Strict control of pulse duration is the defining factor in laser safety and efficacy. To treat pigmented lesions effectively, the laser pulse must be shorter than the target’s thermal relaxation time, often requiring precise regulation in the 20-30 ms range. This ensures that the heat generated is confined strictly to the melanin, destroying the lesion through coagulation and necrosis while preventing thermal damage to the surrounding healthy skin.
The Core Insight Success in laser therapy relies on the principle of thermal confinement. By keeping the pulse duration shorter than the time it takes for the target to cool, you ensure energy destroys the specific defect (the lesion) rather than diffusing outward to burn the adjacent healthy tissue.
The Mechanism of Action
The Principle of Thermal Relaxation Time (TRT)
The fundamental rule of treating pigmented lesions is based on the Thermal Relaxation Time (TRT).
TRT is the time required for a specific target, such as a granule of melanin, to lose 50% of its heat.
To effectively damage the target without harming the surrounding area, the laser pulse duration must be shorter than the TRT of the target chromophore.
Confining Thermal Effects
When the pulse duration is properly regulated (e.g., 20-30 ms), the laser energy builds up heat inside the pigmented lesion faster than it can escape.
This rapid accumulation of energy leads to coagulation and necrosis restricted solely to the lesion.
Because the energy delivery is so swift, the heat does not have time to conduct into the surrounding collagen or epidermis.
The Risks of Improper Duration
Excessive Heat Dissipation
If the pulse duration exceeds the TRT, the target absorbs energy but immediately begins to transfer that heat to its neighbors.
Instead of destroying the pigment, the laser effectively acts as a radiator, warming the surrounding tissue.
Collateral Damage
This process, known as thermal diffusion, is the primary cause of side effects in laser treatments.
When heat dissipates into adjacent healthy skin tissue, it can cause unnecessary burns, scarring, or hyperpigmentation in the surrounding area.
Understanding the Trade-offs
Speed vs. Diffusion
Adjusting the pulse duration (or dwell time) fundamentally changes how energy interacts with tissue.
Short pulse durations with high power allow for rapid energy release. This is conducive to precise ablation or destruction of specific targets because the energy is delivered before it can spread.
Long Pulse Implications
Longer dwell times increase thermal diffusion. While this is dangerous for small pigmented lesions, it can be useful in other contexts where bulk heating is desired.
Clinical practitioners must customize this balance. You must weigh the need for ablation depth against the risk of thermal damage, adjusting for patient skin type and specific conditions like acne scars or pigmentation depth.
Making the Right Choice for Your Goal
To achieve optimal aesthetic results, you must match the pulse duration to the specific characteristics of the target.
- If your primary focus is treating distinct pigmented lesions: Ensure the pulse duration is shorter than the target's thermal relaxation time (typically <30 ms) to lock heat inside the pigment and spare the skin.
- If your primary focus is bulk heating or modifying deeper tissue: Longer pulse durations may be utilized to intentionally allow thermal diffusion, provided the surrounding tissue can withstand the thermal load.
Precision in timing is not just a setting; it is the boundary between effective treatment and accidental injury.
Summary Table:
| Factor | Short Pulse Duration (<30 ms) | Long Pulse Duration (>30 ms) |
|---|---|---|
| Mechanism | Thermal Confinement (Heat stays in target) | Thermal Diffusion (Heat spreads to neighbors) |
| Effect | Selective Coagulation & Necrosis | Bulk Heating of surrounding tissue |
| Target | Precise Pigmented Lesions | Deeper tissue modification / Acne scars |
| Safety | High safety for surrounding skin | Risk of burns/scarring if used on pigment |
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References
- Amir Koren, Ofir Artzi. Evaluation of subject response following treatment for pigmentation or wrinkles using a diode laser. DOI: 10.1111/jocd.13200
This article is also based on technical information from Belislaser Knowledge Base .
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