A brief review of the principles of photothermolysis – from Einstein’s theory of stimulated emission to modern laser therapy.
While Albert Einstein didn’t invent the laser per se, he certainly paved its way into modern society and medicine. Expanding on physicist Max Planck’s quantum hypothesis, in 1905, Einstein proposed how light delivers its energy in chunks, which are represented by photons-discrete quantum particles.1 Later, in 1916, he introduced the concept of stimulated emission: photons, by interacting with excited atoms or molecules, could stimulate the emission of new photons having the same frequency, phase, polarisation and direction of the first one.2
This process, which thus behaves like an amplification process, was termed ‘light amplification by stimulated emission of radiation’ or, for short, ‘LASER’.
In 1954, the first stimulated emissions of radiation were demonstrated for microwave radiation (MASER) by J P Gordon and C H Townes. The possibility to extend the principle into the infrared and optical regions was proposed by Schawlow and Townes in 1958 and the first laser was constructed by Theodore Maiman in 19603, who used a ruby crystal stimulated by high intensity flashtubes to emit millisecond pulses of coherent 694nm (near-infrared) ruby laser (red) light. The neodymium doped yttrium-aluminium–ga-net laser (Nd: YAG) was constructed by Johnson in 1961, the argon ion laser (Ar+) was invented by Bennett et al in 1962 and the carbon dioxide (CO2) laser was created by Patel in 1964.
Different types of gas lasers, solid- state lasers and metal vapour lasers were invented later. The wavelength domain was gradually developed into the ultraviolet range, which made ionisation and bond-rupturing of target molecules possible.3
In 1962, the American dermatologist Leon Goldman, a pioneer in applying lasers to dermatologic conditions, was the first physician to test the ruby laser on human skin; in 1963, he reported on the effects of Maiman’s ruby laser in the selective photodestruction of pigmented skin elements such as black hair.4 He also described the potential use of ruby laser and the more innovative Q-switched device in tattoo removal and the possible treatment of other pigmented lesions, such as nevi and melanomas. Moreover, Goldman investigated the use of Argon laser in the treatment of vascular malformations, and the use of carbon dioxide laser for the photo-excision of skin lesions. In 1973, he also introduced the neo dymium: yttrium- aluminium garnet (Nd: YAG) laser in the treatment of vascular lesions.1
The next major leap in the field of cutaneous lasers was the concept of “pulsing” the laser beam, which allowed selective destruction of abnormal or undesired tissue, while leaving surrounding normal tissue undisturbed.
This “selective photothermolysis” theory was developed by dermatologists 5 Rox Anderson and John Parrish .
Their 1983 article elucidated the tissue-laser interaction leading to selective destruction of an intended target structure, termed a chromophore. Laser energy of a predetermined wavelength was preferentially absorbed by a chromophore, creating thermal absorption by the target more so than surrounding structures, leading to selective tissue heating and destruction.
An exciting more recent development was the introduction of fractionated thermolysis in the early 2000s. The advent of fractional lasers, which use narrow, microscopic columns of light to treat a specific portion of the skin and leave surrounding tissue intact, has transformed the way in which skin resurfacing is performed, offering more accessible and safe treatments with low downtime and comparable results to traditional resurfacing lasers.
Today, lasers are widely used in almost all fields in medicine, with a broad range of wavelengths, fluence rates and pulse length and frequency. It has become a fundamental, irreplaceable and omnipresent device of modern science. Of course, the medical potential of lasers is continuing to be explored.
Basic parameters of a laser
Wavelength
A laser’s wavelength is the distance between two peaks measures in nanometres. Wavelengths must be consistent with the target tissue/colour (heomoglobin, melanin, water, etc). The shorter the wavelength, the more superficial the penetration and the higher the energy; the longer the wavelength, the deeper penetration and the lower the energy.
The lasing medium or energy source – eg, gas (CO2), solid (Ruby, Alexandrite, Nd:Yag) or liquid (dye) – determines the wavelength of the laser.
Fluence
Fluence is the energy per area measured in Joules, typically ranging from 3-150J/ cm2. As fluence increases, so too does the destructive force of the energy.
Spot size
Typically, a large spot size equates to a deeper penetration and decreased scatter. Conversely, a small spot size means more energy is absorbed in superficial structures and there is increased scatter.
Pulse duration
This refers to how quickly the energy is delivered to the tissues and is measured in milliseconds. Generally, the longer the pulse, the more gentle the heating of the target.
Quality switched (Q-switched) lasers allow for the generation of nanosecond- range laser pulses. The pulses result in rapid thermal expansion and fragmentation of the target.
Newer picosecond technology delivers an ultra-short pulse width in picoseconds. It generates higher mechanical stress in the target without increased heating.
The wavelength peaks of the laser light, pulse durations and how the target skin tissue absorbs this, determine the clinical applications of the laser types.
| Laser type | Laser source | Wavelength peaks |
| CW: emits a constant beam of light with long exposure durations | CO2 | 10,600 nm |
| Argon | 488/514 nm | |
| Quasi-CW: shutters the CW beam into short segments, producing interrupted emissions of constant laser energy | Potassium-titanyl-phosphate (KTP) | 532 nm |
| Copper bromide/vapour | 510/578 nm | |
| Argon-pumped tunable dye (APTD) | 577/585 nm | |
| Krypton | 568 nm | |
| Pulsed*: emits high-energy laser light in ultrashort pulse durations with relatively long intervening periods between each pulse | Pulsed dye laser (PDL) | 585–595 nm |
| QS Ruby | 694 nm | |
| QS Alexandrite | 755 nm | |
| QS neodymium (Nd):yttrium-aluminum-garnet (YAG) | 1064 nm | |
| Erbium:YAG | 2940 nm | |
| CO2 (pulsed) | 10,600 nm | |
| Picosecond | (Nd):yttrium-aluminum-garnet (YAG) | 532/1064 nm |
| Alexandrite | 755 nm |
* Pulsed laser systems may be either long-pulsed such as PDL with pulse durations ranging from 450ms to 40millisec, or very short-pulsed (5-100ns) such as the quality-switched (QS) lasers.
Source: DermNet NZ
Top 10 tips for choosing Laser and Intense Pulsed Light equipment
- Get educated on the theory of lasers, IPL, safety and clinical presentations. If you understand wavelengths, principles of photothermolysis, pulse width, fluence, spot size, clinical end points, Fitzpatrick Skin Types, expected side effects and possible adverse outcomes then you will be able to be discerning and make a well-educated decision. Choose theory courses that are accredited, endorsed or recognised by radiation health departments or health professional bodies to ensure they are quality and evidence based.
- Check out your state/territory regulations. Are you able to operate a Class 4 laser in your state? Are there licensing requirements? Did you know that IPL devices are only regulated in Tasmania?
- Be a critical reader. Examine the claims and the clinical papers that are associated with them. Don’t take anything for face value and get independent information, too.
- Decide what services you’d like to deliver. Will it be hair removal, skin rejuvenation, skin resurfacing, tattoo removal or all of these? There is no one machine (yet) that does it all. How much can your population afford to pay? Where will you find your customers?
- Machine settings should be customisable. You want the machine with parameters that are able to be adjusted to suit the different targets and skin types to get the results you really want.
- Ongoing education and support from the manufacturer is essential. After they show you
the basics of how to use the machine, will they return to show you advanced applications? Do they have marketing in place that supports your practice? They should provide signage for the door for controlled access and laser safety eye wear as standard. Just because you have been orientated on the use of the machine and its accessories, doesn’t mean you have completed a Laser Safety Officer course. If you don’t have one, get one (see number 1). - Consider the costs – both initial outlay and ongoing. The initial cost of your equipment isn’t the end of the story. Consider the costs of disposable heads, handpieces that need refurbishment or repair and other disposables such as gel, spatulas and linen. Ask for information on the numbers of treatments needed to start to get a return on your investment.
- Practice. Provided consent is obtained, work within your scope of practice/skill set under supervision and wnsure safety protocols are adhered to. Colleagues, friends, family, volunteers, paid models or paying clients can all help you hone your skills. Some of the RTOs have short practical workshops that you can do as stand-alone modules. Choose practical workshops that are accredited, endorsed or recognised by radiation health departments or health professional bodies to ensure they are quality and evidence based.
- Determine your budget. Shop around; there are hundreds of choices on offer. Some are TGA approved, some are not. Some are in the hundreds of thousands of dollars and some are not. Consider if you will buy outright, rent to buy, seek finance or how you will pay your machine off.
- Cover your legal bases. Have adequate professional liabilities insurance. Work within your legal scope. Develop your practice based on exceptional education, evidence- based practice and the discerning choice of equipment.
Source: Bravura Education
Laser Regulations By State
QLD
- You do not need any pre-qualifications for laser use across all treatments
- You will need a laser safety certificate and an infection control certificate
- You will then apply for a trainee licence; while on that licence you will need to complete a logbook of supervised hours*:
- 25 hours for hair removal
- 50 hours for skin rejuvenation (including superficial pigmentation adjustment)
- 50 hours for superficial capillary reduction
- 100 hours for tattoo removal
- Once this has been completed, you may apply for your user licence
WA
- You do not need any pre-qualifications for laser use in hair reduction, pigmentation and/or vascular treatments
- You will need qualifications as a Doctor, Beauty Therapist or a Registered Nurse for laser use in tattoo removal as well as a tattoo removal course (this one INCLUDES the laser safety certificate).
- You will need to complete a logbook of supervised hours*:
- 25 hours for hair removal
- 50 hours for skin rejuvenation (including superficial pigmentation adjustment)
- 50 hours for superficial capillary reduction
- 100 hours for tattoo removal
- Once this has been completed, you may apply for your user licencee
TAS
- You do not need any pre-qualifications for laser use across all treatments
- You will need a laser safety certificate for use with a laser and/or IPL
- You will need to demonstrate to the Radiation Health Department that you have adequate clinical supervision from a full licence holder to be able to practice independently
NSW, SA, NT, VIC, ACT
You do not need any pre-qualifications for laser use across all treatments.
You do, however, need a laser safety certificate. This is the minimum requirement across the whole of Australia to comply with laser standards and to secure insurance.
*supervised hours must be done with a person who already holds a full licence in your state.
Source: Bravura Education.
For more on online laser and IPL safety course and certificates, visit bravura.edu.au
References:
1. Gianfaldoni S, Tchernev G, Wollina U, Fioranelli M, Grazia Roccia M, Gianfaldoni R, et al. An overview of laser in dermatology: the past, the present and … the future (?) Open Access Maced J Med Sci. 2017 Jul 25; 5(4):526–530.
2. Einstein A. Zur Quantentheorie der Strahlung. Physikalische Gesellschaft ZÃ1⁄4rich. 1916;18:47-62.
3. Peng, Qisheng & Juzeniene, Asta & Chen, Jiyao & Svaasand, Lars & Warloe, Trond & Giercksky, Karl-Erik & Moan, Johan. (2008). Lasers in medicine. Reports on Progress in Physics. 71. 056701. 10.1088/0034- 4885/71/5/056701.
4. Karen Appold. Dermatology Times, April 2019 (Vol. 40, No. 4), Volume 40, Issue 4.
5. Anderson RR, Parrish JA. Selective photothermolysis: precise microsurgery by selective absorption of pulsed radiation. Science. 1983 Apr 29;220(4596):524-527.
