Selective Photothermolysis serves as the fundamental operating logic for treating pigmented lesions safely. It dictates that laser equipment must be configured with a specific wavelength, pulse duration, and energy density (fluence) to match the target. By strictly adhering to these parameters, the laser destroys the pigment while preserving the surrounding normal tissue.
The core objective of this principle is "thermal confinement"—delivering energy faster than the target pigment can cool down, ensuring heat stays within the lesion rather than spreading to healthy skin.
The Three Pillars of Precision Treatment
To successfully treat conditions like age spots, freckles, or birthmarks, the laser system must align three specific variables with the physical properties of the pigment.
1. Wavelength Selection
The laser must emit a specific color of light that is efficiently absorbed by the target chromophore.
In the case of pigmented lesions, the target chromophore is melanin.
If the wavelength is not highly absorbed by melanin, the energy will pass through the lesion without effect, or be absorbed by other tissue elements, causing unnecessary damage.
2. Pulse Duration and Thermal Relaxation Time (TRT)
This is the most critical factor for safety. The pulse duration—the length of time the laser is actually firing—must be shorter than the target’s Thermal Relaxation Time (TRT).
TRT is the time it takes for the target object to cool down by 50%.
If the pulse duration exceeds the TRT, the pigment particles cannot cool down fast enough. This causes heat to diffuse laterally into the surrounding normal tissue, resulting in burns or scarring.
3. Energy Density (Fluence)
The system must deliver sufficient energy density, known as fluence, to destroy the target.
While wavelength ensures the right target is hit, and pulse duration ensures safety, fluence ensures the blow is powerful enough to be effective.
The energy converts to intense heat within the pigment, shattering or vaporizing the pathological tissue.
Understanding the Trade-offs
While Selective Photothermolysis provides a roadmap for safety, it requires precise calculation.
The Risk of "Heat Leakage" The primary trade-off involves the balance between energy delivery and speed. If you extend the pulse duration to deliver more energy, you risk exceeding the TRT.
Once the pulse duration surpasses the TRT, the principle of selective destruction fails. The procedure shifts from a controlled removal of pigment to a non-selective thermal injury of the surrounding skin.
Depth vs. Absorption High-absorption wavelengths, such as the 532nm (green light) from a KTP laser, are excellent for superficial lesions like freckles.
However, because they are absorbed so quickly by surface melanin, they may not penetrate deeply enough to treat deeper pigment issues without damaging the surface skin first.
Making the Right Choice for Your Goal
When selecting or configuring laser equipment for specific patient needs, apply the principle as follows:
- If your primary focus is treating superficial pigmentation (freckles, age spots): Prioritize systems like the Frequency-Doubled Nd:YAG (KTP) or IPL that emit wavelengths (e.g., 532nm) highly absorbed by superficial melanin.
- If your primary focus is patient safety and preventing scarring: Ensure the pulse duration is strictly set below the Thermal Relaxation Time of the specific lesion size you are treating to prevent lateral heat damage.
- If your primary focus is efficacy: Verify that the fluence is high enough to damage the chromophore, but never at the expense of the pulse duration constraints.
By mastering the relationship between pulse duration and the target's cooling time, you transform a laser from a blunt instrument into a precision tool.
Summary Table:
| Key Parameter | Role in Selective Photothermolysis | Clinical Objective |
|---|---|---|
| Wavelength | Targeted absorption by melanin | Ensures energy hits the lesion, not healthy tissue |
| Pulse Duration | Must be shorter than Thermal Relaxation Time (TRT) | Prevents heat diffusion to avoid burns and scarring |
| Fluence | Sufficient energy density delivery | Provides the power necessary to shatter pigment particles |
| Thermal Confinement | Restricting heat within the target | Maximizes treatment efficacy while protecting skin integrity |
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References
- Firly Clarissa Suyanto, Putu Dyah Ayu Saraswati. Terapi Laser untuk Lesi Hiperpigmentasi. DOI: 10.55175/cdk.v50i8.757
This article is also based on technical information from Belislaser Knowledge Base .
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