When laser light is absorbed what are the three types of biological tissue effects that can occur

Índice Show

Light Dispersion in Tissue
Interaction of Light with Tissue
Lasers and Their Mechanisms
Skin Profile
Laser Tissue Interactions
Absorbing Chromophores
Absorption Zones
Penetration Depth
Diode Lasers in Dermatology

Dr. Karl Stock from ILM at the University of Ulm. If you want to describe the biological and physical effect of light on tissue, you must first understand light dispersion in tissue to consequently understand the different interactions between light and tissue.

Light Dispersion in Tissue

The majority of light that meets tissue is reflected, transmitted, scattered, or absorbed. If light is absorbed, the absorbed light energy is either transmitted in the form of heat, fluorescence, or phosphorescence. Depending on the wavelength of the incoming light and the tissue type, the aforementioned effects occur in different amounts.

The reflection proportion largely depends on the refractive difference between air and tissue, as well as on the angle of incidence. Light that penetrates the tissue is either absorbed or scattered by microscopic structures such as, for example, cell components.

This scattering is responsible, for example, for the fact that a laser beam cannot be focused as needed in the tissue but rather that the spot diameter increases in size.

Absorption is the crucial mechanism in being able to use the applied laser energy in therapeutics. The probability at which radiated light is absorbed is described by the absorption coefficient µa. The reciprocal of µa is the mean free path that a photon travels in the tissue until it is absorbed. [1].

Important absorbers in tissue include:

In the UV range: Peptide bonds and nucleic acids
In the Vis range: Bilirubin, carotine, melanine, and hemoglobin
In the IR range: Water and hydroxyl apatite.

As the blue curve shows in Fig. 2, the absorption in water in the infrared spectral range is particularly high (a depth of penetration of only 1 µm at a wavelength of 3 µm). This is why the 2.94 µm Er:YAG laser and the 10.6 µm CO2 laser are particularly well suited for cutting and removing soft tissue: it consists largely of water.

Interaction of Light with Tissue

The characteristics of the tissue and the radiation parameters (wavelength, intensity, pulse energy, duration of radiation) lead to different effects:

Low Laser Power
In low laser power, fluorescence, for one, can be used to diagnose bladder tumors, for example. For another, photochemical processes are used in low-level laser therapy (LLT) and photodynamic therapy (e.g., in combination with methylene blue to kill bacteria).

Higher Laser Power
In higher power, thermal effects play an increasingly important role. In thermotherapy, the tissue does not sustain thermal damage. At approx. 60°C, the tissue coagulates (e.g., during ablation of the blood vessels) and at approx. 300°C the tissue vaporizes (which is referred to as so-called tissue vaporization). The latter is the effect used, for example, in surgery to cut soft tissue with a CO2 laser or diode lasers.

Lasers and Their Mechanisms

High-Power Pulsed Lasers
One particularly efficient type of tissue ablation is thermomechanical ablation, which is used in connection with pulsed lasers and high absorption in water. The high absorption and high power of the laser pulse causes the tissue to heat up suddenly. At approx. 100°C, the water vaporizes and the tissue rapidly increases in pressure, which can cause explosive tissue ablation. Due to fast and efficient ablation, the thermal damage of the tissue is significantly lower than with vaporization. In hard tissue, bones, teeth, and bladder and kidney stones, efficient and precise ablation can also be achieved, especially with Er:YAG lasers (see Fig. 3).

Excimer Lasers

Excimer lasers are used in the UV range with short pulses and high intensity. Thus, not only the absorption in tissue but also the energy of the single photon is so high that ablation occurs with single atoms. This photoabalation is used in particular in ophthalmology to correct corneal curvature.

Ultra-short Pulsed Lasers
In photodisruption, the atoms in focus are ionized with ultra-short pulsed lasers in the nano, pico, or even the femtosecond range. This produces microplasma that can expand extremely fast and create an acoustic shock wave. This shock wave leads, for example in LASIK surgery, to high-precision ablation that is also used to correct ametropia. In deeper tissue, plasma and the pigments in tattoos, for example, can also be destroyed.

[1] Lasertherapie der Haut, S. 26, R. Steiner, Springer-Verlag Berlin Heidelberg, 2013

Karl Stock, a doctor of human biology who also studied engineering, is the associate director of the Institute of Laser Technology in Medicine and Measurement Technology (ILM) at the University of Ulm and head of the equipment development workgroup. This workgroup primarily develops units and applicators for medical and dental applications – most often for industrial partners, such as, for example, laser methods for surgical and diagnostic applications, including those in the specialist areas of otorhinolaryngology (ENT medicine), urology, general surgery, and ophthalmology.

Fig. 1: Light Propagation in Tissue

Fig. 2: Wavelength dependent absorption coefficients (water

When laser light impinges on tissue, it can reflect, scatter, be absorbed, or transmit to the surrounding tissue. Absorption controls to a great degree the extent to which reflection, scattering and transmission occur, and wavelength is the primary determinant of absorption. The CO2 laser is consistently absorbed by most materials and tissues and the Nd-YAG laser wavelength is preferentially absorbed in pigmented tissues. The factors which determine the initial tissue effect include the laser wavelength, laser power, laser waveform, tissue optical properties, and tissue thermal properties. There are almost an infinite number of combinations of these factors possible, many of which would result in unacceptable damage to the tissues. This underscores the need to thoroughly test any particular combination of these factors on the conceptual, in-vitro, and in-vivo level before a treatment is offered.

Last updated on February 12, 2019

In 1963, Leon Goldman became the first person to report using lasers in dermatology. Goldman reported the effects of Ruby laser in the selective destruction of cutaneous pigmented structures, like black hairs. Since then, lasers have been used for various skin treatments such as vascular lesions and cosmetic surgery. Studying laser tissue interactions is crucial for making advances in new treatments.

Depending on the application, such as removing skin tattoos or stimulating the dermis, various laser choices exist. For example, high energy CO2 lasers can be used to remove tattoos, whereas Nd:YAG and diode lasers can be used to stimulate the dermis. Knowing the biological factors that are at play in laser tissue interactions can help practitioners choose their lasers carefully.

In this article we will discuss the factors that influence laser interactions with skin such as penetration depth, absorption chromophores, and ablation profiles. The use of various diode lasers for targeting chromophores are also discussed. The specific applications of lasers in tattoo removal, and other dermatological applications can be found in other FindLight articles.

Skin Profile

The human skin has the epidermis which is about 100 um thick, with the dermis extending to 1.6 to 1.9 mm. The depth is not the only anatomical factor that makes penetration hard. One must also consider the distribution of light absorbing molecules such as chromophores in different layers. This can vary for different people, which makes uneven delivery of laser energy for different skin profiles. Nevertheless, treatment has still been found to be effective for surface skin treatments, with care to be taken for monitoring deep skin treatments.

Relative depth of various layers in human skin.

Laser Tissue Interactions

Laser tissue interactions can be classified into four types on contact: reflection, absorption, scattering, and transmission. Reflection wastes laser energy and makes it unsuitable for medical applications. Absorption at the surface is influenced by various chromophores as we will in detail later in this article. Transmission helps the beam to penetrate the surface tissue to reach layers underneath. For example, choosing visible light instead of UV helps light penetrate the cornea, lens, and image the retinal layer. Finally scattering opposes precise delivery of the laser energy to a spot. In this blog post we will focus on absorption profiles and transmission depths, as these are the main forms of laser tissue interactions.

Laser tissue surface interactions. Courtesy pocketdentistry.

Absorbing Chromophores

Absorption in tissues occurs because of certain molecules that absorb strongly at a particular wavelength. Specifically, melanin, hemoglobin and water are the main molecules that absorb light in the skin. The absorption profiles for these molecules are shown in the image below. Consequently, this can be a useful guide in choosing a laser wavelength based on the molecular content of the tissue.

Absorption strength of various chromophores in biological tissue. Courtesy Optics&Photonics.

Absorption Zones

Apart from the absorption wavelength, the profile of surrounding tissue affected is important to achieve precise effects. The following diagram shows the various zones of affected tissue at the region of laser application.

Absorption zones depending on contact and pressure for various lasers. Courtesy ResearchGate.

The contact tip, pressure, pulse energy and beam profile all play a significant role in this process. Applying pressure with the contact can achieve photoablation effects as shown in the right. On the other hand, a flat beam profile removes a flat tissue region. If a non-flat beam is used, then the profile of the beam influences the profile of tissue ablated. The laser beam profile can meticulously be monitored using beam profilers. The pulse energy and mode of operation (pulsed or CW) can determine whether the application is for heating or ablating a tissue. Moreover, using small pulses can reduce the thermal conduction to surrounding tissues.

The profile of the tip can also influence the surrounding regions affected. The contours of heat conduction is shown in the image below.

Tip geometry and contact pressure influencing energy distribution.

For pointed tips, the conduction is elliptical, and reaches a higher depth. Flat tips conduct in concentric circles. The depth and the ablation profile determine the type of tip used.

Penetration Depth

Wavelength choice plays a crucial role in the penetration depth of laser light. As shown in the diagram below, wavelengths close to red penetrate the deepest, and light close to UV remain within the epidermis.

Laser tissue penetration depths for various wavelengths. Courtesy ResearchGate.

Shorter wavelengths are preferred for applications limited to the epidermis, while near red wavelengths are optimal for dermal applications. Nd:YAG and diode lasers exist for near red spectrum, which makes them more suitable than CO2 lasers for deep skin applications.

Diode Lasers in Dermatology

Diode lasers can emit wavelengths in the visible and infrared region. One can use a specific diode laser depending on whether the intended absorbing molecule is hemoglobin, melanin, or water. For example in blood flow measurements of blood vessels close to the surface, a technique called Laser Doppler Flowmetry (LDS) is used. This can be a useful technique in dermatology to learn about the blood flow close to the skin surface. In this technique, the light reflected from hemoglobin molecule is used to detect the velocity of the red blood cells. Since hemoglobin reflects red strongly, diode lasers with emitting red light can be used in LDS.

Diode lasers for various wavelengths vs the absorption profiles of various chromophores. Courtesy BioOpticsWorld.

Diode lasers are becoming more popular in dermatological applications because of higher penetration depth, low cost, and compact size. Check out FindLight’s collection of diode lasers to use in your projects.