Radiology and Physical Medicine

Laser Radiation. Biological Effects

March 30, 2014

The term “laser” is an acronym for “Light Amplification by Stimulated Emission of Radiation”1. They are used in medicine for laser surgery and skin treatments.

Lasers are distinguished from other light sources by their coherence:

– Spatial coherence is typically expressed through the output being a narrow beam which is diffraction-limited (“pencil beam”). Laser beams can be focused achieving a very high irradiance or they can be launched into beams of very low divergence and concentrate their power at a large distance.

– Temporal (or longitudinal) coherence implies a polarized wave at a single frequency whose phase is correlated over a relatively large distance along the beam2.

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Laser radiation provokes some physical effects depending on its power and on the surface on which it is focused:

– Thermal effects occur when laser radiation is absorbed by the obstacle. They induce tissue reaction, related to the organism temperature elevation and to the duration of the heating process. Different reactions can occur :

– Hyperthermia: The temperature rises a few degrees. A 41°C temperature during a few tens of minutes can induce cellular death.

– Coagulation: It corresponds to an irreversible necrosis without immediate tissular destruction. The tissue reachs temperatures between 50°C and 100°C during about 1s. Tissues will be eliminated and the wound will scar.

– Ablation: It corresponds to matter loss. This occurs at temperatures higher than 100°C. In these conditions, the cell constituting elements evaporate within a brief time.

– Mechanical effects: They are caused by the creation of a plasma (vaporisation or cavitation). These effects are mainly related to the expansion of a shock wave which in turns has destructive effects. When ejecting matter from the substrate, the latter moves backward due to the energy/momentum conservation, and a part of the electromagnetic energy is converted into kinetic energy.

When these effects accidentally happened they are harmful. However, when used appropriately, they can have therapeutic effects3.

Biological Effects of Laser Radiation4:

– Laser effects on the eye

The human eye is extremely sensitive to laser radiation and can be permanently damaged from direct or reflected beams.

a) Retina: Visible and near infrared (400 – 1400 nm) laser light pose a critical hazard on the retina. Since the retina are unable to undergo any repair, lesions may be permanent. The most critical area of the retina is the central portion, the macula, and the fovea.

b) Cornea and lens: Laser light in the ultraviolet or far infrared spectrum can cause damage to the cornea or the lens.

– Ultraviolet (180 nm to 400 nm): Photochemical damage is caused by the absorption of UV light by selective sensitive portions of cells of the cornea (proteins, DNA, RNA) are “denatured” by the radiation.

– Far infrared (1400 nm to 1 mm; CO2 lasers, 10600 nm): Thermal damage is caused by the heating of the tears and tissue water of the cornea. This results in a loss of transparency or irregularities of the cornea.

– Laser effects on the skin

Thermal injury is the most common cause of laser induced skin damage and 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.

 The thermal effects of laser exposure depend on:

– The absorption and scattering coefficients of the tissues.

– Radiant exposure of the laser beam.

– Duration of the exposure and pulse repetition characteristics.

– Extent of the local vascular flow.

– Size of the area

 Tissue damage may also be caused by thermally induced acoustic waves following exposures to sub-microsecond laser exposures. The effects of the pulses are additive

Author: Sara Darwish Mateos

2º Course, Medicine. Granada University

References:

1.- Gould, R. Gordon (1959). “The LASER, Light Amplification by Stimulated Emission of Radiation”. In Franken, P.A. and Sands, R.H. (Eds.). The Ann Arbor Conference on Optical Pumping, the University of Michigan, 15 June through 18 June 1959. p. 128. OCLC 02460155.

2.- Conceptual physics, Paul Hewitt, 2002

3.-http://www.optique-ingenieur.org/en/courses/OPI_ang_M01_C02/co/Contenu_04.html

4.- http://blink.ucsd.edu/safety/radiation/lasers/effects.html

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