CHAPTER 2 LASER - TISSUE INTERACTIONS AND ITS APPLICATION TO MEDICINE AND BIOLOGY

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1 CHAPTER 2 LASER - TISSUE INTERACTIONS AND ITS APPLICATION TO MEDICINE AND BIOLOGY The use of lasers in medicine began soon after the first laser was invented in In recent years, the number and variety of applications of lasers to biology and medicine have increased rapidly. All biomedical applications of laser are based on the fact that lasers could produce high photon flux (of MW cm"2) on a localized spot (of micron diameter). Such high power density causes a broad spectrum of effects and one can utilize these effects for different medical applications. This is shown schematically in Figure 2.1. Out of many properties of the laser such as monochromaticity, spatial and temporal coherence, directionality and brightness, monochromaticity of laser has only minor importance for most of the medical applications. This is because the action spectrum («10 nm) of all biomolecules is many times greater than the spectral width of laser (» 0.1 nm). Of all the properties, the spectral brightness associated with laser has the greatest impact, since laser beam with or without focusing, produces internal local heating and localized specific photochemical reaction. In addition, the directionality of the beam which is intimately related to spectral brightness, helps to pass the beam into the optical fiber. Further they can be operated either in cw mode or pulsed mode and can produce ultrashort pulses of femto second pulses. Over and above the lasers

2 39 Biomolecule Laser Diagnosis Photobiochemical process (Therapy) Macrodestruction (surgery) micro macro iagnosis diagnosis one photon multiphoton processes processes thermal hydro photoablation destruction dynamical destruction Figure 2.1 Various degrees of laser interaction with biomolecules V

3 40 with wavelengths varying from UV to IR are now available and hence one can choose a certain type of laser to bring about an optimum interaction with biomolecules. Though a single laser can provide a combination of several of the above properties, only one or two of the above properties are essential for a particular application, depending upon the specific modes of interaction (i.e. various process of conversion) of the incident electromagnetic energy with biomolecules. From the understanding of mechanisms of laser-tissue interactions, one can initiate selective laser interaction to use it for a particular type of application. 2.1 LASER - TISSUE INTERACTION The effects of lasers on living tissue can be analyzed by considering two separate events, a photophysical event and a photobiological host response. All phases of the photophysical event usually occur in less than a second while the host response may involve over months Photophysical Process When light photons fall on the tissue, four basic optical processes may occur [10]: i. Direct reflection at the boundaries of the layer due to change in the refractive index; ii. Scattering by molecules, particles, fibers, cell organelles and cells within the layer;

4 41 iii. Absorption (which may lead to Photochemistry or dissipation of the absorbed energy via heat, fluorescence, or phosphorescence); and iv. Direct transmission through the layer. Once energy has been absorbed by various biomolecules, it can be transferred to other molecules by chemical energy transfer, conduction, transfer of particles, charge transfer, or by movement of further electromagnetic waves. The energy absorbed by a molecule may raise its kinetic energy (electronic) sufficient to cause unimolecular or bimolecular reactions. Photochemical alterations with breaking or alterations of bonds may also occur. Thermochemical and thermal alterations may result in changes in the primary, secondary, tertiary, or quarternary configuration of macromolecules within the tissue. This may result in alteration of membranes, denaturation, dehydration or carbonization of molecules. These changes may be accompanied by breaks in tissue continuity. Both sonic and supersonic waves can be propagated within the tissue and when these exceed the elastic limits of the tissue further breaks in continuity or changes in membrane function may occur. These alterations may be expressed microscopically and macroscopically as acute changes in the physical properties of tissues such as changes in viscosity, density, optical properties and state of hydration. They may also be measured as ablation, cutting, drilling, shearing, or discontinuous phases within the tissue. Tissue optics are acutely altered which leads to the whitening of tissues sometime seen after laser impact.

5 Photobioloaical Processes The photophysical events described above may lead to activation or deactivation of enzymes or may alter the physical and chemical properties of key macromolecules as shown in Figure 2.2. This may set off complex chemical cascades which cause changes in viscosity, clotting, sealing, and change in membrane potentials. Rapid physiological responses lead to changes in blood flow and vessel permeability. These mechanisms may serve to amplify the impact of the photophysical event on the tissue. Injured cells react with changes in function, metabolism, and replication kinetics. Cell death may occur and changes in DNA may lead to mutations in surviving cells. If enough cell injury occurs, chemical mediators are released and primitive yet complex tissue and host responses are noticeable histologically and grossly over the next few hours. Redness, swelling, heat, pain and loss of function signify inflammation. Repair, increased cell proliferation, regeneration, fibrosis and changes in vasculature are long term responses to cell injury manifested over days to weeks. Certain components of these delayed responses may be therapeutic in some pathological conditions. All the above processes (i.e., photophysical events and hence biological responses) depend on i. wavelength of laser ii. energy density iii. pulse duration iv. irradiation time and v. absorption characteristics of target molecule.

6 43 < 1 to 103s 10 to 105s 102 to 107s "Immediate" Delayed Tissue Macromolecular -- > Cellular ---> and Host Alterations Changes Responses Enzyme Activation Clot Agglutinate Seal i / / Metabolism Function Replication Kinetics Inflammation Repair Regeneration Rapid Physiological Responses Altered Blood Flow Permeability change Figure 2.2 Schematic diagram of various photo-biological processes

7 Absorption of Biological Tissues In this context, it is worth mentioning the relationship between the nature of the laser (whether cw or pulsed and when pulsed, duration of the pulse) and the absorption capability, of the biomolecules and hence its application in medicine and biology. Figure 2.3 and Table 2.1 show that in the ultraviolet region from 250 to 350 nm, proteins and DNA dominate the absorption. Oxyhemoglobin, and melanin are the absorbers in the visible portion of the spectrum. At wavelengths longer than 2 pm, water, the principle component of most tissues, is the chief absorber. The wavelength interval between nm forms a window of low optical absorption; laser photon of such wavelength can penetrate into tissue to a depth of more than a centimeter [11]. The window exists because electronic transitions of tissue absorbers have become weak, but the IR absorption of water ih not yet strong. It provides an important entree for phototherapy and selective surgery (Figure 2.4). Besides this dependency of wavelength and absorption characteristics of the target molecule, Jean-Luc Boulnois [12] classified the laser - tissue interactions into four groups, which were distinguished by the order of increasing time scale (from sec to 103 sec) versus power density (from 10 3 to 102 W/cm2). This is schematically represented in Figure 2.5, which indicates that for most of the photomedical applications, lasers such as NdiYAG, Ar ion, Kr, C02, excimer, dye laser or He-Ne are used. Damage processes in pulsed systems differ from those associated with continuous wave (cw) lasers, in that time-

8 45 c (10 M cm ');ah,0(n ' Figure 2.3 Absorption coefficient spectrum of biological tissues

9 46 Table 2.1 Selected absorption coefficients of biological tissues Tissue Wave1ength (nm) a (cm' DNA, RNA Blood, oxygenated (normal hematocrit) Blood, reduced (normal hematocrit) Epidermis, white Fat, subcutaneous Liver (rat) Kidney (rat) Retinal pigment Epithelium

10 47 Penetration depth (pm) Figure 2.4 The penetration depth of optical radiation in skin and blood a,t different wavelengths

11 48 Power density (W/cm*) Figure 2.5 Medical laser interaction map

12 49 constants are substantially different. From Figure 2.5 it is found that, i. The reciprocal correlation between intensity and time over a wide range demonstrates that the specific energy dose required to achieve a laser-induced biological transformation is nearly constant. Consequently, the time of exposure during which this energy dose is delivered appears to be the single parameter distinguishing the transformation process entirely. ii. Also, four separate groups of transformations, which share this common fluence, can be distinguished, along a diagonal on this chart, according to the duration of interaction: they correspond precisely to the characteristic timescale of the respective photobiological damage involved. This distinction into four photomedical families serves as the basis for the classification into four photobiological laser processes which are : i. Thermal processes due to continuous or quasi- ii. Photochemical processes continuous wave irradiation iii. Electromechanical processes due to high power short- iv. Photoablative processes pulse regions

13 50 A brief description of the above processes are given below, based on the mapping and the time-scale distinction. 2.2 ANALYSIS OF DIFFERENT INTERACTIONS Thermal Interaction All the surgical applications of lasers, whether in cutting or in hemostasis, rely on the conversion of electromagnetic energy into heat. The laser beam is potentially sharp enough for surgical use. It is intrinsically sterile and has heomostatic properties. All these make laser surgery relatively blood - free, and the lack of trauma to surrounding tissues lead to less edema and faster post - operative recovery. The laser beam is highly directional and the angle of divergence '9' is of the order of milliradian for surgical lasers. Hence, unlike radiation from conventional sources, all the power can be easily focused by a lens into a very small spot of width fe, where "f" is the focal length of the imaging lens (Figure 2.6). Besides this focusing and collimation due to spatial coherency, efficient coupling into the optical fibers of diameter around 100 jum allows easier delivery of energy to the target site, which results in thermal injury, tissue removal or control of bleeding. The choice of wavelength and tissue determines the depth of penetration and this influences the interplay between tissue removal and hemostasis [12]. The characterization of the photothermal-biological response following laser irradiation depends, however, on the structural level at which it is targeted. At the microscopic level, the photothermal process originates from

14 Asymptotic divergence Figure 2.6 A Gaussian-mode laser beam focused by a lens

15 52 the bulk absorption occurring in molecular vibration - rotation bands or, perhaps, in the vibrational manifold of the lowest electronic excited state, followed by subsequent rapid thermalisation through non-radiative decay. The reaction with a target molecule A proceeds in two steps : first, the absorption of a photon of energy, hy, promoting A to a vibrational excited state A* and second, an inelastic scattering with a collisional partner M, belonging to the surrounding medium. On colliding with A*, M increases its kinetic energy into H' by carrying away the internal energy released by A*. The microscopic origin of the temperature rise results from the amount of energy released to M*; the two - step reaction can be schematically represented as : Absorption : A + hv > A* (vibration - rotation) Deactivation : A* + M(E) > A +-M' (E+ E) In contrast to other photobiological laser processes in which the choice of photon energy is usually selected to access a specific reaction channel, the biological effects of heating to first order are non-specific. The scattering and absorption properties of the medium may influence the wavelength selection and, to some extent, the depth of penetration. However, the characteristic heating effects are largely controlled by molecular target absorption, essentially from free water, haemoproteins, pigments (e.g., melanin) and other macro molecules such as nucleic acids and aromatics. The common mechanism by which tissue is thermally affected is by molecular denaturation ( e.g., proteins, collagen, lipids, hemoglobin). Table 2.2 summarizes the temperature ranges of successive transformations [13].

16 53 Table 2.2 Physical principles of photothermal processes in tissues: Temperature Effects of tissue 43-45*C Conformational changes, Hyperthermia (cell mortality) 50 * C Reduction of enzyme activity 60 *C Protein denaturation Coagulation O o o CO Collagen denaturation Membrane permeabilization 100 C Vaporization and abalation 300 C Carbonization 500 C Tissue burning

17 54 Heating, in addition to causing ablation, can lead to alteration of the tissue left behind. These alterations can be desirable when the laser stops bleeding by coagulation of blood vessels adjacent to the cut. But at the sane tine it is deleterious when laser radiation causes destruction of adjacent nornal tissue. Thermal danage can interfere with healing. Thermal damage can be minimized by confining the laser generated heat to the region in which it was deposited, which is approximately one optical absorption length deep. One way of achieving such confinement is by using pulsed radiation whose duration is less than the thermal relaxation time corresponding to an optical absorption length. For example, water relaxation time (325 fis) at 10.6 Mm suggests an interesting operation mode for C02 lasers in microsurgery which could be called real super pulse mode. The high temperature needed for phase change ( steam formation ) without appreciable heating of adjacent tissues is reached only when the exposure duration is shorter than 325 /xs. Consequently by pulsing the C02 laser with pulse duration of about 200 ms, it is possible to vaporize tissue directly and still obtain an extremely small amount of necrosis to the adjacent tissues. For comparison purposes, a summary of thermal relaxation times corresponding to various biological media is presented in Table Photochemical Processes Photochemical based therapies are not recent ones. The ancient Egyptians used the sun to initiate the photochemical reaction of naturally occurring plant

18 55 Table 2.3 Thermal relaxation times (in s) for various biological media at wavelengths for different lasers Biological Ultra Argon He-Ne Nd-YAG co2 medium violet (200) (488/514) (633) (1060) (10.6) nm nm nm nm /zm Water 30xl0"6 20X X10 Oxygenated blood 0.2xl0**3 15xl Plaque in o Melanin

19 56 chemical, psoralen to treat depigmented skin, leukoderma. Photochemical processes are based on the electronic excitation of chromophores of endogenic or exogenic sensitizing molecules by the photons. Biomolecule such as hemoglobin, or melanin is a polyatomic complex organic molecule. Figure 2.7 gives the energy level diagram, which shows that the ground state is a singlet state SQ. When an electron of the molecule is excited by absorption of a photon, it may reach any of the excited electronic energy, say, S-j^ or S2. In all these levels there is spin parity. There is another set of levels, Ti,T2 etc., which are triplet levels, in which the spin are parallel. Further each electronic level is associated with a set of vibrational manifold. A molecule excited to S2 or upper vibrational level of reach the lower vibrational level of in ps. The life time in is about 5 ns. Transition from S-^ to SQ is called fluorescence and non-radiative transition to T-^ is called intersystem crossing. The triplet state Tj is a metastable level having a lifetime of 100 ns to 1 ms depending upon the environment. Most of the photochemical reactions take place effectively in this triplet(t) state than in singlet state(s) because of its longer life time. When being acted on by low intensity light, a molecule can absorb no more than one photon because the excitation rate Wexc = cr I is much lower than the relaxation rate 1/Trel f the excited molecule (where a is the excitation cross - section, I is the radiation intensity in photons cm-2 s 1 and Trel is the excitation relaxation time) [14].

20 57 Figure 2.7 Schematic representation of the energy level diagram of a polyatomic complex organic molecule.,4

21 58 The excited molecule either takes part in a chemical reaction or transfers its excitation energy to another molecule participating in a chemical transformation as shown in the Figure 2.8. This is the basis of singlet oxygen generation which will be discussed later in detail in chapter 3. However, under the effect of short laser pulses of intensity I which is so high that Wexc» 1'/Trel' then the molecule can absorb a second photon either from a triplet state or from an excited singlet state. The excited molecule again can suffer both a photochemical transformation and transfer excitation to another photochemically active molecule. The first two schemes form the basis for linear one photon photobiology, while the third involving the participation of more than two photons underlies nonlinear, multiphoton photobiology [15]. These two approaches impose different requirements on the laser intensity One-Photon excitation One-photon photobiochemical processes, such as phototherapy of neonatal jaundice, photochemotherapy of various skin diseases and cancers with the aid of dyes are well known in photobiology and this is discussed in Chapter 3. Many PCT do not require lasers and so they can be performed under the effect of light from incoherent sources. Nevertheless, the use of laser is extremely useful for the following reasons: i. Laser radiation is easily delivered to the internal organs through optical fibers without significant loss of intensity. ii. The high intensity laser radiation ensures a high rate of photochemical processes. This laser provides

22 59 A etc + M Chemical Vm* reactions Figure 2.8 Schematic representation of primary photoprocesses occuring in picosecond tvo-quantum excitation of biological molecules

23 60 a therapeutic dose over a shorter time span than is possible with radiation from conventional sources. It is for this reason that lasers have given a fresh impetus to the development and improvement of established cancer PCT techniques using UV-Vis. radiation [14]. iii. The necessary radiation dose can be provided by a single pulse, which is of particular use in those cases where the effect of light on subsequent secondary chemical reactions must be excluded Multiphoton excitation The multiphoton excitation of biological molecules can be produced by ultrashort pulses of low energy but high peak power. Biological molecules rapidly decay from their excited states non radiatively. This situation is different if the molecule relaxes through a triplet state with a comparatively long life time ( s ). In this case, a two-step excitation can be effected by means of ultra- short pulses (Figure 2.8). However, for the molecule to absorb a second photon from a singlet state, its excitation rate must be close to the relaxation rate that lies in the picosecond range l/rrei « s-1. Such a two photon excitation can, therefore, be effected only by means of ultrashort pulses. This fact is used to advantage the laser therapy of tumours, for it provides the possibility of locally producing water radicals that kill tumour cells but no effect on nearby normal cells, by means of specific sensitizing molecules.

24 Electro- mechanical Processes In this mode of interaction, a fluence about 100 J/cm2 is delivered to possibly transparent tissues by an Nd-YAG laser in extremely short time of exposures by means of either mode-locked 30 ps pulses or 10 ns Q-switched pulses. The process is not maintained by linear absorption and is consequently not thermal. Rather, the high-peak power laser pulse, when focused at a target, creates high irradiances (approximately 1010 W/cm2 for ns pulses and 1012 W/cm2 for ps pulses) which locally generate high electric field ( V/cm) comparable to average atomic or intramolecular Coulomb electric fields. Such large fields induce a dielectric breakdown of the target material resulting in the formation of a microplasma, i.e., an ionized volume with a very large free electron density. The shock wave associated with the plasma expansion generates a localized mechanical rupture for spatial extensions where the pressure rise is superior to the yield strength of encountered tissue. Table 2.4 summarizes the overall sequence of physical processes involved [16]. This photodistruption process is of particular interest in several pathological conditions of the eye for the non-invasive treatment of capsulotomies, certain iridectomies, the removal of vitreous strands or the dissociation of opacified membranes which frequently develop after cataract surgery. Since this technique is simple it can be used in ambulatory treatments. Recently, optical breakdown techniques have been used also in Cardiovascular models to investigate possible disintegrations of atheromatous plaques in small arteries by the use of pulsed photodistruption.

25 62 Table 2.4 Physical principles of laser induced breakdown Short laser pulse focused at target High power density (EpAp) I « w2 s I» 1012W/cm2 High electric field E = (21/cCo)1/2 * 106 V/cm Dielectric breakdown (multiphoton process) Elaser 58 Eionization molecules Plasma formation Free electron Ne * 1021/cm3;T>20000 C Spherical shock wave propagating at sound velocity P(bars) «13Ep/R3; Ep(mJ); r(mm) A «15x10 mm/s Localized mechanical rupture for small radius P > yield strength of tissues Ne : Electron number density A : Shock wave velocity Tp : Pulse duration wg : Spot radius

26 63 2«2.4 Photoablative Processes Tissue ablation is the basis of much laser surgery. It can involve many forms of material removal, from evaporative to explosive. Most ablative processes using visible or infrared lasers are thermal in nature. However, tissue can be removed by mechanisms involving plasma formation, and photochemical processes play a role in the ablation of tissue by pulsed ultraviolet excimer lasers. Since UV radiation is strongly absorbed by most biological molecules as shown in Figure 2.3 and Table 2.1 in a band between 250 and 350nm, absorption coefficients as large as 104 cm"1 are common and absorption depths are consequently very small, a few /xm at most. This feature has recently been exploited experimentally to produce well defined, non-necrotic photoablative cuts of very small width («50 /xm) by exposure to excimer lasers at several U.V. wavelengths, with short pulses (»10 ns) focused on tissue ( 108W/cm2). Similar sharp cuts (2-3/xm) with minimal thermal damage can also be obtained with the fourth harmonic of the Nd-YAG laser at 266nm? the excellent cutting quality is due to the high spatial quality of the beam [13]. This process may prove to be unique in its ability to produce sharp incisions with minimal thermal damage to adjacent normal structures. Table 2.5 summarizes the pathway of the processes involved. Photoablative techniques with excimer lasers have been applied to various microsurgical models such as skin removal and ablation of atheromatous plaques in human vascular tissue in vitro. But a promising area appears in the treatment of several ocular disorders because of the potential of changing the eye's refractive

27 64 Table 2.5 Physical principles of laser photoablation Short u.v. laser pulse ( 10 ns ) focused on tissue I «108 W/cm2 Strong absorption in the u.v. Absorption depth-l/m (6ev) ( Proteins; amides; peptides) Promotion to repulsive excited state Photodissociation Desorption No Necrosis

28 65 power (e.g., reducing myopia) by making radial corneal incisions. Ablation of corneal stroma with 193nm radiation seems to produce most precise cuts, as narrow as 20/xm, without the ragged edges produced at 248nm [17]. Further investigations are in progress, in particular to answer questions about possible mutagenic or carcinogenic effects in the interaction with DNA in neighboring cells; this is of the utmost importance because,in vivo, UV photochemistry may not only alter DNA bases but may also modify RNA structure photochemically, as well as enzymatic proteins and perhaps membranes; hence it may eventually change cell functions. Further work is needed to demonstrate whether photoablative techniques might become useful as microsurgical tools. A comprehensive analysis and comparison of different biomedical applications of lasers are presented. The photophysical steps of the process involved have been emphasized. Four groups of interactions are distinguished according to their radically different tissue reactions, which depends on the duration of irradiation. The review on PCT, pulsed lasers in the breakdown mode, or U.V. photoablative techniques has been made to demonstrate the versatile potentiality that lasers may offer in photomedicine in the near future.