Interaction of Laser with Biological Tissues: Principles and Applications

Author: Markolf H. Niemz (German)
Translator: Zhang Zhenxi / et al.
Editor-in-Chief: Wu Shoukui
Publisher:
Publish Date: 1999-04-01
Features:
The first two chapters describe and explain the interaction between light and biological tissues, as well as the fundamental physical concepts of light reflection, absorption, and scattering in tissues. In Chapter 3, based on existing theories, the transmission of photons and their interaction with tissues, such as photochemical interactions, thermal interactions, and the physical processes of photobleaching, plasma-induced ablation, and photodestruction, are described, along with an analysis of different laser characteristics. In Chapter 4, the applications of lasers in various medical disciplines and clinical settings are discussed. The final chapter introduces the basic concepts of laser safety, which are of great significance for laser applications. The rich literature makes this work more comprehensive, and the important data listed in the appendix facilitates easy reference. The book also has distinctive pedagogical features, making it an excellent textbook for students and a valuable reference for medical and scientific workers, as well as suitable for self-study. Numerous photographs, illustrations, tables, and in-depth interpretations make this book a useful guide for graduate students and researchers.
Excerpt:
Continuous-wave (CW) lasers and pulsed lasers. Most gas lasers and some solid-state lasers belong to the class; pulsed lasers mainly include solid-state lasers, lasers, and certain dye lasers. The types of laser medicine and their two characteristic parameters—wavelength (wavelength) and pulse duration (pulseduration)—are listed in Table 1.1. The table is arranged according to pulse duration because, as we evaluated in Chapter 3, exposure time marks the type of interaction with biological tissues. Wavelength is the second most important laser parameter. It determines the depth of penetration of laser radiation into tissues, i.e., the efficiency of absorption and scattering. The third parameter, energy density, is also generally considered important. However, its value is merely a necessary condition for producing a specific effect, and its degree is secondary. In Chapter 3, it will be explained that all medically relevant effects are achieved within an energy density range of 1 J/cm2 to 1000 J/cm2. This is quite narrow compared to the potential pulse durations spanning 15 orders of magnitude. The fourth parameter, intensity, is given as the ratio of energy density to pulse duration. Detailed discussions of the basis for all these parameters can be found in Chapter 3. As described in Chapter 4, each type of laser listed in Table 1.1 is used for specific clinical applications. Newly developed diode lasers and free-electron lasers have become increasingly important in medical research. Diode lasers can emit continuous or pulsed radiation and are very compact. Free-electron lasers can provide extremely short laser pulses, but they require large electron accelerators to operate and can only be used in specific locations. The progress of laser surgery is mainly attributed to the rapid development of pulsed laser systems. As mentioned earlier, pulse duration ultimately determines the effect on biological tissues, particularly the distinction between thermal (thermal) and non-thermal (non-thermal) effects. A rough approximation is the "1 μs rule," which states that pulse durations greater than 1 μs often accompany measurable thermal effects, while pulse durations shorter than 1 μs, if the repetition rate is appropriately chosen, typically result in negligible thermal effects (see Section 3.2 for further details). Without additional equipment, most lasers can emit continuous radiation or pulses with durations greater than 1 μs. Therefore, the study is limited to potential thermal effects. Other interactions may only occur when shorter laser pulses are generated. Among these, the ablation (ablation) mechanism is particularly effective, such as photobleaching, plasma-induced ablation, and photodestruction (photodisruption) occurring at the nanosecond or picosecond level. Even femtosecond-range short pulses can now be achieved. As will be elaborated, when comparing the mechanisms of plasma-induced ablation with photodestruction, their clinical advantages are quite questionable. Both are physically referred to as photodisruption. As the theoretical analysis in Section 3.4 shows, the threshold parameters for photodisruption do not further decrease when transitioning from picosecond to femtosecond pulses. However, overall, as laser systems improve their ability to provide shorter pulses, new meaningful applications often emerge. Figure 1.1 illustrates the development of pulsed laser systems. In solid-state lasers, there are two milestones: one is achieved through the development of mode-locking technology, and the other is achieved through the development of new laser media with extremely wide bandwidth, which will be discussed in detail below. These two milestones are represented as two jumps in the corresponding curves in Figure 1.1. Another type of laser capable of providing ultrashort pulses includes dye lasers. They were invented after solid-state lasers. Their development has been gradual rather than abrupt. The development of new technologies such as collision-pulse mode-locking can also produce pulses with durations much shorter than those of solid-state lasers. However, the medical applications of dye lasers remain quite limited because they are inconvenient to use and require complex maintenance. Compared to long-lived solid crystals, dyes need to be recycled, and this is typically not possible through simple button operations for regular replacement. The earliest lasers were ruby lasers pumped by flash lamps. The output of such lasers contains several spikes. The total duration of these spikes is determined by the flash itself, which corresponds to the lifetime of the laser transition to a high-energy state, about 1 ms for ruby. The invention of Q-switches made it possible to obtain pulses as short as 50 ns. Mechanical devices (rotating holes or laser mirrors) or optical devices (electro-optic or acousto-optic Pockels crystals) can be used as Q-switches. In both cases, the loss in the resonator is artificially maintained high until a significant inversion is achieved. When the artificial loss is eliminated, all the energy stored in the laser medium is instantly converted through stimulated emission. Shorter pulses can be obtained when mode-locking is initiated in the laser cavity. In mode-locking, rapid modulation of the crystal (active mode-locking) or selective saturation filters (passive mode-locking) can modulate the electromagnetic field. This ensures that all axially oscillating laser modes are in phase, producing picosecond pulses. A typical example is the Nd:YAG laser, whose optical bandwidth is on the order of 1 nm. This bandwidth limits the shortest pulse duration that can be achieved to several picoseconds.