Lecture 9 Flashcards
Properties of the tissue that light interaction depends on
Optical properties
Thermal properties
Chemical properties
Light interactions depend on
Laser irradiance (1j/cm^2 - 1000 j/cm^2)
Exposure time
In biological tissues absorption is caused by
Water molecules
Proteins
Pigments
Light tissue interaction mechanisms
Photochemical interaction
Thermal interaction
Photo ablation
Plasma-induced ablation
Photodisruption
Medical application of photochemical effect
Photodynamic therapy (PDT)
Chromophores capable of causing light-induced reactions in non-absorbing molecules
Photo sensitizers
Irradiance and exposure time of photochemical effect
Irradiance: 1 W/cm^2
Exposure time: seconds or continuous
PDT depends on
The excitation of oxygen molecules from the triplet ground state to a singlet excited state leading to the formation of ROS which is an aggressively reactive species that causes necrosis of affected tissues
Disadvantages of PDT
- Long time of tissue decay and removal from body
- Patients become photosensitive for several weeks or days
- Might be painful and toxic
Clinical uses of PDT
Cancers: Lung/ skin/esophagus/ head and neck/post-radiation therapy
Non-cancer: Arterial diseases/ ophthalmology
PDT is similar to
SDT (sonodynamic therapy, uses ultrasound instead of light)
2 processes important for photothermal interaction to happen
- Absorption –> excitation
- Deactivation (nonradiative decay) –> inc in kinetic energy which leads to increasing tissue temperature
Irradiance and exposure time of photothermal interaction
Irradiance: 10-10^6 w/cm^2
Exp time: 1 min - 1 ms
Photothermal interaction depends on which tissue optical properties
- Absorbed power density (H)
- Probability per unit length of absorbed photon (absorption coeff)
- Irradiance or fluence
Photothermal interaction depends on which tissue thermal properties
- Heat capacity
- Thermal conductivity
Leads to non-radiative processes such as internal conversion and vibrational relaxations
Thermal absorption
Thermal effects of the tissue depend on
Wavelength of beam and exposure time
Heating of tissue leads to
- Coagulation
- Vaporization
- Carbonization
- Melting
Irreversible necrosis without immediate tissue destruction
Coagulation
Temperature required for coagulation
50-100°C for even a second
Coagulation happens due to
Denaturation of proteins and collagen as a result of increase in temperature
A process which produces thermal ablation of the tissue
Vaporization
Temperature required for Vaporization
at least 100°C (as water turns into steam)
Thermo-mechanical effect caused by pressure build-up due to steam formation
Thermal ablation
A process where tissue chars and its organic constituents become carbon
Carbonization
Temperature required for carbonization
at least 150°C
Why should carbonization be avoided
As it leads to irreparable damage and has no benefit
A process used for tissue welding
Melting
Temperature required for melting
Must be higher than the melting point of the tissue (for milli or nanoseconds)
Melting happens due to
High power density of a pulse laser beam for just a few milli or nanoseconds
Lasers used for thermal applications depend on
- Wavelength of the beam
- Absorption coefficient for each tissue type
Applications of photothermal interactions
- Minor skin surgery
- Cosmetic applications
- Hair/ tattoo removal
- Photothermal treatments in ophthalmology
When the whole of the retina is coagulated
Panretinal photocoagulation
Happens when the laser beam exceeds the bond energy between atoms of a molecule
Photoablation
2 steps of photoablation
1- Excitation by UV light
2- Dissociation
Irradiance and exposure time of photoablation
-Irradiance: 10^7 - 10^8 w/cm^2
-Exp time: nanoseconds
Photoablation will only take place if
Intensity of absorbed light is higher than or equal the threshold intensity of photoablation
A process by which cellular and extracellular components are photochemically decomposed by intense UV light
Photoablation
Why is UV light used in photoablation
- It has higher electric field intensity than the binding energy between molecules
- Molecular bonds break and vaporize without generation of heat
Difference between thermal ablation and photoablation
Thermal ablation: uses kinetic energy to increase temperature of the tissue
Photoablation: Uses kinetic energy to break the bonds between atoms and vibrate rather than cause heat
Applications of photoablation
Refractive surgery (eye correction surgeries) –> reshaping cornea and implanting lenses to reduce dependency on eyeglasses and contact lenses
Photomechanical effects
- Plasma-induced ablation
- photodisruption
Gas is converted into plasma by
ionization
The mixture of electrons and nuclei resulting from ionization
plasma
Advantages of plasma-induced ablation
highly precise, well-defined removal of tissue
Photoablation and photodisruption depend on
Optical breakdown
Plasma generation by intense electric field
Dielectric breakdown
Generation of electric field using light
Plasma-induced ablation
Irradiance and exposure time needed for plasma-induced ablation
irradiance: 10^11 - 10^12 w/cm^2
exp time: 500 ns - 1 fs
Application of plasma-induced ablation
Posterior capsulotomy after eye cataract surgery
Causes cavitation bubbles and jet formation
Photodisruption
Irradiance and exposure time needed for photodisruption
irradiance: 10^11 - 10^16 w/cm^2
exp time: 100 ns - 100 fs
Difference between photodisruption and plasma-induced ablation
in photodisruption, laser beam is pointed inside the tissue, not on the surface leading to formation of cavitation bubbles that consist of gaseous vapors which diffuse again in surrounding tissue
Jet formation is achieved when
cavitation bubbles collapse in fluids and near a solid boundary
a kind of visible light waves medical application on tissues. It is a therapy that uses monochromatic light and adapted chromophores which are injected into the body and light may then trigger photochemical reactions, resulting in certain biological transformations
PDT
