Basic Tissue Optics and Laser Tissue Interactions

Question 1

What is tissue?

Answer 1

The intermediate level between individual cells and whole organs. (Made up of cells with similar functions)

Question 2

What does ‘in vivo’ mean?

Answer 2

Within the body, so an in vivo procedure is done inside the body.

Question 3

What tissue properties are important from a physics perspective?

Answer 3

The optical, thermal, acoustic and mechanical properties.

Question 4

What are the components of the total attenuation coefficient (μ_t)?

Answer 4

μ_t = μ_a + μ_s (absorption + scattering)

Question 5

Possible light interactions with tissue?

Answer 5

Elastic & inelastic scatter, reflection, absorption and fluorescence.

Question 6

What is the net flux vector?

Answer 6

The radiance integrated over the solid angle of interest.

Question 7

What is the physical meaning of the source term S(r)

Answer 7

Power per unit volume deposited by a laser in the tissue.

Question 8

What happens to the radiance for diffuse illumination of tissue?

Answer 8

It is constant in all directions.

Question 9

What is the penetration depth δ?

Answer 9

The distance into the tissue where the irradiance falls by a factor e.

Question 10

What does an albedo of 1 indicate?

Answer 10

Scatter in the tissue dominates over absorption.

Question 11

What is reflectance a measure of?

Answer 11

What fraction of incident light is reflected = reflected irradiance / incident irradiance.

Question 12

What type of light does Beer’s Law apply to?

Answer 12

A single λ collimated laser.

Question 13

What is the ‘therapeutic window’ from the Boulnois plot?

Answer 13

Between 600nm and 1200nm, absorption by both haemoglobin and water is low. Below 600nm, haemoglobin absorption dominates and above 1200nm water dominates.

Question 14

When considering scattering, what are the two components of the radiance L?

Answer 14

1) from the primary beam
2) from all other scatter components

Question 15

What does the anisotropy factor g represent?

Answer 15

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

Question 16

What are the 4 types of scatter?

Answer 16

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

Question 17

Profiles for Top Hat and Gaussian beams?

Answer 17

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.

Question 18

How to create an approximate point source?

Answer 18

Placing a small spherical diffuser on the end of a fibre optic to create uniform illumination

Question 19

Steps of Monte Carlo Modelling for Diffusion?

Answer 19

1) Inject a photon at a particular position and direction.
2) The step length it takes is determined by a random number weighted by μ_t.
3) At the end of the step, the weight is reduced by μ_a.
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.

Question 20

Diffusion Theory?

Answer 20

For high albedo (high scatter and low absorption), irradiance through a tissue follows a decaying exponential, with transmission coefficient = (μ_a/D)^1/2 where D is the diffusion coefficient.

Question 21

Properties of lasers needed for photochemical interactions?

Answer 21

Long irradiation time ~ 1s - 20min (>relaxation time)
Low power CW sources (~1Wcm^-2)
λ in transmission window

Question 22

Steps for PDT?

Answer 22

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.

Question 23

Types of reactions in PDT

Answer 23

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.

Question 24

Properties of lasers needed for photothermal interactions?

Answer 24

Medium irradiation time ~ μs - 100s
Medium power pulsed or CW lasers (10 - 1000000Wcm^-2.

Question 25

Key temperatures within a tissue and their effects?

Answer 25

<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

Question 26

Stages in photothermal treatment?

Answer 26

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

Question 27

Relation between heat and specific heat capacity?

Answer 27

dQ = mCdT

Question 28

Equation for specific heat capacity and what does it represent?

Answer 28

C = (1.55 +2.8ρ_W/ρ_t) kJ k^-1K^-1
- The energy required to raise the temperature of one kg of tissue by 1°

Question 29

Equation for thermal conductivity and what does it represent?

Answer 29

k ~ (0.06 +0.57ρ_W/ρ_t) Wm^-1K^-1
- The rate at which heat flows through a tissue.

Question 30

Equation for thermal diffusivity?

Answer 30

χ = k/ρC m²K^-1

Question 31

What is thermal relaxation time?

Answer 31

An estimate of the time required for heat to conduct away from a directly heated tissue region. (Assuming diffusion length = absorption length)
τ_r = (4χμ_a²)^-1

Question 32

Equation for absorption length L?

Answer 32

L² = 4χt

Question 33

Condition for heating just a small volume of tissue?

Answer 33

Pulse duration must be less than the thermal relaxation time and pulse interval must be greater than the thermal relaxation time.

Question 34

What is photoablation?

Answer 34

Tissues absorb UV photons and high energies lead to breaking of molecular bonds and photodecomposition

Question 35

Properties of lasers needed for photoablative interactions?

Answer 35

Short pulse time ~ ns - μs
High power pulsed lasers (10⁷ - 10¹⁰Wcm^-2)

Question 36

What is plasma induced ablation?

Answer 36

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.

Question 37

What is plasma shielding?

Answer 37

As more plasma forms, photon scattering increases and more photons are absorbed into the plasma, making adding more laser energy less efficient.