3.1 Introduction

Many different interactions might happen when a laser is impinging onto biological tissues. The laser parameters as well as tissue characteristics play a critical role in this diversity. In this chapter, we discuss the tissue optical properties which are essential for laser–tissue interaction. Tissue thermal properties—such as heat conduction and heat capacity—are also discussed in this chapter. On the contrary, the laser parameters, such as wavelength, exposure time, applied energy, focal spot size, energy density, and power density, are also discussed. As we will find later on in this chapter, the exposure time is a critical parameter when choosing the type of interactions. There is an unlimited number of possible combinations for the experimental parameters. However, mainly five categories of interaction types are classified today. These are photochemical interactions, thermal interactions, photoablation, plasma-induced ablation, and photodisruption. In this chapter, we thoroughly discuss each of these interaction mechanisms.

Figure 3.1 shows a double-logarithmic map with the five basic interaction types. The y-axis expresses the applied power density or irradiance in W/cm². The x-axis represents the exposure time in seconds. Two diagonals show constant energy fluences at 1 J/cm² and 1000 J/cm², respectively. According to this chart, we can roughly divide the timescale into four sections: continuous wave or exposure times >1 s for photochemical interactions, 1 min to 1 μs for thermal interactions, 1 μs to 1 ns for photoablation, and <1 ns for plasma-induced ablation and photodisruption. The difference between the latter two is attributed to different energy densities. They will be addressed separately in Sects. 3.6 and 3.7 since one of them is solely based on ionization, whereas the other is primarily associated with a mechanical effect.

3.2 Optical Properties of Tissue

In laser–tissue interaction, it is important to know about the absorbing and scattering properties of tissues. The purpose is to have better prediction of successful treatment. When we apply laser onto a highly reflecting materials, the index of refraction might be of interest. In general, we do not assume any tissue optical properties unless specified in tables or graphs. We emphasize more in the general physical interaction which mostly apply to solid and liquid. In reality, there are limitations given by the inhomogeneity of biological tissue to predict the optical properties.

3.2.1 Absorption

The intensity of light is attenuated during absorption by the biological tissue. The absorbance of tissue is defined as the ratio of absorbed and incident light intensities. A partial conversion of light energy into heat motion or certain vibrations of molecules of the absorbing material governs the process of absorption. A perfectly transparent medium which has no absorption will transmit the total radiant energy entering into such medium. In visible range of light, the cornea and lens can be considered as transparent media. In contrast, when the media absorb all the incident radiation, it is called opaque .

The terms “transparent” and “opaque” are very wavelength-dependent. This term depends on the main absorber inside the biological tissue. The cornea and lens, for instance, mainly consist of water which is highly absorbing in the infrared region, will appear opaque in the infrared region but transparent in the visible region. There is no medium known to be either transparent or opaque to all wavelengths of light.

General absorption is being considered if the substance reduces the intensity of all wavelengths by a similar fraction. If we considered the visible region, this substance would appear gray to our eyes. Colors actually originate from selective absorption. Basically, we can divide color as surface and body colors. Surface color is originated from surface reflection. Body color is originated from backscattering light that experiences multiple absorption and scattering inside subsurface of the substance.

The ability of a medium to absorb electromagnetic radiation depends on many factors, mainly the electronic constitution of its atoms and molecules, the wavelength of radiation, the thickness of the absorbing layer, and internal parameters such as the temperature or concentration of absorbing agents. Two laws, which describe the effect of either thickness or concentration on absorption, are commonly called Lambert’s law and Beer’s law and are expressed by:

where z is the sample optical thickness, I(z) is the intensity at a distance z, I₀ is the incident intensity, and μ_a is the absorption coefficient of the medium.

In biological tissues, absorption is mainly caused by either water molecules or macromolecules such as proteins and pigments, whereas absorption in the IR region of the spectrum can be primarily attributed to water molecules, proteins, and pigments mainly absorb in the UV and visible range of the spectrum. Proteins, in particular, have an absorption peak at approximately 280 nm [1].

Absorption spectra of two elementary biological absorbers—melanin and hemoglobin (HbO₂)—are shown in Fig. 3.2. Melanin is the basic pigment of the skin and is the most important epidermal chromophore. Its absorption coefficient monotonically decreases across the visible spectrum toward the infrared [2]. Hemoglobin is predominant in vascularized tissue. It has relative absorption peaks around 420, 540, and 580 nm and then exhibits a cutoff at approximately 600 nm. Most biomolecules have their complex band structure between 400 and 600 nm. Macromolecules or water is not highly absorbed in the near-infrared region. Thus, a “therapeutic window” is ranged between roughly 600 and 1200 nm. In this spectral range, biological tissues have a lower absorption, thus enabling treatment of deeper tissue structures.

As already previously stated, hemoglobin is predominant in vascularized tissue. Krypton ion lasers at 531 nm and 568 nm, respectively, have almost perfectly matched wavelength with the absorption peaks of hemoglobin. Thus, these lasers can be used to coagulate blood and blood vessels. Dye lasers may also be a choice for laser treatment since their tunability can be advantageously used to match particular absorption bands of specific proteins and pigments. In some applications, special dyes and inks are used to provide enhanced absorption. Thus, we can increase specific tissue absorption which leads to better laser treatment. Moreover, we will have less damage to the adjacent tissue due to this enhanced absorption.

3.2.2 Scattering

Refractive index mismatches cause scattering of light in biological tissue at microscopic boundaries such as cell membranes and organelles. The scattering coefficient describes the scattering properties of a medium. The scattering coefficient is the product of the scattering cross section of the particles and the number density of scattering particles. The scattering cross section is an area which describes the likelihood of light being scattered by a particle. Therefore, μ_s represents the probability per unit length of a photon being scattered [3]. In the same manner as for absorption, one can define a scattering coefficient, μ_s, for a collimated source, such that [3]:

where z is the sample optical thickness, I(z) is the intensity at a distance z, I₀ is the incident intensity, and μ_s is the scattering coefficient of the medium.

The exact origins of scattering in tissue are not well known. Biological tissue is acomplex and highly heterogeneous material. There are a number of hypotheses identifying contributions to tissue scattering from various biological and biochemical microstructures, both extracellular such as collagen fibers [4, 5] and intracellular such as mitochondria [6], cell nuclei [7], and possibly a large variety of other structures such as cell membranes [8].

The angular distribution of scattering is described by the scattering phase function, p(θ), which gives us the probability of a photon to be scattered at an angle, θ, with respect to its initial direction. The phase function is normalized in such a way that ∫p(θ) dω = 1 (with dω denoting integration over solid angle ω). The Henyey–Greenstein (HG) phase function, which is originally introduced to describe scattering of light by interstellar matter [9], provides a satisfactory description of the angular patterns arising from tissue scattering [10, 11]:

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