The clinical significance of adjusting pulse width lies in synchronizing the laser's energy release with the "thermal relaxation time" of the specific target structure.
By tuning this duration—such as selecting between 10 ms and 45 ms—you ensure that thermal energy is delivered efficiently to destroy the target (like a vein or hair follicle) while allowing enough time for the surrounding healthy tissue to cool down. Shorter pulses are clinically necessary for confining heat to small, fine targets, whereas longer pulses are required to generate the sustained, deep heat needed to coagulate larger, thicker structures.
The core objective of pulse width adjustment is to balance efficacy and safety: the pulse must be long enough to gently heat the epidermis without damage, yet short enough to thermally destroy the target before it dissipates the heat.
The Science of Target Size and Thermal Relaxation
The fundamental principle governing pulse width selection is the Thermal Relaxation Time (TRT). This is the time it takes for a target tissue to lose 50% of its heat.
Matching Pulse to Anatomy
To destroy a target without burning the surrounding skin, the laser pulse duration should roughly match the target's TRT.
If the pulse is too long, the target cools down during the laser shot, preventing it from reaching the destruction threshold.
If the pulse is significantly longer than the TRT, heat diffuses outward, potentially damaging adjacent healthy tissue.
The Role of Diameter
The physical size of the target dictates the required setting. Small objects cool rapidly, while large objects retain heat longer.
Therefore, fine vessels require shorter pulse widths to "catch" them before they cool, while large vessels require longer pulse widths to build up heat gradually throughout their volume.
Clinical Applications by Pulse Duration
Adjusting the milliseconds on a Long-Pulse Nd:YAG laser changes the biological interaction with the tissue.
Treating Vascular Lesions
For vascular therapy, the primary reference highlights that pulse width must vary based on vessel diameter.
Shorter pulses are utilized for fine, small vessels. This prevents excessive heat diffusion into surrounding healthy tissue, which is critical for precision.
Longer pulses are essential for vessels with thicker walls and larger diameters. These targets require a sustained energy delivery to ensure uniform heating and effective coagulation (ablation) of the entire vessel structure.
Laser Hair Removal
In hair removal, the target is the follicle, which generally has a longer TRT than the epidermis.
Pulse widths in the 35 ms to 65 ms range are commonly used to match the TRT of the hair follicle. This duration allows energy to accumulate and destroy the germinal center.
Crucially, this longer duration allows the melanin-rich epidermis (the skin surface) to dissipate heat while the laser is firing, reducing the risk of side effects like erythema or burns.
Skin Rejuvenation
Intermediate pulse widths (often 15–25 ms) are used for bulk heating of the dermis.
This approach avoids the "snap" of high-peak power, providing a gentle, sustained release of energy.
The goal here is to stimulate biological responses—such as collagen contraction and regeneration—without compromising the integrity of the skin surface.
Understanding the Trade-offs
Selecting the wrong pulse width leads to two distinct clinical failures: lack of efficacy or collateral damage.
The Risk of Short Pulses on Large Targets
If you use a short pulse on a large vessel or deep follicle, you may cause a rapid spike in temperature at the surface of the target.
However, because the energy delivery ends too quickly, the heat may not penetrate to the center of the structure, resulting in incomplete coagulation and eventual recurrence of the issue.
The Risk of Long Pulses on Small Targets
Using a long pulse (e.g., 50 ms) on a very fine vessel is often ineffective.
The tiny vessel dissipates the heat into the surrounding skin as fast as the laser delivers it.
This results in the vessel surviving the treatment, while the surrounding tissue suffers from unnecessary non-specific heating.
Making the Right Choice for Your Goal
The "best" pulse width is entirely dependent on the physical characteristics of the anatomy you are treating.
- If your primary focus is Fine Vessels: Select a shorter pulse width to confine heat to the small diameter and prevent diffusion into healthy tissue.
- If your primary focus is Large/Deep Vessels: Select a longer pulse width to ensure sustained heat penetrates the thick vessel walls for uniform ablation.
- If your primary focus is Hair Removal: Select a pulse width (typically 30-50 ms) that matches the follicle's size, allowing the epidermis to cool while the follicle reaches destruction temperature.
- If your primary focus is Skin Rejuvenation: Select a medium-to-long pulse (15-25 ms) to deliver gentle, cumulative heat that stimulates collagen without damaging the epidermis.
Successful Long-Pulse Nd:YAG therapy relies on the clinician's ability to assess target size and adjust the time domain to maximize damage to the target while sparing the skin.
Summary Table:
| Pulse Width Range | Clinical Application | Target Goal | Mechanism of Action |
|---|---|---|---|
| Short (<15ms) | Fine Vessels / Telangiectasia | Precision destruction | Confines heat to small diameters before diffusion occurs |
| Medium (15-25ms) | Skin Rejuvenation | Collagen stimulation | Gentle bulk heating of the dermis without epidermal damage |
| Long (30-65ms) | Hair Removal / Large Vessels | Thermal ablation | Sustained heat for deep follicles/thick vessel walls |
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References
- Christopher N. Nguyen, Michael H. Gold. Vascular uses of <scp>LP1064</scp> applicator on the harmony <scp>XL</scp> pro device. DOI: 10.1111/jocd.15756
This article is also based on technical information from Belislaser Knowledge Base .
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