Laser and its medical applications

A wide application of laser in medicine and beauty therapy Surgical laser: removing tumors, making incisions. Cosmetic treatments: resurfacing, removal of birth mark, age spots, spider veins, hair, tattoos, Ophthalmology: inner eye surgery in removing cataract, repairing retina, correct nearsightedness.

FIRST OFF WHAT DOES LASER STAND FOR? LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION

Basic Concepts: Laser is a narrow beam of light of a single wavelength (monochromatic) in which each wave is in phase (coherent) with other near it. Laser apparatus is a device that produce an intense concentrated, and highly parallel beam of coherent light.

Basic theory for laser (Einstein 1917) : Atom composed of a nucleus and electron cloud If an incident photon is energetic enough, it may be absorbed by an atom, raising the latter to an excited state. It was pointed out by Einstein in 1917 that an excited atom can be revert to a lowest state via two distinctive mechanisms: Spontaneous Emission and Stimulated Emission.

electron orbits displayed as an energy level diagram energy is plotted vertically with the lowest (n=1) , or ground stat, and with exited states (n=2, 3,4,…) above. n is called orbit number and can be only positive integer.

The electron can absorb energy and jump to a higher level, the process is called excitation. (b) A photon is emitted when an electron change from a higher orbit to a lower orbit with a characteristic emission spectrum. This process is called de-excitation. (c) If an atom absorbs a photon, an electron jumps from a lower orbit to a higher orbit with a characteristic absorption spectrum.

where E is the change in energy between the initial and final orbits. Atom will absorb and emit light photons at particular wavelength corresponding to the energy differences between orbits. The wavelength l of emitted or absorbed photon can be obtained by the formula: where E is the change in energy between the initial and final orbits. A variety of biological molecules have notable absorption spectra in the visible, IR, and UV. This has many clinical application. e.g. Oximeter. The level of oxygen in the blood can be determined by measuring the absorption spectrumof hemoglobin in the blood at several wavelengths. Its absorption spectra can be used to compare with those of oxyhemoglobin and deoxyhemoglobin can be determined and used to compute the oxygen level by a device called an oxumeter.

Spontaneous emission: Each electron can drop back spontaneously to the ground state emitting photons. Emitted photons bear no incoherent. It varies in phase from point to point and from moment to moment. e.g. emission from tungsten lamp.

Stimulated emission: Each electron is triggered into emission by the presence of electromagnetic radiation of the proper frequency. This is known as stimulated emission and it is a key to the operation of laser. e.g. emission from Laser Excited state Ground state hν

Absorption: Let us consider an atom that is initially in level 1 and interacts with an electromagnetic wave of frequency n. The atom may now undergo a transition to level 2, absorbing the required energy from the incident radiation. This is well-known phenomenon of absorption. E1 E2 hn=E2 – E1

According to Boltzmann's statistics, if a sample has a large number of atoms, No, at temperature T, then in thermal equilibrium the number of atoms in energy states E1 and E2 are: N1 = No e-E1/kT N2 = No e-E2/kT If E1 < E2 Then N1 > N2 If E1 < E2 and N1 < N2 This is called "Population Inversion".

Population inversion: Generally electrons tends to (ground state). What would happen if a substantial percentage of atoms could somehow be excited into an upper state leaving the lower state all empty? This is known as a population inversion. An incident of photon of proper frequency could then trigger an avalanche of stimulated photon- all in phase (Laser).

Consider a gas enclosed in a vessel containing free atoms having a number of energy levels, at least one of which is Metastable. By shining white light into this gas many atoms can be raised, through resonance, from the ground state to excited states.

Output (amplification) Population Inversion E1 = Ground state, E2 = Excited state (short life time ns), E3 = Metastable state (long life time from ms to s). Life times E3 10-9 sec E2 10-3 -1 sec Output (amplification) hn =5500 Ao E1 Excitation

To generate laser beam three processes must be satisfied:- Population inversion. Stimulated emission. Pumping source. MEDIUM PUMP MIRROR COLLIMATED BEAM

Pumping Sources Optical Pumping: Suitable For Liquid And Solid Laser Because They Have Wide Absorption Bands. Electric Pumping: Suitable For Gas Laser Because They Have Narrow Absorption Band. Chemical Reaction.

High and Low Level Lasers High Level Lasers –Surgical Lasers –Hard Lasers –Thermal –Energy (3000-10000) mW

Types of lasers According to the active material: solid-state, liquid, gas, excimer or semiconductor lasers. According to the wavelength: Infra-red (IR), Visible, Ultra-violet (UV) or X-ray Lasers.

Types of lasers Solid-state lasers have lasing material distributed in a solid matrix (such as ruby or Nd-YAG). Flash lamps are the most common power source. The Nd-YAG laser emits infrared light at 1.064 nm. Semiconductor lasers, sometimes called diode lasers, are p-n junctions. Current is the pump source. Applications: laser printers or CD players.

Types of lasers Dye lasers use complex organic dyes, such as Rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths. • Gas lasers are pumped by current. Helium- Neon (He-Ne) lasers in the visible and IR. Argon lasers in the visible and UV. CO2 lasers emit light in the far-infrared (10.6 mm), and are used for cutting hard materials.

Excimer lasers: (from the terms excited and dimers) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as argon, krypton, or xenon. When electrically stimulated, a pseudo molecule (dimer) is produced. Excimers laser in the UV.

Solid-state Laser Example: Ruby Laser Operation wavelength: 694.3 nm (IR) 3 level system: absorbs green/blue Gain Medium: crystal of aluminum oxide (Al2O3) with small part of atoms of aluminum is replaced with Cr3+ ions. Pump source: flash lamp The ends of ruby rod serve as laser mirrors.

Ruby Laser

How Ruby laser works? 1. High-voltage electricity causes the quartz flash tube to emit an intense burst of light, exciting some of Cr3+ in the ruby crystal to higher energy levels.

How Ruby laser works? 2. At a specific energy level, some Cr3+ emit photons. At first the photons are emitted in all directions. Photons from one Cr3+ stimulate emission of photons from other Cr3+ and the light intensity is rapidly amplified.

How Ruby laser works? 3. Mirrors at each end reflect the photons back and forth, continuing this process of stimulated emission and amplification

How Ruby laser works? 4. The photons leave through the partially silvered mirror at one end. This is laser light.

Low Level Lasers –Medical Lasers –Soft Lasers –Subthermal –Energy (1-500) mW –Therapeutic (Cold) lasers produce maximum output of 90 mW or less (600-1000) nm light

Parameters Laser –Wavelength –Output power – Average power – Intensity –Dosage

Wavelength Nanometers (nm) Longer wavelength (lower frequency) = greater penetration Not fully determined Wavelength is affected by power

Power Intensity Output Power –Watts or milliwatts (W or mW) –Important in categorizing laser for safety Intensity Power Density (intensity) –W or mW/ cm2 – Takes into consideration – actual beam diameter If light spread over lager area – lower power density – Beam diameter determines power density

Average Power Knowing average power is important in determining dosage with pulsed laser If laser is continuous – average power = peak output power If laser is pulsed, then average power is equal to peak output power X duty cycle.

Energy Density Dosage (D) Amount of energy applied per unit area Measured in Joules/square cm (J/cm2) – Joule – unit of energy – 1 Joule = 1 W/sec Dosage is dependent on: –Output of laser in mW. – Time of exposure in seconds. – Beam surface area of laser in cm2

Laser Treatment & Diagnostics Treatment cover everything from the ablation of tissue using high power lasers to photochemical reaction obtained with a weak laser. Diagnostics cover the recording of fluorescence after excitation at a suitable wavelength and measuring optical parameters.

Laser Tissue Interaction:

2. How laser works

Spontaneous emission and stimulated emission An excited electron may gives off a photon and decay to the ground state by two processes: spontaneous emission: neon light, light bulb stimulated emission : the excited atoms interact with a pre-existing photon that passes by. If the incoming photon has the right energy, it induces the electron to decay and gives off a new photon. Ex. Laser.

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

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

Characteristics of laser The second photon has the same energy, i.e. the same wavelength and color as the first – laser has a pure color It travels in the same direction and exactly in the same step with the first photon – laser has temporal coherence Comparing to the conventional light, a laser is differentiated by three characteristics. They are: Directionality, pure color, temporal coherence.

Characteristics of laser Pure color Directionality Temporal coherence

The power and intensity of a laser The power P is a measure of energy transfer rate; where the unit of power is Joules/s or W. The energy encountered by a particular spot area in a unit time is measured by the intensity (or power density):

laser versus ordinary lights: The directionality of laser beam offers a great advantage over ordinary lights since it can be concentrate its energy onto a very small spot area. This is because the laser rays can be considered as almost parallel and confined to a well-defined circular spot on a distant object.

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

Fluence, F is defined as the total energy delivered by a laser on an unit area during an expose time TE, F(J/cm2)=I(watts/cm2) x TE(s) The advantage of directionality of a laser : we can focus or defocus a laser beam using a lens. This can be used to vary the intensity of the laser. f 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. Incoming parallel ray Focused spot Diverged beam

continuous wave (CW) lasers versus pulsed lasers CW lasers has a constant power output during whole operation time. pulsed lasers emits light in strong bursts periodically with no light between pulses usually T>>Tw

The tw may vary from milliseconds (1ms=10-3 s) to femtoseconds (1fs=10-15s), but typically at nanoseconds (1ns=10-9s). energy is stored up and emitted during a brief time tw, this results in a very high instantaneous power Pi the average power Pave delivered by a pulsed laser is low. Instantaneous power Pi Average power Pave Where R is the repetition rate

Example A pulsed laser emits 1 milliJoule (mJ) energy that lasts for 1 nanoseconds (ns), if the repetition rate R is 5 Hz, comparing their instantaneous power and average power. (The repetition rate is the number of pulses per second, so the repetition rate is related to the time interval by R=1/T).

Mechanism of laser interaction with humen tissues

When a laser beam projected to tissue Five phenomena: reflection, transmission, scattering, re-emission, absorption. In most cases, the photons energy of laser light is transferred to blood, tissues or bone in the form of heat. In other cases laser can also transfer photon energy to chemical bond energy of the molecules in human body, such as DNA. This risks the modification of genetic information. There are generally three interaction mechanisms involved. Laser light interacts with tissue and transfers energy of photons to tissue because absorption occurs.

Photocoagulation What is a coagulation? A slow heating of muscle and other tissues is like a cooking of meat in everyday life. The heating induced the destabilization of the proteins, enzymes. 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 of tissues above 50 oC but below 100oC induces disordering of proteins and other bio-molecules, this process is called photocoagulation.

Consequence of photocoagulation When lasers are used to photocoagulate tissues during surgery, tissues essentially becomes cooked: they shrink in mass because water is expelled, the heated region change color and loses its mechanical integrity cells in the photocoagulated region die and a region of dead tissue called photocoagulation burn develops can be removed or pull out,

Applications of photocoagulation destroy tumors treating various eye conditions like retinal disorders caused by diabetes hemostatic laser surgery - bloodless incision, excision: due to its ability to stop bleeding during surgery. A blood vessel subjected to photocoagulation develops a pinched point due to shrinkage of proteins in the vessel’s wall. The coagulation restriction helps seal off the flow, while damaged cells initiate clotting.

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 possible for : hemostatic incision,or excision. complete removal of thin layer of tissue. Skin rejuvenation, resurfacing

Conditions for photo-vaporization the tissue must be heated quickly to above the boiling point of the water, this require very high intensity lasers, a very short exposure time TE, so no time for heat to flow away while delivering enough energy, highly spatial coherence (directionality) of lasers over other light sources is responsible for providing higher intensities

Intensity requirement Intensity (W/cm2) Resulting processes Low (<10) General heating Moderate (10 – 100) Photocoagulation High (>100) Photo-vaporization

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

Selective absorption of laser light by human tissues

Selective absorption Selective absorption occurs when a given color of light is strongly absorbed by one type of tissue, while transmitted by another. Lasers’ pure color is responsible for selective absorption. The main absorbing components of tissues are: Oxyhemoglobin (in blood): the blood’s oxygen carrying protein, absorption of UV and blue and green light, Melanin (a pigment in skin, hair, moles, etc): absorption in visible and near IR light (400nm – 1000nm), Water (in tissues): transparent to visible light but strong absorption of UV light below 300nm and IR over 1300nm

Selective absorption

Applications of lasers

Lasers in beauty therapy Lasers application in beauty therapy are based on: selective absorption of absorbing components. photo-vaporization process for removal of the treated components. pulsed lasers are used.

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

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

Laser removal of port-wine stain Yellow laser is absorbed by the presence of hemoglobin in blood vessels.

Laser removal of tattoo tattoo can be removed with variety of laser depending on the presence of inks in the tattoo.

Lasers in ophthalmology For retina operation, visible laser can be used. Visible light is transparent to the cornea and crystalline lens, and can be focused with eye’s lens on the retina. The most popular visible laser is the green argon laser. Treatment of glaucoma: Argon laser is focused externally on iris to make incision, creating drainage holes for excess aqueous humors to release pressure, Retina tear: photocoagulation burn to repair retina tears due to trauma to the head. 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.

Lasers in ophthalmology For cornea and lens, UV light emitted by the excimer laser is strongly absorbed by water and proteins, so their energy can be absorbed by transparent cornea and lens, permitting laser surgery on these areas. Cataracts: a milky structure in the lens of the eye. Photo-vaporization by using UV laser to remove the obaque regions. Correction of myopia: over focusing of the lens. Excimer laser removal of surface of cornea to make it flatten.

Laser hazards and protections

Absorption of the eye

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. 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.

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

Protection to the eye 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.

LASER Regulation Lasers are classified according to the hazard; * Class 1 and 1M (magnifier) lasers are considered safe * Class 2 and 2M (magnifier) - emit visible light at higher levels than Class 1, - eye protection is provided - can be hazardous if the beam is viewed directly with optical instruments;

* Class 3R (Restricted) Laser - produce visible and invisible light that are hazardous under direct viewing conditions; * Class 3B lasers - produce visible or invisible light that is hazardous under direct viewing conditions - they are powerful enough to cause eye damage in a time shorter - Laser products with power output near the upper range of Class 3B may also cause skin burns;

* Class 4 lasers - high power devices capable of causing both eye and skin burns, - heir diffuse reflections may also be hazardous - the beam may constitute a fire hazard;