Lasers in Medicine

LASERS IN MEDICINE

LASERS (Light Amplification by Stimulated Emission of Radiation)

Characteristics of laser sources

•Tissue optical properties

•Laser/tissue interactions

•Some diagnostic applications

Components of a Laser

a)  Lasing Medium: provides appropriate transition and determines wavelength.Solid: Ruby, v Nd:YAG, Ti:Sapphire, etc. Liquid: Organic dyes, e.g. rhodamine Gas: Ar, CO₂, HeNe, ArF, etc.

b) Pump: provides energy necessary for population inversion.

E.g. electric discharge, flashlamp, another laser.

c) Cavity: provides opportunity for amplification and produces a directional beam.

Useful Characteristics of Output Beam

a) Coherence

b) Collimation

c) Monochromaticity

d)Widerangeofpulsestructure

e) High power

Optical Properties of Tissues

Scattering

·        Elastic (i.e. no energy loss), although Doppler shift and Raman shift have been exploited for diagnostic information.

·        Mean free path for scattering is typically 100 microns.

·        Scattering is forward peaked, typically the average cosine of the scattering angle is > 0.9 (for isotropic scatt

·        Scattering coefficient decreases slowly as a function of wavelength.

Absorption

Depends on concentration and absorption spectra of specific molecules in the tissue. Highly dependent on wavelength. UV - high absorption by proteins.Visible - can identify specific features of absorption by hemoglobin, melanin, and other pigments.700 - 900 nm - the “optical window” where tissue absorption is low, maximum light penetration in tissue.IR - absorption is mainly due to water, highest at 2.95 microns.

Distribution of Light in Tissue

The quantity we are usually interested in is the fluence rate. This is defined as the ratio of total power incident on an infinitesimal sphere to the cross sectional area of that sphere. The SI unit is W m^-2. It is a measure of how many photons are available per unit volume in the tissue.The fluence rate distribution in tissue is highly dependent on the absorption and scattering coefficents of the tissue.

The beam is incident on tissue at two different wavelengths:300 nm and 700 nm. At 300 nm the “effective” scattering coefficient is 1 mm^-1 and the absorption coefficient is 10 mm^-1. At 700 nm, let us assume the scattering is the same but the absorption coefficient is only 0.005 mm^-1.

Mechanisms of interaction

In order for light to affect tissue, absorption must take place. The rate at which energy is deposited in the tissue is given by the product of the fluence rate (W cm^-2) and the linear absorption

coefficient (cm^-1). The rate of energy absorption largely determines whether photochemical, thermal, or photomechanical effects are dominant.

Photochemical

Initial absorption by specific molecules.If photon energy is high enough (UV, excimer laser), direct bond- breaking is possible.Alternatively, the molecule can be raised to an excited state from which a variety of chemical reactions are possible such as the generation of free radicals and reactive oxygen species.

Photomechanical

For very high rates of energy deposition, shock waves can be generated in the tissue by mechanisms such as bubble expansion/collapse or plasma formation.The mechanical properties of the tissue govern the propagation of these waves and their biological effect.

Tissue can be ablated (i.e. physically removed from the surface, torn or, in the case of “brittle” tissue, shattered. Interestingly, these two quantities span many orders of magnitude but their product (the light fluence), varies over a much smaller range. This emphasizes the point that is is the rate of energy absorption that determines the nature of the light-tissue interaction.

Selected Applications of Lasers in Medicine

Diagnostic: Goal is to learn something about the tissue

Therapeutic: Goal is to modify the tissue, e.g. kill malignant cells.

Optical spectroscopy

Endogenous absorbers: Hemoglobin, proteins, melanin, water

Endogenous fluorophores: Collagen, elastin, NADH

Fluorescence Spectroscopy

Noninvasive tissue characterization to replace or guide physical biopsy, e.g. early diagnosis of lung cancer.

Images are acquired at the two wavelengths shown, and a ratio image is computed and displayed to the physician in real time. This application uses coherence and collimation of the laser toachieve efficient coupling to the fiber and endoscopic light delivery. In addition, the choice of laser (HeCd) provides optical power at the optimum wavelength for fluorescence excitation.

Photodynamic Therapy

Use chemical reactions initiated by light absorption to kill cells. Original application in oncology but is applicable to other diseases, including age-related macular degeneration caused by a proliferation of new blood vessels in the retina.

Process:

1. Inject photosensitizer or apply topically. 

2. Possibly wait for biodistribution.

3. Irradiate with light of appropriate wavelength.

Recent advances:

1. Long wavelength photosensitizers.

2. Reliable clinical diode lasers.

3. Better targetting of photosensitizers

Selective Destruction of Blood Vessels

Port Wine Stain: Congenital hypervascularization of the dermis.Could just ablate the epidermis and dermis but this would result i n unacceptable scarring. Instead, develop a strategy to target the blood vessels:

Wavelength;

The vessels are filled with hemoglobin - most of it oxygenated. Oxyh emoglobin has a strong absorption peak at 577 nm.

Thermal confinement:

Use a pulsed laser to heat the blood in a time short compared with the “thermal relaxation time of the vessel.

Solution:

Pulsed dye laser (ms pulses) tuned to 577 nm.