Laser Tissue Interaction

Light Interacts Matter

• Interaction will be based on• Light source (wave length, spot size, pulse

duration)• Tissue characters (optical properties of tissue

such as absorption coefficient, scattering coefficient, refractive index)

• Each tissue has specific absorption characteristics based on composition and chromophore contents

• Major Chromophores are • Hemoglobin• Melanin• Water• Protein

ABSORPTION• During absorption, the intensity of an incident

electromagnetic wave is attenuated in passing through a medium.

• The absorbance of a medium is defined as the ratio of absorbed and incident intensities.

• Absorption is due to a partial conversion of light energy into heat motion or certain vibrations of molecules of the absorbing material.

• A perfectly transparent medium permits the passage of light without any absorption, i.e. the total radiant energy entering into and emerging from such a medium is the same.

• Among biological tissues, cornea and lens can be considered as being highly transparent for visible light.

• Laser light entering the biological tissue is either scattered or absorbed.

• Scattering is a process by which energy in a beam is redirected without a change in its wavelength.

• Generally scattering and absorption affect the distribution of photons in the tissue target, but absorption alone determines the effect of the radiation.

• Absorption of a photon may alter the electronic structure of a molecule. The electrons on the outer shell will jump to a higher energy level.

• An excited molecule usually looses the excess energy by• Chemical reaction (photochemical ) • Collides with another molecule or atom • Releases a photon in a spontaneous emission • The excess electron energy is converted into increased relative

motion which in turn • leads to heat production – thermal effect

• Laser effects in biological tissues may be divided into three general categories

1. Photochemical2. PhotoThermal3. Photo Ionization

Flowchart of photothermochemical processes in thermal interaction of laser light

with biological tissues.

Penetration power of Medical Laser

Photoablation

• It is a Photochemical effect• The surface will be removed layer by layer in a very

precise manner. • Photons from a laser have sufficient energy to break

chemical bonds between molecules at the target site• The photon energy breaking the bond and ejects

molecular fragments from the target site at supersonic velocities (2000 m/s)

• E.g • For corneal Photoablation: ArF excimer laser 193 nm and 248nm

Krypton KrF

Photocoagulation

• Photocoagulation is thermal effect• Temperature rise in irradiated tissue is

proportional to light absorption in that tissue• Closure of blood vessels• Heats hemoglobin, collagen and surrounding

connective tissues• Caution: thermal injuries

Photo disruption• Photo disruption is ionizing effect• Irradiate the tissue by laser• This irradiances are obtained by using a very small spot sizes

and extremely brief durations ranging from 30 to 20 ps (Pico seconds).

• The high irradiance ionizes material• Collections of ions and electron to form plasma• The plasma absorbs or scatters radiation and mechanically

disrupts tissue• Additional disruption occurs because of latent stress present

in the tissue when the laser incision occurs.• E.g. ND-YAG 1064 nm

Pathology of laser reaction in skin

• Tissue necrosis• Steam bubbles in superficial portion of tissues• Thrombosis and a decrease in number of vascular channels in

the target areas• No specific changes in blood vessel wall other than rupture• Circulation changes• No specific effect on enzymes but temperature may affect• Non-specific fibrosis (but Ruby, Neodymium pulsed Laser,

Argon & CO2 cw laser are not produce fibrosis )• Dermatitis, …. Etc (different laser produce different

pathological reaction)

Thermal and non-thermal effects of laser irradiation

Thermal Interactions

• Thermal interactions are most important aspect

• Effects will be based on • Time duration• Area of exposure

• 37 ˚C +21 ˚C = 58 ˚C --- produce cell destruction for exposure longer than 10sec

• Tissue can withstand temp up to 70 ˚C if the duration of exposure very short less than 1 sec

Factors for the Thermal Effects

• The absorption and scattering coefficients of the tissues at the laser wavelength.

• Irradiance or radiant exposure of the laser• beam.• Duration of the exposure and pulse repetition

characteristics.• Extent of the local vascular flow.• Size of the area irradiated.

Basic mechanisms

• The basic mechanisms of thermally induced tissue destruction result from

• Denaturation of cell protein

• Interference with basic cell metabolism

• Interference with vascular blood supply

Thermal effect on the tissue

• Reversible hyperthermia (T>31o C) – some functions of the tissue can be perturbed but the effect is reversible

• Irreversible hyperthermia (T>42o C) – some fundamental functions of the tissue can be destroyed irreversibly

• Coagulation (T>60o C) – the tissue becomes necrotic• Vaporization (T≥100o C)• Carbonization (T>150o C) [conversion of an organic

substance into carbon or a carbon-containing residue]• Pyrolysis (T>300o C) (Pyrolysis is a thermochemical

decomposition of organic material at elevated temperatures without the participation of oxygen)

Parameters to be considered

• Source:– Energy– Focus optics– Beam divergence– Pulse duration, etc

• Target area:– Absorption coefficient– Scattering coefficient– Refractive index– Reflection, etc

Non-Thermal interactions

• Without producing heat• ex:– Photo Dynamic Therapy (PDT)– Biostimulation

• Photo chemical interations

Bio-stimulatory effects of laser

• The promotion of healing of wounds.

• Treatment of skin infections.

• Treatment of ulcers.

• Laser may have an enhancing effect on healing

wherever inflammation is present.

Effects of Laser Light on Tissue

• Accelerated Tissue Repair (Stimulate removal and regeneration by stimulation of ATP)

• Rapid Formation of Collagen (stimulate fibroblast cells)

• Beneficial Effect on Nerve Cells and the Production of B-Endorphins, enchephalins

• Accelerated Lymphatic System Activity and Reduction in Edema

• Formation of New Capillaries and Increased Blood Flow