Laser is an artificial light source. This radiation does not occur in nature. The physical-technical knowledge of lasers is a prerequisite for their adequate use in medicine.
The term “laser” can be found in many areas today, it has become part of everyday language. In addition to imaginative ideas, there is also great respect for the subject. However, in order to use this technique for yourself, a precise understanding and correct estimation are very important.
In the course of my professional life, I have always worked in the medical technology sector and know the special conditions that are encountered here, particularly in the use of lasers.
Legal conditions
Basically, we have to fulfill certain legal requirements for all lasers – no matter where we use the laser.
Health protection is the top priority, and the specifications that are binding for occupational health and safety must be observed. These laws integrate both national and international law in Switzerland. SUVA promotes prevention and information on the relevant legal basis. In their publication “Attention: Laser Beam” (Order No. 66049), the use of lasers and the organizational necessities are described in more detail. A responsible person must be appointed for laser operation who holds the title “Laser Safety Officer”. The necessary expertise must be available. If lasers are used for medical applications, the manufacturer must have produced the device in accordance with the Medical Devices Ordinance MepV or Directive 93/42/EEC. These specifications also regulate operation and use.
Currently, a review, revision and addition to the current laws are taking place. A draft law on protection against non-ionizing radiation “NIR and sound” is being prepared. This law is intended to regulate the use of NIR in tanning beds or cosmetic applications. The population must be protected from damage to health.
As the last link in the safety chain, SUVA prescribes personal protective equipment (PPE). When using lasers, the most important measure is laser safety goggles, and with more powerful lasers, protective clothing. The DIN EN 207 standard is the legal basis for this.
Basics
So much for the regulations that we have to observe, or the laws that we have to comply with. The information described here is intended to provide you with knowledge, but is not a substitute for a course to obtain laser expertise.
Laser is a light source that has special properties and can only be generated artificially. This radiation does not occur in nature.
The name “laser” is derived from the first letters of the English description of this effect (overview 1). From the first letter we recognize that it is light, i.e. non-ionizing radiation. The range of optical radiation extends from 100 nm to 1,000,000 nm, with a further division into subgroups here:
- UV radiation: 100-380 nm
- Visible optical radiation: 380-780 nm
- IR radiation: 780-1,000,000 nm.
With a few exceptions, lasers are in the visible or infrared range. Linguistically, we use terms that are commonly known but have a special meaning for us:
- With transmission, we are talking about the depth of penetration into the tissue without any change. Comparable to the view through a window pane.
- By reflection we mean the partial or total back reflection of radiation falling on a surface. Comparable to looking into a mirror.
- When we speak of diffuse scattering, we mean the more or less uniform spatial distribution of radiation from the point of entry, e.g., into tissue. Comparable to the view through a pane of frosted glass.
- Absorption is the absorption of power or energy to convert it to heat, i.e., to denature, vaporize, or disintegrate tissue.
When dealing with laser systems, we also like to refer to lasers by the laser medium, which is the element used to generate the laser light. This designation also indicates the generated color of the laser light. Since the color is monochromatic, one can also speak of the wavelength of the color (Tab. 1) . However, actually only penetration depth or absorption are important for the user.
Through the considerations and researches of our famous physicists, many foundations and prerequisites were created to better understand light and make it controllable. Based on the quantum hypothesis of M. Planck and the later addition by A. Einstein with the photoelectric effect, on May 16, 1960, T. Maimann was the first person to generate artificial laser light. It was a ruby laser that emitted a red light at 694 nm.
Artificial light
Since we’ve talked about “artificial light” several times now, let’s take a look at what it means and what’s special about lasers. The three basic principles and properties of this light are:
- Monochromatic light, i.e. light with only one color, wavelength, frequency (compared to white or sunlight, which has many colors, wavelengths and frequencies). The popular rainbow results from the different refraction of the sunlight in the raindrops and the resulting different deflection of the individual colors. If we were now to replace the sun with a laser, only one color would be visible in the rainbow. Due to this effect, when using the laser, we have an intentional effect without the potentially disturbing or harmful wavelengths.
- Parallel beam path. Due to the generation of the light, the beam aligns itself in parallel in the resonator and leaves it in this way. In theory, this beam is parallel. Depending on the resonator length, there is some beam expansion, but it is very small compared to the normal light source. Laser sources have strong focus and low divergence. Light sources, on the other hand, have a strongly divergent beam.
- Coherent light is synchronized in time and space (same phase and same amplitude). Light quanta are created at the same time and move in the same direction. I also like to describe it with long distance runners and marching soldiers. Long-distance runners take steps of different lengths and step on the ground at different times – similar to incoherent light that is not synchronized in time and space. Marching groups, on the other hand, all have the same stride length, lift and lower their feet at the same time – like the laser (same phases, same amplitude).
These three basic properties make the laser something extraordinary, which, as I said, can only be generated artificially in this way.
Emergence of (laser) light
If we now look at the formation of light and include the special conditions for lasers, we quickly recognize the laser light characteristics (the illustration is very simplified and is only intended to describe the formation).
What we see as light are the photons (light quantum). They are the smallest unit of action of an electromagnetic interaction.
First we select an element that we want to bring to the laser. This determines the subsequent wavelength, since each element has more or less only one main wavelength (although there are so-called harmonic wavelengths, which either do not support the wavelength due to coating of the optics or can be “switched on” if necessary). The element consists of the atoms, these consist of the atomic nucleus and the atomic shell. The positive core and the negative shell are in a tight bond due to the electrostatic attraction. Now, as soon as we add an energy, the electrostatic balance is changed. The electrons in the shell can move further away from the nucleus due to the added energy and rise to a higher shell. The atom is now positively or negatively charged and is called an ion. The ion dwells in this state for a short time, but tries again to reach the original state and then releases the excess energy in the form of a photon. Since this rebound occurs unstimulated, light is emitted.
Now, if we put a mirror on each of two opposite sides, a photon may hit that mirror and be reflected back in the same direction where it came from. Since both mirrors are plane-parallel to each other, this photon would now be reflected from one side to the other. Since other atoms also have a higher occupation level and the charged electrons have not yet fallen back, they would be nudged by the reflected photon and stimulated to take the resulting photon with them in the same direction and oscillation. Now, when this process starts like this, the photons amplify themselves by an avalanche-like effect due to the stimulated discharge of the other atoms. As long as energy is now supplied, the stimulated discharge takes place, the laser process is active.
Due to the plane-parallel design of the two mirrors and the laser medium in between, we have a resonator in which a single-color parallel light is now generated and a coherent laser radiation is generated by the stimulation.
In order to be able to use the laser beam, one of the resonator mirrors is made partially transparent. Partial permeability is usually quite low, about 5-10%. However, this transmission is sufficient to obtain useful values for treatment.
The wavelength is specified by the laser medium and is thus permanently assigned. The laser medium can have different states. Laser media are:
- Gas laser, e.g.CO2 or argon
- Solid, e.g. ruby or alexandrite
- Colorant, e.g. rhodamine 6G or coumarin
- Semiconductors, e.g. GaAs, or GaAlAs.
The supply of energy (also called “pumping”) can be done by applying an electric voltage, DC voltage or radio frequency, or in the form of optical pumping by means of lamps filled with xenon and krypton.
Due to the design of the laser resonator, the laser beam is generated in such a way that the distribution of energy in the beam corresponds to the Gaussian profile, the beam shape has a TEM 00 – in the center the highest possible energy, which slowly decreases towards the edge.
Operating modes
Lasers are divided into different operating modes depending on their design and application. There are:
- “continuous wave laser” (cw laser), i.e. continuous wave laser. In this case, the laser medium works longer than 250 ms and thus has a thermal effect on the tissue.
- A suborder is the “pulse mode”, here the laser is operated with a fixed frequency but with variable pulse width control. The advantage is that the fabric has short thermal recovery times and there is less thermal edge zone stress on the fabric.
- The other special form in cw operation is the super pulse, ultra pulse, shar pulse, etc. Here, the maximum power of the laser is always activated for a very short time, the number of pulses per second then results in the average output power. The advantage is that the tissue evaporates abruptly and one acts on the tissue almost without carbonization.
Pulsed lasers are only ever fired for one flash and emit the laser beam accordingly. Long-pulsed lasers operate in the ms range and thermally to denature tissue. Sometimes it is necessary from a medical point of view to deliver a short pulse sequence, this is electronically controlled and is often referred to as “burst mode”. This is a pulse train that delivers two to five pulses in very rapid succession with a single triggering. It is used to destroy a pigmented layer without burning. Short pulsed or QS lasers operate in the range of nano- or pico-seconds. The effects here are no longer linear and they thus already have a mechanically explosive effect on the target tissue (also called optical breakthrough).
Energy density
Power or energy density is the value that expresses the power or energy delivered to a defined area (Tab. 2) . The table shows that the diameter has a high impact on the effect on the tissue. Accidentally changing the distance can extremely change the result. With some lasers the set area/spot size is not monitored electronically, here it can easily happen that the settings are not synchronized and an over- or under-effect occurs.
Transmission systems
In order to bring the laser light to the surgical field, we need transmission systems that can transport this light. Depending on the type of laser, we have different necessities.
Fiber optic cables: Fiber optic cables with diameters ranging from 50 µm to 1 mm can be used very elegantly and can thus also be introduced into the body via endoscopes. At the output of the optical fibers, the light emerges divergently and is used in either contact or non-contact mode. There are also various handpieces that deliver the laser light to the tissue, according to the application.
Hollow waves: Rarer is the transmission via flexible tubes. These have a reflective layer inside to reflect the laser light through these conditionally durable and costly hollow waves. At the output, the laser light emerges divergently and is made available for the application using the contact method or via additional optics.
Mirror joint arm: This is an arrangement of longer tubes that have a built-in mirror in each joint, thus coupling the light into the next tube. The adjustment must be very precisely aligned, but the advantage is a very high beam quality, because the laser light can be transmitted optimally. Caution is advised, however, as the laser light continues to emerge from the arm in parallel and is therefore dangerous regardless of distance.
Free beam projection: Due to compact design, it is possible in some cases to project the laser light directly from the resonator onto the surgical field. Here, the beam is specially prepared for the application and is usually designed for a single application only.
Due to the miniaturization of components, it is now possible to build laser handpieces that then only need to be connected to the basic device via connecting lines. The ability to operate additional handpieces with other lasers via the basic unit expands the range of different treatment options.
Laser classes
Not all lasers are equally dangerous, therefore there are subdivisions into laser classes which express the danger in ascending order (Tab. 3).
Areas where class 3 and 4 lasers are used and therefore hazardous radiation is emitted must be demarcated at the access area. In other words, each door to the laser room must be marked by warning signs and, for Class 4, also by warning lights.
Dangers when using the laser radiation
The substances in the operating field can be excited to various reactions by the laser radiation. Alcoholic liquids, gases, respiratory gases or even endogenous gases can cause fires or explosions.
Material decomposition products in smoke or vapors, e.g. during wart treatment, give rise to gases, dust or explosive mixtures that are hazardous to health. Chemical and toxic substances are also produced, among other things, when tubes, gauzes or covers are irradiated. A suitable fume extraction system should be mandatory.
The maximum permissible irradiation (MTA) represents the limit value for harmless irradiation of the eye or skin. Internationally, this value is also referred to as NOHD. This distance is specified individually for each accessory in the operating instructions. The MZB (NOHD) distance can go from a few meters to infinity. After this removal, eye protection is no longer necessary.
The biological effects on the eye and skin differ somewhat. Since the eye is much more sensitive in its structure than the skin, damage occurs much more quickly here. Damage can occur to the eyes at wavelengths below 400 nm or above 2500 nm at the anterior segment of the eye and between 400 nm to 2500 nm at the posterior segment of the eye. On the skin, if the wavelength is below 300 nm or above 2500 nm, the skin surface can be damaged and between 300 nm to 2500 nm, the skin can be damaged to a depth of about 6 mm.
Each wavelength has a typical penetration depth and a specific absorption behavior. Lasers are very often used in dermatology today. The absorption coefficient is crucial here in order to achieve the correct wavelength (penetration depth) and the appropriate absorption to the target tissue.
Since, especially in the medical field, people cannot be protected from laser equipment by structural measures, this is where the last link in the safety chain comes in: PPE. Laser safety goggles are usually obligatory. They are defined according to DIN EN 207 and must match the laser in terms of wavelength, operating mode and protection level (e.g. for a Nd:YAG laser DI 1000-1100 LB4 RH DIN S).
- For the mode (first digit) we have the abbreviation D for continuous wave, I for pulsed, RI for giant pulse and MI for mode-coupled, found individually or in combination on the glasses.
- The wavelength (second digit) is given in nm and is written as a number or range of numbers on the glasses.
- For the protection level (third digit), 1 is the weakest filter and 9 is the strongest. Depending on the laser power/energy, the safe filter glass must be used. The operating instructions must indicate the correct protection level. Older goggles may still have the previous “Lx” marking for the protective filters (“LBx” is the current marking). As long as they comply with the manufacturer’s specifications, these can also be used without hesitation. Goggles with OD marking do not comply with any European standard and therefore must not be used.
Conclusion
The laser is a wonderful tool with which we can achieve a lot if we know how to deal with its peculiarities. I hope I was able to make these peculiarities a little more familiar to you. So that you use the laser with pleasure and success in the future!
Take-Home Messages
- Laser is a light source that has special properties and can only be generated artificially. This radiation does not occur in nature.
- Artificial light is characterized by the following basic principles: It is monochromatic, coherent with parallel beam path.
- Laser classes indicate in ascending order the possible hazards when using laser radiation.
- If structural measures to protect against laser radiation are not possible, this is where the last link in the safety chain comes in: personal protective equipment (PPE). Obligatory we use mostly laser safety goggles or protective filters.
