CGIER 19-21 - Principles of Laser Action & Laser-Tissue Interactions

Question 1

What does LASER stand for?

Answer 1

Light 
Amplification by 
Stimulated
Emission of 
Radiation

Question 2

What happens when photons move to a higher or lower energy level?

Answer 2

• When electrons move to a higher energy level, they absorb an incident photon with the same amount of energy as the difference between the two energy levels.
• When electrons move to a lower energy level, they emit a photon with the same amount of energy as the difference between the two energy levels.
• The energy difference between the two energy levels determines the wavelength and frequency of the emitted photon and its colour.

Question 3

When does stimulated emission occur?

Answer 3

1. An electron that absorbs a photon and moves into the high energy state, they remain there for a short-lifetime until they drop to a metastable state which has a longer lifetime. This creates a build-up of electrons in this state called a POPULATION INVERSION.
2. Stimulated emission can now occur from the metastable state. An incident photon can stimulate the de-excitation of an electron from the metastable to a lower energy state, but in the process, the electron emits an identical stimulated photon that is coherent, collimated, and has the same energy as the incident one (Hence who laser radiation is one colour).
3. The two photons then travel in phase and in the same direction.

Question 4

What do all lasers consist of?

Answer 4

• An active medium from which the laser will be produced (solid, liquid, or gas).
• A source of energy to cause stimulated emission
• A set of mirrors (one 100% reflective & one 95% reflective to allow some radiation to pass through, forming the laser beam)

Question 5

How does a laser work?

Answer 5

1. A photon has to be spontaneously produced to start stimulated emission.
2. The incident photon then travels along the axis of the crystal causing a cascade of stimulated emission and forming many photons that are reflected back and forward by the mirrors along the crystal to cause even more stimulated emissions.
3. The laser beam then forms by allowing some radiation through the 95% reflective mirror.

Question 6

What are the different types of lasers?

Answer 6

1. Solid-state lasers (infrared to red)
2. Gas Lasers (infrared to Ultraviolet)
3. Dye Lasers (visible)
4. Diode Lasers (infrared)
   (Based on the active medium)

Question 7

What type of em radiation do lasers produce?

Answer 7

Clinically useful lasers produce em radiation within the infrared-visible-ultraviolet region of the em spectrum.

Question 8

What happens to the laser’s intensity as it penetrates tissue?

Answer 8

According to the Lambert-beer law, as tissue depth increases, laser intensity decreases.

Question 9

What are the three optical properties that determine absorption or scattering within tissue?

Answer 9

1. Absorption coefficient
2. Scattering coefficient
3. Anisotropy factor

Question 10

Place in order how deep different types of laser em radiation will penetrate tissue.

Answer 10

Generally, longer wavelengths will penetrate tissue further than shorter ones, therefore in order of decreasing tissue depth:

1. infrared
2. visible
3. ultraviolet

Question 11

What does different type of radiation do to the eyes?

Answer 11

1. Ultraviolet radiation (shorter wavelengths, less penetration, more absorption) causes radiation to be absorbed in the cornea.
2. Red or infrared radiation (longer wavelengths, deeper penetration, less absorption) causes radiation to be focussed on the retina causing retinal burns.

Question 12

What determines the interaction that takes place within the tissue?

Answer 12

Laser parameters:

• Radiant Exposure: the amount of total energy per unit area delivered to the tissue in a given time
• Irradiance: the amount of power received per unit area

Question 13

What are the three types of interactions that can occur in the tissue?

Answer 13

1. Photochemical Interactions (resulting from exposure to low irradiances) (Biostimulation & photoactivation of drugs)
2. Thermal Interactions
3. Photomechanical Interactions (resulting from exposure to high irradiances) (Photoablation, optical breakdown)

Question 14

What happens at different temperatures for thermal interactions?

Answer 14

37-45 degrees (no irreversible damage to tissue)
45-60 degrees (oedema)
60-100 degrees (necorsis)
100-300 degrees (vaporization of water)
> 300 degees (vaporization of solid material)

Question 15

What is specular and diffuse reflection?

Answer 15

• Specular reflection is when the radiation wavelengths are much greater than the size of the surface irregularities, therefore making the angle of incidence equal to the angle of reflection.
• Diffuse reflection occurs when the radiation wavelength is less than the size of rough surface irregularities.

Question 16

List clinical applications of lasers.

Answer 16

1. Low power treatment (Photochemical interactions):
   - Photodynamic Therapy - PDT (destroys cancers cells by combining light and a photosensitive drug to produce singlet oxygen that is toxic to malignant cells)
2. Thermal treatment (Thermal Interactions):
   - Laser Interstitial Thermal Therapy - LITT (causes necrosis in tumors)
   - repair of a detached retina (laser welds)
   - bloodless surgery (CO2 dye laser cuts tissue and the heat seals blood vessels, preventing blood loss)
   - Port Wine Stain (laser causes necrosis in blood vessels)
   - Excessive snoring (damages uvula to reduce snoring)
3. High power treatment (Photomechanical Interactions):
   - Laser Angioplasty
   - Tattoo Removal & Nevus Ota (a specific wavelength of a laser is chosen that the ink from the tattoo will absorb, causing it to break up into tiny fragments which are ingested by the tissue and removed by the dermis)
   - Vision correction

Question 17

What is Transcutaneous Pulse Oximetry (TPO) and how does it work?

Answer 17

It used for detecting the percentage of hemoglobin that is saturated with oxygen. Oxygenated hemoglobin absorbs more infrared light than red light, while deoxygenated hemoglobin absorbs more red light than infrared light. The oximeter cycles from shining infrared and red light and then the absorption levels of hemoglobin will tell you the portion of hemoglobin that is oxygenated. If more infrared light is absorbed, higher hemoglobin saturation. If more red light is absorbed, lower hemoglobin saturation.