CLT Laser Physics & Safety Principles 3 — Questions and Answers
Question 1: Selective photothermolysis requires that the pulse duration be:
- Longer than the thermal relaxation time of the target chromophore
- Equal to or shorter than the thermal relaxation time of the target chromophore (Correct answer)
- At least 10 times the thermal relaxation time
- Independent of the thermal relaxation time
Correct answer: Equal to or shorter than the thermal relaxation time of the target chromophore
For selective photothermolysis, the pulse must confine heat to the target before it diffuses to surrounding tissue, requiring pulse duration ≤ thermal relaxation time.
Question 2: Which primary chromophore in skin has strong absorption peaks around 532 nm (green light)?
- Water
- Melanin
- Oxyhemoglobin (Correct answer)
- Collagen
Correct answer: Oxyhemoglobin
Oxyhemoglobin has strong absorption peaks at approximately 418 nm, 542 nm, and 577 nm, making 532 nm lasers highly effective for vascular lesion treatment.
Question 3: In laser-tissue interaction, 'absorption' refers to:
- The bending of the laser beam as it enters tissue
- The conversion of laser photon energy into heat or photochemical reactions within tissue (Correct answer)
- The reflection of the laser beam off the tissue surface
- The passage of laser energy through tissue without interaction
Correct answer: The conversion of laser photon energy into heat or photochemical reactions within tissue
Absorption occurs when tissue molecules take up photon energy and convert it to heat or initiate photochemical reactions, producing the desired therapeutic effect.
Question 4: Water is the dominant chromophore for laser wavelengths in which range?
- 300–400 nm (UV range)
- 400–600 nm (visible range)
- 2,000–3,000 nm (mid-infrared) (Correct answer)
- 630–700 nm (red range)
Correct answer: 2,000–3,000 nm (mid-infrared)
Water has very high absorption coefficients in the mid-infrared range, with peaks near 2,940 nm (Er:YAG) and 10,600 nm (CO2), driving tissue ablation.
Question 5: Which tissue effect is most associated with ultra-short (nanosecond to picosecond) pulses at high peak power?
- Photocoagulation
- Photovaporization
- Photoacoustic/photomechanical effect (Correct answer)
- Low-level photochemical stimulation
Correct answer: Photoacoustic/photomechanical effect
Ultra-short pulses create rapid thermoelastic expansion generating mechanical shockwaves, the mechanism underlying Q-switched laser tattoo removal.
Question 6: Melanin's absorption spectrum across wavelengths is best described as:
- Narrow peak at 694 nm only
- Broad absorption from UV through visible (~250–1100 nm) with decreasing absorption at longer wavelengths (Correct answer)
- Limited to infrared wavelengths above 1000 nm
- Exclusively in the UV range below 400 nm
Correct answer: Broad absorption from UV through visible (~250–1100 nm) with decreasing absorption at longer wavelengths
Melanin has broad, gradually decreasing absorption from UV through the near-infrared (~1100 nm), which is why multiple laser wavelengths can target pigmented lesions.
Question 7: The depth of laser penetration in tissue is primarily determined by:
- The output power of the laser source
- Wavelength and the tissue's absorption and scattering coefficients (Correct answer)
- The diameter of the laser aperture
- The temporal coherence length of the beam
Correct answer: Wavelength and the tissue's absorption and scattering coefficients
Tissue penetration depth is governed by wavelength-dependent absorption and scattering coefficients, not by power level alone.
CLT Laser Physics & Safety Principles 3
Selective photothermolysis requires that the pulse duration be: