J So the number of photons per second is:

0.005 J / s = 1.6 ×10 16 photons/s −19 3.16 ×10 J

The amount of photons encountered by a particular unit area in a unit time is measured by the intensity (or power density). For an area illuminated, the relation between the power and intensity can be expressed by:

Intensity (W/cm 2 ) =

Power (W) spot area (cm 2 )

The directionality of laser beam offers a great advantage over ordinary lights since it can be concentrate its energy onto a very small spot area. This is because the laser rays can be considered as almost parallel and confined to a welldefined circular spot on a distant object. Thus even a small power of laser can achieve high intensity.

Sample problem: we compare the intensity of the light of a bulb of 10W and that of a laser with output power of 1mW (10-3 W). For calculation, we consider an imagery sphere of radius R of 1m for the light spreading of the bulb, laser beams illuminate a spot of circular area with radius r of 1mm. Then:

I bulb =

Pbulb 10W 10W = = = 8 ×10 −5W / cm 2 2 2 A 4πR 4π (100 cm )

and

I laser =

Plaser 10 −3W 10 −3W = = = 3 ×10 −2 W / cm 2 A πr 2 π (0.1cm ) 2

I laser 3 ×10 −2W / cm 2 = ≈ 400 I bulb 8 ×10 −5W / cm 2 The intensity of a surgical laser determines how much light energy a body cell would encounter in a unit time illuminated by the laser. This determines the laser‘s effectiveness for surgery or therapy. On the other hand, Fluence, F is defined as the total energy delivered by a laser on a unit area during an expose time TE,

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F(J/cm2)=I(watts/cm2)xTE(s) This can be thought as a dose of light energy delivered to region on a unit area. Another advantage of the directionality of a laser is that laser light can be focused down to a extremely small spot with a lens, this is because that a perfect parallel beam can be focused into a single point at the focal point. So, extremely high intensity can be obtained with a laser beam. Then away from the focal point, the laser light diverges and spreads out and corresponding intensity decreases. This property can be used to vary the intensity of a laser by 100~10000 times.

f Incoming parallel ray Focused spot Diverged beam 3.4. CW laser vs. pulsed lasers Laser has two modes of operations: continuous wave (CW) or pulsed mode. A CW laser has a constant power output during whole operation time; while a pulsed laser emits light in strong bursts, or pulses periodically with no light emitted in the intervals between bursts.

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The energy emitted in each bursts, Epulse, and the pulse width tw, are known for each laser. The instantaneous power Pi can be calculated as

Pi =

E pulse tw

The tw may vary from milliseconds (1ms=10-3 s) to femtoseconds (1fs=10-15s), but typically at nanoseconds (1ns=10-9s). In pulsed lasers energy is stored up and emitted in a very short time, this results in a very high power during a brief time tw. Two ways to achieve this energy storage and delivery: • Q-switching: an optical technique to compress the laser output into 10 to 20 ns.

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Mode-locking: a technique further compress the laser into a shorter pulse of 10 to 40 ps (1ps=10-12s).

In fact the average power delivered by a pulsed laser is very low because in between pulses no energy emitted. The average power Pave can be obtained by

Pave =

E pulse

= E pulse ( J ) × R ( Hz ) T Where R is the repetition rate, i.e. the number of pulses per second, so the repetition rate is related to the pulse period by R=1/T.

Examples: A pulsed laser emits 1 milliJoule (1mJ=10-3J) energy that lasts for 1 ns, if the repetition rate R is 5 Hz, comparing their instantaneous power and average power. E 1mlliJoule 10 −3 J Pi = pulse = = −9 = 10 6W tw 1ns 10 s

Pave =

E pulse T

= E pulse ( J ) × R ( Hz ) = 10 −3 J × 5 Hz = 5 ×10 −3W

The instantaneous power determines the intensity for heating during instance, while average density determines the fluence during a total time of operation.

4. Mechanisms of laser interaction with human tissues In order to understand how to choose correctly a laser for application purpose, we need to understand further how laser light interacts with body tissue. When a beam of laser projected to a body tissue, basically fives phenomena happens, they are reflection, transmission, scattering, re-emission, and absorption.

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When a laser beam hits the target, it may be (a) absorbed, reflected and scattered or (b) transmitted, re-emitted

Laser light interacts with tissue and transfers energy of photons to tissue because absorption occurs. In most cases, the photons energy of laser light is transferred to blood, tissues or bone in the form of heat. 4.1 Photocoagulation A slow heating of muscle and other tissues is like a cooking of meat in everyday life. The heating induced the destabilization of the proteins, enzymes, etc. This is also called coagulation. Like egg whites coagulate when cooked, red meat turns gray because coagulation during cooking. Proteins are body’s most important structural and functional chemicals. They form the muscles, connecting tissues and blood vessels, they transport oxygen necessary for the metabolism. When temperature is much higher than the body temperature, proteins are destabilized: their complex structures begin to uncoil and losing their natural order and forming dense tangled network. A Laser heating above 50 oC but below 100oC induces disordering of proteins and other biomolecules, this process is called photocoagulation. When lasers are used to photocoagulate tissues during surgery, tissues essentially becomes cooked: they shrink in mass because water is expelled, the heated region change color and loses its mechanical integrity and can be moved or pull out, cells in the photocoagulated region die and a region of dead tissue called photocoagulation burn develops. Application: destroy tumors, treating various eye conditions like retinal disorders caused by diabetes. Photocoagulation also accounts for laser surgery’s excellent hemostatic property blood less incision, excision, due to its ability to stop bleeding during surgery. A blood vessel subjected to photocoagulation develops a pinched point due to shrinkage of proteins in the vessel’s wall. This coagulation restriction helps seal off the flow, while damaged cell initiate clotting. 4.2 photo- vaporization With very high power densities, instead of cooking, lasers will quickly heat the tissues to above 100o C , water within the tissues boils and evaporates. Since 70% of the body tissue is water, the boiling change the tissue into a gas. This phenomenon is called photo-vaporization. Photo- vaporization results in complete removal of the tissue, making incision or complete removal of thin layer of tissue possible. Due to coagulation around the edges of the heated tissue, bloodless operation is possible. 8

Two conditions must be met in order that photovaporization possible,

1. the tissue must be heated quickly to above the boiling point of the water, this require very high intensity lasers,

2. a very short exposure time T , so no time for E

heat to flow away while delivering enough energy, For condition 1, the advantage of lasers over other light sources is their highly spatial coherence (directionality) and can provide higher intensities over conventional lamp. In practice, we can either vary the laser spot size or the laser power to change the intensity of the laser to achieve various laser intensities.

Powers density (intensity) required for different processes. Power density (W/cm2) Low (<10) Moderate (10 – 100) High (>100)

Results General heating Photocoagulation Photo-vaporization

For condition 2, a short exposure time TE is important because during heating, a competing process called thermal relaxation also occurred. The thermal relaxation time TR is the time for thermal energy flow away from heated region. In order to control the heating within the targeted area, TE should be smaller than TR, while sufficient energy should be supplied by the laser within a short time. This can be achieved only with high power density pulsed lasers.

4.3 Photochemical ablation When using high power lasers of ultraviolet wavelength, some chemical bonds can be broken while without causing local heating; this process is called photo-chemical ablation. The photo-chemical ablation results in clean-cut incision. The thermal component is relatively small and the zone of thermal interaction is limited in the incision wall.

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5. Selective absorption of laser light by human tissues Selective absorption occurs when a given color of light is strongly absorbed by one type of tissue, while transmitted by another. Lasers’ pure color is responsible for its selective absorption by body tissues. This selectivity provides a handle for laser surgery and therapy. Each type of tissue has its specific absorption characteristics depending on its components, and the main absorbing components of tissues are:

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Hemoglobin (in blood): the blood’s oxygen carrying protein, absorption of UV and blue and green light,

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Melanin (a pigment in skin, hair, moles, etc): absorption in visible and near IR light (400nm – 1000nm),

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Water (in tissues): transparent to visible light but strong absorption of UV light below 300nm and IR over 1300nm

Absorption spectrum for hemoglobin and melanin.

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6. Applications of lasers

6.1 General purpose: surgery: incision, ablation, coagulation • CO2 laser operating at 10.6 mm in the far – infrared region, most effective for •

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incision while minimizing bleeding. Tissue at the focus plane of the laser is subject to high irradiance and being ablated for incision, while by defocusing, coagulation can be made. The Nd:YAG laser operates at 1.064mm in the near infrared and employed where heavy tissue coagulate is desired, electrocautery is replaced by laser now.

6.2 Lasers in beauty therapy • Skin rejuvenation: IR lasers are used to remove extremely thin layer of skin (<0.1 mm). In the absence of pigment in general, they take advantage of the presence of water in the skin to provide an ability to remove skin and body tissue.

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Hair removal: here selective absorption with absorbing component being melanin pigment in hair and follicle, it is best worked with a red light ruby laser. White hair can not be treated with any laser due to the lack of absorbing component.

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Port-wine stains: this is made with a yellow dye pulsed laser 585nm. It corresponds to the peak absorption of the hemoglobin.

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Tattoo Removal: tattoo can be removed with variety of laser depending on the absorption of the ink in the tattoo. All these applications are based on the selective absorption and photo-vaporization of the treated components and in general pulsed lasers are used. 6.3 Lasers in ophthalmology Visible light is transparent to the cornea and crystalline lens, and can be focused with eye’s lens on the retina. So for retina operation, visible laser can be used. The most popular visible laser is the green argon laser.

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Treatment of glaucoma: due to excess pressure builds up. Argon laser is focused externally on iris to make incision, creating drainage holes for excess aqueous humors Diabetic retinopathy: inadequate blood supply to the retina due to diabetes. Small photocoagulation burn by green argon laser to repair the retina due to vessels leakage. Retina tear: photocoagulation burn to repair retina tears due to trauma to the head.

UV light emitted by the excimer laser is strongly absorbed by water and proteins, so their energy can be absorbed by transparent cornea and lens, permitting laser surgery on these areas. • •

Cataracts: a milky structure in the lens of the eye. Photo-vaporization by using UV laser to remove the obaque regions. Correction of myopia: over focusing of the lens. Excimer laser removal of surface of cornea to make it flatten.

7. Laser hazards and protections 7.1 Hazards to the eye The retina The directionality of a laser beam permits the ray to be focused to an extremely small spot on the retina. A collimated laser will be concentrated by a factor of 100,000 when passing from cornea to retina. So Visible or near IR lasers (400 nm to 1400nm) are particularly dangerous to the retina and always requires eye-protection when working with these kind of lasers.

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The cornea and lens • Cornea is accessible to danger of UV and most of IR lasers, • UV-A, UV-B (between 295nm and 320 nm) and IR-A (between 1 to 2 mm) are dangerous for lens, • 308-nm (UV-B) excimer XeCl laser is particular dangerous because of it can simultaneously damage the lens, the cornea and the retina.

Eye protection Eyewear (goggles) is the most common laser protective measure, especially for open laser beams. It should be good design with all around shielding and adequate visible light transmission. Identification of the eyewear : All laser protective eyewear shall be clearly labelled with information adequate to ensure the proper choice of eyewear with particular lasers. Required optical density (OD)

7.2 Hazards to the skin Unless for high power laser, the skin is less vulnerable to laser hazards than is the eye, because that skin is rarely at the focal point of the laser spot. Skin protection •Skin protection is normally not required with the use of most non-UV lasers •With the use of UV lasers, ordinary street -wear or surgical gowns would normally provide a protective factor of 1000, •Gloves will provide sufficient protection too. 7.3 Laser classification Class I -Exempt from laser control measures Lasers are incapable of producing damaging radiation levels, no control measures are required, including the use of warning labels, Good practice and good common sense dictate that unnecessary and prolonged gazing at the laser beam must be avoided.

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Class II - Low power laser CW Lasers of output less than 1 mW, incapable of causing eye injury accidentally, but can produce retinal injury with continuous intra-beam viewing. Control measures: • avoid staring at the beam for a long time. • "caution" label should be placed prominently on the laser housing and/or control panel. Class III (a) - Medium Power laser CW Laser (visible) of output up to 5 mW, incapable of injuring the eye within the duration of the blink or aversion response (0.25 second). Injury is, however, possible using binoculars or similar optical devices, or by continued staring at the beam. Control measures: •warning label should be placed prominently on the laser housing and/or control panel. •suitable engineering controls should be adopted. •suitable administrative controls should be adopted. •wearing protective eyewear and introduction of the laser control area are advisable. Class III b - Medium Power laser Lasers output in any part of spectrum (200nm to 1mm), CW power up to 500mW. Capable of causing accidental injury to the eye if the beam is viewed directly from a specular reflection. Control measures: •warning label should be placed prominently on the laser housing and/or control panel. •suitable engineering controls should be adopted. . •Suitable administrative controls should be adopted. •Wearing protective eyewear and introduction of the laser control area are advisable Class IV – High power lasers : CW power > 0.5 W All viewing is hazardous. Risk of skin burns. Extreme caution essential.

Most of medical lasers are class 4 lasers.

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Optional reading : Oxymetry A oximeter displays the percentage of arterial hemoglobin in the oxyhemoglobin configuration. Acceptable normal ranges are from 95 to 100 percent. A pulse oximeter is a particularly convenient noninvasive measurement instrument. Typically it has a pair of small light-emitting diodes facing a photodiode through a translucent part of the patient's body, usually a fingertip or an earlobe. One LED is red, with wavelength of 660 nm, and the other is infrared, 910 nm. Absorption at these wavelengths differs significantly between oxyhemoglobin and its deoxygenated form, therefore from the ratio of the absorption of the red and infrared light the oxy/deoxyhemoglobin ratio can be calculated.

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