Dr. Hossam Eldin Sayed Ali

Lecturer of medical Biophysics

The Research Institute of Ophthalmology

An equal flow of energy can be supplied by changing the exposure time and wavelength (λ) of the laser radiation so that we can reach the high intensities like 1016 W/cm2.

There are 4 types of interactions of the laser beam with tissues:

Photochemical.

Photothermal.

Photoablative.

Photomechanical

interactions.

PHOTO-CHEMICAL laser causes chemical change or response; the light has a chemical impact on the tissue being treated. Examples include: Photo Dynamic Therapy (PDT) Cancer treatment, ophthalmic treatments, and Laser therapy in pain relief.

PHOTO-THERMAL occurs when we use long pulses, biological effect due to heating such as hair removal, and most surgical lasers.

PHOTO-MECHANICAL: Short-pulsed (q-switched) lasers cause pressure waves that can stimulate the lymph draining system leading to dissolution of inflammatory mediators.

Photoablation such as in tattoo removal.

The interactions are based on the absorption of radiation by :

Water contained in the tissues.

Hemoglobin in the blood

Pigments or chromophores in some tissues normally present (or externally administered).

1-The photochemical (Photodynamic) interaction:

occurs when the energy of photons is greater than that of chemical energy bond, typically greater than 5 eV;

Occurs at very low levels of intensity or irradiance (I ~ 1 W/cm2)) and for high exposure times (duration time greater than a second).

The field of action is that of ultraviolet radiation that generates chemical fragmentation effects.

(If the energy density deposited is very high it is possible to achieve an ablation with high spatial control) (KrF laser, ArF). In this case the contours of the spots must be well-defined for the prevention of thermal spreads.

The energy absorbed in the tissue is used for structural modifications of the existing molecules, in fact the reaction: hν + A + B →(AB) …. a molecule A and another one B receives energy hν, a new product AB will be produced.

Fluence rates below the hyperthermia threshold can be used for PDT (PhotoDynamic Therapy) , a two-step modality in which the delivery of a light activated and lesion-localizing photosensitizer is followed by a low, non-thermal dose of light irradiation to kill cancer cells.

Almost all photochemical reactions of biological relevance are dependent on generation of reactive oxygen species (ROS). The most important is 1O2

1O2 (Singlet oxygen) is a high energy form of oxygen

It is the lowest excited state of the dioxygen molecule. Its lifetime in solutions is in the microsecond range (3 µsec in water).

It undergoes several reactions with organic molecules.

It is believed to be the major cytotoxic agent involved in PDT.

It is not possible to generate 1O2 by light of higher wavelengths than about 800 nm. Thus, for PDT one has to use radiation in the UV or visible range.

Because of the optical window of tissue ‘between’ 620 and 1100 nm, yielding optimal penetration depths (∼1–3mmat 630 nm), red light is most frequently used for PDT.

The red band of heme proteins is at 620 nm, so chemicals (sensitizers) such as Porphyrins, chlorines and phthalocyanins that absorb beyond 620 nm are being used for laser PDT.

Several lasers can be applied, notably dye lasers and diode laser.

Absorption of 10 light quanta can give as many as six 1O2

molecules, so the process is extremely efficient, and sub-hyperthermal fluence rates can be satisfactory.

Singlet oxygen has a short lifetime of 10–40 ns in cells and tissues, and its radius of action is only about 10–20 nm.

This means that PDT acts selectively on targets with high concentrations of sensitizer . PDT Tumor selectivity is based on this principle.

The tumor selectivity of sensitizer accumulation can be related to a number of factors:

-Low tumor pH (several sensitizers protonate and get more lipophilic below pH 7),

-High concentration of lipoprotein receptors in tumours(many sensitizers are bound to lipoprotein in the blood),

-Presence of leaky microvessels with low lymphatic drainage in tumors, and a high concentration of macrophages (taking up aggregated sensitizers) in tumors

Diagnostic uses of PDT:

Based on the concept of photochemical reactions of light, there are some biomedical techniques that allow us to diagnose cancer of the larynx, esophagus and bladder, they can show the differentiations of the tissues, and their metabolic activities, which gives a precise diagnosis of the disease by the analysis of the fluorescence spectrum of the organ.

Some examples of diagnostics based on the fluorescence of different tissues as a function of the wavelength emitted by them when they are hit by UV light are shown here.

Fluorescence spectrum emitted from different tissues irradiated with UV light (a) and tumor tissues of the bladder (b) which is injected with Photofrin 48 hours before the investigation.

By irradiating healthy tissue and the tumour with radiation of a wavelength 405 nm, we observe a very strong reduction of fluorescence in the blue-green region with respect to the healthy tissue.

2-Photothermal interactions: Occur when the energy of the used photons is lower

than the binding energy and by using infrared lasers, such as Nd: YAG or CO2 lasers, and using power densities above 100 W/cm2, or by irradiation with pulsed lasers of durations between 1 ms and 5 sec.

In infrared and visible ranges these interactions produce thermal effects on the irradiated tissues.

Chromophores in the tissue absorb the irradiation photons, which bring it into an excited rotovibrationalstate, this subsequently damps the inelastic collisions with the surrounding molecules leading to an increase of its own kinetic energy or thermal energy.

Once the laser light is absorbed, a consequent rapid non-radiative decay occurs inducing a local temperature increase.

The effects of temperature can cause different degrees of damage as a function of irradiation time.

For temperatures from 45°C - 50 °C the thermal damage involves heating and hyperthermia of healthy tissue, and may result in necrosis of the tissue.

Denaturation occurs at temperatures between 50° and 100° C the of the biomolecules and their aggregates (proteins, collagen, lipids, hemoglobin) or irreversible coagulation of proteins.

High heating causes an irreversible deformation of these structures and loss of protein function. The denatured cellular material is absorbed by the body and replaced with scar tissue. These processes of photocoagulation are used for example in eye surgery for the reduction of retinal detachments, and in dermatology for the treatment of vascular lesions by using continuous Argon laser (488 and 514,5 nm).

Scheme of the thermal interaction in biological tissue for different temperatures.

When the temperature of the tissue reaches about 100 °C the water constituting most of the soft tissues, vaporizes, dehydrating the tissue. When the water present in the tissue is completely evaporated, the tissue temperature rapidly increases up to about 300 °C and the tissue burns. In this case the vaporization together with the carbonization gives rise to the decomposition of the constituents tissue.

3- The fotoablative interactions: Occurs when the intensity and wavelength of the laser exceed

certain threshold values that allow the removal of layers of material from the irradiated target;

Requires high power density (107-1010 W/cm2) and pulse durations ranging typically from 10 to 100 ns.

Many biomolecules absorb strongly in the UV band (200-320 nm) and in the visible band (400-600 nm); such strong absorptions involve a localized molecular dissociation that may be accompanied by thermal effects like evaporation. Ionization with the formation of plasma and removal of material from the target usually happens with high power densities.

The photoablation process is therefore the photodissociation of macromolecules in repulsive photoproducts :

hν + AB → (AB)* → A + B*and the transfer of energy to atomic and molecular species that are emitted at high speed from the irradiated target.

In the photoablation process the residual energy that is not used to break molecular bonds in the photoproducts remains in in tissue the form of translational kinetic energy.

This translational kinetic energy causes the instant high-speed ejection of photoproducts from the area irradiated by the laser beam. Similar effects may also be obtained by visible lasers with intensity of the order of 1010 W/cm2.

The process of photoablation is related to the product of laser intensity multiplied by the square of the wavelength (I λ2 )

Laser pulses are focused on tissue with intensities greater than 108 W/cm2; the radiation is strongly absorbed by the molecules, such as protein, starche and peptides, until a penetration depths of about 1 μm. It is followed by a high excitation of macromolecules with the formation of photodissocitation in repulsive photoproducts; finally an expulsion of photo products at supersonic speed without tissue necrosis or thermal effects are produced in the case of UV band.

The removal of atoms, molecules, clusters from the tissue, through laser irradiation, can be described by the curves of the ablation rate, which represent the removal rate as a function of the used laser fluence.

The ablation threshold is the minimum level of radiation above which the removal of tissue is produced and below which only fluorescence effect and heating, without any ablative effect occur. This parameter is important for the analysis of the ablation process, and it is generally expressed by the laser fluence threshold (J/cm2).

4-The Photomechanical interaction:

Takes place when the laser pulses impinging the material have high intensities greater than 1010 W/cm2 and the tissue absorbs them.

Ionization effects are induced; they generate a plasma which causes cavitation effects

In addition to the direct dynamic effect of shock waves on interfaces, so-called cavitation occurs in certain media such as water and sometimes body tissue as well. Cavitation bubbles occur directly after the pressure/tension alternating load of the shockwaves has passed the medium. A large number of bubbles grow and then violently collapse while emitting secondary spherical shock waves.

Cavitation is a biologically effective mechanism produced by shockwaves, which can be selectively used in localized areas, even in deeper tissue layers. The physically induced energy causes biological reactions via different mechanisms.