CLT Laser Modalities & Applications 3 — Questions and Answers
Question 1: According to the theory of selective photothermolysis, the pulse duration should be:
- Longer than the thermal relaxation time of the target
- Equal to or shorter than the thermal relaxation time of the target (Correct answer)
- Set independently of the target's thermal properties
- As long as possible to maximize energy delivery
Correct answer: Equal to or shorter than the thermal relaxation time of the target
Pulse duration at or below the target's thermal relaxation time confines heat to the chromophore, preventing collateral tissue damage.
Question 2: Which parameter describes the total energy delivered per unit area during a laser treatment?
- Power density
- Fluence (J/cm²) (Correct answer)
- Irradiance
- Pulse repetition rate
Correct answer: Fluence (J/cm²)
Fluence, measured in joules per square centimeter, represents the total energy deposited per unit area and is a key determinant of treatment efficacy.
Question 3: Increasing the spot size of a laser beam while maintaining the same fluence will generally:
- Decrease depth of penetration
- Increase depth of penetration due to reduced scatter effects (Correct answer)
- Have no effect on depth of penetration
- Increase surface temperature only
Correct answer: Increase depth of penetration due to reduced scatter effects
Larger spot sizes reduce lateral scatter at the beam edges, allowing photons to penetrate deeper into tissue at equivalent fluences.
Question 4: Q-switching in laser technology achieves extremely short, high-peak-power pulses primarily for:
- Continuous wave hair removal
- Tattoo and pigmented lesion removal via photomechanical effect (Correct answer)
- Vascular coagulation
- Laser resurfacing
Correct answer: Tattoo and pigmented lesion removal via photomechanical effect
Q-switched lasers generate nanosecond pulses with extreme peak powers that shatter pigment particles via photomechanical (acoustic) waves rather than thermal heating.
Question 5: A laser's thermal relaxation time is primarily determined by:
- The wavelength of the laser
- The size of the target chromophore (Correct answer)
- The skin type of the patient
- The fluence applied
Correct answer: The size of the target chromophore
Thermal relaxation time scales with the square of the target's diameter, so larger targets (like hair follicles) have longer thermal relaxation times than smaller targets (like melanosomes).
Question 6: Dynamic cooling devices (DCD) spray cryogen on skin immediately before each laser pulse primarily to:
- Increase laser transmission into deeper tissue
- Protect the epidermis from thermal injury during treatment (Correct answer)
- Enhance melanin absorption of laser energy
- Reduce the required fluence for efficacy
Correct answer: Protect the epidermis from thermal injury during treatment
Pre-pulse cryogen spray cools the epidermis selectively, allowing higher fluences to be used safely while protecting melanin-rich epidermal cells.
Question 7: When treating tattoos with a Q-switched Nd:YAG 1064 nm laser, which ink color responds least effectively?
- Black
- Dark blue
- Green and yellow (Correct answer)
- Red
Correct answer: Green and yellow
Green and yellow inks absorb poorly at 1064 nm; they require different wavelengths such as 532 nm or 694 nm for effective fragmentation.
CLT Laser Modalities & Applications 3
According to the theory of selective photothermolysis, the pulse duration should be: