LASER APPLICATIONS TO MEDICINE AND BIOLOGY Prof Dr

LASER APPLICATIONS TO MEDICINE AND BIOLOGY Prof. Dr. Moustafa M. Mohamed Biophysics Department, Medical Research Institute, Alexandria University

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

Basic Concepts: n n 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) : n n n 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: n spontaneous emission and n stimulated emission.

Spontaneous emission: n n n 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: n n 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 hν Ground state

Absorption: n 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 E absorption. 2 hn=E 2 – E 1

n n n 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 E 1 and E 2 are: N 1 = No e-E 1/k. T N 2 = No e-E 2/k. T If E 1 < E 2 Then N 1 > N 2 If E 1 < E 2 and N 1 < N 2 This is called "population inversion".

Population inversion: n 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).

n n 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.

Population Inversion n E 1 = Ground state, E 2 = Excited state (short life time ns), E 3 = Metastable state (long life time from ms to s). Life times hn =5500 Ao E 3 10 -9 sec E 2 10 -3 -1 sec Output (amplification) E 1 Excitation

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

Pumping sources: n Optical pumping: suitable for liquid n and solid laser because they have wide absorption bands. Electric pumping: suitable for gas laser because they have narrow absorption band. n Chemical reaction.

Types of lasers: Lasers are classified according to laser active medium into: n Solid: for example : n n n Ruby (Cr: Al 2 O 3) Neodymium- glass (Nd-Glass). Nd- YAG (Nd-Yttrium, Aluminum granite) Liquid lasers: (Dyes). Gas lasers: He-Ne, Ar, CO 2, He-Cd, N 2, Kr, Excimer (Ar. F, Xe. F, HF, DF).

Laser Beam n n Coherent (in phase) Monochromatic (single wavelength) Collimated (highly parallel) Intense (Concentrated)

USES OF LASER TECHNOLOGY INCLUDING: n n n SCIENCE MACHINING COMMUNICATIONS SECURITY/MILITARY MEDICINE

Historical introduction n n 1946: A German physician, Gerd Meyer, used the sun to treat detached retinas and destroy tumors in some of his patients eyes. 1948: High intensity xenon lamp used for photocoagulation 1961: one year after Maiman built the first laser, Milton Zaret used laser to produce ocular lesions in animals. 1963: Chris Zweng treated retinal disease in his patients using laser beam

Heat By Laser n n Intense Heat Destructive effects can be extremely selective and precisely controlled Reversible effect 37 C Protein Denaturation 60 C Coagulation 80 C Vaporization and ablation 100 C Homeostasis Welding Cutting

Laser Tissue Interaction: b REFLECTION SCATTERING LASER BEAM a TARGET TISSUE Transmitting

c d e f g FLUORESCENCE For diagnostic PHOTOCHEMISTRY Destroy the target e- e - HEAT PHOTODISSOCIATION (Break molecular bond) SHOCK WAVE (Breaks mineralized deposits)

TREATMENT & DIAGNOSTIC BY LASER n n n PHOTOCOAGULATION OF THE RETINA Heating a blood vessel to a point where the blood coagulate and block the vessel. Photocoagulation can be done by: n n 1 - Xenon lamp 2 - Laser

Photocoagulation Xenon lamp: Laser Spot size 750 m m High energy deposited in the eye: 20 -50 times greater than deposited treatment by laser beam Spot size 50 mm low energy deposited in the eye Longer exposure (1 sec) than laser, so local anesthesia must be used Short exposure (ms to ms) So local anesthesia are not needed

Laser Treatment & Diagnostics n n 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.

Diagnostic Laser System Several factor have to be consider in designing a diagnostic laser system: 1 - A suitable excitation wavelength. 2 - Knowledge about fluorescence properties of different chromospheres in tissue is needed. 3 -Origin of the fluorescence spectra must be identified. 4 - Tumor seeking drugs (e, g. hematoporphoryin) is used to enhance the optical demarcation of malignant tumors.

Surgical Application of Laser n n n n Tissue heating (Skin rejuvenation & tissue welding) Coagulation Vaporization Fragmentation of tattoo pigment Cold cutting Photoacoustic (lithotripsy) Photodissociation (non-thermal ablation of the cornea in ophthalmology).

Retina Treatment n n The dark brown melanin pigment of the retina absorb the green beam of the argon laser. The argon laser can destroy specific regions of the retina without harming the other area of the eye, which absorb different wavelength of light.

Red birthmarks (Port wine stains) n n Red birthmarks also absorb the argon laser, which could be blue or green depending on its wavelength. The absorbed light destroys hundreds of the extra blood vessels that beneath the skin’s outer layer and discolor it.

Disadvantages n n The heat generated by the beam can sometimes spread to parts of the skin other than the abnormal blood vessels and cause scarring or loss of pigments. R. Rox Andrson and A. Jhon (1983) (Harvard University) suggested that short exposure less than 1 ms – to intense light would destroy the absorption site but produce little or no damage to adjacent tissue.

Advantage of wide damage n n Wide damage caused by the longer slower heating of tissue can be turned advantage. Removing of a damaged portion of the liver cause extensive bleeding. The long exposure to a continuous wave laser reduces bleeding because heat spreads to the capillaries nearby. A CO 2 laser with a wavelength 10. 6 microns may be used because it is absorbed by the compound most common to tissue: Water

Intraocular Nd: YAG Laser Damage Mechanisms of intraocular Nd: YAG Laser Surgery (Single laser pulse: n Plasma formation and expansion n Emission of acoustic transient n Cavitations with jet formation

Pulsed lasers can also remove tissue n n Er-YAG Laser (Erbium Yttrium-Aluminum Garnet) which has a wavelength of 2. 9 micron and pulse duration of 200 us, can cleanly ablate calcified bone. Xenon chloride excimer laser (0. 308 microns and pulse duration of 10 ns) can vaporize bone with little or no associated thermal damage.

Laser and Fiber Optics n Coupling lasers with other technologies such as fiber optics, one can achieve non- thermal, as well as thermal, results in previously inaccessible parts of the body.

Photodynamic Therapy of Cancer n n n A dye selectively concentrates in cancerous tissue 48 to 72 hours after it is injected. Blue-violet light from krypton laser, administrated through an optical fiber, causing dye to fluorescence, so it can easily be observed and diagnosed. The optical fiber then drives laser light of another wave length, which destroys the tumor.

Laser Angioplasty n n The removal of plaque in obstructed vessel by laser, administrated through a fiber optics. Fluorescence characterization of the vessel wall could be performed via the same fiber as that used for the delivery of high-power pulses for plaque removal.

Ultrastructural changes of Staph. aureus by laser irradiation:

There are many benefits of laser dentistry. They include: n n n Faster healing. Reduced risk of infection Decreased Sensitivity. Less time in the dental chair. Less bleeding. Less post-treatment discomfort

laser Doppler velocimeter n n n The laser Doppler velocimeter sends a monochromatic laser beam toward the target and collects the reflected radiation. According to the Doppler effect, the change in wavelength of the reflected radiation is a function of the targeted object's relative velocity. Thus, the velocity of the object can be obtained by measuring the change in wavelength of the reflected laser light, which is done by forming an interference fringe pattern.

Typical Laser Doppler Velocity meter (Velocimeter) n n n A laser power source is the essential part a Helium-Neon (He-Ne) or Argon ion laser with a power of 10 m. W to 20 W is used. Lasers have many advantages over other radiation/wave sources, including excellent frequency stability, small beam diameter (high coherence), and highly-focused energy.

Doppler Effect

Comparison Between the Single Beam and Cross-Beam Systems n single-beam system, - employs a focused laser beam which is scattered from particles. - n A portion of the scattered light is sampled and mixed with a portion of the unscattered laser beam and collected by an o p t i c a l - p h o t o m u l t i p l i e r system The two l i g h t beams heterodyne to yield the difference frequency between the two light beams.

This difference frequency, or Doppler frequency, is related to the particle velocity in the flow. Df. D = (vs/l)(cos(a-q’)-cos a) where Df. D Doppler frequency, Hz vs particle velocity, m/s l wavelength of laser beam, m a angle between particle velocity vector and laser beam, deg q¢ angle between laser beam and scattered light, deg n

Lasers are classified according to the hazard • • Class 1 and 1 M (magnifier) lasers are considered safe Class 2 and 2 M (magnifier) • • Class 3 R (Restricted) Laser • • produce visible and invisible light that are hazardous under direct viewing conditions; Class 3 B lasers • • 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; 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 3 B 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;