Laser Technical Discussion

My name is Stuart Mohr and I am Cutera's Technical Marketing Manager.  I hold degrees in Biomedical Engineering and Electrical Engineering as well as an MBA.  In addition to scholastic credentials, I designed aesthetic laser systems and other laser based technology devices for 10 years prior to going into marketing.

In this blog, I intend to describe technical and biologic descriptions behind many of our products from Pearl to Laser Genesis to laser hair removal and everything in-between.  I also hope to serve as a resource for anyone that has questions regarding Cutera products or aesthetic systems in general.

Now on to the fun stuff...

The reason for selecting YSGG over CO₂ is only strengthened when comparing YSGG to low power CO₂ lasers. Low Power CO₂ lasers (typically 30 watts and below) exaggerate the problems of CO₂ and have several independent problems as well. Histology obtained from data sheets for Deka’s SmartXide DOT and Alma’s Pixel CO₂ are included to illustrate the problems with devices that do not have sufficient power and energy density in each spot. 

Many people believe wavelength determines depth. For ablative lasers, this is not true. Wavelength determines the thermal characteristics of the laser, but depth of ablation is determined by power and energy density. With the right amount of power, all ablative wavelengths can ablate equally deep. Their thermal damage zones, however, vary significantly across devices.

A first typical problem of lower power CO₂ lasers is ablation depth. To ablate deep, lasers must have high energy densities delivered in short time periods. This requires high power. Depth of ablation is directly proportional to treatment results up to about 400-800 µm deep. Beyond this depth, there are two schools of thought. One thought is that greater depth is better, the other is that greater density is better. There is likely some truth to both. Either way, however, device should be able to treat at least 400-800 microns deep. 

In the histology image below, the image on the left shows a lesion from a single laser pulse. The ablation depth in this image is ~200µm (based on epidermal thickness of ~70µm). The histology image on the right shows a deeper treatment, but as can be seen, this required stacking 3 pulses (i.e. firing three pulses in the same location). Stacking pulses is challenging as even minor movements by the patient or practitioner can misalign the treatment holes as each spot is only several hundred microns in diameter. Further, it takes 3X longer to perform a treatment if stacked pulses are required. This means the patient experiences 3X as many shots per pass, which can reduce the tolerability of the treatment.

The second problem with low powered CO₂ devices is pulse duration. The primary problem with any CO₂ laser is excessive thermal damage. This thermal damage leads to increased potential for pigmentary complications and slows wound healing time. This is the reason all low power CO2 units were long ago abandoned for full resurfacing. This thermal damage is also a limiting factor or fractional CO₂ devices and can lead to the same complications as observed with full resurfacing CO₂ lasers if greater depth or density treatments are attempted. To properly perform a resurfacing procedure with a CO₂ laser total exposure time should be less than ~600µs to confine thermal damage. 

·        This is documented in the text “Cutaneous Laser Surgery” Chapter 6, Carbon Dioxide Laser Surgery.  In this text, it states that the thermal relaxation time for the volume of ablated tissue during CO₂ resurfacing is about 695 us and pulse durations of less than 950 us (0.95 ms) are sufficient to prevent clinically significant thermal damage

Because the laser is low powered, long pulse durations must be used to deliver sufficient energy to ablate tissue. In fact, many pulse durations as much as 100X longer than was experimentally and clinically proven optimal for CO₂ lasers.

The long pulse durations used with low power CO₂ lasers can be seen histologically in the two images above. In both cases, the coagulation / thermally damaged tissue zone is ~250-300 microns wide and deep. This means the treatment diameter of the laser is not just the spot size, but the spot size plus ~250 µm on each side of thermally damaged tissue, meaning the spot size is ~500µm wider than stated – leading to greater downtimes if treatments are performed to equivalent depths for equivalent treatment results.

The end result of minimized ablation depth, excessive thermal damage, and large effective spot sizes is that treatments are predominately thermal (not ablative), and are not very deep. Fractional non-ablative devices were completely thermal, and deeper, but required 4-6 treatments. Low powered CO₂ lasers do have maximized ablation (the only difference between ablative and non-ablative fractional lasers) meaning these treatments and results are closer to fractional non-ablative devices. Further, it is often necessary to perform multiple treatments to provide equivalent end results as highly ablative fractional devices (like Pearl Fractional). Many before and after pictures actually show results pre and post 3 treatments each spaced roughly 1 month apart.

Pearl Fractional was designed to reproducibly deliver deep fractional ablation with variable density to provide single-treatment results with maximum results and consistency. Accomplishing this and limiting downtime and complication risks required a wavelength capable of providing controlled thermal coagulation in a pulse-duration that is thermally confined. Pearl fractional was designed with sufficient power to ablate to necessary depths (typically 600-800 microns, which is ½ of maximum power). See comparison histology images below – image sizes are scaled to maintain approximately equal magnification. Histology images for the SmartXide DOT and the Alma Pixel CO₂ were obtained from product brochures from each company.

In a simple answer, Cutera selected YSGG (Pearl and Pearl Fractional) for thermal control with the desire to maximize ablation and control, but not eliminate coagulation (thermal damage). A little thermal damage during ablative procedures is a benefit, but too much can be a liability. Laser wavelength is the primary determinant of thermal damage. Power determines how deep an ablative laser vaporizes – and necessary power is determined by water absorption. At the edges of the ablation column though (sides and bottom) there is not sufficient power to continue ablating tissue. From this point outward, the zone of thermal damage is determined by how far sub-ablative intensity light penetrates through tissue – creating a coagulation zone. Cutera selected YSGG because this zone is 40-60 microns thick. Conversely, Er:YAG is about 10-20 microns and CO₂ is about 75-150 microns.

The coagulation zone (thermally denatured tissue) with YSGG is thick enough to coagulate normal microvasculature in dermal tissue, thus preventing significant operative bleeding. It is small enough though to be able to maximize ablation area without unduly increasing the size of the lesion. Note: thermally denatured tissue is dead / necrotic tissue that the body must also clear for the wound to heal. Additionally, excess thermal damage is associated with increased risks of pigmentary complications. With the thermal control of YSGG, Pearl fractional can maximize treated area and tissue vaporization beyond that of CO₂, yet still provide coagulation and thermal stimulation that is lacking with Er:YAG.

A frequent question regarding Pearl and Pearl Fractional for resurfacing is which product is more aggressive? The common misconception is that Pearl Fractional is less aggressive than Pearl. This misconception comes from old technologies like CO₂ traditional resurfacing vs. fractional – where the fractional therapy is less aggressive. 

Pearl (confluent) is not designed to create deep full coverage resurfacing like old CO₂. This is not due to its ability, but rather due to the market desire. There are very few deep full coverage resurfacing procedures performed due to the prolonged downtime and high risk of complications. Pearl (with YSGG) is able to perform a treatment that no other wavelength can accomplish. That is to treat to the D/E junction with a thermally confined pulse to provide full surface improvement and collagen stimulation without weeping, oozing or long downtimes and risk. 

Pearl Fractional is designed to be the aggressive tool. Because it treats fractionally, it is possibly to create deep ablation without high risk and downtime. When performing Pearl Fractional procedures, the energy delivered from the laser is significantly more intense than a Pearl laser shot, resulting in ablation in excess of 1mm. Most treatments are performed at 600 to 800µm, which is 1/3 to ½ of the maximum settings on the device.  . Density is then selected by the user to determine the degree of coverage – and subsequent intensity desired. Pearl Fractional is actually one of the most powerful and potentially aggressive devices available on the market today. The aggressiveness of Pearl Fractional treatments (depth and coverage percentage) is limited by physician choice, not by the device.

So why use both? Pearl performs a treatment no fractional laser can accomplish. It treats the entire surface of the face to the D/E junction. This improves universal skin tone, texture, quality, pigment, etc. Pearl Fractional is aggressive to treat significant damage with minimal downtime, but does so by being fractional. This can be used to create greater dermal injuries in focal regions for greater stimulation – but it, like all fractional lasers, intentionally does not treat the majority of the skin’s surface.

The goal during any Laser Genesis procedure is to methodically move the laser across a region of the face (typically ¼ to 1/8th of the face at a time) creating a localized zone of heated tissue with vasodilation.  This is visible as blushing or reddening in the area and should be maintained for a period of time.  Additionally, the patient will notice the increase in tissue temperature and the greater efficacy of each pulse.  By the end of treating any region, the patient should definitely feel the treatment.  The procedure does not require any topical anesthetic or other numbing agents meaning that it is not a painful procedure, but if the patient does not experience some discomfort toward the end of treating a region, the results may less than desired.  Once the treated region is sufficient treated (treat to erythema and vasodilation - typically ~2000 pulses), move to the next region.

To achieve proper fluence delivery, the handpiece should remain within about 1cm of the surface of the skin (about the width of your index finger).  Further away leads to beam degradation and can affect treatment.

Laser Genesis is a very deliberate procedure and should be administered in a methodical manner to achieve optimum results on wrinkles, texture, pore size, and diffuse vascular redness (blush or rosacea).  The treatment depends on both the fluence / pulse duration settings and bulk heating, which can only be obtained by focusing on regions of the face delivering uniform energy across the entire area. 

Treatments are extremely safe on all skin types, but to ensure optimal safety, always move the handpiece to prevent delivery of multiple sequential pulses to the same location.  Allow at least 1-2 seconds after delivering a pulse in any location before hitting that same location again.

Laser Genesis works through two parallel actions to stimulate collagen, improve texture and pore size, reduce wrinkles, and improve redness and fine facial veins—direct absorption and bulk heating.  The first part (direct absorption) requires the use of 0.3ms (or 300 us) pulses.  As described in my blog about pulse duration, short pulses target small objects. 

The target for Laser Genesis procedures is the microvasculature in the papillary dermis.  The laser’s 0.3 ms pulse durations enable Laser Genesis to target these vessels while completely sparing the epidermis.  This enables extremely safe treatments at high rep-rates on all skin types.  Each individual pulse specifically heats up this vasculature closing the larger microvasculature that leads to the appearance of blush.  These treated vessels are then cleared away by the body and evoke a healing response with collagen stimulation.

The second mechanism of action occurs through repetitive pulsing.  The laser operates using a 5 mm spot size pulsing at 10 Hz.  This creates bulk heating in each treatment area leading vasodilation and an influx of blood and healing factors.  Vasodilation also increases the amount of target (blood) in the vessels improving the efficacy of each individual pulse.  In addition to the healing mechanism stimulated by treating the microvasculature, extended bulk tissue elevation also leads to collagen stimulation. 

Achieving optimal results with Laser Genesis is easy, and the device is exceptionally safe on all skin types.  Focusing on the mechanism of action, and treating to achieve these biologic responses will optimize results. 

Any devices that claim to have the ability to “mimic” Laser Genesis, must meet all the requirements of wavelength, pulse duration, fluence, spot size and rep-rate.  If pulse duration or fluence is compromised, the laser does not have sufficient energy to treat the microvasculature.  Smaller spot sizes mean less area is treated per pulse, so less heat is applied to the skin per unit time meaning that bulk heating is compromised.  A change from a 5mm spot size to a 3mm spot size reduces bulk heating by almost 3X even when using the same rep-rates.  If the rep-rate is reduced, the laser does not fire as many pulses per unit of time, therefore reducing the heat delivered to the skin compromising the effect created by bulk heating.

Pulse duration is one of the most significant laser parameters to understand. Proper pulse duration is key to a safe, consistent and effective laser hair removal or vascular treatment. For this reason, it is typically the first parameter set by practitioners. Vein treatments include facial veins, leg veins and an occasional varicose vein treatment--although varicose veins are part of a larger medical condition not treated by a non-invasive aesthetic laser system.

Pulse duration determines the volume of tissue treated by the laser. Long pulse durations treat large volumes, short pulse durations treat small volumes. As an analogy, imagine a hot iron. A person can tap the surface of a hot iron that is several hundred degrees and not feel pain or get a blister. The reason is only the most superficial cells heated up during the time their finger contacted the iron. The heat did not have time to spread deeper into the finger. Conversely, it's extremely difficult to hold a hot cup of coffee even though it's far less hot than the iron. The difference is contact time. The cup is in contact for a long time so heat has time to conduct into the finger. Another analogy that compares to laser treatments is a pot full of hot soup. The soup stays hot in the pot for a long time, but if it's poured into 10 separate bowls, the bowls of soup cool down rapidly. This is because small objects give away heat to their surroundings more rapidly than large objects.

These same concepts are used when performing a laser vein treatment or laser hair removal treatment with an aesthetic laser system. The goal of every treatment is to heat the target you want to treat, but do as little collateral damage to surrounding tissue as possible. Large vessels (like leg veins) are like a pot of soup. They heat up slowly, and also don't give away their heat to their surroundings as well as small veins. This means long pulse durations will heat the vein, but still not heat surrounding tissue. In contrast, small veins, like facial veins, are very small and give away heat to their surrounding tissue quickly. This means energy must be applied in a short time--faster than they can give away their heat. If the pulse duration is too long, some of the energy (heat) ideally meant to treat the vein is lost into surrounding tissue heating the surrounding tissue, but not the vein. So, if the pulse duration is too long, more fluence (power) is required to achieve a clinical endpoint, but some power was wasted in surrounding tissue increasing the risk of complications. If the pulse duration is too long, it becomes impossible to treat the target regardless of the amount of fluence.

Using the correct pulse duration results in the most consistent outcomes with the lowest risk of complication. This allows the lowest amount of fluence to have the greatest impact on the target with the greatest degree of protection to the epidermis. In a nutshell, every treatment wants to use a pulse duration shorter than the time it takes for the target (hair or vein) to give away heat (thermal relaxation time) and longer than the time required for the epidermis to give away heat. Pulse durations longer than 5-10 ms provide epidermal safety by the same principal that prevents a long pulse duration from efficiently treating a small vein. The longer the pulse duration, the safer it is to the epidermis, but if it's too long, it will cease to be effective on the target.

So, in summary, how does someone select the correct pulse duration? Small targets require short pulse durations, large targets respond to long pulse durations. Longer pulse durations maximize epidermal safety. The following are approximate times to help select the correct pulse duration. If the duration is within about 5-10 ms of the ideal pulse duration, the treatment will achieve maximum efficacy and safety with minimum fluence. Remember, cooling is also a critical factor effecting epidermal protection and is also required for safe treatments.

Hair Treatment:

• Fine = 5-20 ms
• Medium = 15-30 ms
• Coarse = 25-50 ms
• Very coarse = 40-80 ms

Vein Treatment (size of vein is measured diameter):

• Typical pulse durations range from 10ms to 100ms for small high pressure telangiectasias to large low flow leg veins.

How does Pearl provide immediate results in one treatment while a Fraxel Laser Treatment typically requires 3 treatments to start seeing results and 5+ treatments to complete? The answer is in the method of delivery. Fractional laser devices only treat a small fraction of the skin with each treatment.  This means even after 3 treatments, only 50% of the skin is actually treated.  Pearl safely and consistently treats both the entire thickness and surface area of the epidermis in one treatment.  Treating the entire epidermis removes pigmented lesions, improves texture, and reduces fine lines and wrinkles - the primary signs of mild to moderate photo aging.

Pearl's wavelength (2790 nm YSGG) is the key that enables it to safely treat the entire epidermis.  This wavelength enables the laser to penetrate through the entire epidermis in a single treatment with the right balance of thermal stimulation.  During delivery, the top 1/3 of the epidermis is immediately removed and the remaining 2/3 is coagulated - meaning 100% of the epidermis is treated.  The controlled depth of heating and coagulation provided by the wavelength provides two additional benefits.  First, it provides a natural protective layer on the skin preventing any bleeding or oozing post treatment. Second, it stimulates collagen growth in the papillary dermis beneath the D/E junction. The coagulated tissue peels off after 3-4 days, unveiling a new completely re-epithelialized epidermis void of many of the signs of mild to moderate photo aging. 

This fresh epidermis is new from the D/E junction up.  This means pigmented lesions stemming from the basal cells at the bottom of the epidermis are replaced with new normal cells.  Pearl's treatment of the entire epidermis, in both coverage and thickness, provides better results on pigment in just one session than multiple IPL treatments.  Further, the body's natural response to laser treatments to the D/E junction is to stimulate new fibroblast growth.  New fibroblast activity results in collagen stimulation helping to eliminate fine lines and wrinkles.  This process occurs more gradually meaning results from a single Pearl treatment continue to improve for as much as 6 to 9 months.

Pearl's depth and three mechanisms of action (immediate ablation, immediate coagulation, and stimulation in the upper dermis) are misunderstood by many people.  Pearl provides immediate visible results in mild to moderate photo damage in just 1 treatment by utilizing these three mechanisms of action while treating the entire surface area of the face.  Further, because of the laser's controlled water absorption, treatments are safe, reproducible and consistent from person to person, removing the guess work.