Photophysical Processes in Recent Medical Laser Developments: a Review

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Photophysical Processes in Recent Medical Laser Developments: A Review

Article in Lasers in Medical Science · August 1986 DOI: 10.1007/BF02030737

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JEAN-LUC BOULNOIS Quante/, BP 23, 91941 Les Ulis-Orsay-Cedex, France Abstract. A single diagram, encompassing most medical applications for all types of laser in current use, forms the basis of this review of recent medical developments. Emphasis is placed on the physical processes that govern different microscopic mechanisms of laser-tissue interaction. Four distinct photophysical groups are considered in a general classification of these specific modes of interaction: for continuous wave exposure, the photothermal and the photochemical transformations; and, for pulsed irradiations, the electromechanical and the photoablative processes.

The principal photobiological interactions ultraviolet lasers (13, 14 and W.S. Grundfest et between non-ionizing electromagnetic radiation al, unpublished observations) constitute yet and living tissues have been extensively another mode of tissue action, which has been reviewed, for radiation in both the submillimetre labelled photoab/,ative interaction. part of the spectrum and in the optical region This paper reviews the photophysical (1-7). For laser radiation, the unique principles governing these applications. characteristic of monochromaticity of the Emphasis is placed on the microscopic incident field, with the ensuing spatial and mechanisms that control various processes of temporal coherence of the emission, is finding laser energy conversion. The review starts with increasing use in medicine, in both diagnosis and the basic observation that these seemingly therapy. Together with the biological response different interaction modes share a unity (15). of the irradiated tissues, these characteristics Owing to molecular saturation effects, optimal determine the specific modes of interaction (i.e. laser parameters associated with a given photo the various processes of conversion) of the medical process can be arranged into four incident electromagnetic energy within distinct families that share a single common biomolecules. datum: a total specific energy dose between 10 2 At present several uncorrelated mechanisms and 1000J/cm • A single variable distinguishes are used in laser photomedicine. The high power these processes - namely, the exposure time densities reached on sub-millimetre spot sizes sufficient for delivery of the foregoing energy provide spatially localized heating used in the dose. thermal mode of interaction, forming the well From these findings, I propose to classify the known basis of all surgical applications (3-5, 7, laser-tissue interaction modes into four groups, 8). The so-called photochemical interaction distinguished by order of increasing time-scale: mode, corresponding to the matching of laser electromechanical interaction (lOps to lOns frequencies with the specific excitation bands of pulses); photoablative interaction (lOns to chromophores, molecules or photosensitizers of 100 ns pulses, ultraviolet); thermal interaction precise cellular structures, has recently opened a (1 ms to 10 s exposure, quasi-continuous wave); spectacular field of applications in photo and photochemical interaction (10 s to 1000 s, dynamic therapy (9-11). The particular continuous wave). combination of temporal coherence in the generation of ultrashort pulses and high peak powers, together with the ability to focus laser PHYSICAL PROCESS radiation (spatial coherence) constitutes the Review of the laser photomedical damage essence of the optically induced dielectric break chart down mechanism used in the electromechanical mode of interaction (12). The recently introduced Owing to the finite number of individual cells to techniques of photodecomposition with pulsed be treated, whether in chemotherapy or in

Paper received 28 August 1985 Lasers in Medical Science I.bl 1 1986 © Bai//i6re Tindall 48 J.-L. Boulnois

' ' ' ' ' 1012 ELECTRO- ' ', 'MECHANICAL', ~ ',, ' ' ' x~ ',,, ' PHOTOABLATIVE ', ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' ' '~'v~ '1--c. '~,,, ,~~ '

Interaction time (s)

2 Fig. 1. Medical laser interaction map. The ordinate shows the irradiance (in W/cm , on a logarithmic scale), and is commonly labelled the power density. The abscissa shows the interaction time. The diagonals show several lines of constant ffuence (in J/cm2). The boxed areas, labelled electromechanical, photoablative, thermal and photochemical (see text) enclose points that correspond to more than 50 experimentally determined optimal variables obtained from most published reports of clinical and experimental applications of lasers in medicine. Nd-YAG, neodymium-doped yttrium aluminium garnet laser; XeCI, xenon chloride laser, ArF, argon fluoride laser; KrF, krypton fluoride laser; Ar, argon laser; Kr, krypton laser; C02, carbon dioxide laser; Lava, laser-assisted vascular anastomosis; He-Ne, helium-neon laser; HP.D, haematoporphyrin derivative. RF, radio frequency; ps, picosecond; ns, nanosecond; opht, ophthalmology; ENT, otorhinolaryngology; gyn, gynaecology; gastr, gastrology; dermato, dermatology; hepat, hepatology. radiation therapy, the determining parameter is (c.w.) lasers, in that time-constants are substan the dose of reactants supplied. For an incident tially different. For c.w. operation, when the radiant flux, the reactants are pho~ons and the pulse time-constant is of the order of the thermal energy dose supplied per unit area (the energy diffusion time or the scattering life-time, 2 fiuence, measured in J/cm ) can measure the damage phenomena are controlled in depth by macroscopic transformation or the 'biological irreversible thermal effects or chemical trans damage' in the general sense (thermal, chemical, formations. For pulsed operation (picosecond or mechanical or electrical) caused to the exposed nanosecond regimes) time-constants are so short and reacting tissues. that radiant electric-field effects predominate in Damage processes in pulsed systems differ a zone of extremely small extent. from those associated with continuous wave A chart that gathers together published

Lasers in Medical Science 1986 © Bailliere Tindall Photophyslcal Processes in Medical Lasers 49 information about the medical uses of lasers (15) irreversibility of the transformation achieved. is shown in Fig. 1. The boxes enclose the points Following this approach, a microscopic model that correspond to more than 50 experimentally based on the foregoing two-step process is determined optimal variables, covering most proposed, assuming a three-level system (15): photomedical applications for a large variety of first, a resonant absorption at the wavelength widely used lasers, such as neodymium-yttrium corresponding to the incoming photon energy; aluminium garnet, argon, krypton, carbon and secondly, a decay towards the lower excited dioxide, excimer (KrF, ArF, XeCl), and dye or state, with a subsequent trigger of a biological helium-neon lasers. Two major features are process and an associated quantum yield. The displayed on this chart. model establishes that molecular transitions First, the data are not scattered uniformly involved in resonant interactions with the laser but, rather, are approximately aligned over 12 field should be saturated. It provides a orders of magnitude on a diagonal within the reasonable estimation of the experimental value 10-1000J/cm2 fluence band. This reciprocal of the energy fluence shown in Fig. 1; and it correlation between intensity and time over a clearly shows this energy to be approximately wide range demonstrates that the specific constant and comparable to the saturation energy dose required to achieve a laser-induced energy of the excited transitions (15). biological transformation is nearly constant. In some sense, these findings extend the Consequently, time - precisely, the time of Bunsen-Roscoe reciprocity law of photo exposure during which this energy dose is chemistry, which states that as long as the delivered - appears to be the single parameter product of the irradiance and the time of distinguishing the transformation process exposure is the same the photochemical effect entirely. will be the same (16). Of course, in exposed Second, four separate groups of transforma biological systems a certain degree of reciprocity tions, which share this common fluence, can be failure is to be expected because repair reverses distinguished, along a diagonal on this chart, some of the radiation-induced damage (17). This according to the duration of interaction: they departure from reciprocity should be related to correspond precisely to the characteristic time the scatter of the data in Fig. 1 (J .L. Boulnois, scales of the respective photobiological damage unpublished observations). involved. Thus, the scheme shown in Fig. 1 and the This distinction into four photomedical associated microscopic model advantageously families serves as the basis for a classification reveal the underlying unity between different into four photobiological laser processes that are photomedical laser applications. A general class analysed in this review: the quasi-continuous ification of laser~tissue interactions is proposed ~ave irradiations - thermal and photochemical below, based on the mapping and the time-scale processes; and the short-pulse regimes - distinction. electromechanical and photoablative processes. However.. this scheme first requires theoretical Analysis of the different interactions confirmation. At the macroscopic level, to achieve a host response, which might vary with the level of Thermal interaction biological organization affected, the laser field should provide a measurable energy dose. Part All surgical applications for lasers, whether in of this energy dose should be converted within cutting or in haemostasis, rely on the conversion the excited biomolecules, so inducing a of electromagnetic energy into thermal energy. 'biological damage' that results, ultimately, in a This is achieved through focusing a beam onto specific excitation of metabolism. In this two spot sizes a few micrometres or millimetres wide: step modelling, the energy conversion path such collimation is possible because of the determines the link between the photophysical spatial coherence of lasers which can, hence, step and the biological step in the response. supply high densities of energy, providing Microscopically, the photo-induced alteration spatially confined heating of target tissues, of individual target macromolecules would then which results in thermal injury, tissue removal require a finite number of precise energy quanta or control of bleeding. The choice of wavelength per molecule, the number being determined by and tissue determines the depth of penetration scattering and absorptive properties of the and thus influences the interplay between tissue irradiated tissue as well as by the degree of removal and haemostasis.

Lasers in Medical Science 1986 © Bail/iere Tindall 50 J.-L. Boulnois

In fact, a vast majority of therapeutic absorption properties of the medium may applications for lasers take advantage of their influence the wavelength selection and, to some capability for some spatial control over the extent, the depth of penetration; however, the degree and extent of tissue injury. The characteristic heating effects are largely characterization of the photothermal biological controlled by molecular target absorption, response following laser irradiation depends, essentially from free water, haemoproteins, pig however, on the structural level at which it is ments (e.g. melanin) and other macromolecules targeted. such as nucleic acids and aromatics. The photo At the microscopic level, the photothermal physical parameter of interest, the absorption 1 process originates from bulk absorption coefficient a (cm- ) which measures the 1 occurring in molecular vibration-rotation bands characteristic absorption length, 0:- , is plotted or, perhaps, in the vibrational manifold of the in Fig. 2 over a wide spectral range for several lowest electronic excited state, followed by sub primary elementary biological absorbers (water; sequent rapid thermalization through non-radia oxyhaemoglobin - Hb02; melanin). tive decay. The reaction with a target molecule The data are a compendium of properly A proceeds in two steps: first, the absorption of a normalized values (J.L. Boulnois, unpublished photon of energy, hv, promoting A to a vibra observations). Most organic molecules absorb tional excited state A*; and second, an inelastic very strongly in the ultraviolet range; water scattering with a collisional partner M, typically reaches l06 cm-1 in the far u.v. belonging to the surrounding medium. On (100 nm). Proteins, which constitute about colliding with A*, M increases its kinetic energy 15-20% of all cells, also absorb in this spectral into M' by carrying away the internal energy region, usually with a peak around 280 nm. released by A•. The microscopic origin of the Consequently, penetration depths in the u.v. are temperature rise results from the amount of extremely small (fractions of µ.m). One notices energy released to M'; the two-step reaction can that Hb02, predominant in vascularized tissue, be schematically represented as: also absorbs in other visible bands (green and yellow), but exhibits a cut-off at about 600 nm. Absorption: A+hv-~A· By contrast, the absorption spectrum of (vibration-rotation) melanin, the basic pigment of the skin and by far Deactivation: A•+ M(E) -----. A+ M'(E+ flE) the most important epidermal chromophore, increases monotonically across the visible range For completeness, it should be mentioned that towards the u.v. A general feature of most bio in normal conditions the kinetic energy per molecules is their complex band structures in molecule, KT, is about 0.025eV, while thermal the high-energy part of the visible spectrum (o: 2 1 lasers such as carbon dioxide, neodymium ranging from approximately 1 to 10 cm- ). yttrium aluminium garnet (Nd-YAG) and argon Infrared (i.r.) radiation, on the other hand, is have corresponding photon energies between absorbed essentially by water with increasingly five times larger (C02: A.=10.6µ.m, e=0.12eV) stronger bands towards longer wavelengths, and 100 times larger (Nd-YAG: A.=1060nm, and typical absorption depths as small as 10 µ.m e=l.17eV; Ar: A.=514nm, e=2.4eV). Two in the far i.r. In Fig. 2, a spectral 'therapeutic factors contribute, then, to thermal efficacy: (a) window' (19) is delineated between 600nm and the rather high probability of deactivation of the 1200 nm: in this range, radiation penetrates excited state A* which, measured in terms of a tissues with less loss because of weaker collision cross-section (18) has values around scattering and absorption, thereby offering the 18 2 17 10- cm to 10- cm2; and (b) the extremely large possibility of reaching deep targets. number of accessible vibrational states of most The wavelengths corresponding to the most biomolecules (103 to 105): consequently the widely used photomedical lasers are also shown channels available for de-excitation and thermal in Fig. 2. C02 lasers yield immediate vaporiza conversion are numerous, and the process is tion in tissues with a high water content and, highly efficient, provided that the durations of hence, they have excellent cutting effects, the laser pulses are properly selected. especially at high power densities ( -10 2 In contrast to other photobiological laser kW/cm ), producing minimal necrosis but poor processes in which the choice of photon energy is haemostasis. By contrast, Nd-YAG laser usually selected to access a specific reaction radiation is capable of deeper penetration, with channel, the biological effects of heating to first thermal exchanges taking place in the bulk, and order are non-specific. The scattering and with most of the absorption occurring with

Lasers in Medical Science 1986 © Bailliere Tindall Photophysical Processes In Medical Lasers 51

Fig. 2. Absorption coefficient spectrum of water, haemoglobin (Hb02l, melanin and haematoporphyrin derivative (HPD). together with the wavelength positions of the most widely used medical lasers. hypervascularized and compactly connected The first mechanism by which tissue is tissue (liver tissue, for example, absorbs much thermally affected is by molecular denaturation more than stomach tissue); the cutting effect is (of, e.g., proteins, collagen, lipids, haemoglobin). less marked but the haemostatic properties are For completeness, Table 1 summarizes the widely recognized. In the visible spectrum, a temperature ranges of successive transforma host of lasers such as argon, copper, Nd-YAG tions (20). (second harmonic) or dye lasers show simul taneous interactions with haemoglobin, melanin Table 1. Physical principles of photothermal and other organic compounds, whereas gold processes: Conversion of electromagnetic radiation into heat increases the tissue temperature lasers fall just outside the Hb02 absorption cut off. Dye lasers are attractive since their tun Temperature Effects on tissue ability can be advantageously used to match 43-45°C Conformational changes particular absorption bands of specific chromo Retraction phores. Being strongly absorbed by haemo Hyperthermia (cell mortality) globin, by red globule pigments and by melanin, 50°C Reduction of enzyme activity argon laser radiation is used in subcutaneous 60°C Protein denaturation vascular coagulations. The u. v. spectral region is Coagulation fairly well covered by excimer lasers (XeCl, KrF, 80°C Collagen denaturation ArF) which are powerful sources, but the fourth Membrane permeabilization harmonic of the Nd-YAG laser might also prove Carbonization to be interesting. 100°c Vaporization and ablation

Lasers in Medical Science 1986 © Bailliilre Tindall 52 J.-L. Boulnois

At around 45°C (hyperthermic range) one being x -1.4Xl0-3 cm2/s, heat diffuses to0.8mm observes a tissue retraction (or shrinkage) re in 1 s. From this relationship, one constructs the lated to macromolecular conformational (wavelength-dependent) relaxation time r which changes, bond destructions, and membrane alt measures the thermal susceptibility of irra erations. Beyond 60°C is the range of protein diated tissue: denaturation: it results in tissue coagulation (2) which is exploited either to destroy small tumours or to stop haemorrhage (haemostasis). For comparison purposes, a summary of thermal This combination of thermal retraction and relaxation times corresponding to various haemostasis induces closing of vessel lumens, biological media is presented for several wave which can subsequently be obstructed by a lengths of interest in Table 2. Thermal properties blood clot (thrombosis). The temperature limit at are assumed to be those of water (21), whereas which tissues become carbonized is about 80°C. collected optical data originate from different Vaporization occurs beyond 100°C, predomin published sources (21-23). The respective roles antly from heated free water; the large heat of of C02 lasers in tissues with a high water vaporization of water (2530J/g) is advant content, and of argon lasers in tissues with a ageous, since the steam generated carries away high blood content are clearly evident. These excess heat, thereby preventing any further d_ata are also consistent with published tissue temperature increase of adjacent tissue. irradiation times which vary between 1 ms and Vaporization together with carbonization yield 1 s, depending on the pigmentation, tissue decomposition of tissue constituents. Laser constituents, width of affected zone and depth ablation, which is used to make incisions or of penetration (5). resections, serves as the basis of all photo The water relaxation time at 10.6 µ.m suggests surgical or photocoagulative applications. an interesting operation mode for C02 lasers in These irreversible structural changes reflect microsurgery which could be called 'real super tissue thermogenesis caused by deep thermal pulse mode'. The high temperatures needed for conduction of the absorbed incident power. The phase change (steam formation), without major problem with removal of material is to appreciable heating of adjacent tissues, are adjust the duration of laser exposure in order to reached only when the exposure duration is minimize thermal damage to adjacent zones so shorter than r: consequently, by pulsing the as to obtain little necrosis. The scaling para laser with pulses shorter than 200 µ.s, it should be meter for this time-dependent problem is the so possible to vaporize tissue directly and still called thermal re/,a,xation time, r, obtained by obtain an extremely small amount of necrosis. equating the characteristic absorption length Typical pulses of lOmJ/50 µ,s, from a C02 laser a-1 to the characteristic thermal diffusion operating at 50 Hz, would vaporize 300 µ,m spots length, L. This latter quantity is proportional to and would cut with a velocity larger than the square root of time together with a factor lOmm/s. involving the tissue diffusivity x (cm2/s), namely In a similar approach, attempts to control a lumped physical parameter characterizing the tissue temperature for selective photothermoly material's thermal response (thermal conduc sis (7) have been made recently with a high tivity, specific heat and density). The diffusion power Nd-YAG laser (400 W), operating in a length, L, is defined through a relationship of repetitively pulsed or burst mode (24); a fast the form: repetition rate (up to 100 Hz) is chosen such that, for a specific tissue with a characteristic cooling L 2 -4xt (1) rate, a constant average energy is permanently For example, the diffusivity of liquid water stored in the material: a constant average temp-

Table 2. Thermal relaxation times (in s) for various biological media at wavelengths for different lasers

Biological Ultraviolet Argon He-Ne Nd-YAG C02 medium (200nm) (488nm/514nm) (633nm) (1060nm) (10.6µm) Water aox10-6 2ox103 o.2x10-3 Oxygenated blood o.2x10-3 15x10-3 9 5 Plaque 0.5 45 90 Melanin 0.1 0.5 4

Lasers in Medical Science 1986 © BailliBre Tindall Photophysical Processes in Medical Lasers 53

P(t)

lii ppk ~ 0..

0 Time(t) P(t)

p pk

Pav2 -- -

0 Time(t)

T(t)

T volatilization

Tav2 "'!!? ::i ! 8. E Tav1 ~ T coagulation

0 Time(t)

Fig. 3. Scheme showing the temporal control of average tissue temperature Tav by adjustment of the repetition rate in a fast-pulsed laser (constant peak power Ppk; adjusted average power P8v)· erature is thus achieved. By adjustment of the brings the local tissue temperature up to around pulse repetition rate, the tissue temperature can 60°C. Such processes are typical of the photo be maintained with precision above the coagula coagulation treatment of diabetic retinopathy, tion temperature and below the carbonization retinal detachment (25, 26) or choroidal lesions limit (24). Fig. 3 shows schematically the (27). working principle of this temporal technique, Power densities in the range 104 W/cm2 are which provides a useful extra degree of freedom routinely used with C02 lasers for consecutive to complement current photosurgical methods. irradiation pulses of, at most, 1 s eatji, in For completeness, I shall now briefly review laryngeal microsurgery (28, 29) or in gynaecolo the well established applications based on photo gical treatment (30). thermal interactions. Various gastrointestinal bleeding lesions are In ophthalmology, short interaction times treated with Nd-YAG lasers operating in the (10-100 ms) are commonly used with argon haemostatic mode (31). Resection of various lasers (1-4W) or krypton lasers (1-3W), which tumours - in gastroenterology, urology, trache are focused on spot sizes of 30-100 µm. This obronchial endoscopy and in general surgery - 5 2 results in power densities up to 10 W/cm , and is usually done with the Nd-YAG, C02 or argon

Lasers in Medical Science 1986 © Bailliere Tindall 54 J.-L. Boulnois

lasers, at typical fluences of 100-1000J/cm2 and vessel walls have been proposed recently (23, 21, with corresponding irradiation times of the 47), to predict the threshold laser powers order of seconds (32-37). necessary to reach 100 °C at the fibre tip; a Dermatology is another field in which the recent review has been devoted to the subject argon laser or the copper (510nm or 578nm) (48). laser prove useful, particularly for cutaneous In a related development, extensive interest is coloured lesions or exophytic lesions (38). currently generated by the use of low-power Selective thermal damage to endogenous tissue thermal lasers for vessel welding. LaseT"-aSsisted chromophores (e.g. haemoglobin or melanin) vascular anastomosis (LAVA) may become a may induce minimal necrosis to healthy major procedure in microvascular surgery (49). adjacent tissue. This type of subcutaneous Although initial repair work on 1 mm vessels vascular coagulation technique, used for was done with Nd-YAG lasers (50), 750mW treating thrombosis of superficial dysplasia argon lasers have been shown to be efficient in such as portwine stains, requires a careful coagulating blood to form an adherent tensile control of the energy dose with c.w. lasers, to sleeve for the anastomosis of small vessels (51).

minimize scarring (39). More recently, pulsed Continuous-wave C02 lasers, operating in the copper vapour lasers at 578nm have been range 50-200mW in a series of short 0.1 s investigated (40) in an operating mode similar to exposure bursts along the anastomotic line, the foregoing 'superpulse mode': pulse duration have also recently proved effective (52-54). In an was about 60 ns (while r for haemoglobin is experimental model, rat femoral arteries about 15ms) and the energy was around 3mJ. (0.8-1.2mm) were exposed at 35J/cm2 fluence The process was presumably thermal (possibly levels to 150 µ.m spot sizes: a first brief pass is via selective heating of blood vessels), owing to made around the entire vessel for sealing

the fast deactivation of the Hb02 excited state purposes and a second for supplementing the and the high average power delivered by such bond strength (stay sutures were used for lasers (with repetition rates of -5 kHz). adjusting the edges). Sequential histological Several recent attempts to broaden the scope findings show the appearance of an initial coa of existing clinical procedures make use of the gulation bond, coapting the edges, followed by a thermal effect with new, drastic approaches. healing process with proliferating intimal cells Four such developments seem promising. and neovascularization at the laser anastomotic The first of these is percutaneous transluminal site over a 2-3 week recovery time. Besides laser angioplasty, used in the treatment of reducing the operating time considerably LAV A, thromboembolic and occlusive vascular dis when developed into an established technique, orders. One of the main objectives is to obtain may offer attractive advantages: it seems to disintegration of atheromatous plaques or perform better seals, with no leakage, in a thrombi with minimal irreversible damage to relatively non-traumatic manner. Potential artery or vessel walls. The technique for clinical applications are in reimplantation, tissue removing obstructions (41) was first transfer, revascularization, circulation improve demonstrated with 4 W argon laser radiation, ment, or in other conditions in which sealing of coupled with a special optical-fibre catheter (42) leaking or damaged vessels is necessary. before being applied to human arteries (43). Waveband interaction is a third barely Although wall perforations have been reported explored aspect of laser interaction that is with Nd-YAG systems (44), the role of perfusate currently attracting attention: a synergistic absorption has been stressed (45). A newly effect is possible when this technique is used for proposed technique, for coaxial positioning of a irradiating a biological sample with two mono fibre-optic balloon catheter within arteries chromatic wavelengths (55). In related clinical together with a flowing perfusate, has demon investigations, various partial resections of the

strated that conditions could be found in which liver have been done with C02 and Nd-YAG no arterial wall perforation or distal emboliza lasers in combination (56). In a small series of tion took place. Angiograms of femoral arteries four patients, successful partial resections of the in vivo, with 4 mm length and 1 mm internal dia liver were achieved with a prototype handpiece

meter stenosis, show efficient recanalization that combined a focused 60 W C02 laser and a after three exposures at 12W (36 OOOJ/cm2 defocused SOW Nd-YAG laser: the necrotic fluence) (46). Owing to the strong interest in zones were significantly reduced, and defocused these procedures, extensive modelling of Nd-YAG radiation appeared to be an efficient thermal and optical interaction with plaques and haemostatic tool (57). Similarly, in lung cancer

Lasers in Medical Science 1986 © Bail/iere nnda/I Photophyslcal Processes in Medical Lasers 55 treatment, bronchial obstructions are first endo shows that radiation distribution is dominated scopically cleared with an Nd-YAG laser, and a by scattering in this wavelength range (59). If radioactive irridium wire is subsequently spectrally adapted chromphores are introduced inserted, causing localized necrosis of the neo and selectively retained in specific cellular sites, plastic tissue (58): damage to healthy tissue narrow-band irradiation can trigger selective seems to decrease in this laser-radiation photochemical reactions in vivo with combination. Photosynergism, when extended subsequent photobiological transformations; to other dual wavelengths, could certainly be hence, energy can selectively be delivered to used in a preconditioning treatment, through a deep target cells. Normal processes such as combination of infrared or visible laser exposure melanogenesis can thus be photoactivated in together with ionizing radiation, for example, vitiligo Psoralen Ultra-violet-A (PUVA) therapy. thereby offering great advantages for the Photodamage to abnormal cells can also be in treatment of tumours. duced such as photochemical modification of The fourth exi>erimental development, nucleic acids in psoriasis PUVA therapy by flashlamp pumped pulsed dye lasers, emitting in furocoumarins, or cytotoxic photosensitization the visible range between 450 and 600 nm and therapy by haematoporphyrin in tumour cells. operating at 5-30 Hz repetition rates, are being A chromophore compound capable of causing investigated in conjunction with a fibre delivery light-induced reactions in molecules that do not system in the fragmentation of kidney stones, at absorb light may be called a 'photosensitizer' (9, fluence levels in the range 20-200J/cm2 (40). 60). Following resonant excitation by a mono Short pulses, lasting from 10 to 400 µ,s, with chromatic source, the photosensitizer undergoes energies ranging from 10 to 200 mJ, locally heat a series of simultaneous or sequential decays, an extremely small volume of the kidney-stone which result in intramolecular transfer reactions porous matrix. This heat is rapidly conducted to and ultimately culminate in the release of highly the interstitial water that is confined inside the reactive cytotoxic species, which cause irrever matrix microcavities, so bringing it to boiling sible oxidation of some essential cellular point: the subsequent pressure rise creates the component and destroy the affected host tissues desired localized fragmentation of the stone. (61). The essence of photochemical interaction Since the water heat of vaporization is about (which should, rather, be called photosensitized 2530J/g, pulses of 250mJ will heat up a focal oxidation) lies in the 'assistance' of the volume at most equal to 0.1 mm3 which, for exogenous chromophore receptor which acts as a typical small kidney stones (a few mm in dia photocatalyst: it is first activated by resonant meter) requires. exposure times of tens of absorption, thereby storing energy in one of its seconds before the fragmentation is complete. excited states; only when it deactivates can a Besides their cost-effectiveness and their reaction take place, but with a reactant that is possibility of operating at higher repetition not the photosensitizer. rates, pulsed dye lasers offer a major advantage Most photosensitizers currently used are because their broad emission spectrum covers a organic dyes and therefore they exhibit the large number of absorption bands for the usual electronic structure of singlet states (total constituents of calculi, which are essentially electron spin momentum S=O) and triplet calcium or magnesium salts of phosphates or excited sta~s (S=l) where the triplet energy is oxalates. accordingly smaller than the corresponding excited singlet state. Each electronic state is further subdivided into a large manifold of vibra Photochemical interaction tional states. At the very end of the exposure scale, for The sequence of molecular reactions can be extremely long interaction times ( -1000 s) and separated into several stages: excitation; decay; 2 low power densities (below 1 W/cm ), lies the substrate reaction or reactant formation; and family of photochemical transformations. In oxidation. With Sas the sensitizer, the reaction most instances, the basic physical channels for kinetics can be schematically separated into two photochemical interactions between laser radia mechanisms which depend on substrate involve tion and cellular structures are only partially ment in the crucial stage of reactant formation elucidated. (62). Table 3 summarizes these pathways, and However, within the 'therapeutic ·window', Fig. 4 shows the energy level diagram of a widely radiation penetrates rather deeply into most used sensitizer (HPD), the absorption spectrum tissue. In fact, careful control of the laser dose of which is given in Fig. 2 (62).

Lasers in Medical Science 1986 © Bsilliere Tindall 56 J.·L. Boulnois

Table 3. Photosensitization kinetics in Type I and Type II mechanisms, and possible carotenoid protection

Photosensitizer excitation Resonant excitation 1. Singlet state absorption 1S+h11 •is* Decays 2. Radiative decay (fluorescence) lS* • 1S+h11' 3. Non-radiative singlet decay is* •is 4. Intersystem crossing lS* •as* 5. Triplet state decay as* •is 6. Triplet phosphorescence decay as• • 1S+h11" Type I mechanisms Free radical derivations 7. Hydrogen transfer as*+RH • SH"+R· 8. Electron transfer as*+RH • s·-+RH"+ Reactant formations 9. Hydrogen dioxide SH"+ao2 • 1s+HOii 10. Superoxide anion s·-+ao2 •1s+o2 Oxidation Type II mechanisms Reactant formation 7. Intermolecular exchange as•+ao2 • is+io~ Oxidation 8. Cellular oxidation io•2+X •x(O) Carotenoid protection 1 1. Singlet oxygen extinction 0*2+CAR ... ao2+aCAR

1S, ground singlet state of sensitizer; 1S*, excited singlet state of sensitizer; 3S*, excited triplet state of sensitizer; 3 , 1 0 2 ground triplet state of oxygen; 0*2, excited singlet state of oxygen; RH, substrate; X, cellular target; X(O), oxidized cellular target; CAR, carotenoid.

Of the electronically excited derivatives of the promoted to its singlet state 10; (first electronic 3 1 photosensitizer, the excited triplet state S* is excited state at 7900cm- ). The reaction be 3 3 generally endowed with the greatest reactivity tween S* and 0 2 corresponds to an electronic because of its favourable spin configuration: exchange collision with a spin rearrangement furthermore, its relatively long lifetime (1 ms to which is all the more probable because it is a a few seconds), when compared to the 1S* quasi-resonant interaction without a net change fluorescence lifetime (0.1-lOOns, depending on of spin (J .L. Boulnois, unpublished observa· the solvent), increases its probability of inter· tions). Owing to its peculiar electronic orbital acting with other molecules (62). A prerequisite and spin configuration, 10; is highly for efficient photodynamic sensitizers is that electrophilic: it efficiently oxidizes electron-rich they possess a high quantum yield 7/ of inter sites in neighbouring biomolecular targets. It is 3 system crossing: the usual value (71 -10- ), which believed that target sites are proteins, nucleic is small when compared to the fluorescence acids and mitochondria, presumably located on yield, is often the rate-limiting factor (15). cell membranes (61, 62). Singlet oxygen lifetimes In the Type I mechanism the 3S* species are are rather long, ranging from 5 to 50 µ,s in most involved directly with the substrate from which solvents, with a ten-fold increase in deuterated neutral or charged free radicals are derived, solvents. Consequently, the limited diffusion giving rise to a wide variety of possible further length of 10; maintains selectivity on a cellular reactions: e.g. promotion of chain processes by basis. In fact, this photodynamic action, in interaction with other substrate targets, or reac· which molecular oxygen is consumed in a photo· tions with oxygen to yield peroxidized products sensitization reaction, has been known by that attack the substrate. chemists and biologists alike for more than 50 In the Type II mechanism, the 3S* species are years (9, 63). involved in transfer reactions to molecular In general both types of mechanism may occur 3 oxygen (triplet 0 2 ground state) which is competitively, the efficiency of either

Lasers in Medical Science 1986 © Bailliilre Tindall Photophysical Processes In Medical Lasers 57

Energy cm-1 ev

4 HPD 30000

1 3 5 2----

20000

lntersystem crossing

10 000 II 1 ID·

I I / E I / c elc, I I / Phosphorescence ~ ii

,s 0 ~-___., _____ ------

Fig. 4. Energy level diagrams of haematoporphyrin derivative (HPD) and molecular oxygen. mechanism being controlled by the nature of S, recently demonstrated to be highly . effective by the relative concentration of oxygen and sub (65). strate, and by rate constants for the Several points can be made about the efficacy substrate-sensitizer and oxygen-sensitizer of photosensitized oxidation. First, the photo interactions. Since 0 2 is much less soluble in sensitizer is a 'true' photocatalyst since it is water than in most organic solvents, decreasing generally not consumed and is returned to its the solvent polarity can be expected to favour singlet ground state 1S (62, J.L. Boulnois, 'fYpe II sequences; on the other hand, the unpublished observations). Second, all decays at complexation of the sensitizer with the substrate one point or another, whether from singlet or prior to irradiation, which often takes place in triplet states, involve inelastic processes, some biological systems, enhances the probability of times with large cross-sections, all yielding a 'fYpe I mechanisms (64). For completeness, it vibrational excited state of the 1S ground state should be mentioned that two-photon laser with efficient coupling to the surrounding production of porphyrin radicals has been thermal heat bath; even radiative processes

Table 4. Physical principles of photodynamic therapy bacteria protector, namely the carotenoids: these agents protect photosynthetic systems Administration of photosensitizer S against photosensitization by their own chlorophyll (69). Their molecular protection Selective tumour retent!n of photosensitizer mechanism is a complete reversal of the singlet oxygen formation, and consists in 1 quasi Irradiation by monochriatic source (laser) 0; resonant extinction, which produces the non .. toxic triplet state of carotenoids 3CAR, which Resonant excitation of photosensitizer deactivate rapidly (see Table 3). This photo S+hv •s• protective property of carotenoids has been + successfully tested in a therapy against ery Simultaneous or sequential decays thropoietic protoporphyria (70). In view of the rather long irradiation times for photodynamic therapy (typically 1000 s), a basic Intramolecular• transfers Thermal deactivation question may be asked about the photochemical process: could the power density be significantly Productiontf reactive species I raised and the interaction time reduced? Owing I • to the small quantum yield, the number of mole Cyto t mac. action'f'. H yperth ernua . ~'~~~~~~~~~~~~' cules directly decaying towards the ground state 3 Tumour era.di cation . is at least 10 times larger than the number involved in singlet oxygen production. In c.w. irradiation, a large thermal component is thus participate (v'(v or v"(v) and therefore contri associated with the process and only at low bute to heat generation in the bulk. power densities can this excess heat be dissi Owing to its selective targeting, this mode of pated so that the photodynamic effect remains action is attractive in photodynamic therapy dominant. The situation is, of course, different in (10, 11). A summary of the photophysical steps pulse configurations; photodynamic therapies involved in this therapeutic procedure is given in are investigated on 100 ns time-scales, with Table 4. Current experimental approaches to metal vapour lasers, in which the pulse repeti cancer management rely on the selective reten tion rates are adjusted to the characteristic tion of haematoporphyrin derivative (HPD) in thermal relaxation time of the irradiated tissue malignant tissue. Several photosensitizers have (71). long been known, such as fluorescein, berberin Another point to consider is the use of a mono sulphate, tetracycline, but certain ones have chromatic source for irradiation. Currently the been found to have a higher affinity for tumour technique relies on argon-pumped continuous cells than for healthy ones, among which are wave dye lasers which emit about 1 Wat 630 nm, HPD, DHE (dihaematoporphyrin ether) (66), but a white light-source such as a xenon lamp and certain groups of phthalocyanins (67). HPD, with an appropriate filter ( -lOnm) would a mixture of products obtained from haema perform similarly. Although they are not toporphyrin stabilized by acetic and sulphuric adequate for large surface treatments, lasers acids, seems to be the most suitable for clinical offer the advantage of high power densities use primarily because of its selective retention in which may reduce treatment times in localized the tumour and its attractive absorption peak at areas since the question is one of dosimetry (15, 630 nm; HPD kinetics have been characterized 59); however, their major attraction stems from by picosecond fluorescence spectroscopy in their ease of coupling to fibre-optic systems. phosphate-buffered saline, yielding measure Irradiation at the appropriate wavelength ments of fluorescence lifetime ( - 240 ps) and (514nm for superficial tumours and 630nm for 9 1 radiationless transition rate ( - 4Xl0 s- ) (68). deeper lesions) with energy doses ranging 2 DHE has also been shown recently to be poten between 10 and 100J/cm , results in tumour tially useful since it appears to be twice more eradication. Current systems may not provide active than HPD and it has a strong tendency the best medical laser combination, and a single for self-association (66). powerful source might be preferable. High It is important to stress that during the weeks repetition-rate (3 to 5 kHz) metal vapour lasers, following photodynamic therapy with HPD it such as copper (A=510nm, 578nm), gold (628 would be possible to protect patients by using nm), manganese (534 nm) or lead (723 nm) may the natural green plant or photosynthetic supersede current systems, since they offer very

Lasers in Medical Science 1986 © Bailliere Tindall Photophysical Processes in Medical Lasers 59 high average power in the visible range (up to Table 5. Physical principles of laser-induced 20 W) with potentially simpler designs: coupled breakdown to slow scanning devices, they could thus facili tate treatment of large areas with little thermal • Short laser pulse component. focused at target Recent measurements have shown that the increase in tissue temperature induced by photo • Highpowe~ dynamic therapy could contribute to cell des density i J-1012W/cm2 truction (72). Such a role for hyperthermia in t enhancing tumour control has been tested by • High electric field -106V/cm E=(~)CEO monitoring sequential photodynamic treat ments and localized microwave irradiation in • Dielectric break-l vivo (73). The origin of this hyperthermic down (multiphoton E1aser - Eionization component in photodynamic therapy is process) ~ molecules naturally linked to the efficient decays of the 21 3 1 • Plasma formation Ne -10 /cm ; T ) 20 000°C excited singlet state S*: an increase in tissue Free electrons temperature to the 45 °C range may prove to be a 3 useful alternative owing to the large induced • Spherical stock P (bars)-13EP1R ; EP(mJ); wave propagating r(mm) death of tumour cells. at sound velocity A -15X106 mm/s The effectiveness of extremely low-intensity laser irradiation (power 1-5 mW) on biological • Localized J,. mechanical P ) yield strength of tissues tissue has been the subject of controversial rupture for small claims. In particular, clinical investigation has radius covered the so-called wound healing or anti inflammatory properties of red light or near in Ne, electron number density; TP' pulse duration; A, shock wave velocity; w , spot railius. frared sources such as He-Ne lasers (74, 75), or 8 GaAs laser diodes, and the possible stimulation of microcirculatocy effects and other features. 2 Typical energy doses ranged from 1to10J/cm • Presumably, observers have noticed improve high electric fields (106-107 V/cm) comparable to ments but reasonable explanations based on average atomic or intramolecular Coulomb systematic experimental protocols have yet to electric fields; such large fields induce a di be proposed. However, the stimulation of DNA electric breakdown of the target material synthesis has been studied (76), and recently the resulting in the formation of a microplasma, i.e. activation of the enzyme-substrate complex and an ionized volume with a very large free electron transformations of prostaglandins have been density. The shock wave associated with the reported but not unequivocally established (77). plasma expansion generates a localized In all cases the controversy stems from the mechanical rupture for spatial extensions where difficulty in specifying the photochemical the pressure rise is superior to the yield strength channels of the reactions involved. Detailed in of encountered tissue (78). Table 5 summarizes vestigations in this area are badly needed. the overall sequence of physical processes involved. At the microscopic level, the mechanism res Electro-mechanical interaction ponsible for optical breakdown is the massive generation of free electrons. It is possible to In this mode of interaction, a fluence of about distinguish the initiation, which corresponds to 100J/cm2 is delivered to possibly transparent a localized electron 'seeding' by ionization tissues by an Nd-YAG laser in extremely short involving a few electrons only, from the massive time exposures by means of either mode-locked subsequent electron photoproduction. 30ps pulses or lOns Q-switched pulses. The In the initial phase, the ionization mech process is not maintained by linear absorption anisms, in which energies of about 7 to 10 eV and is consequently not thermal. must be supplied to bound electrons to separate Rather, the high-peak-power laser pulse, when them from individual molecules, differ focused at a target, creates high irradiances depending on pulse duration (78, 79). For Q· (approximately 1010 W/cm2 for ns pulses and switched pulses (3 to lOns duration), ionization 1012 W/cm2 for ps pulses) which locally generate is caused by thermionic emission resulting from

Lasers in Medical Science 1986 © Bailliere Tindall 60 J.-L. Boulnois focal heating of the target, where local 'tempera created in the focal volume of the pulsed laser tures' (i.e. specific molecular energies) exceeding beam. This is called laser-induced breakdown of several tens of thousands of degrees are reached, the dielectric medium. all the more rapidly when impurities are The condition for plasma growth and sustain present. For very short mode-locked 20 ps pulse ment is that losses such as inelastic collisions or trains, molecular ionization is obtained by a non free electron diffusion do not quench the inverse linear process called multiple photon absorption: Bremstrahlung avalanche (81, 82). A general because of the extremely high irradiance at the macroscopic model which encompasses creation focal spot, photons can add up their energies and loss mechanisms, and which predicts the coherently and provide the ionization energy temporal evolution of all geometric and thermo that produces free electrons. dynamic parameters can be constructed (83, J.L. The threshold for optical breakdown is higher Boulnois, unpublished observations). Fig. 5 for mode-locked ps pulses than for Q-switched ns represents on a logarithmic time-scale the pulses: in air, the threshold for a single 25 ps typical evolutions of the principal macroscopic pulse is about 1014 W/cm2 whereas it is only parameters (plasma radius, electron number 1011 W/cm2 for a lOns pulse (79). In biological density and pressure) following irradiation by a solutions, these numbers are reduced by a factor 25 ps pulse. If the rise time is sufficiently short, of 100, the total energy density delivered in irrespective of pulse duration, a permanent state 2 achieving optical breakdown ( -10 J /cm ) being is reached in about 100 ps or less: the origin is the ultimately the same for a mode-locked pulse so-called 'plasma shielding effect'. As the train and a Q-switched pulse, even though peak plasma number density N. increases, the power density is, on average, 100 times higher characteristic plasma frequency wP' proportional for ps pulses (80). to N:, increases, and so does photon scattering. The key physical element of the electro Consequently, progressively less energy is mechanical mode of interaction is rooted in the coupled from the laser field to free electrons; the massive subsequent photoproduction of extra critical density N; at which incident energy is neous free electrons. The process is termed not converted any further is obtained when the 'electron avalanche growth' or 'inverse plasma frequency wP becomes equal to the pulsa Bremstrahlung effect' (18). In the field of mole tion w of the incident electromagnetic wave (84): cules or ions, free electrons already seeded in terms of the radiation wavelength, this condi absorb incoming photons and convert this tion is written: energy into kinetic energy, thereby increasing N• their velocity: on colliding with neighbouring e molecules, rapid free electrons ionize collision partners when their relative kinetic energy is Here r0 is the classical electron radius sufficient and, hence, this process creates extra (2.8x10-13 cm): for Nd-YAG laser radiation, the 21 3 neous free electrons. upper bound electron density is N; -10 cm- • The detailed microscopic process is basically a When this density is reached (in less than 100 ps free-free absorption (i.e. a transition in which a with a 25 ps pulse), the plasma radius is about free electron is present in the initial and final 50 µ.m, and pressure and temperature have states) which must necessarily take place in the reached their maxima As the plasma shock field of an ion A+ or in that of a neutral atom. wave expands, it cools, and the pressure falls This process can be schematically written: accordingly. For a mode-locked picosecond pulse train with pulses typically separated by 7 ns h11 e A+--. e A+ + + + intervals, the cooling rate between pulses is The particularly important feature of this faster than with Q-switched nanosecond pulses, process is that there is no restriction of the since N. remains approximately constant photon energy: in fact the cross-section is largest throughout the exposure time, but the process is for small photon energy h11, which corresponds qualitatively similar. to an extremely large efficiency of the inverse In the permanent regime, once the critical Bremstrahlung effect (18). Each electron in turn density N; has been reached, the plasma expan absorbs more photons, accelerates, strikes more sion problem is quite similar to the well known atoms or molecules and ionizes them in an 'strong explosion in a homogeneous medium' avalanche process with exponential growth: in a with constant density Clo (85). At time t the wave few hundred picoseconds a very large free reaches a radius R and encompasses a volume 21 3 electron density (typically 10 cm- ) is thus whose mass is 411' R3 Qj3; assuming an instan-

I 3 Growth I Permanent regimen R(JUTI) p(bars) Ne(cm- ) - r---- 105 103

104

103

E=0.5mJ (new pulse) 102 1Q2

Time 2 103 104 10 10 t(ps)

Fig. 5. Temporal evolution of plasma parameters (electron number density N8 , plasma radius R, and pressure p), following a laser induced breakdown with 0.5 mJ energy in a 25 ps pulse (J.L. Boulnois, unpublished observations). taneous release of an energy amount E, the wave retina (80, 86). The real question is: in how short expands adiabatically and the pressure behind a time is the critical plasma electron number this shock wave is proportional to the average density N; reached? 3 energy per unit volume p -EIR • It is then quite First, it should be recalled that when the simple to verify that the shock-front velocity is plasma is highly ionized, the absorption co 312 proportional to R- , and from there to check efficient, being proportional to N;, is very large, that the front radius time evolution is typically in the range l-lOcm-1 (87); 116 6 R - (E/eo) t:2' , whereas for pressure consequently, one should expect a large absorp 215 615 p - (E/e0) t- • These results closely match the tion of the incident beam. In the case of pico data in Fig. 5 for times greater than 100 ps. Such second mode-locked pulse trains, a typical 25 ps temporal scaling laws are essential for pulse contains about 3X1015 photons which are predicting the limits of plasma expansion and absorbed in less than 1 µm because the growth for modelling the spatial extent of injury. rate of N. is extremely large, about 1031 s-1 as Clearly, inside the spherical volume where the seen in Fig. 5: hence the fraction of incident pressure is superior to the yield strength of bio energy transmitted to the retina is small during logical structures encountered, a localized mech optical breakdown, the same sequence of anical rupture will take place, resulting in an processes being reproduced 7ns later. For Q opening or disruption of these structures: this is switched nanosecond pulses, the answer to the the physical basis of new ophthalmic surgical question depends fundamentally on the pulse techniques (12). rise time: if the rise time is of the order of 100 ps, The plasma shielding effect, mentioned above, then the process is quite similar to that for pico is very important in retinal protection during second pulses, but if the rise time is longer pulsed Nd-YAG laser ophthalmic treatments in ( -1 ns) then significant energy transmission is the anterior segment of the eye. Published work possible because the growth rate of N. is not confirms the current considerable confusion sufficient to ensure strong absorption and to about the role of plasma as a shield to protect the overcome scattering of the incident photon flux

(J.L. Boulnois, unpublished observations). It Ablation of material with u. v. lasers has been should be noticed that by coincidence, 100 ps is well investigated in polymer films used as photo approximately the time during which light resists in semi-conductor processing (92, 93) and travels the distance from the eye's anterior takes place at well defined wavelength-depen segment to the retina. Practical prevention of dent fluence thresholds related to the absorption retinal injury in the course of intraocular cross-section of the polymer constituents. surgery is achieved by magnifying the laser Recently, systematic ultrastructural explora· beam and then focusing with a short focal lens: tions of the etched substrate and chemical with a sufficiently large converging cone angle, analysis of the effluent have been done in a irradiance decreases rapidly beyond the focal comparative study between a synthetic polymer point owing to the increased area of exposed sur· and bovine cornea in vitro; remarkably similar face. u.v. photoreactions in both materials have been This photodisruption process is of particular attributed to the similarity of their respective interest in several pathological conditions of the polymeric structure. eye for the non-invasive treatment of capsuloto The photoablative process consists in a photo mies (12, 88, 89), certain iridectomies, the dissociation, i.e. the direct breaking of intra· removal of vitreous strands-or the dissociation molecular bonds in polymeric chains, caused by of opacified membranes which frequently absorption of incoming photons, followed by develop after cataract surgery. Since this tech effluent desorption: in the photon energy range nique is simple it can be used in ambulatory of 5-7 eV, biopolymers sucli as collagen may treatments. Recently, optical breakdown tech dissociate by absorption of a single photon. The niques have been used also in cardiovascular microscopic mechanisms correspond to transi· models (90) to investigate possible disintegra tions of a macromolecule AB that is promoted to tions of atheromatous plaques in small arteries a repulsive electronic state that yields photo· by the use of pulsed photodisruption. Although products A and B. This can be written: promising preliminary results in vitro suggest hv+AB-__,••A+B that this technique is feasible, a main difficulty remaining is the transport within fibres of the What is the probability of such a process taking necessary high peak powers if the model is to be place? For typical photon energies around 6 eV, of any future therapeutic interest. absorption cross-sections for organic polymers 18 2 are rather small (10- cm ), but electronic transi tions that satisfy the Franck-Condon principle Photoablative interaction are known to be significantly favoured (18). One further problem concerns the fate of molecular As already stated, u.v. radiation is extremely photofragments: how do they leave the surface? strongly absorbed by most biological molecules It is currently assumed that desorption is an (see Fig. 2) in a band between 200 and 360nm: important secondary mechanism, but the absorption coefficients as large as 104 cm-1 are dynamics of nuclear motion are not yet clearly common and absorption depths are con ascertained. However, radiative dissociation in sequently very small, a few µm at most. repulsive electronic excited states yields This feature has recently been exploited products whose kinetic energy is necessarily the experimentally to produce well defined, non· difference between hv and the bound-state mole necrotic photoablative cuts of very small width cular energy; not every excited state may ( -50 µm) by exposure to excimer lasers at several achieve photodissociation and a quantum yield T/ u.v. wavelengths (ArF: 193nm/6.4e V; KrF: is thus associated with the process. 248nm/5eV; XeCl: 308nm/4eV), with short The chemical structures of both collagen and 8 2 pulses ( -lOns) focused on tissue ( -10 W/cm ) the organic polymer polymethyl methacrylate (13, 14, W.S. Grundfest et al, unpublished (PMMA) contain monomer units of the same observations). Similar sharp cuts (2-3 µrn) with average molecular weight (100) with one chromo minimal thermal damage are also obtained with phoric group per monomer. Measurements of the fourth harmonic of the Nd-YAG laser at the etch depth per pulse in bovine cornea and in 266 nm (4. 7 eV); the excellent cutting quality is PMMA show remarkably similar behaviour as a due to the high spatial quality of the beam (91). function of fluence (94). Nearly identical etch Control of thermai damage is clinically impor· thresholds are obtained with both materials, the tant since it generally produces undesirable photofragments being methyl methacrylate biological effects. (MMA) and, presumably, amides or peptides

4sers in Medical Science 1986 © Bail/iere Tindall Photophysical Processes in Medical Lasers 63 from the polypeptic triple helix of procollagen. Table 6. Physical principles of laser photoablation With increasing fluence, a steeply rising photo production is observed and a saturation reached Short u.v. laser pulse (lOns) 8 2 when recombination rates overtake photo focused on tissue J-10 W/cm dissociation rates: u.v. absorption at 193nm Strong absorption in the Absorption depth-1 µm being at least tenfold stronger than at 248 nm, u.v. (6eV) (proteins;• etch depths per pulse are inversely proportional amides; peptides) as expected ( - 0.5 µ.m and 5 µ.m respectively). Saturation is achieved at nearly 0.3J/cm2 Promotion to+ repulsive fluence for 193nm irradiation and at 3J/cm2 for excited state 248 nm, when the photon per monomer ratio ~ reaches unity (94). For a monomer with a mole Photodissociation cular weight of 100, this limit corresponds to an •. ablated mass nearly equal to 2X10-4 g per Joule: Desorption this is totally consistent with quantum yield ~ . measurements, which give approximately N o necrosis 2.4X10-4 g/J for MMA and bovine cornea photo fragments, irrespective of wavelength (94). Such comparative studies unravel some essen· of changing the eye's refractive power (e.g. tial steps in photoablative processes. Laser reducing myopia) by making radial corneal inci induced photodissociation is an extremely rich sions. Ablation of corneal stroma with 193 nm field: a comprehensive photophysical analysis radiation seems to produce most precise cuts, as for polyatomic molecules would have to con narrow as 20 µm, without the ragged edges sider how structured is their vibrational mani produced at 248 nm (13). Such an effect might be fold, whether the process is single photon or attributed to the difference in absorption multiphoton, whether it is resonant or non (a=2700cm-1 at 193nm; but a=210cm-1 at resonant, whether the incoming field is weak or 248 nm). However, beam quality might also be strong and whether it is single mode or multi crucial, as suggested by the 3 µm cuts achieved mode (95). One can thus anticipate that u.v. with the fourth harmonic of an Nd-YAG laser photodissociation, when applied to biomole (91). Further investigations are in progress, in cules, will prove to be even more complex. particular to answer questions about possible Recalling the characteristic relaxation times mutagenic or carcinogenic effects in the inter given in Table 2, we notice that u.v. photoabla action with DNA in neighbouring cells; this is of tion is inherently limited by pulse width. The the utmost importance because, in vivo, u. v. limitation is determined either by characteristic photochemistry may not only alter DNA bases de-excitation times, of the order of 100 ns at but may also modify RNA structure photo· most, arising from recombination, attachment chemically, as well as enzymic proteins and per· or other quenching mechanisms, or it is deter haps membranes; hence it may eventually mined by water thermal diffusion occurring on a change cell functions. Further work is needed to 30 µs time-scale. Long u. v. exposures would, demonstrate whether photoablative techniques hence, be associated with a large thermal com might become useful as microsurgical tools. ponent; this explains the position of the photo ablative interaction family in Fig. 1. When validated, this experimental surgical CONCLUSION technique may prove to be unique in its ability to produce sharp incisions with minimal thermal A comprehensive analysis and comparison is damage to adjacent normal structures. Table 6 presented of different biomedical applications summarizes the pathways of the processes for lasers. The photophysical steps of the involved. Photoablative techniques with processes involved have been emphasized. Four excimer lasers have been applied to various groups of interactions may be distinguished microsurgical models such as skin removal (14) according to their radically different tissue reac and ablation of atheromatous plaques in tions, which depend on the duration of irradia human vascular tissue in vitro ('W.S. Grundfest tion: the proposed argument is based on the et al, unpublished observations). But a magnitude of the energy dose supplied, which promising area appears in the treatment of has been shown empirically to be approximately several ocular disorders because of the potential constant in all instances, over more than 12

Lasers in Medical Science 1986 © Bail/tere Tindall 64 J.-L. Boulnois decades of time. Absorption characteristics of ablation of the skin. Lasers Surg Med 1984 4:201 primary absorbers, kinetics, and associated 15 Boulnois JL. A general classification of laser-tissue relaxation times have been analysed for each interactions. Lasers Surg Med 1985:in press 16 Giese AC (ed) Photophysiology, Vol 6. New York: group, and they support the proposal. This Academic, 1971 review of the relatively young fields of laser 17 Smith KC. Photobiology of ultraviolet radiation. In: photodynamic therapy, pulsed lasers in the Pratesi R, Sacchi CA (eds) Lasers in Photomedicine and breakdown mode, or u.v. photoablative tech Photobiology. New York: Springer, 1980 niques has attempted to demonstrate the versa 18 Bond JW, Watson KM, Welch JA. In: Atomic Theory of Gas Dynamics. Massachussets: Addison-Wesley tile potential that lasers may offer in photo Reading, 1965 medicine in the near future. More innovations 19 Parrish JA. New concepts in therapeutic photomedicine: are likely to come, but one cannot hold an un photochemistry, optical targeting and the therapeutic bridled optimism about the immediate applica· window; J Invest Dermatol 1981, 77:45 tions. The complexity of the problems 20 Brunetaud JM, Mordon S, Bourez J et al Therapeutic applications of lasers. Optical Fibers in the Biomedical encountered underlines the need for careful Field (Proc SPIE 405) May 1985:2 evaluation of results before instruments become 21 van Gemert MC, Schets GA, Stassen EG, Bonnier JJ. accepted in an operational clinical environment. Modelling of coronary laser angioplasty. Lasers Surg Med 1985 5:219 22 van Gemert MC, de Kleijn WJ, Hulsbergen JP. Temperature behavior of a model portwine stain during ACKNOWLEDGEMENTS argon laser coagul,tion. Phys Med Biol 1982, 27:1104 23 Welch AJ. The thermal response of laser-irradiated The author is indebted to Professor R. Sultan, Rueil tissue. IEEE (Inst Eke Electron Eng) J Quantum Malmaison Hospital, who suggested an invited paper at the Electron QE 20 1984, 12:1471 Second International Congress of the European Laser 24 Mordon S, Brunetaud JM, Mosquet L et al Effets Association, Brussels, January 1985: his keen arguments thermiques des lasers: etude par camera thermique were particularly stimulating. infrarouge. Laser Medical. Opto 82. Paris: Masson 1984:58 25 L'Esperance FA. Ocular photocoagulation. Saint-Louis: REFERENCES Mosby, 1975 26 Little HL, Zweng HC, Peabody RR. Argon laser slit lamp retinal photocoagulation. Thzns Am Acad 1 Goldman L, Rockwell J, Jr. Lasers in Medicine. New Ophthalmol Oto-Laryngol 1970 74:85 York: Gordon and Breach, 1971 27 Coscas G. Le laser a krypton en ophtalmologie: premiers 2 Adey WR. Tissue interactions with nonionizing electro essais experimentaux et cliniques. Bull Mem Soc Fr magnetic radiation. Physiol Rev 1981, 61:435 Ophtalmol 1981, 87:100 3 Hayes JR, Wolbharsht ML. 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