LASER APPLICATIONS TO MEDICINE AND

Prof. Dr. Moustafa. M. MohamedVice Dean

Faculty of Allied Medical SciencePharos University

Alexandria

Dr. Mervat Mostafa

Department of Medical Biophysics

Pharos University

FIRST OFF WHAT DOES LASER STAND FOR?

LLIGHTIGHT AAMPLIFICATION BY MPLIFICATION BY SSTIMULATED TIMULATED EEMISSION OF MISSION OF RRADIATIONADIATION

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.

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

h=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-E

1/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.

Population Inversion E1 = Ground state, E2 = Excited state (short life time ns), E3 = Metastable state (long life time from

ms to s).

hE1

E2

E3

10-3 -1 sec

10-9 sec

Output (amplification)

Life times

Excitation

To generate laser beam three processes must be satisfied:- Population inversion. Stimulated emission. Pumping source.

MEDIUM

PUMPMIRROR

COLLIMATEDBEAM

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.

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.

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.

Types of lasers

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.

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.

How Ruby laser works?

High and Low Level Lasers High Level Lasers

–Surgical Lasers –Hard Lasers –Thermal –Energy (3000-10000) mW

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 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:

What Does Laser Do? Laser light waves penetrate the skin with no

heating effect, no damage to skin & no sideeffects.

Laser light directs biostimulative light energy to the body’s cells which convert into chemical energy to promote natural healing & pain relief.

Stimulation of wound healing – Promotes faster wound healing/clotformation –Helps generate new & healthy cells & tissue

Increase collagen production –Develops collagen & muscle tissue

Increase macrophage activity – Stimulates immune system

Alter nerve conduction velocity – Stimulates nerve function

What Does Laser Do?

Improved blood circulation & vasodilation – Increases blood supply • Increases ATP production • Analgesic effect – Relieves acute/chronic pain • Anti-inflammatory & anti-edematous effects – Reduces inflammation

What Does Laser Do?

Tissue & Cellular Response Magnitude of tissue’s reaction are based on

physical characteristics of:

–Output wavelength/frequency –Density of power –Duration of treatment – Vascularity of target tissues

Direct and indirect laser effects Direct effect - occurs from absorption of photons Indirect effect – produced by chemical events caused by interaction of photons emitted from laser and the tissues

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;