Understanding lasers, lights, and tissue interactions

1 Understanding lasers, lights, and tissue interactions



Fernanda H. Sakamoto, H. Ray Jalian, R. Rox Anderson




Summary and Key Features




• Lasers and flashlamps can destroy histological targets using the concept of selective photothermolysis (SP)


• Ablative lasers vaporize tissue; non-ablative lasers heat tissue without vaporization


• Selective histological damage requires heat confinement to desirable target structures. Selective photothermolysis combines appropriate wavelength (‘color’ of light), fluence (‘dose’ of light), pulse duration, and protective skin cooling for the treatment of a variety of diseases


• Understanding the optical and thermal properties of skin and its histological targets allows safe and optimal treatments using light sources



Light


Light is a fundamental form of energy with numerous medical applications. At the quantum level, light is composed of packets of energy, known as photons. Each photon carries a discrete amount of energy. Light is also an electromagnetic wave. The electromagnetic spectrum extends from low frequency radio waves to ultra-high-energy gamma rays. The energy carried by each photon is determined by its wavelength, which for visible light (400–700 nm) corresponds to its color. Laser is an acronym for light amplification by the stimulated emission of radiation. Stimulated emission is a quantum process by which one photon can stimulate the creation of another photon, by interacting with an excited atom or molecule. Lasers work by pumping many atoms into the excited state, from which a very large amount of stimulated emission can occur. Laser light is typically monochromatic, meaning that the output is composed of a single wavelength of light. A second characteristic of lasers is coherence, meaning that all waves of light travel in phase spatially and temporally. Laser light is also highly collimated, which allows the laser beam to travel long distances without divergence, and to be focused to a spot about equal to its own wavelength. These properties of lasers allow for unique forms of in vivo imaging, such as confocal microscopy and optical coherence tomography.


Lasers are also capable of producing extremely intense, short pulses of light. In dermatology and ophthalmology, pulsed lasers have become mainstream tools for precise surgery and target-selective treatments. Prior to 1983, lasers in dermatology were used primarily for non-specific tissue destruction. With the description of the theory of selective photothermolysis (SP) by Anderson & Parrish in 1983, applications of lasers in dermatology have evolved to a host of devices for more precise, targeted thermal damage, while minimizing non-specific tissue destruction. Non-laser flashlamp sources called intense pulsed light (IPL) have also been developed for some of the applications of SP that use millisecond pulses of light. Understanding the theory of SP is vital for making sense of the large number of laser and IPL devices and applications. An understanding of the optical properties of skin is also needed, since the whole endeavor of laser treatment starts with the absorption of light energy, inside the skin.


Lasers that vaporize a thin layer or column of tissue have also been developed. The concept of fractional photothermolysis (FP), reported by Manstein and colleagues in 2004, recently launched another era of lasers in dermatology, in which patterns of very small non-selective thermal damage zones are used to stimulate skin remodeling without scarring. Laser-stimulated remodeling is a complex process that mimics large wound healing in some aspects, with epidermal regeneration, induction of metalloproteinases, and formation of new dermal matrix including elastin fibrils and collagen types I and III. Compared with gross wound healing, there is minimal inflammation and no scarring. A ‘cookbook’ approach should be avoided when choosing among these devices for various applications. When treating a particular patient with a particular device, a combination of fundamental understanding, careful observation of the appropriate clinical end points, dexterity, and clinical experience is far better than a set of instructions (Box 1.1).



Box 1.1


How to choose a light source?




1. Define clinical indication


2. Choose correct light wavelength (nm) based on histological target chromophore


3. Observe whether continuous wave (CW) or pulsed source is required


4. Choose right pulse width if necessary (seconds)


5. Choose pulse frequency (Hz), if necessary


6. Set skin cooling parameters


7. Choose appropriate light dose


8. Test laser to check whether it is working properly


9. Trigger single pulse on target skin and observe clinical end point


10. Adjust dosimetry if necessary


11. If no unwanted sign is observed, continue with treatment


Always:



a. Anesthetize area if necessary (e.g. tattoos, ablative lasers)


b. Wear appropriate personal protection (e.g. wavelength specific eye goggles / glasses, fume-resistant mask, gloves)


c. Turn smoke evacuator on if performing ablative procedures


d. Dress the area with petrolatum ointment and protect from sun exposure until treated area is healed



Light interactions with skin


Photons can be absorbed (giving up their energy to matter) or scattered (changing their direction of travel). Light that is scattered back from skin is called reflectance. For a given skin layer, light that passes through it is called transmittance. Scattering is inversely wavelength dependent, such that shorter wavelengths are scattered more and longer wavelengths (such as infrared) are scattered less. We are all familiar with these events – black objects become hot when placed in sunlight due to absorption and water droplets (clouds) or crystals (snow) appear bright white because they strongly scatter light, with little or no absorption. Similarly, light is both absorbed and scattered within the skin. Thus, skin layers are cloudy and colored depending on the mix of scattering and absorption. Penetration of light into (and beyond) skin is limited by both absorption and scattering. All effects of light on the skin begin with photon absorption, and the molecules that absorb light are called chromophores. Ablative lasers are those that vaporize tissue by rapidly boiling water inside the tissue. It should come as no surprise therefore, that the lasers intended for skin ablation are at wavelengths strongly absorbed by water. Non-ablative lasers do not vaporize tissue. There are many non-ablative lasers in dermatology, some of which are at wavelengths absorbed by water and some of which are absorbed by other chromophores such as melanin and / or hemoglobins.



Pearl 1


Ablative light sources vaporize the skin while non-ablative and selective laser treatments can be performed only with selective photothermolysis.


Laser dosimetry is extremely important for safe and effective results. In order to remove tissue, ablative lasers must raise local tissue temperature beyond the boiling point of 100oC, plus add much more energy needed for changing water into steam. The fundamental unit of energy is a joule (J). It takes 4.2 J to heat 1 cm3 of water by 1oC. In order to vaporize the same 1 cm3 of water, more than 2000 J are required. An ablative laser must deliver about 2500 J of energy per cm3 of vaporized tissue. Not only is a lot of energy required to ablate skin tissue – the energy must be delivered quickly to remove the hot tissue before heat is conducted deeply into the skin, causing a burn. The standard ablative lasers in dermatology are erbium (2940 nm) and CO2 (10 600 nm). The desired interaction of these ablative lasers is to precisely remove a thin layer for resurfacing or narrow column for fractional treatment of skin, leaving behind minimal residual thermal damage. A thin residual thermal damage layer, typically about 0.1 mm, is useful in practice for hemostasis. Minimum residual thermal injury is achieved with ablative lasers by a combination of wavelength, pulse duration, and power density (W/cm2) at the skin surface. A common mistake made by beginning laser users is to ‘turn down’ the power of a surgical CO2 laser in a misguided attempt to exercise caution. Unfortunately, turning down the power can cause burns because the process turns from rapid, precise vaporization with minimal thermal damage to bulk heating of the skin from unwanted residual heat. Fortunately, many of the ablative lasers made specifically for dermatology are designed to stay within a range of dosimetry for rapid tissue ablation, making this scenario less likely. The safest erbium and CO2 lasers are those emitting high power, high energy, and short (less than a few ms) pulses, designed specifically for dermatologic use with minimal residual thermal damage. Despite whatever safeguards an ablative laser may offer, the most reliable safeguard is an ability to recognize the desired and undesired immediate response end points. For example, immediate contraction of the skin is always a sign that substantial thermal injury of the dermis has occurred (Fig. 1.1).


image

Figure 1.1 Scheme of ablative laser vaporizing skin, leaving a residual thermal damage layer.


Fluence is defined as the energy delivered per unit area of skin, and its units are typically expressed in J/cm2

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Apr 2, 2016 | Posted by in Laser surgery | Comments Off on Understanding lasers, lights, and tissue interactions

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