Biomedical Lasers: Dr. N. Venkatanathan

BIOMEDICAL LASERS Dr. N. venkatanathan

Femto – Second Lasers The development and commercialization of

femto - second lasers has opened up new applications in biomedicine. Particularly in surgery, Femto-second pulses allow for much more precise cutting than do nanosecond lasers. Two-dimensional images of biological tissue can be recorded using an ultrafast electronicgated imaging camera system.

This imaging typically uses 120 fs pulses at a 1

kHz repetition rate from ultrafast laser amplifiers. The biggest advantage of ultrafast lasers in surgical applications is limiting biological tissue damage. The pulse interacts with the tissue faster than thermal energy can diffuse to surrounding tissues.

It simply means it hardly burns

or destructs the neighboring tissue. The main components of biological tissue that contribute to the absorption are melanin, hemoglobin, water and proteins.

Absorption spectra of main absorbers in biological tissue.

The absorption in the IR region (2000–10000 nm)

originates from water. Which is the main constituent of most tissues. Proteins absorb in the UV region (mainly 200–300 nm). Pigments such as hemoglobin in blood and melanin, the basic chromophore of skin, absorb in the visible range. The absorption properties of the main biological absorbers determine the depth of penetration of a laser beam.

Properties of Nd:YAG Laser The

Nd:YAG laser (1064 nm) for which absorption of water, pigments and proteins is low. This property is therefore obviously important for medical applications. For example, the Nd:YAG laser can penetrate deeper A cut made with the Nd:YAG laser will not bleed due to tissue coagulation.

Properties of CO2 Laser The CO2 laser (10.6 µm) does not

penetrate deeply because of water absorption in this region. The CO2 laser which is a better ‘‘knife’’ For precise thermal cutting of tissue due to vaporization by focusing on the tissue along a short optical path.

INTERACTION MECHANISMS

Double logarithmic plot of the power density as a function of exposure time. The circles show the laser parameters required from a given type of interaction with biological tissue.

All types of interactions can be placed between

two diagonals illustrating the energy fluences of 1 J/cm2 and 1000 J/cm2. This indicates that lasers used for medical applications must have fluences ranging from 1 J/cm2 to 1000 J/cm2. The fluences are controlled both by the energy (controlled by exposure time) and degree of focusing the laser beam on the tissue.

Various interactions and Laser Sources With cw lasers or exposure time >1 s, only

photochemical interaction can be induced. Powers of only a few mW can be used for these purposes. For thermal interactions shorter exposure times (1 min–1 ms) and higher energies must be used. Thermal effects can be induced both by cw or pulsed lasers of 15–25W power.

Photoablation occurs at exposure time between

1 ms and 1 ns. In practice, nano-second pulses of 106 –109 W/cm2 irradiance should be employed. Plasma-induced ablation and photo disruption occur for pulses shorter than nano seconds. In practice, Pico - and femto second lasers with an irradiance of 1012 W/cm2 should be used.

Note that although both phenomena occur at a

similar time exposure and irradiance. They differ according to the energy densities that are significantly lower for plasma-induced ablation. The plasma-induced ablation is solely based on tissue ionization. Whereas photo disruption is primarily mechanical disruption.

PHOTOCHEMICAL INTERACTIONS Photochemical interactions do not need a

high power density. Lasers of 1 W/cm2 power density and long exposure times ranging from seconds to cw light are sufficient. For this category of interactions, a laser induces chemical effects by initiating chemical reactions in tissue.

For

example, vision processes in rhodopsin or proton pumping in bacteriorhodopsin are initiated by a laser beam from the visible range. Photochemical interactions are used in photodynamic therapy (PDT). Another application of very low irradiance of a laser beam is biostimulation.

Photodynamic Therapy Photodynamic therapy utilizes the laser light

effect on various chemical substances in an oxygen-rich environment. Light induces a sequence of reactions that produce toxic substances such as singlet oxygen or free radicals. These substances are very reactive and can damage proteins, lipids, nucleic acids as well as other cell components.

It is found that some porphyrins are accumulated at

much larger concentrations and for a much longer time in cancer cells compared to healthy cells. It was suggested that this effect can be used for killing cancer cells. If porphyrins are transferred to toxic states in any way, e.g., by light, cancer cells would be damaged first. The method is very selective because it does not destroy healthy cells.

THERMAL INTERACTION Thermal interactions are induced in a

tissue by the increase in local temperature caused by a laser beam. In contrast to photochemical interactions, thermal interaction may occur without only specific reaction path and is highly non-selective and nonspecific.

THERMAL EFFECT ON THE TISSUE Reversible hyperthermia (T>31o C) – some functions

of the tissue can be perturbed but the effect is reversible, Irreversible hyperthermia (T>42o C) – some fundamental functions of the tissue can be destroyed irreversibly, Coagulation (T>60o C) – the tissue becomes necrotic, Vaporization (T≥100o C), Carbonization (T>150o C), Pyrolysis (T>300o C).

PHOTOABLATION A molecule is promoted to the repulsive

excited state or to the Franck-Condon vibrationally hot state and followed by dissociation. The chemical bond is broken, leading to the destruction of biological tissue. As electronic transitions occur usually in the UV range, the photo-ablation process is usually limited to UV lasers.

Therefore,

excimer lasers (ArF, KrF, XeCl, XeF) are mainly employed. But higher harmonics of other lasers can also be applied.

PLASMA-INDUCED ABLATION An ultra short pulse from a Q-switched or

modelocked laser ionizes biological tissue. This generates a very large density of free electrons in a very short period of time with typical values of 1018 cm-3 due to an avalanche effect. Free electrons from ionization accelerate to high energies and collide with molecules, leading to further ionization.

Typical lasers used for plasma-induced

ablation are Nd:YAG, Nd:YLF, Ti:sapphire with Pico- or femto second pulses generating irradiance at about 1012 W/cm2. Such a high power density leads to fields of 107 V/cm, comparable with the energies of electrons revolving in atoms.

Therefore,

the Q-switched or modelocked lasers can ionize molecules in biological tissue. Light electrons and heavy ions move at different velocities, leading to the effect similar to that in the acoustic wave with areas of compression and dilation.

APPLICATION OF LASERS IN MEDICINE Lasers have many applications

§ DENTISTY, § CARDIO VASCULAR MEDICINE, § DERMATOLOGY, § GASTROENTEROLOGY, § GYNECOLOGY, § NEUROSURGERY, and § OPHTHALMOLOGY

BIOLOGICAL EFFECTS OF THE LASER BEAM. EYE INJURY: Because of the high degree of

beam collimation, a laser serves as an almost ideal point source of intense light. A laser beam of sufficient power can theoretically produce retinal intensities at magnitudes that are greater than conventional light sources, and even larger than those produced when directly viewing the sun. Permanent blindness can be the result.

THERMAL INJURY The most common cause of laser-induced

tissue damage is thermal in nature. Where the tissue proteins are denatured due to the temperature rise following absorption of laser energy. The thermal damage process (burns) is generally associated with lasers operating at exposure times greater than 10 microseconds.

In

the wavelength region from the near ultraviolet to the far infrared (0.315 µm-103 µm). Tissue damage may also be caused by thermally induced acoustic waves following exposures to sub-microsecond laser exposures. With regard to repetitively pulsed or scanning lasers, the major mechanism involved in laserinduced biological damage is a thermal process wherein the effects of the pulses are additive.

Factors for the Thermal Effects The absorption and scattering coefficients

of the tissues at the laser wavelength. Irradiance or radiant exposure of the laser beam. Duration of the exposure and pulse repetition characteristics. Extent of the local vascular flow. Size of the area irradiated.

The biological effects of non-ionizing laser radiation

Effect

Non Ionizing laser radiation include the action of visible, ultraviolet (UV), or infrared radiation upon tissues. Generally, lasers in the UV region induce photochemical reactions. Lasers in the infrared region induce thermal effects.

Damage can occur when a laser beam

encounters tissue, depending combined characteristics of both

on

the

the incident laser beam and the properties of the tissue involved. Laser wavelength, power density, and pulse

duration. Tissue property to reflect, transmit, or selectively absorb the laser radiation.

Effect on Eye The retina, cornea, and lens are the areas

most commonly damaged. Laser light in the visible to near infrared spectrum can cause damage to the retina. These wavelengths are also know as the "retinal hazard region. Laser light in the ultraviolet or far infrared spectrum can cause damage to the cornea or the lens.

Laser effects on the skin Thermal (burn) injury is the most

common cause of laser induced skin damage. Thermal damage is generally associated with lasers operating at exposure times greater than 10 microseconds and in the wavelength region from the near ultraviolet to the far infrared.

Low level laser therapy Photo bio-modulation, also known as low

level laser therapy (LLLT), cold laser therapy, and laser biostimulation. It is an emerging medical and veterinary technique in which exposure to low-level laser light or light emitting diodes might stimulate or inhibit cellular function possibly leading to beneficial clinical effects.

Certain

wavelengths of light at certain intensities will aid tissue regeneration, resolve inflammation, relieve pain and boost the immune system. Observed biological and physiological effects include changes in cell membrane permeability, up-regulation and downregulation of adenosine triphosphate and nitric oxide.

Bio-stimulatory effects of laser

The

promotion of healing of wounds. Treatment of skin infections. Treatment of ulcers. Laser may have an enhancing effect on healing wherever inflammation is present.

Effects of Laser Light on Tissue Accelerated Tissue Repair Rapid Formation of Collagen Beneficial Effect on Nerve Cells and

the Production of B-Endorphins Accelerated Lymphatic System Activity and Reduction in Edema Formation of New Capillaries and Increased Blood Flow

Accelerated Tissue Repair Photons of light from a laser penetrate

deeply into tissue This power the synthesis of adenosine triphosphate (ATP). ATP is a molecule that is a major carrier of energy from one reaction site to another in all living cells. So that the cell can take in nutrients faster and get rid of waste products.

Rapid Formation of Collagen Collagen is the most common protein found

in the body. Connective tissue is the most widely distributed. In connective tissue, fibroblast cells produce the ground substance and tissue fibre. The “extra” energy from laser light is used by fibroblasts to increase collagen production.

Collagen is the essential protein required to

replace old tissue or to repair tissue injuries. Perhaps the most common example of collagen is the clear sticky substance found around open wounds. Wounds are healed or closed over very rapidly by the application of laser light. There is also less scar tissue formed when laser light is applied to the area.

Beneficial Effect on Nerve Cells and the Production of B-Endorphins Laser light has a highly beneficial effect on nerve cells

which block pain transmitted by these cells to the brain. In this case, laser light increases the potential difference across the cell membrane, thus, decreasing nerve ending sensitivity. Pain blocking mechanism involves the production of high levels of painkilling chemicals such as endorphins and enkephelins from the brain, adrenal gland and other areas as a result of exposure to laser light.

Accelerated Lymphatic System Activity and Reduction in Edema

The problem is that the veins in the leg

are only capable of removing one component of the swelling. Blood vessels can remove the water but not the dirty protein solution that is present. The lymphatic system is required to take away dirty proteins from edema.

Laser light is capable of doubling the size of

the lymphatic ducts in the area of exposure and rapidly removing the protein waste. Another important aspect of the study showed that laser light was capable of “perfect” regeneration of the lymphatic system in the immediate area with no leakage and no confused network of ducts.

Formation of New Capillaries and Increased Blood Flow

Laser light does this extremely well in

increasing blood flow. The laser light will significantly increase the formation of new capillaries in damaged tissue. It is the formation of new capillaries that speeds up the healing process, closes wounds quickly and reduces scar tissue.

Nano Manipulation Using

Nanotechnology and Laser technology, manipulation of individual atoms and molecules to build structures to complex, atomic specifications is possible. The atomic force microscope (AFM) is one of the foremost tools for imaging, measuring and manipulating matter at the nano scale.

AFM The AFM consists of a micro scale cantilever with a

sharp tip (probe) at its end that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law.

Forces that are measured in AFM include

mechanical contact force, Van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces etc. As well as force, additional quantities may simultaneously be measured through the use of specialized types of probe. Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes.

If the tip was scanned at a constant height, a risk

would exist that the tip collides with the surface, causing damage. In most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample. Traditionally, the sample is mounted on a piezoelectric tube, that can move the sample in the z direction for maintaining a constant force, and the x and y directions for scanning the sample.

The AFM can be operated in a number of

modes, depending on the application. In general, possible imaging modes are divided into static modes and a variety of dynamic modes where the cantilever is vibrated. The static mode can also be called as Contact mode. The dynamic mode is otherwise called as Non Contact Mode.

AFM cantilever in the Scanning Electron Microscope, magnification 3,000 x

Static mode The static tip deflection is used as a feedback

signal. Because the measurement of a static signal is prone to noise and drift. Low stiffness cantilevers are used to boost the deflection signal. However, close to the surface of the sample, attractive forces can be quite strong, causing the tip to 'snap-in' to the surface.

Thus static mode AFM is almost always

done in contact where the overall force is repulsive. Consequently, this technique is typically called 'contact mode'. In contact mode, the force between the tip and the surface is kept constant during scanning by maintaining a constant deflection.

Dynamic Mode The cantilever is externally oscillated at or

close to its fundamental resonance frequency or a harmonic. The oscillation amplitude, phase and resonance frequency are modified by tipsample interaction forces. These changes in oscillation with respect to the external reference oscillation provide information about the sample's characteristics.

Schemes for dynamic mode operation

include frequency modulation and the more common amplitude modulation. In frequency modulation, changes in the oscillation frequency provide information about tip-sample interactions. In amplitude modulation, changes in the oscillation amplitude or phase provide the feedback signal for imaging.

Identification of individual surface atoms The

AFM can be used to image and manipulate atoms and structures on a variety of surfaces. The atom at the apex of the tip "senses" individual atoms on the underlying surface when it forms incipient chemical bonds with each atom. Because these chemical interactions delicately alter the tip's vibration frequency, they can be detected and mapped.

Application in Cell Biology

Forces corresponding to

§the unbinding of receptor ligand couples §unfolding of proteins §cell adhesion at single cell scale have been gathered.

Biomedical Laser Beam Delivery Systems

Beam delivery systems for biomedical

lasers guide the laser beam from the output mirror to surface of the tissue. Beam powers of up to 100 W are transmitted routinely. All biomedical lasers incorporate a coaxial aiming beam, typically from a HeNe Laser (632.8 nm) to illuminate the tissue.

BEAM – GUIDING METHODS A flexible fused silica (SiO2) optical fiber

generally available for laser beam wavelengths between ≈400 nm and ≈2.1 µm, where SiO2 is essentially transparent. An articulated arm having beam guiding mirrors Can be used for wavelengths greater than ~2.1 µm (e.g. CO2 lasers), for the Er:YAG (erbium doped YAG laser) and for pulsed lasers having peak power outputs capable of causing damage to optical fiber surfaces due to ionization by the intense electric field (e.g. pulsed ruby).

The arm comprises straight tubular sections

articulated together with high quality power handling dielectric mirrors. The mirrors are present at each articulation junction to guide the beam through each of the sections. Fused Silica optical fibers usually are limited to a length of 1 – 3 m and to wavelengths in the visible - to – low midrange IR (< 2.1 µm).

Fused silica fibres cannot be used for longer

wavelengths of IR radiation, since longer radiation are absorbed by water impurities (<2.9 µm) and by the SiO2,lattice itself (wavelengths > 5 µm).

Since the flexibility, small diameter, and small

mechanical inertia of optical fibers allow their use in either flexible or rigid endoscopes. They offer significantly less inertia to hand movement, fibers for use at longer IR wavelengths.

Material systems showing promise are fused Al2O3

fibers in short lengths for near - 3 – micro meter radiation of the Er:YAG laser Ag halide fibers in short lengths for use with CO2 laser emitting at 10.6 µm. A flexible hollow Teflon waveguide 1.6 mm in diameter having a thin metal film overlain by a dielectric layer has transmitted 10.6 µm CO2 radiation with attenuation of 1.3 and 1.65 dB/m for straight and bent (5 – mm radius, 90 degree bend) sections, respectively.