Basic Tissue Optics and Laser Tissue Interactions Flashcards
What is tissue?
The intermediate level between individual cells and whole organs. (Made up of cells with similar functions)
What does ‘in vivo’ mean?
Within the body, so an in vivo procedure is done inside the body.
What tissue properties are important from a physics perspective?
The optical, thermal, acoustic and mechanical properties.
What are the components of the total attenuation coefficient (μt)?
μt = μa + μs (absorption + scattering)
Possible light interactions with tissue?
Elastic & inelastic scatter, reflection, absorption and fluorescence.
What is the net flux vector?
The radiance integrated over the solid angle of interest.
What is the physical meaning of the source term S(r)
Power per unit volume deposited by a laser in the tissue.
What happens to the radiance for diffuse illumination of tissue?
It is constant in all directions.
What is the penetration depth δ?
The distance into the tissue where the irradiance falls by a factor e.
What does an albedo of 1 indicate?
Scatter in the tissue dominates over absorption.
What is reflectance a measure of?
What fraction of incident light is reflected = reflected irradiance / incident irradiance.
What type of light does Beer’s Law apply to?
A single λ collimated laser.
What is the ‘therapeutic window’ from the Boulnois plot?
Between 600nm and 1200nm, absorption by both haemoglobin and water is low. Below 600nm, haemoglobin absorption dominates and above 1200nm water dominates.
When considering scattering, what are the two components of the radiance L?
1) from the primary beam
2) from all other scatter components
What does the anisotropy factor g represent?
The size and direction of scatter. g=1 indicates complete forward scatter and g=-1 indicates complete backscatter.
In tissue, g ~ 0.7 to 0.99
What are the 4 types of scatter?
Rayleigh - Elastic and particle diameter < λ
Mie - diameter > λ and main mechanism is biological tissue
Brillouin - Inelastic scatter off phonons
Raman - Inelastic scatter and exchange of vibrational energy
Profiles for Top Hat and Gaussian beams?
A Top Hat Laser is uniform across its width. A Gaussian beam is most intense in the centre and drops off according to a Gaussian distribution.
How to create an approximate point source?
Placing a small spherical diffuser on the end of a fibre optic to create uniform illumination
Steps of Monte Carlo Modelling for Diffusion?
1) Inject a photon at a particular position and direction.
2) The step length it takes is determined by a random number weighted by μt.
3) At the end of the step, the weight is reduced by μa.
4) Remaining weight scattered in a random new direction weighted by the scattering phase function.
5) Process repeats.
6) If it hits a boundary, use Fresnel equations to work out how much is reflected back into the tissue etc.
Diffusion Theory?
For high albedo (high scatter and low absorption), irradiance through a tissue follows a decaying exponential, with transmission coefficient = (μa/D)1/2 where D is the diffusion coefficient.
Properties of lasers needed for photochemical interactions?
Long irradiation time ~ 1s - 20min (>relaxation time)
Low power CW sources (~1Wcm^-2)
λ in transmission window
Steps for PDT?
1) A chromophore drug is introduced to the body in a specific target area.
2) Light applied, triggering selective photochemical interactions in the photosensitiser.
3) Excitation is followed by subsequent decays resulting in intramolecular transfer reactions.
4) Photosensitised oxidation leads to cell death in the surrounding target area.
Types of reactions in PDT
Type I - Triplet state interacts with a target molecule, resulting i the creation of free radicals.
Type II - Triplet state interacts with an oxygen triplet state, creating reactive excited singlet oxygen molecules.
Properties of lasers needed for photothermal interactions?
Medium irradiation time ~ μs - 100s
Medium power pulsed or CW lasers (10 - 1000000Wcm-2.
Key temperatures within a tissue and their effects?
<45°C - Only reversible damage
50-90°C - Denaturation of protein and coagulation of blood.
100°C - Vaporisation of water in tissue.
>150°C - Dehydration and carbonisation.
>300°C - Melting and vaporisation of hard tissues
Stages in photothermal treatment?
1) Incident lasers cause non-radiative decay in tissue –> generates heat
2) Transport mechanisms within the tissue move the heat away.
3) Heat effects eg tissue damage
Relation between heat and specific heat capacity?
dQ = mCdT
Equation for specific heat capacity and what does it represent?
C = (1.55 +2.8ρW/ρt) kJ k-1K-1
- The energy required to raise the temperature of one kg of tissue by 1°
Equation for thermal conductivity and what does it represent?
k ~ (0.06 +0.57ρW/ρt) Wm-1K-1
- The rate at which heat flows through a tissue.
Equation for thermal diffusivity?
χ = k/ρC m2K-1
What is thermal relaxation time?
An estimate of the time required for heat to conduct away from a directly heated tissue region. (Assuming diffusion length = absorption length)
τr = (4χμa2)-1
Equation for absorption length L?
L2 = 4χt
Condition for heating just a small volume of tissue?
Pulse duration must be less than the thermal relaxation time and pulse interval must be greater than the thermal relaxation time.
What is photoablation?
Tissues absorb UV photons and high energies lead to breaking of molecular bonds and photodecomposition
Properties of lasers needed for photoablative interactions?
Short pulse time ~ ns - μs
High power pulsed lasers (107 - 1010Wcm-2)
What is plasma induced ablation?
A photomechanical effect, very high power lasers cause dielectric breakdown in molecules. This increases the temperature and leads to the formation of a plasma, which cleanly removes tissue without thermal or mechanical damage.
What is plasma shielding?
As more plasma forms, photon scattering increases and more photons are absorbed into the plasma, making adding more laser energy less efficient.
