LASERS
in Dermatological Practice
LASERS
in Dermatological Practice
Editors
Kabir Sardana
MD DNB MNAMS ProfessorDepartment of Dermatology and STD Maulana Azad Medical College
New Delhi, India
Vijay K Garg
MD MNAMS Director–Professor and Head Department of Dermatology and STDMaulana Azad Medical College New Delhi, India
Forewords
Ganesh S Pai
B Krishna Rau
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First Edition: 2014 ISBN 978-93-5152-300-0
Dedicated to
My colleagues, friends and foes, the last of which goad us to better ourselves constantly……
My wife Dr Supriya, who helps me to keep the balance between family and academics
My daughter Zoya, who is the ‘zing’ in my life
My parents, Mrs Amba Sardana and Major General Sardana who have instilled discipline in my life
and
Lastly, the Department where over the years we have honed the skills in laser intervention
My family and friends
My wife Mrs Manju Garg, who has stood by me through times of strife My son Devansh, who is pursuing his MBBS
and
My daughter Dr Ekta, who is a dentist
—Kabir Sardana
Anil Aggrawal MD Forensic Medicine (AIIMS)
Director-Professor Forensic Medicine
Maulana Azad Medical College New Delhi, India
Anil Ganjoo MBBS MD
Senior Consultant Dermatologist and Head of Dermatology
Sunderlal Jain Hospital Saroj Hospital and INMAS New Delhi, India
Anjali Madan MD
Senior Resident
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Anuj Tenani MBBS PGY-II
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Anusha H Pai MD
Consultant Dermatologist Derma-Care Skin and Cosmetology Center Mangalore, Karnataka, India
Atul M Kochhar MD DNB MNAMS FAAD
Senior Specialist–Grade I
Department of Dermatology and STD Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Banwari Jangid MD
Department of Dermatology and Venereology
All India Institute of Medical Sciences New Delhi, India
Dharmendra Karn MD Dermatologist Dhulikhel Hospital Kathmandu University Teaching Hospital Kavre, Nepal Ganesh S Pai MD DVD
Senior Consultant Dermatologist Derma-Care Skin and
Cosmetology Center Mangalore, Karnataka, India
Inder Raj S Makin
MBBS (India) Dipl-Ing (Germany) RDMS PhD (USA)
Associate Professor AT Still University
School of Osteopathic Medicine in Arizona (SOMA)
Arizona School of Dentistry and Oral Health (ASDOH)
Mesa, USA
Jaspriya Sandhu MBBS PGY-I
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Kabir Sardana MD DNB MNAMS
Professor
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Khushbu Goel MD
Pool Officer
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Narendra Kamath MD DVD
Consultant Dermatologist Cutis Skin Care Center Mangalore, Karnataka, India
Pavithra S Bhat MD
Kovai Medical Center and Hospital Coimbatore, Tamil Nadu, India
Payal Chakravarty MD
Senior Resident
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Rashmi Ranjan MD
Senior Resident
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Rashmi Sarkar MD MNAMS
Professor
Department of Dermatology Maulana Azad Medical College and LN Hospital
New Delhi, India
Chief Founder and Honorary Secretary Pigmentary Disorders Society
New Delhi, India
Shahin S Nooreyezdan MBBS MS MCh (Plastic Surgery) PGIMER Chandigarh
Senior Consultant
Department of Plastic, Cosmetic and Reconstructive Surgery
Indraprastha Apollo Hospitals New Delhi, India
Shikha Bansal MD DNB MNAMS
Specialist
Department of Dermatology Safdarjung Hospital
New Delhi, India
Shivani Bansal MD
Senior Resident
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Simal Soin
PG Dermatology (St Johns Institute of Dermatology) London
MPhil Cambridge University UK
Medical Director and Chief Cosmetic Dermatologist Three Graces
New Delhi, India
Soni Nanda MD (Dermatology)
Shine and Smile Skin Clinic Max Super Specialty Hospital New Delhi, India
Sujay Khandpur MD DNB MNAMS
Professor
Department of Dermatology and Venereology
All India Institute of Medical Sciences New Delhi, India
Twinkle Daulaguphu MBBS PGY-I
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Vanya Narayan MBBS PGY-III
Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Vijay K Garg MD MNAMS
Director-Professor and Head Department of Dermatology Maulana Azad Medical College and Lok Nayak Hospital New Delhi, India
Vivek Nair MBBS MD
Consultant Dermatologist Dr Nair’s Skin Clinic (Palam Vihar) Clinic Dermatech (Vasant Vihar and Gurgaon)
Metro Hospital (Palam Vihar) New Delhi, India
Lasers have moved from the fringe of dermatology to a more centrist path over the past decade. Fifteen years ago, when lasers trickled into our country, they were considered to be exotic and perhaps accessible to a select few. Cosmetic dermatology and lasers have grown by leaps and bounds and that necessitates that they are absorbed in the mainstream.
With close to half of the dermatologists now owning or having access to lasers, it is important that our younger generation of dermatologists have access to good practical textbooks as well as high quality equipment. This book, Lasers in Dermatological Practice is best suited to educate our specialty about the perils and pitfalls of using lasers.
Indian skin is unique since it comes commonly in 3 types—IV, V, VI. Parameters will therefore vary depending on the skin types, a dilemma that western books do not address. Postinflammatory hyperpigmentation will vary in each skin type and even show variation among patients in a single skin type. Such unpredictability and perplexing results are a cause of anxiety in a cosmetologist at an inflexion point in his career. A comforting thought is that our patients, except for a miniscule minority, are forgiving and compliant. Most cases of tissue damage by laser will heal over time, nature coming to our rescue. Our patience and reassurance will comfort patients in the interim period.
In clinical dermatology, we have a chance to assess, judge and treat patients. If there is an error of management, we can apply a midcourse correction and modify therapy. Unfortunately, this is not true of lasers. A mistake made, a poor assessment, using more or less power than required can lead to laser burns and scarring. If it is on the face, as it is most of the time, the consequences are not difficult to portend. Since there is no second chance to repair damage, it is important to understand the basics of lasers and the specifics of equipment much like reading a car manual before driving your new car. This book does both and will hopefully lead to confident cosmetologists and happy patients.
Foreword
Ganesh S Pai MD DVD FAAD
Medical Director Derma-Care Skin and Cosmetology Center, The Trade Center Director-Professor, Department of Dermatology KS Hegde Medical College, Deralakatte
I thank the authors for giving me the opportunity to write the Foreword to this excellent book, Lasers in Dermatological
Practice. The editors along with the co-authors have put
down their vast experience in the use of laser in various dermatological conditions. It is a book of international standards and, in particular, reference to the application
of lasers in brown and dark skin patients. Basics of laser in relation to skin lesions are well-written.
The use of the different lasers in different dermatological lesions and the step-by-step approach to each and every lesion is superb. The practical tips to avoid wrong outcome is well-documented. The use of non-laser energy sources in dermatological practice is very illuminating. The references at the end of each chapter are apt and to the point.
The chapter on medicolegal aspects is pertinent and informative. On the whole, it is the end result of the vast experience over the years that the editors have acquired to write this book. I am confident that this book will find a place in all dermatologists library.
Foreword
B Krishna Rau
MS FRCS (Eng and Edin) FRCS (Thailand) (Hon) FIAMS FACG FICS FIGSC
Professor-Emeritus, Dr MGR Medical University Honorary Fellow, American Surgical Association President, World Federation of Society for Laser and Surgery Medicine
The genesis of this book arose from the common mechanistic approach where we learn which buttons to push, in courses provided by the more reputable device manufacturers just after a laser is purchased. This approach is foolish beyond words, and can harm patients, and worse create medicolegal hazards. There are some excellent books that we have referred to but most of them deal with technologies that are nice to hear but too expensive to use in India. Our book was initially planned as a companion to the hands on workshop where the nitty gritty was left out while the topic in focus was discussed. Thus the first edition was done with the help of Sun Pharmaceuticals. This edition is the combined effort of Abbot and the vision of Shri Jitendar P Vij, who convinced us to make it an elaborate yet compact book.
The book answers the three basic questions, what to do, why to do it and how to do it? But our basic target is the dermatologists who need a step-by-step approach to the technology commonly used and not the laser that a speaker in most conferences uses, which as a thumb rule is expensive, the reason why the company sponsors the talk in the first place! Though the FDA gives clearance of a device for a particular labeled indication, this cannot be taken as any assurance that it will work safely and effectively enough to satisfy the patients. Tragically, it may not be an understatement that a majority of lasers bought in this country are not US FDA approved in the first place!
The book will also look at some questions that we rarely ask. What is the histological depth of fractional lasers? Which type of atrophic scar actually responds? Is Fr CO2 superior to Er:Glass? And many others.
As the field of cosmetic intervention usually encompasses indications where novel non-laser technologies are used, we have covered radio-frequency, focused USG, plasma resurfacing and LED.
The book is planned in such a way where the commonly performed procedures are discussed which gradually move on the advanced techniques. Practical aspects like medicolegal hazards and pearls are discussed in the latter half of the book. Some very useful information is provided in the appendices.
Our contributors are largely those who are experts in their field of interest. Our own work spanning over 8 years, with almost 5,000 procedures helped us to bridge the gap between theory and practice.
But this is not a “Cook Book” and only a guide on the best approach is provided. Individual laser parameters can vary, thus there is no substitute for hands-on training, which cannot be obtained in this book or sitting in a lecture hall more so when there are hundreds sitting in it!
Hope you like the effort. More will follow soon…
Preface
Kabir Sardana Vijay K Garg
We would like to thank our faculty residents and students of the department some who have left to join other institutions, for their role in establishing and developing the Laser Clinic at Maulana Azad Medical College (MAMC), New Delhi, India.
Special thanks to Dr Vijay K Garg, Director-Professor and Head, Department of Dermatology and STD, MAMC, who through his administra-tive acumen, managed to get the lasers. He has given me great support and has served as a mentor throughout my professional career. His guidance and encouragement over the years have influenced my efforts.
A special thanks to the team at M/s Jaypee Brothers Medical Publishers (P) Ltd, New Delhi, India, especially Shri Jitendar P Vij (Group Chairman) and Mr Ankit Vij (Managing Director), for latching on to the project, Mr PN Venkatraman (Vice President-International), Mr Shashikumar Sambhoo, for handling the publicity and sales and Mr Tarun Duneja (Director-Publishing), Mr Subrata Adhikary (Commissioning Editor), Mr Lalit kumar (DTP Operator) for helping with the deadlines.
A big thanks to our contributors, some of whom who have worked on their chapter on a one month deadline! Each of them is an expert in their field.
Dr Simal Soin, Dr Shahin Nooreyezdan, Dr Inder Raj S Makin and Dr Vivek Nair have worked on such a deadline. Dr Inder Raj S Makin has also been kind enough to review two chapters for us and his comments have been an asset to the chapters.
Dr Khandpur and Dr Anil Agarwal have also contributed after taking out time from their busy schedule. Dr Atul M Kochhar who is also the Purchase Officer at our Hospital has given nuances of buying lasers.
A big thanks to Dr Antje Katzer (Ascepelion), for letting us use the images of the company’s devices.
And lastly, our tributes to the countless patients who have taught us dermatology and helped us to learn and relearn lasers!
Real knowledge is to know the extent of one’s ignorance
—Confucius
Never sacrifice your dignity to make money, but charge what you are worth
Contents
Section 1: Conventional Laser Interventions
1. Basics of Laser-Tissue Interactions 3
2. Ablative Lasers 25
Overview 25
Ablative Laser Treatment of Common Conditions 52 Step by Step Approach 86
Atlas 93
3. Pigmented Lesions and Tattoos 101
Overview 101
Lasers For Tattoo Removal 115
Laser Treatment of Common Pigmented Conditions 131 Step by Step Approach 160
Atlas 163
4. Fractional Photothermolysis 172
Overview 172
Laser Treatment of Common Conditions 204 Step by Step Approach 229
Atlas 233
5. Vascular Lasers 236
6. Lasers for Hair Removal 252
Section 2: Advanced Laser Interventions
7. Nonablative and Subsurface Rejuvenation 275 Step by Step Approach 291
8. Nonsurgical Tightening 294
9. Aesthetic Intense Focused Ultrasound (IFUS): Clinical Perspective
on Fitzpatrick Skin Types III–VI 319
10. Noninvasive Body Contouring 336
11. Lasers for Scars, Keloids, and Stretch Marks 361
Section 3: Practical Aspects and Advances
12. Miscellaneous Laser Responsive Disorders 379 13. How to Start a Laser Practice (Private Setup) 416 14. How to Set up a Laser Clinic in a Public Funded Institution 421
15. Therapeutic Pearls in Lasers 432
16. Medicolegal Aspects of Lasers in Dermatological Practice 441
17. Complications and their Management 455
Appendices
Appendix 1: Laser Safety/Eye Care 493
Appendix 2: Consent Form 504
Appendix 3: Procedure Checklist 506
Appendix 4: Postoperative Care 507
Appendix 5: Sample Operative Note 512
Appendix 6: Sample Postoperative Instructions (Ablative Lasers) 513
Appendix 7: Patient Information Sheet 514
Appendix 8: Local Anesthetics 528
Appendix 9: Select Bibliography 538
Laser and Medical Devices (Index) 541
Section
1
Conventional
Laser Interventions
chapter
1
Basics of
Laser-Tissue Interactions
Kabir Sardana, Vijay K Garg, Shivani Bansal, Jaspriya Sandhu, Twinkle Daulaguphu
Medical lasers have evolved over the years with numerous applications. Dermatologic laser surgery is regarded as one of the fastest growing areas in the emerging fields of photomedicine and biomedical optics. As with any device, the most efficacious and appropriate use requires an understanding of the basic photobiological and photophysical principles of laser-tissue interaction as well as the properties of the laser itself. This chapter provides a brief description of the nature of the laser, how it works, and the fundamental mechanisms of its interaction with human skin.
Light
Light represents one portion of a much broader electromagnetic spectrum. Light can be divided into the UV (200–400 nm), VIS (400–700 nm), NIR “I” (755–810 nm), NIR “II” (940–1,064 nm), MIR (1.3–3 mm), and Far IR (3 mm and beyond) (Fig. 1.1).
Normally, the percentage of incident light reflected from the skin surface is determined by the index of refraction difference between the skin surface (stratum corneum n = 1.55) and air (n = 1). About 4–7% of light is typically reflected and is called the Fresnel reflectance because it follows Fresnel’s equations relating reflectance to the angle of incidence, plane of polarization, and refractive index. The angle between the light beam and the skin surface determines the percentage of reflected light. More light is reflected at “grazing” angles of incidence. It follows that, to minimize surface losses, in most laser applications, one should deliver light approximately perpendicular to the skin. One can deliberately angle the beam, on the other hand, to decrease penetration depth and also attenuate the surface fluence by “spreading” the beam.
On the other hand, the surface of dry skin reflects more light because of multiple skin-air interfaces (hence the white appearance of a psoriasis plaque). The light penetration into the epidermis depends on the wave-length dependent absorption and scattering. Because of scattering, much incident light is remitted (remittance refers to the total light returned to the
environment due to multiple scattering in the epidermis and dermis, as well as the regular reflection from the surface). In laser surgery, light reflected from the surface is typically “wasted”. This “lost” energy varies from 15% to as much as 70% depending on the wavelength and the skin type. For example, for 1,064 nm, 60% of an incident laser beam may be remitted.
Tissue effects occur only when light is absorbed. The absorption coefficient is defined as the probability per unit path length that a photon at a particular wavelength will be absorbed and it depends on the concentration of chromophores (absorbing molecules) present. The three primary skin chromophores are water, hemoglobin and melanin (Fig. 1.1). Chromophores exhibit characteristic bands of absorption at certain wavelengths. For example, melanin absorbs broadly across the visible and ultraviolet (UV) spectrum, the oxyhemoglobin and reduced hemoglobin in blood exhibit strong bands in the UV, blue, green and yellow regions. Water has strong absorption in the infrared (IR) region (Fig. 1.1).
Optical properties of the epidermis and dermis are different. In pigmented epidermis, melanin absorption is usually the dominant process over the majority of the optical spectrum (200–1000 nm) (Fig. 1.1). In the dermis, there is strong, wavelength-dependent scattering by collagen fibers, which attenuates penetration of light. This scattering varies inversely with wavelength. Thus as a thumb rule, between 280 nm and 1300 nm, the depth
Fig. 1.1: Absorption spectrum of various lasers in
of penetration increases with wavelength. Above 1300 nm, penetration decreases due to the absorption of light by water. The most deeply penetrating wavelengths are 650–1200 nm, while the least penetrating wavelengths are within the far-UV and far-IR regions.
types of Light Devices
Lasers contain four main components, the lasing medium, the excitation source, feedback apparatus and an output coupler. The amplifier of a laser is the laser material that can be a solid, a gas, or a liquid. The feedback mechanism is produced by the resonator, where the light is reflected by two mirrors so that the photons pass several times through the laser material. The number of photons within the resonator increases exponentially due to the stimulated emission (Fig 1.2).
With respect to lasing media, there are diode lasers, solid-state lasers, dye and gas lasers.
Solid-state lasers include the Nd:YAG laser, Er:YAG laser, alexandrite laser and the ruby laser. The gas lasers include the carbon dioxide (CO2) laser, argon ion laser and the excimer lasers, while the diode and dye lasers are singular in their class.
Light Device terminology
Basic parameters for light sources are power, time and spot size for continuous wave lasers and for pulsed sources, the energy per pulse, pulse duration, spot size, fluence, repetition rate and the total number of pulses (Table 1.1).
At least for most ablative lasers, the effect of the laser beam on human skin can be affected by any of three variables: power, time and spot size. The effects of power and time are proportional whereas that of spot size (radius) is an inverse square. If either the power or time is doubled, fluence increases by a factor of 2. However, if the spot size is decreased by a factor of 2, fluence increases by a factor of 4. Doubling the spotsize results in a four-fold reduction in fluence.
Energy: Measured in Joules (J).
Fluence: The amount of energy delivered per unit area is the fluence, sometimes called the dose or radiant exposure, given in J/cm2
Power: The rate of energy delivery is called power, measured in watts (W). One watt is one joule per second (W = J/s).
Density: The power delivered per unit area is called the irradiance or power density, usually given in W/cm2.
Pulse width :Laser exposure duration (called pulse width for pulsed lasers) is the time over which energy is delivered.
Thus the lasers may be continuous, pulsed, quasi continuous and Q-switched (Fig. 1.3).
The older lasers had pulse durations that varied from seconds to milliseconds (0.01s/10-3). Millisecond CO
2 lasers are gated lasers but largely
continuous wave in nature. The CO2 laser is a classic example of a continuous mode laser. Microsecond lasers (0.000001 sec/10-6) are the ideal ultrapulse
lasers. Most Er:YAG lasers are also microsecond lasers. Another example is that of the PDL where a single or a train of pulses is emitted.
Pseudocontinuous lasers (KTP) have very short pulses of light repeated at very high repetition rates. Extremely short pulses are achieved by Q-switching. These nanosecond lasers (0.000000001 s/10–9) are used in pigmented lesions
(Q-switched lasers). Recently picoseconds (0.000000000001 s/10-12) have
been used in tattoos.
Power density: It is a critical parameter, for it often determines the action mechanism in cutaneous applications. For example, a very low irradiance emission (typical range of 2–10 mW/cm2) does not heat tissue and is
associated with diagnostic applications, photochemical processes and biostimulation. On the other extreme, a very short nanosecond (ns) pulse can generate high peak power densities associated with shock waves and even plasma formation.
Table 1.1 Various terminologies used in lasers
Power P (W) For Cw lasers
Energy E = (J) For Cw lasers
Power density W/A (irradiance) (W/cm2) (A = effective area)
For Cw lasers
Peak power P max (W) For pulsed lasers
Energy E per pulse (J) For pulsed lasers
Pulse duration t [fs (10−15) to ms (10−3)]
For pulsed lasers Energy density E/A (radiant
exposure) (J/cm2) (A = effective area)
For pulsed lasers
Spot Size: Another factor is the laser exposure spot size (which greatly affects the beam strength inside the skin).
Other important factors include aspects of the incident light (convergent, divergent or diffuse) and the uniformity of irradiance over the exposure area (spatial beam profile). The pulse profile, that is, the character of the pulse shapes in time (instantaneous power versus time) also affects the tissue response.
Operational modes: The Operational modes of lasers are Cw, pulsed as interrupted radiation (in ms), pulsed free running (in hundreds of ms), Q-switched (in ns) or mode-locked (in fs).
Continuous wave (Cw) laser may be differentiated from a pulsed laser,
which provides bursts of energy. In the Cw mode, the laser delivers a continuous beam of light with little or no variation in power output over time (Fig. 1.3). In Cw operation, laser output is controlled by the physician, typically by depressing a foot pedal.
Interrupted radiation of a Cw laser is done by mechanical or electronic
switching with modification of the pulse length. The pulse frequency is low to moderate, up to 100 Hz. Flash lamp pumped solid-state lasers in the free-running mode have pulse lengths of 50 ms up to several hundred micro-seconds. Pulses of medical dye lasers systems can vary from microseconds to 50 ms. Superpulse is a term specific to some carbon dioxide lasers that
Fig. 1.3: A figurative depiction of the energy and duration of
have been modified to produce very short pulses with high peak powers in a repetitive fashion, commonly several hundred pulses per second.
Q-switching: Shorter pulses with very high intensities in the nanosecond
range are produced by Q-switching of the laser. The single, intense pulse with a duration on the order of nanoseconds is produced. With Q-switching (the Q-factor stands for “quality factor,” used in electronics theory terminology), a fast electromagnetic switch (Pockel cell) in the laser cavity causes excitation of the active medium to build-up far in excess of the level of the medium when the shutter is open. In operation, the flashlamp is turned on and the population inversion gradually grows. Lasing is prevented by the shutter. When the population inversion is at a maximum, the shutter is opened so that lasing occurs and a large burst of energy is emitted as the cavity rapidly depletes the population inversion. The net result is an extremely high peak power (greater than 106 W) nanosecond duration pulse or series of pulses.
Ultrashort laser pulses are generated by mode-coupling due to the
coherent properties of the laser. Compared to Q-switching, where the shortest pulse durations are in the range of the resonator period, mode-coupling can generate even shorter laser pulses.
Beam ProfiLes: toP hat versus gaussian
Laser beam profiles vary based on intercavity design, lasing medium and the delivery system. A common profile is Gaussian or bell-shaped (Fig. 1.4). For
Fig. 1.4 : Comparison of the beam types of lasers. In most indications, the top hat
profile is preferred. The lower half of the figure demonstrates the conversion of a Gaussian beam into a top hat beam, which can be achieved in certain laser
many lasers, this profile represents the fundamental optimized “mode” of the laser. This shape is usually observed when the beam has been delivered through an articulated arm. For some wavelengths, this is an effective way to deliver energy (CO2 and Erbium). The disadvantage of the rigid arm is limited flexibility, the typically short arm length, the possibility of misalignment from even minor impact and a tendency for nonuniform heating across the spot. The top hat beam ideally is better as there is uniform heating of the tissue.
Sometimes a bell-shaped profile is desirable, for example, when applying a small spot FIR beam with a scanner. In this scenario, the wings of the beam allow for some overlap without delivering “too much” energy at points of overlap. The Gaussian profile can be modified outside the cavity, which is desirable in many applications. With a fiber equipped delivery system, the beam is mixed within the fiber and can be shaped to be more flat-topped.
WaveLength ranges and CLiniCaL aPPLiCation
A useful way of understanding the effects and clinical application of wavelength is to understand the interaction and depth of the different wavelength in relation to the primary chromophores (Fig. 1.5).
1. UV laser and light sources have been used primarily for treatment of
inflammatory skin diseases and/or vitiligo, as well as striae. The XeCI excimer laser emits at 308 nm, near the peak action spectrum for psoriasis. The penetration is restricted to the epidermis (Fig. 1.5). 2. Violet IPL emissions, low power 410 nm LED and fluorescent lamps are
used either alone or with ALA.
3. Green Yellow (GY): These wavelengths are highly absorbed by hemoglobin (Hgb) and melanin and are especially useful in treating epidermal pigmented lesions and superficial vessels (Figs. 1.1 and 1.5). There are two issues concerning these lasers, one is their poor penetration
in skin (and the even poorer penetration in blood) which makes them poor choices for treatment of deeper pigmented lesions or deeper larger vessels. Similarly they are not useful for permanent hair reduction (with the possible exception of very large spots (i.e., IPL) that enhance light depth). The effective portions of many IPL spectra include the GY range. By the proper manipulation of a laser delivery device, one can optimize parameters for selective heating of pigmented versus vascular lesions. Practical aspects of GY laser manipulation:
A. Applying a compression handpiece without cooling with 595 nm, blood is depleted as a target and pigment is preferentially heated. B. If the pulse duration is reduced to the nanosecond range,
melanosomes are preferentially heated over vessels. For example, extremely short Q-switched 532 nm pulses will cause fine vessels to rupture, but inadequate heat diffusion to the vessel wall precludes long-term vessel destruction. On the other hand, melanosomes are sufficiently heated for single-session lentigo destruction. By choosing specific wavelengths with respect to hemoglobin and melanin, one can achieve some degree of selective melanin or hemoglobin heating.
4. Red and Near IR (I) (630, 694, 755, 810 nm): Deeply penetrating
red light (630 nm) continuous wave devices are efficient activators of protoporphyrin after topical application of ALA. The 694 nm (ruby) laser is optimized for pigment reduction and hair reduction in lighter skin types. The 810 nm diode and 755 nm alexandrite laser, depending on spot size, cooling, pulse duration and fluence can be configured to optimize outcomes for hair reduction, lentigines or blood vessels. They are positioned in the absorption spectrum for blood and melanin between the GY wavelengths and 1,064 nm (Fig. 1.5). They will penetrate deeply enough in blood to coagulate vessels up to 2 mm; also, they are reasonably tolerant of epidermal pigment in hair reduction (with surface cooling) as long as very dark skin is not treated. By decreasing the pulse width into the nanosecond range, the alexandrite laser is a first line treatment for many tattoo colors.
5. Near IR (II) 940 and Nd:YAG (1,064 nm): These two wavelengths
have been used for a broad range of vessel sizes on the leg and face. They occupy a unique place in the absorption spectrum of the “3” chromophores, that is blood, melanin and water. Because of the depth of penetration (on the order of mm), they are especially useful for hair reduction and coagulation of deeper blood vessels. By varying fluence and spot size, reticular ectatic veins, as well as those associated with
nodular port wine stains or hemangiomas, can be safely targeted. On the other hand, they are not well-suited for epidermal pigmented lesions. 6. Mid infrared lasers and deeply penetrating halogen lamps: These
lasers and lamps heat tissue water. The absorption coefficients for the 1320, 1450, and 1540 nm systems are –3, 20 and 8 cm-1, respectively
and the corresponding penetration depths are –1500, 300 and 700 mm. It follows that for equal surface cooling and equal fluences, the most superficial heating will occur with the 1450 nm laser, followed by the 1540 and 1320 nm lasers. The MIR spectral subset has become the mainstay for fractional non-ablative technologies.
7. Far infrared systems: The major lasers are the CO2, Erbium YAG and
Erbium:YSGG (chromium:yttrium-scandium-gallium-garnet) lasers. Overall, the ratio of ablation to heating is much higher with the erbium YAG laser.
However, one can enhance the thermal effects of the Er:YAG laser by extending the pulse or increasing the repetition rate and likewise one can decrease residual thermal damage (RTD) of the CO2 laser by decreasing pw (pulse width). Details of the two lasers are given in the chapter on ablative lasers.
Laser-tissue interaCtions
The actual laser interaction is characterized by a dissipation of energy though an ideal situation is characterized by a direct straight line transfer of energy (z) (Fig. 1.6). When photons strike the surface of the tissue, because of the refractive index change, a portion (4–10%) of the photons are reflected
Fig. 1.6: Laser tissue interaction. Ideal laser penetration is a straight line (z) which is
according to the angle of incidence. Photons penetrating the surface initially are refracted, obeying the law of Snellius, which states that photons entering a medium with a higher refractive index are refracted towards the vertical axis to the surface.
Of all the different interactions, the most important is absorption or scattering.
absorption
The coefficient μa (cm-1) characterizes the absorption. The inverse, Ia,
defines the penetration depth (mean free path) into the absorbing medium and is typically given as cm–1. The absorption coefficient is chromophore
and wavelength-dependent. Absorbing molecular components of the tissue are porphyrin, hemoglobin, melanin, flavin, retinol, nuclear acids, deoxyribonucleic acid (DNA)/ribonucleic acid (RNA) and reduced nicotinamide adenine dinucleotide. The absorption spectra of different chromophores of biological tissue and water are plotted in Figure 1.1 while the penetration is shown in Figure 1.5.
Chromophores
Blood, water and melanin are the main absorbing components in the tissue (Fig. 1.1). Therefore, dye lasers and diode lasers effectively interact with blood, the alexandrite laser with melanin and MIR lasers with the water content of the tissue.
Hemoglobin: There is a large HgbO (oxyhemoglobin) peak at 415 nm, followed by two smaller peaks at 540 and 577 nm. An even smaller peak is at 940 nm. For deoxyhemoglobin (Hgb), the peaks are at 430 nm and 555 nm. The discrete peaks of hemoglobin absorption allow for selective vessel heating. Although the 410 nm peak achieves the greatest theoretical vascular to pigment damage ratio among the other peaks, scattering is too strong for violet light to be a viable option for vascular applications.
Melanin: Most pigmented lesions result from excessive melanin in the epidermis. By choosing almost any wavelength (< 800 nm), one can pre-ferentially heat epidermal melanin. Shorter wavelengths will create very high superficial epidermal temperatures, whereas longer wavelengths tend to bypass epidermal melanin (i.e. 1,064 nm).
Fat: Fat shows strong absorption at 1,200 and 1,700 nm. Although the ratios of fat to water absorption are small, the small differences are exploited with the proper choice of parameters.
1,200 nm might represent the best choice due to decreased overall water absorption and therefore, increased penetration. Sebum is similar to fat but also is comprised of wax esters and squalene.
Once carbon is formed at the skin surface, the skin becomes “opaque” to most laser wavelengths (that is, most energy will be absorbed very superficially. It follows that the dynamics of surface heating changes immediately once carbon is formed. This can be used creatively as an advantage. For example, one can convert a deeply penetrating laser to one that would only affect the surface by using a carbon dye. This has been accomplished with a laser peel using a Q-switched Nd:YAG laser, though is is not commonly used now. Collagen: Dry collagen has absorption peaks near 6 and 7 mm. With a free electron laser operating at these wavelengths, collagen can be directly heated.
Scattering
The scattering behavior of biological tissue is important because it determines the volume distribution of light intensity in the tissue. This is the primary step for tissue interaction, which is followed by absorption and heat generation. Scattering of a photon is accompanied by a change in the propagation direction without loss of energy.
Scattering leads to an increase in the light intensity directly below the tissue surface is enhanced by a factor of 2–4 as compared with the intensity of the incident beam. The increased fluence rate is caused by scattered photons overlapping with the incident photons. Another observation is that due to the scattering effect, the penetration depth depends on the irradiated area. Practical Implications
It has been shown that the light intensity directly below the tissue surface is enhanced by a factor of 2–4 as compared with the intensity of the incident beam. The increased fluence rate is caused by scattered photons overlapping with the incident photons. Because of the scattering effect, the penetration depth depends on the irradiated area. Thus, the penetration depth will double if for the same irradiance, the beam diameter increases from 1 mm to 5 mm. Thus for treating port wine stains or for hair removal, 10 mm to 15 mm spot diameters of the laser are recommended as it increases the depth of the laser beam. In tattoos and nevus of Ota in case there is inadequate response , it is wise to increase the diameter of the probe to increase the depth.
reaction Mechanisms
The first systematic presentation of the reaction mechanisms of lasers with tissue was by Boulnois and is depicted in the Figure 1.7. This highlights the different tissue effects and thus smaller the pulse duration of the interaction more the energy. Thus the Q-switched lasers like Nd:YAG can generate photodisruptive fluencies due to the short time of impact.
The various tissue reactions include, Nonthermal reaction, chemical reactions, thermal reactions (based on relaxation time), tissue ablation or photodisruption.
Once the local subsurface energy density has been determined, heat generation can be predicted by energy balance (conservation of energy), pulse duration, thermal relaxation time and the wavelength specific absorption for that target.
We will focus largely on the interactions relevant to commonly used medical lasers.
1. Photothermal Reactions
Photothermal effects (1 ms–100 s; 1–106 W/cm2; Fig. 1.7)
The energy of the laser irradiation is transferred into heat due to the absorption of the photons by tissue components, DNA/RNA, chromophores, proteins, enzymes and water. According to the degree of heating, stepwise and selective thermal damage can be achieved:
¾ 42–45°C: Beginning of hyperthermia, conformational changes and
shrinkage of collagen;
¾ 50°C: Reduction of enzymatic activity;
¾ 60°C: Denaturation of proteins, coagulation of the collagens, membrane
permeabilization;
¾ 100°C: Tissue drying and formation of vacuoles;
¾ >100°C: Beginning of vaporization and tissue carbonization;
¾ 300–1,000°C: Thermoablation of tissue, photoablation and disruption.
Thermal diffusion is responsible for heat flow into the tissue. If the exposure time with a laser pulse, tp, is short compared to the diffusion time, td, then we have “thermal confinement” and the pulse energy is converted into heat.
Thermal diffusion and the extent of tissue necrosis are related. With low laser power and long irradiation time, thermal necrosis is large. Shortening the laser application time reduces the time for thermal diffusion and the zone of necrosis becomes smaller. Minimum thermal necrosis is reached when the irradiation time is equal to the thermal diffusion time or thermal relaxation time. This is demonstrated by the laser interactions with pulsed CO2 lasers (Fig. 1.8).
Thermal damage of the tissue is described by the Arrhenius rate equation (Fig. 1.9). The consequence of this equation is that the threshold for tissue damage depends on the laser power and the application time. This threshold can be reached with high laser power in a very short time, resulting in a higher temperature or with low power but long irradiation, where the threshold is reached with lower temperature.
2. Tissue Ablation
The preconditions for tissue ablation are high absorption and very short laser pulses. Analogous to the thermal confinement, one can define a “stress confinement” when tissue is heated up so fast that the pulse duration is
Fig. 1.8: Example of the effect of pulse duration on tissue effect. (A) 3 J/cm2; 0.01 sec (whitening); (B) 3 J/cm2; 0.40 sec (coagulation); (C) 9 J/cm2; 0.50 (cogulation with ablation). Lower the irradiation time, lesser the coagulation the thermal necrosis
shorter than the propagation time. When the stress wave with velocity “c” cannot leave the heated volume during the laser pulse, then it is removed with the ablation of the material and the surrounding tissue is not damaged. Only UV lasers (ArF excimer laser) and pulsed MIR lasers have such high tissue absorption that they are effective ablating lasers.
Application: The threshold behavior of highly absorbed laser radiation,
e.g., the erbium-doped yttrium-aluminium-garnet (Er:YAG) laser with a 2,940 nm wavelength, can be used to modulate the thickness of necrosis in soft tissue. Operation of the laser in normal ablation mode does not produce effective thermal necrosis; therefore, no coagulation can stop bleeding. The advantage is that the healing is fast with minimal scarring. However, for precise superficial surgical interventions, it would be helpful if the Er:YAG laser could be modulated to coagulate the tissue by a series of high-frequency sub-threshold laser pulses. The energy of such pulses is below the ablation threshold and therefore, is transferred into heat. The heat causes thermal necrosis. The thickness of the necrotic tissue layer can be modulated by the number of sub-threshold pulses.
3. Photodisruption
(10 ps–100 ns:108–1010 W/cm2)
Focused laser pulses in the nanosecond region (e.g. with a Q-switch neodymium (Nd):YAG laser), or with a picosecond or femtosecond durations (titanium (Ti) sapphire laser) develop power densities of 1012 W/cm² and
more (Fig. 1.7). The electric field strength of this focused radiation is high enough to pull electrons out of the atoms, forming a plasma and producing an optical breakdown with shockwaves disrupting the tissue.
Fig 1.9: Time-temperature characteristic of tissue damage. Thus a 1s pulse reaches
Application
A simple overview of laser tissue interactions as a function of time and depth is given in Figure 1.8.
Lasers like Cw CO2 and Er:YAG classically have little thermal confinement. Ultrapulse CO2 lasers, some Er:YAG lasers, long pulse Nd:YAG and alex achieve thermal confinement , wherein the pulse duration is shorter than the thermal diffusion length or thermal relaxation time. The Q-switched lasers and PDL achieve stress confinement.
Effect of Cooling
Surface cooling enhances efficacy and safety in skin laser surgery, especially for visible light technologies, (i.e., green-yellow light sources such as IPL, KTP laser, and PDL) that are popular in cutaneous laser surgery. This is also the wavelength range where epidermal damage is most likely. The epidermis is an innocent bystander in cutaneous laser applications where the intended targets, such as hair follicles or blood vessels, are located in the dermis. Specifically, absorption of light by epidermal melanin causes skin surface heating. The first goal is of surface cooling is preservation of the epidermis. The second and related goal of surface cooling permits higher fluences to the intended target (i.e., the hair bulb and/or bulge or a subsurface blood vessel). Another benefit of surface cooling is analgesia, as almost all cooling strategies will provide some pain relief.
4. Selective Photothermolysis (SPT)
Selective photothermolysis offered a mathematically rigorous rationale for tissue-selective lasers. As described by Dr Anderson, extreme localized heating relies on: (1) A wavelength that reaches and is preferentially absorbed by the desired target structures; (2) An exposure duration less than or equal to the time necessary for cooling of the target structures; and (3) sufficient energy to damage the target. The heterogeneity of the skin allows for selective injury in microscopic targets.
Thermal Relaxation Time and Pulse Duration
The thermal relaxation time (t) is the interval necessary for a target to cool to a certain percentage of its peak temperature. Given that one goal of treatment is the precise control of thermal energy, the pulse duration of laser irradiation is just as important as optical and tissue factors. One way to maximize the spatial confinement of heat is to use a laser with a pulse duration on the order of the thermal relaxation time (TRT) of the target chromophore (Fig. 1.10).
TRT is defined as the time required for the heat generated by the absorbed light energy within the target chromophore to cool to half of the original value immediately after the laser pulse. During a lengthy laser exposure, most of
the heat produced diffuses away despite its origin in the target structure. The target does not become appreciably warmer than its surroundings because the absorbed energy is invested almost uniformly in heating of the tissue during exposure. As a result, longer pulse durations offer a more generalized heating and therefore, less spatial selectivity resulting in nonspecific thermal damage to adjacent structures regardless of how carefully one has chosen a wavelength (Fig. 1.10). However, if the laser pulse is suitably brief, its energy is invested in the target chromophore before much heat is lost by thermal diffusion out of the exposure field (Fig. 1.10). A transient maximum temperature differential between the target and adjacent structures is then achieved. Shorter pulse durations confine the laser energy to progressively smaller targets with more spatial selectivity. The transition from specific to nonspecific thermal damage occurs as the laser exposure equals and then exceeds TRT.
When defining thermal relaxation time, the target size and geometry are important (Box 1.1). For most targets, a simple rule can be used: The thermal
relaxation time in seconds is about equal to the square of the target dimension
in millimeters. Thus a 0.5 mm melanosome (5 × 10-4 mm) should cool in about
25 × 10-8 s, or 250 ns, whereas a 0.1 mm PWS vessel should cool in about 10–2 s,
or 10 ms. This provides an approximate pulse width for varying degrees of thermal confinement.
The often used term “thermal relaxation time of the skin” is meaningful only when used for specific wavelengths (or specific skin structures, i.e., the epidermis). With a ubiquitous absorber such as tissue water, it should be considered within the context of the laser source. For example, if one uses
Fig. 1.10: Relation of pulse duration (p) and TRT (thermal relaxation time). p = TRT,
the 1,540 nm laser, the entire epidermis and large portions of the dermis are heated and the TRT is on the order of seconds, because the thickness is several hundred millimeters. So even though TRT of the epidermis is about 10 ms based on its thickness, a thicker slab of skin is heated at 1,540 nm, the epidermis will take several seconds to cool because there is no temperature gradient between it and that of the dermis. A summary of the TRT of major target tissues is given in Box 1.1.
Application
With a very short pulsewidths (pw), lasers vaporize targets. For example, in treating blood vessels, rapid heating results in acute vessel wall damage and petechial hemorrhage (with Q-switched 532 nm). With intermediate length pulses (0.1–1.5 ms), one can gently heat targets without immediate rupture of the vessels. But intravascular thrombosis can create purpura and delayed hemorrhage. With longer pulses (6–100 ms), the ratio of contraction to thrombosis increases and side effects are less likely. Too long pulses with very small targets can create two problems. With highly absorbing targets, (i.e., tattoo inks), the heat generation is so great and long-lived that significant diffusion occurs to the surrounding dermis. On the other hand, using a long pulse YAG for a nevus of Ota results in an insufficient temperature rise as the pigmented nevus cells cool off too fast during the delivery of the pulses (also melanin absorption is much weaker than black ink).
Selective Photothermolysis of Tattoos
Amorphous carbon, graphite, India ink and organometallic dyes, typically found in dark blue-black amateur and professional tattoos, have a broad absorption in the visible and near-infrared portions of the spectrum. At visible
Box 1.1 TRT of potential targets used in dermatology
Melanosome (0.5 μm) 0.25 μs
Melanocyte (7μm) 1 μs
Nevus cell (10 μm) 0.1 ms
Collection of nerves cells (100 μm) 10 ms
Epidermis (100–200 μm) (dermoepidermal junction 10 μm) 10 ms Erythrocyte 2 μs Hair follicle (200 μm ) 40 ms Vessel (0.1 mm diameter) 10 ms Vessel (0.8 mm diameter) 300 ms Vessel (0.1 mm diameter) 10 ms Tattoo (0.5–100 μm) 20 ns–3 ms
wavelengths longer than 600 nm, hemoglobin and melanin light absorption is minimized and tattoo dyes can be targeted selectively.
The pigment granules characteristically found in tattoos have diameters of 0.5–100 mm, which correspond to TRT of 20 ns to 3 ms. With the development of the Q-switched ruby (694 nm), alexandrite (755 nm), and Nd:YAG (1.06 mm) lasers, tattoo removal without scarring can be achieved. The frequency-doubled, Q-switched Nd:YAG laser (KTP laser) emits at a wavelength of 532 nm, which provides improved removal of red dye. Recently picosecond lasers have been used for tattoos.
Selective Photothermolysis of Pigmented Lesions
Pigmented lesions can be divided in to epidermal and dermal. Although highest in the ultraviolet portion of the spectrum, melanin absorption is also significant in the visible and near-infrared wavelengths. The diameters of individual melanosomes (0.5–1.0 μm) and melanocytes (7 μm) correspond to TRT of 20–1,000 ns. Therefore, Q-switched green, red, and near-infrared wavelengths have been utilized for this indication. Though Q-switched lasers are used most commonly the gentle heating by the millisecond laser can also treat epidermal disorders. With longer pulses (ms), the dermal melanocyte does not become hot enough to achieve pigment reduction, thus ensuring selective epidermal damage.
Selective Photothermolysis and Laser Assisted Hair Removal
The human hair follicle is a complex structure derived from both epidermal and dermal components. The target chromophores, primarily melanin-rich hair shafts, are located deep in human skin (bulge around 1.5 mm and bulb at 2–7 mm). At this depth, only red and near-infrared wavelengths are useful (690–900 nm). The follicular structure responsible for regeneration has not been conclusively identified and therefore, current systems target the entire follicle. As a result, long pulse widths on the order of milliseconds and high fluences capable of heating large volumes of tissue are required. Millisecond-domain ruby, alexandrite, diode and Nd:YAG lasers using high light doses can produce selective injury to human hair follicles resulting in prolonged growth delay and in some cases, permanent hair loss after a single treatment. Selective Photothermolysis of Cutaneous Blood Vessels
The pulsed dye lasers at 577–595 nm wavelengths well absorbed by the targeted hemoglobin molecule relative to other optically absorbing structures, cause selective thermal damage to dermal blood vessels while minimizing epidermal melanin absorption. Furthermore, because the TRT for cutaneous blood vessels varies between 10–300 ms a variable pulse duration is required for optimal results.
But there are numerous variations in pulse duration and absorption of various chromophores (bloodless dermis, oxyhemoglobin and deoxyhemoglobin) that can complicate this simplistic interpretation.
practical clinical applications
There are numerous clinical applications that have been given in the text above and the chapters that follow. Two examples are given below:
1. The geometry (and therefore the microscopic characteristics) of lesions is important. For example, in the treatment for a nevus versus a lentigo, the nevus is composed of melanocytes in aggregates as (collectively of a size of 100 μm in diameter) whereas the lentigo is a mere sheet of melanocytes some 10 μm thick. So the TRT of the nevus cell is about 10 ms while that of the lentigo is about 0.1 ms (Box 1.1).
Thus, in treating nevus with a long pulsed alexandrite laser with a high fluence, the TRT will approach a second. From the above equation, it follows that thermal confinement will be high, and the peak temperature will rise accordingly. More importantly, the thick slab of melanocytes will take long to cool, such that there will be considerable heat diffusion away from the target. On the other hand, the lentigo represents a slab only tens of microns thick; there will be heat diffusion during the long pulse and rapid cooling after the pulse. Thus, with ms-domain fluences, the nevus case might result in scarring and a lighter lentigo might not become hot enough for clearance. If one applies ns pulses to the two lesion types, the lentigo shows a good response with possibly complete clearing, whereas the nevus will require multiple sessions, as each laser application will result in heat confined to the most superficial part of the lesion. Conversely a microsecond laser might work for nevi.
2. Spot diameter: In general, the spot size should be 3–4 X > d (target
diameter), as larger spots make it more likely that photons will be scattered back into the incident collimated beam. Photons scattered out of the beam are essentially wasted. Larger beams (with the same surface fluence as smaller beams) create deeper subsurface cylinders of injury because there is less surface versus volume for photons to escape. Basically, for small beams (narrow), scattered photons are carried out of the beam path after only a few scattering events. Thus, as a thumb rule, larger the spot, more the dermal/epidermal damage ratio but also higher is the epidermal damage thus the fluence should be reduced. For shallow penetrating lasers such as CO2 and erbium where the d << spotsize (all cases except for the fractional devices), the diameter of the beam does not affect the tissue response. That is why equivalent results can be obtained for skin resurfacing using pulsed CO lasers versus scanned, tightly focused Cw CO2 lasers.
how to use the Knowledge practically?
The vast amount of data is of little meaning if used correctly. Thus in practice, three principles have to be remembered on which laser applications are applied.
1. Absorption spectra of various chromophores: It is important to
understand the wavelength that is absorbed by the target chromophore. This is specially relevant in tattoos, thus accounting for the use of 532 nm (green) for a red tattoo and 1,064 nm (blue) for a black pigment. This also accounts for the use of Er:YAG as an ablative tool for dermal tumors where the target chromophore is water.
2. Pulse duration of the lasers: This is directly dependent on the size of the target. This explains, why a Q-switched laser is used for a nanosized tattoo and this also explains the logic of using a microsecond laser (Er:YAG) is used for epidermal ablation (TRT 10 ms).
3. Penetration depth of laser: The optical penetration depth is a important
consideration specially in pigmented skin. As melanin has a wide range of absorption spectrum, most lasers can be used, based on the above two principles. The reason most of us use the Q-switched Nd:YAG is as it penetrates deeper and thus would not interact with the competing epidermal pigment, which is a competing factor in pigmented skin. A summary of the indications and lasers used that largely conform to the above principles is given Table 1.2.
Table 1.2 A summary of lasers used for common disorders
Vascular lesions Pigmented lesions Tattoo
removal Photoepilation Resurfacing Ablation PDL* (585–600 nm) QS Ruby (694 nm) QS Ruby (694 nm) Long-Pulse Ruby (694 nm) Carbon dioxide * (10,600 nm) Long-pulse Nd:YAG (1,064 nm) QS Nd:YAG * (532, 1,064 nm) QS Nd:YAG* (532, 1,064 nm) LongPulse Nd:YAG * (1,064 nm) Er:YAG * (2,490 nm) Long-pulse KTP (532 nm) QS Alexandrite (755 nm) QS Alexandrite * (755 nm) LongPulse Alexandrite (755 nm) Fractional* (1540 nm)
IPL† IPL † Long Pulse
Diode * (800 nm) IPL*
* Used preferentially in pigmented skin †Not very effective
Exceptions
Inspite of these principles numerous laser applications are there which do not always conform to these basic principles. This includes plasma resurfacing, laser lipolysis, use of Q-switched lasers for melanocytic nevi and ablative fractional lasers. In some, the target size have changed like for hair removal lasers where the initial work focused on the bulge area but it now targets the diameter and the hair shaft. In PWS, multiple issues arise including the size of the vessels, the presence of deoxy or oxyHb and the depth of the vessels. The pulse duration is being tweaked to adapt to the needs of the PDL. In Tattoo cases, after the first few sessions, the optical property of the pigment changes and macrophage engulfment changes the target size. The use of the R20R technique for tattoos is a example where in all likelihood after the first impulse the optical properties of the particle changes and the repetitive impaction do not conform to the basic principles of laser tissue interaction. One illustrative example is of the AFR (ablative fractional) lasers. The approximate optical penetration depth (OD) in water for such lasers is minimal, e.g., 1µm for the Er:YAG laser (2,940 nm) and 10 µm for the CO2 laser (10,600 nm). But they are used for acne scars which can involve the lower dermis. This is as high volumetric energy densities are reached virtually instantaneously within the focus of the laser beam, and therefore such lasers can quickly advance a cavity deep into the tissue during the pulse. Due to this process, it is possible that the resulting depth of an MTZ can greatly exceed the optical penetration depth of any particular laser wavelength. Also the optical penetration depths provided are approximations of the penetration depth in water which can vary substantially and is approximately 30% for the epidermis and 70% for the dermis. As the optical properties of water are also temperature, dependent, it has been reported that the rapid change of tissue temperature during a laser pulse can dynamically alter the penetration depth substantially.
ConCLusion
There are numerous other interesting scenarios that can affect the laser tissue interactions like optical clearing with hyperosmolar solutions, photon recycling, using a polarizing lamp to enhance illumination and selective cell targeting. But the basic principles that are used are an understanding of the laser wavelength and chromophore interaction, the TRT and the pulse duration.
Books
1. Nelson JS. An Introduction to Lasers and Laser-Tissue Interactions in Dermatology. Principles and practices in cutaneous laser surgery. Editor: Arielle NB Kauvar; Associate editor, George J Hruza; 2005.
2. Ross EV, Anderson RR. Laser Tissue Interactions. Cutaneous And Cosmetic laser Surgery, Ist Edn. Mitchel P Goldman; 2006.
3. Ronald G. Wheeland Basic Laser Physics and Safety. In: Goldberg DJ (Ed). Laser Dermatology. 2005.
4. Steiner R. Laser-Tissue Interactions. In: Raulin C, Karsai S (Eds). Laser and IPL Technology in Dermatology and Aesthetic Medicine; 2011.
BiBLiograPhy
1. Boulnois JL. Photophysical processes in recent medical laser developments: a review. Lasers Med Sci. 1986;1:47-66.
Kabir Sardana, Vanya Narayan, Rashmi Ranjan
OVERVIEW
Skin resurfacing is not a new concept; various methods have been used extensively for almost a century. Dermabrasion, in its various forms, has been used successfully for treating wrinkles and acne scars for several decades. It has recently lost popularity due to the introduction of laser resurfacing, the difficulty in obtaining precise depth control, and the release of blood-borne viruses into the aerosol.
Chemical peeling, which still enjoys popularity, is largely a blind procedure and is greatly dependent on multiple variables to obtain desirable penetration depths. Although superficial (i.e., epidermal) peeling is very safe and predictable, deeper chemical peels are less precise and can lead to either inadequate or excessive penetration depth. Phenol and augmented phenol peels can produce spectacular results in removing heavily sun-damaged skin but are less suitable for darker and skinned patients, males and younger patients.
Carbon dioxide laser resurfacing was favored as the preferred method of skin resurfacing by many experts in aesthetic surgery. Although the short and long-term improvement in sun damage and wrinkles can be extremely dramatic, carbon dioxide laser resurfacing has significant morbidity, even when performed by well-trained doctors. This includes redness, temporary hyperpigmentation, permanent hypopigmentation and dermal pacification.
Though fractional lasers have largely replaced ablative laser resurfacing, out interest is in using these lasers for common epidermal and dermal disorders, where their role is paramount.
At present, three laser are used for ablative indications and as the chromphore in all is water the difference lies in their relative affinity. As shown in the Figure 2.1, the Er:YAG has a higher affinity than Er:YSSG with the CO2 having the least absorption for water. Thus the safest of all is the Er:YAG laser.
CARBON DIOXIDE LASERS
The continuous-wave carbon dioxide laser, producing infrared light with a wavelength of 10,600 nm, was the first to be used for resurfacing procedures.
CHAPTER
2
Its wavelength is strongly absorbed by water, which is the most abundant chromophore in the skin and comprises approximately 70 percent of its total volume. This seemed to make it an ideal tool for generalized superficial ablation. But its tissue-dwell time could not be precisely controlled and far exceeded the 1 millisecond thermal relaxation time of the 20 to 30 μm of cutaneous tissue that absorbs CO2 light. Excessive thermal diffusion and concomitant unintended tissue damage were the common results.
However, in the early 1990s, new pulsed and scanning CO2 lasers were developed that could deliver very high peak fluences of at least 5 J/cm high enough to vaporize cutaneous tissue in less than 1–2 milliseconds (Fig. 2.2). The energy required for vaporization of the epidermis is 5 J/cm. For the thickness of the tissue (20–60 μm), the TRT is about 800 μs. This is achieved by the ultrapulse lasers where a 250 mJ pulse using a 2.5 mm probe size achieves much higher fluencies in a shorter time as compared to the continuous wave (Cw) CO2. The superpulse laser is a mechanically shuttered laser whose peak power is higher than Cw lasers but the average power over time is the same. These pulsed systems can precisely and safely remove thin layers of skin, between 20 μm and 30 μm with each pass, while leaving an acceptably narrow zone of residual thermal damage: 25 μm to 70 μm, in contrast to the 200 μm to 600 μm zone produced by the continuous-wave CO2 laser.
Principles of Carbon dioxide Lasers
R Rox Anderson and Parrish coined the term ‘selective photothermolysis’ in 1983 to describe the process by which a chromophore is heated by laser light absorption in a time period shorter than its thermal relaxation time. The latter is the amount of time required for a material to lose 50% of its heat by conduction to its surroundings. Thus, when a chromophore is heated by selective photothermolysis, only the intended target is damaged and there is minimal diffusion of heat and no consequent injury to the surrounding
structures. The mechanism of injury involves both thermal coagulation and/ or photoacoustic injury in the form of supersonic high pressure shock waves. For both the CO2 and Er:YAG lasers, the predominant mechanism is photothermal. The pulse fluence necessary to achieve vaporization and thus ablation of skin tissue with the CO2 laser is 5 J/cm2 with a calculated TRT of
800 µs. The unique aspect of CO2 laser is that for each 20 µm that is ablated, 3–4 times this amount is damaged. It is this latter effect that allows for the purported collagen remodeling and wound healing. This zone of coagulation is modest compared to 1000 µm layer of damage that results from Cw CO2 lasers.
Pulse Duration of Carbon Dioxide Lasers
Laser dwell time is the amount of time that the beam is on in one location. Low power densities require longer dwell times to achieve the same effect as high power densities. The longer the dwell time and the slower the heating, the more desiccation and charring of tissue that results. Further heating of charred tissue results in extremely high temperatures of 300–600°C. This is because carbonized and desiccated tissue acts as a heat sink for laser absorption. There is no buffer of water to absorb the heat and thus temperatures escalate rapidly. The significance is that if a non-pulsed laser is used, a low pulse duration should be used to minimize thermal injury.
Types of Lasers
Most clinicians use the superpulsed CO2 lasers, which deliver pulse energies in the 10–50 mJ range. The peak power per pulse is 2–10 times higher than Cw CO2 lasers, but the average power over time is similar (Fig. 2.2).
Fig. 2.2 : A comparison of the waveform of CO2 laser. Note that for the same energy (X) generated by a ultraPulse (<1 ms) waveform, 5 superPulse waves are generated. A continous wave is seven times longer than the ultraPulse for the same energy
