Overview of lasers

Address for correspondence: Dr. Uddhav A. Patil, LakshyaKiran Therapeutic Lasers and Research Institute Pvt. Ltd., 10-11, Trade Centre, Station Road, Kolhapur - 416 001, Maharashtra, India. E-mail: ni.tenrahcnas@5vahddu_rpk

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Laser surgery of skin conditions having cosmetic implications has revealed the profound psychological benefits which are unmatched by any other modality of treatment either with or without a knife. An increasingly sophisticated understanding of the biophysics of laser-tissue interactions has lead to a more efficient utilization of the present technology on the clinical side and at the same time is helping the physicists to add more and more highly selective laser systems in the armamentarium of aesthetic laser surgeons.

This article provides a general overview of lasers in skin and cosmetology and discusses its current clinical applications from Plastic Surgeon's point of view.

HISTORY

Laser is an acronym for “Light Amplification by Stimulated Emission of Radiation.” Stimulated emission was based on Einstein's quantum theory of radiation..[1] The first laser was produced by Theodore H. Maiman on 7 th July 1960 using ruby as a lasing medium that was stimulated using high energy flashes of intense light.[2] The Decade of 1960s will always be remembered in the history of lasers as more than ten different lasers were invented using solid, gaseous, semi-conductor as well as liquid lasing media. The refinement of technology along with invention of newer lasers has continued till date and will keep on doing so in the future as well.

A significant understanding of lasers and light sources is required for their optimal use. Also a basic understanding of laser physics is mandatory to carry out an efficient laser treatment.

LASER CHARACTERISTICS

Laser light is monochromatic, bright, unidirectional and coherent.

Monochromaticity:

The luminous waves emitted come out with the same wavelength and energy. A single wavelength or a narrow band of wavelengths emitted allows precise targeting within tissue, while sparing adjacent structures.

Brilliancy:

The light beam emitted is extremely intense and angularly well centred. The brightness or intensity is one of the important properties and can be enhanced by techniques like pulsing and Q-switching where extremely high peak power can be delivered in nanoseconds.

Coherency:

All the photons emitted vibrate in phase agreement both in space and time. Coherence is a measure of precision of the waveform. Highly coherent laser beam can be more precisely focused.

Directionality:

All the photons travel in Uni direction. Directionality of the laser correlates with the emission of an extremely narrow beam of light that spreads slowly. Within the laser apparatus, efficient collimation of photons into a narrow path results in a divergence factor of approximately 1 mm for every metre travelled. Directionality allows the laser beam to be focused on a very small spot size.

Terminology and measurements

IPL: Intense Pulsed Light where peak optical power per pulse is up to 20,000 watts achieved with capacitor banks. All bright light sources are not called IPL, they are just light sources. Wavelengths emitted range usually from 400 nm to 1200 nm and the lower wavelengths can be eliminated by various cut off filters which usually range from 515 to 755 nm.

Chromophore: Chromophore is a material, present either endogenous in the tissues or exogenous i.e. brought from outside, which absorbs particular wavelengths depending on its absorption coefficient. Examples of endogenous chromophores are melanin, haemoglobin, (oxy haemoglobin, de-oxyhaemoglobin and meth haemoglobin), water, protein, peptide bonds, aromatic amino acids, nucleic acid, urocanic acid and bilirubin.[3] Exogenous compounds like different colors of tattoo ink also act as chromophores.

Parameters: Parameters are the values of wavelength, fluence (see below), number of pulses, pulse duration, pulse delay, repetition rate and spot size which are set on laser or IPL systems to treat a particular condition.

Wavelength: The distance between two subsequent peaks or troughs of a light wave. Usually it is expressed in nm (nanometre i.e. 10 −9 metres)

Frequency (V or f) ∝ (1/ wavelength (Hz) Therefore shorter the wavelength, higher is the frequency and longer the wavelength, lower is the frequency.

Photon: Photon is an elementary particle responsible for electromagnetic phenomena. It is the carrier of electromagnetic radiation of all wavelengths, including in decreasing order of energy, gamma rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. The photon differs from many other elementary particles, such as the electron and the quark, in that it has zero rest mass; therefore, it travels (in a vacuum) at the speed of light.

Energy: Each photon carries a ‘quantum’ of energy (E), whereby: E=hV (h – Planck's constant) Therefore:

Measurements used routinely in laser applications include wavelength, frequency, energy, fluence, power, and irradiance.

LASER TISSUE INTERACTION[3]

Laser beam that encounters skin surface may be reflected, transmitted, scattered or absorbed at each layer. Once the laser beam falls on the skin, from this point onwards, we should think about it not as a light but as a continuous or pulsed source of photons. Photon as a particle can only interact with matter by transferring the amount of energy. Therefore, only absorbed photons can produce tissue effect. For photon absorption in a tissue, a chromophore is required. Therefore, our aim is to increase the photon absorption by reducing its reflectance, scattering and transmission.

Transmission:

If there is no chromophore then all the photons will pass through the tissue without producing any effect. This is total transmission. Therefore, selection of a proper chromophore in or near the target tissue is a first important step in laser therapy.

Reflection:

Reflection occurs at all interfaces of media through which the laser beam is travelling, such as optical glass or sapphire tip, air, water jelly and skin surface. For example, the stratum corneum reflects approximately 4% to 7% of visible light that encounters the skin surface. Reflection is minimized by either a firm contact between the laser head of contact lasers or a light guide of I 2 PL system and skin or by using a layer of optically transmissible clear jelly between them in case of IPL systems.. In case of focused and collimated beams, reflection can be minimized by holding the hand piece exactly perpendicular to the skin surface.

Scattering:

Scattering is due to lack of homogenety in the skin's structures, such as molecules, organelles, cells or larger tissue structures. In the dermis, scattering has been shown to occur predominantly from inhomogeneities in structures whose size is of the order of the wavelength or slightly larger e.g. collagen fibres. It, therefore, appears to act as a turbid matrix in which scattering is an approximately inverse function of wavelength (shorter wavelength, greater scattering). The greater the scattering, less will be the depth of penetration, and more possibility of absorption.[4]

In the tissues more scattering occurs when a small spot is used. In case of large spot, after scattering, photons hit each other and get recollected and redirect themselves in the direction of the beam thereby increasing the depth of penetration. Therefore, larger the spot deeper is the penetration. However the selection of spot size also depends on the power generation of the system as well as on the depth of the chromophore from the skin surface.

Absorbed photons can produce thermal, mechanical, or chemical changes in and around the chromophore. Out of these, thermal changes are most useful, as in hair reduction, skin rejuvenation and in vascular lesions. Physical or mechanical tissue changes, known as photoacoustic changes occur when high energy photons are delivered in ultra short pulses of nanoseconds. This is made use of in removing tattoos and clearing certain pigmented lesions.

Example of light energy causing a chemical reaction is photosynthesis in plants. The UV irradiation induced chemical changes in deoxyribonucleic acid (DNA) may be responsible for cell death and neoplastic transformation.

Selective photothermolysis

In 1983, Anderson and Parrish[5] described the theory of selective photothermolysis, which revolutionized laser therapy by explaining a method of producing localized tissue damage sparing the surrounding tissues. For understanding this we need to understand certain terms.

Threshold fluence of a tissue is a fluence, which if equaled or exceeded leads to the tissue destruction.

Thermal relaxation time (TRT) is defined as the time required by an object to cool down to 50% of the initial temperature achieved.

For tissue damage to ensue a wavelength should be preferentially absorbed by the chromophore in the target tissue and not absorbed by the surrounding tissue, it therefore needs to be delivered in a pulse duration which is less than or equal to the thermal relaxation time (TRT) of the target. If delivery time exceeds the TRT then the target does not get damaged instead the energy dissipates to the surrounding tissues inflicting injury there. Even if energy delivery occurs within the TRT limits, the fluence reaching the target after subtracting reflection and scattering in the path needs to equal or exceed the threshold fluence to cause tissue destruction. This seemingly difficult task can be achieved by manipulating three variables: wavelength, pulse duration, and fluence.

Wavelength:

Wavelength has a dual impact attributable to its absorption coefficient in different chromophores [ Figure 1 ][6] as well as the depth of penetration from the skin surface, which roughly increases as the wavelength increases in the visible and near infra-red spectrum. After carefully choosing a proper wavelength for a particular chromophore, the laser surgeon has a difficult task of delivering maximum number of photons to the target chromophore before they are snatched by competing chromophores which are present before the target. By manipulating the other two variables i.e. pulse duration and fluence, this can be done effectively. Not only this, but by choosing proper pulse duration a smaller or a larger target having the same chromophore can be selected.

The absorption of various chromophores as a function of wavelength.[6]

Pulse duration:

Selection of pulse duration is mainly guided by the TRT, which in itself is related to the size of the target. As a general rule, larger the chromophore, longer is the TRT as large objects take long time to cool. More difficulty is encountered while lasing the smaller targets as the short pulse width required makes the epidermis full of melanin more vulnerable (TRT 10 ms). This is achieved by dividing this short pulse into still shorter rations of pulses which are delivered in succession. This form of energy delivery is called multiple synchronized pulsing.

Multiple Synchronized Pulsing:

[ Figure 2 ] Even though the epidermis is a strong competing chromophore for smaller targets, it can be spared as long as the TRT of the target is longer than that of epidermis. Longer TRT means the target takes longer time to cool to 50% of the temperature achieved. By multiple synchronized pulsing, which are usually two to three and maximum five pulses, with delays in between, both the target and epidermis are heated by the first pulse just within the threshold limit of the epidermis, therefore the epidermis is not damaged. After the first pulse there is a delay, during which both epidermis and target start cooling, but as the target cools slowly at the end of the delay it still retains some heat while the epidermis has cooled down completely. During the second pulse and the delay thereafter, the same sequence repeats but now the target gets heated to a higher level as it is starting from an elevated base line. This pulsing is repeated till such time as the temperature in the target at the end of last pulse exceeds its threshold limit.[7]

Multiple synchronized sequential pulsing.[7]

CLINICAL APPLICATIONS

The present clinical applications in skin conditions and cosmetology which most concern a plastic surgeon can be grossly divided into following five categories:

Unwanted hair

During the last decade permanent reduction of unwanted hair by lasers and light sources has been established as a quick, safe and sure method of choice over all earlier modalities of hair removal which were temporary. For permanent reduction the hair root should be destroyed. The term hair root refers to hair within the hair follicle. Here it will be appropriate to quickly review the relevant anatomy.

Hair follicle anatomy

Hair follicle is divided into three units:

Infundibulum includes the region from the hair follicle orifice to the sebaceous duct entrance.

Isthmus encompasses the region between the entrance of the sebaceous duct and the attachment of the arrector pili muscle.

Inferior segment is the region from the insertion of the arrector pili muscle to the base of the follicle and includes the hair follicle bulge and the bulb.

The hair follicle bulge is the extended lower portion of the hair follicle between the insertion of the arrector pili muscle to the hair bulb. The bulge has more or less constant distance of 1.5 mm from the skin surface.

The hair follicle bulb is the lowest portion of the hair follicle and is composed of matrix cells, interspersed by melanocytes, which produce hair. The distance of the bulb from the skin surface varies with the different stages of hair growth. It is shallowest at early anagen and deepest at late anagen stage. In anagen itself coarser the hair, deeper will be the follicle [ Table 1 ].

Table 1

Corresponding hair shaft diameter with anagen hair follicle depth. (Exceptions exist)

Melanin of hair root is the chromophore which absorbs photons and gets heated. Melanin has a wide and gradually sloping down absorption coefficient curve spanning from ultraviolet to infrared spectra giving a wide choice of wavelengths to chose from [ Figure 1 ]. This liberty of choosing wavelengths is restricted by the presence of the same melanin as a competing chromophore in variable quantities in the epidermis of different skin types. An ideal target is an extra coarse, dark black hair root in anagen growth phase on a lighter skin type. The most difficult is a fine, lightly pigmented hair of a darker skin type. In any case, if the target is lighter than the epidermis, no available laser can act on it at present. However, attempts are underway to enhance the absorbability of such lighter targets by incorporating external pigment in them. Meladine™[8] is one such exogenous chromophore enhancer.

Hair root is one example where target and chromophore are not exactly the same. Melanin content of the hair bulb and bulge is the chromophore which absorbs photons and gets heated. The target is follicular epithelium which is at some distance surrounding the bulb. Therefore, the bulb needs to be heated long enough to allow sufficient heat to get conducted to the target. Though the chromophore in case of extra coarse, medium and fine hairs is melanin, they can not be lased with the same parameters as the size of the chromophore in the bulb and bulge is not the same in three. The root of an extra coarse hair will never get destroyed, however high the energy, if you use a shorter pulse. You are just not allowing sufficient time for a larger chromophore to get sufficiently heated and dissipate its heat to the surrounding target. Extra coarse hair needs longer pulse duration than medium, and fine hair needs the shortest. Therefore, the pulse duration should be primarily guided by the size of the chromophore and only secondarily by the competing chromophore. Epidermal melanin poses as a competing chromophore in case of all lasers in the visible and infra red spectrum of light [ Figure 1 ], therefore it will be worth to consider a classification of skin based on epidermal melanin concentration and its response to sun [ Table 2 ].

Table 2

Fitzpatrick's skin photo types[9]

Shorter wavelengths like Ruby 694 nm and Alexandrite 755 nm were the first ones to be used for hair reduction. Though they have higher affinity to melanin, they can not penetrate deeply in the skin. In darker skin types V and VI they led to unacceptable rate of complications like epidermal burns and post treatment hyper pigmentation due to strong competing epidermal melanin. In contrast longer wavelengths like Diode 800 nm and Nd:YAG 1064 nm though having lesser affinity to melanin as compared to shorter ones are found to be more effective due to deeper penetration to the deeply placed hair follicles which at times are 3 to 4 mm deeper. At present both these wavelengths are found effective for hair reduction in darker skin types. Therefore the reach of the wavelength to the target is also an important consideration. Remember, photons have to first reach the target to get absorbed.

Vascular lesions, acne and scars

In vascular conditions, haemoglobin is the chromophore and the vessel wall is the target. Water contained in the cells of the vessel wall also acts as an additional chromophore. Haemoglobin has absorption coefficient curve with peaks at 418 nm, 524 nm, 577 nm and again at 1064 nm [ Figure 1 ]. At the first and second peaks the melanin absorption is also very high and therefore these wavelengths can not be used. However, the spectrum between 580 nm and 590 nm is useful clinically. Pulsed dye laser (PDL) with a wavelength of 595 nm is at present considered as a gold standard for vascular lesions. Some advanced systems have Dynamic Cooling Device (DCD™).[10] cooling to safeguard the epidermis. Here a cryogen spray falls on the skin milliseconds before the laser pulse and cools the epidermis by rapid evaporation thus protecting it.

Port wine stains (PWS)

Flash lamp pumped dye laser was the first example of a laser which was specifically built to treat PWS after understanding the principles of selective photothermolysis having 577 nm wavelength to match 3 rd peak of the absorption curve of haemoglobin and 450 micro second pulse duration which was less than the TRT of the smaller vessels of PWS. Later on it was replaced by yellow light at 585 nm and 595 nm having deeper penetration and giving better results. PDL treatment is the method of choice for most PWS in children [ Figure 3 ]. The IPL sources with a cut off filter of 590 nm are also quite effective and are being routinely used for a variety of vascular lesions including PWS [ Figure 4 ].

PWS before and after 6 treatments with PDL 595 nm

PWS before and after 10 treatments of IPL with 590 nm filter

By extending the principles of selective photothermolysis and by making use of multiple synchronized pulsing, the laser surgeon is now able to selectively target a larger or a smaller vessel in the same lesion having the same common chromophore haemoglobin. The only limiting factor in laser treatment of vascular lesions at present is the reach of the lesion (i.e. its depth from the skin surface. Currently a wavelength of 1064 nm can reach a maximum of 8 mm) and the flow in the lesion (only slow flow lesions can retain the generated heat long enough for the tissue destruction).

Haemangioma

The commonly used 590 and 595 nm wavelengths, being shorter, can act only on superficial lesions up to 1.2 mm depth. PWS, superficial haemangiomas and other superficial vascular lesions are treated by these. For deeper vascular lesions like deep haemangiomas, 1064 nm wavelength is used [ Figure 5 ].

Haemangioma in a five month old baby in rapid proliferative phase before and 1 year after 5 treatments with Nd:YAG 1064 nm. Note undamaged eyebrow and eyelashes, example of chromophore specificity. Eye was protected by a special metal corneal shield.[11]

Acne

Non intense Light Heat Energy (LHE) sources, IPLs and Lasers are playing an important role in multi-modality approach to management of acne and at present their role is three fold: in active acne with haemoglobin and water as chromophores, [ Figure 6 ] in acne induced pigmentation and in post acne scarring. Diode 1450 nm with water absorption is found effective in inflammatory acne. This as well as Erbium: glass 1540 nm Lasers are found promising in contouring atrophic acne scars.

Severe pustular acne before and after 2 IPL treatments (590 filter) as a part of multimodality approach

Scars

In scars role of lasers and IPL systems spans from fibroblast modulation in fresh scars, clearing scar pigment, removing traumatic tattoos to remodelling of collagen in hypertrophic scars [ Figure 7 ] and even older scars. Fresh scars are hypervascular, haemoglobin is the abundant chromophore and fibroblast is the target. Fresh scars can be made inconspicuous. PDLs, IPLs with 590 cut off filter have shown good results [ Figure 8 ]. Recently introduced Er: Glass 1540 nm fractional lasers also have shown promising results.

Post traumatic hypertrophic scar before and after 1 PDL and 4 IPL treatments (590 filter.)

(A) Extensive windshield injury of face. (B) Result after primary plastic surgical repair. (C) Result after 4 IPL treatments (590 filter) at monthly interval

Pigmented lesions and tattoos

As melanin absorbs light at a wide range of wavelengths, from 250 nm to 1200 nm [ Figure 1 ] almost any Laser with sufficient power causing thermal denaturation can be used to remove benign pigmented lesions of the epidermis. In dermal pigmented lesions the target chromophore is intracellular pigment melanosomes or tattoo particles. To make such sub-micrometer particles absorb photons, the energy delivery needs to be in nanoseconds (TRT) as compared to milliseconds for hair root and microseconds for capillaries. This has been made possible by a technique invented in 1962 called quality switching or Q-switching. Normally in a lasing chamber, once population inversion occurs i.e. the number of stimulated photons exceeds resting photons, the laser beam emerges through the partially reflective mirror. In Q-switching, the energy or number of stimulated photons is deliberately allowed to build up to a higher level by blocking the partially reflective mirror with a Pockels cell. Pockels cell contains laminar opaque crystals which become transparent only when an electric current is applied for a few nanoseconds controlled by an electronic switch. When transparent, the built up photons get released just for a few nanoseconds and blocked again to get rebuilt up. These ultra short high energy bursts of pulses lead to mechanical photo acoustic damage in the target cells. Q-switching is available with Ruby 694 nm, Alexandrite 755 nm, Nd:YAG 1064 nm and frequency doubled FD Nd:YAG 532 nm (i.e. in the path of QS Nd:YAG 1064nm beam, a KTP crystal is brought which changes the frequency to double so the 1064 nm wavelength becomes half i.e. 532.) KTP laser is also available as a non QS laser with 532 nm wavelength. This is an example of a laser beam being used as a source of energy (instead of a flash lamp) to stimulate another lasing medium. The rate of heating and rapid material expansion with Q-switched Lasers can be so severe that tissues are torn apart by shock waves, cavitation or rapid thermal expansion. The immediate effect on the pigmented skin is whitening, due to melanosome rupture and pyrolysis leading to formation of gas bubbles that scatter light. Immediate whitening offers a clinically useful treatment end point.

Selective photothermolysis with various Lasers is highly useful for epidermal and dermal pigmented lesions in which cellular pigmentation is the cause, however lasers and IPLs have variable usefulness for dermal melasma and post inflammatory hyper pigmentation because the etiological factors may still persist and lead to recurrence.

Treatment

The approach to the treatment depends on location of pigment (epidermal, dermal or mixed), the way it is packaged (intracellular or extracellular) and the nature of pigment (melanin or tattoo particle). Continuous wave lasers like CO₂ (10,600 nm) or Er-YAG (2940 nm) with water as a target chromophore in epidermis can be used for removing the superficial pigmented skin, especially seborrheic keratosis and ‘Q-switched resistant’ Café-Au-Lait macules. But the non selective thermal injury may lead to some erythema and possible pigmentary and textural changes. In many pigmented lesions however, the melanosomes and melanocytes are clustered so compactly that they act as a larger body of chromophore. In this situation, melanin specific wavelengths even in millisecond domain also lead to lesion clearance. These long pulsed lasers are only suitable to treat nevocellular nevi while IPL (500 nm to 1200 nm) treats photo damaged pigmentations like solar lentigines, dyschromia and melasma [Figures ​ [Figures9 9 ​ 910 10 ].

Congenital melanocytic nevus before and after 8 IPL treatments. First 6 treatments with 645 filter for deeper pigment and last 2 with 615 filter for superficial pigment. Eye was protected by a special metal corneal shield.[11]