Understanding laser tissue interactions is vital for proper patient care. Even more so at a time when laser therapy is being revolutionized with the development of new laser devices.

Properties of lasers

The four key properties of lasers – monochromicity, coherence, collimation and high power - form the basis of laser therapy.

Monochromicity, which means light of a single wave length or a narrow band of wave lengths, determines skin absorption of specific light wave lengths. The specific wave length selected decides the depth of penetration of the laser. Generally, the higher the light wave length, the more the penetration deeper into the skin. Coherence allows all laser rays to act together. Collimation is where all the light rays travel in the same direction with very low divergence (dispersal of the light), this allows laser light to travel long distances through optic fibers for example.

In addition, the laser beam parameters like power density and energy density are critical. By adjusting these parameters and the exposure time the laser can be tailored for specific uses.

Laser tissue interactions

Lasers, like ordinary light, are reflected, transmitted, scattered and absorbed by the body tissues. Transmission means passage of light through the tissues without damaging them or altering the laser properties. Reflection means the light bounces off the tissue without penetrating it. The amount of reflection increases with increase of the angle of incidence of the light hitting the tissue surface. Maximum reflection occurs when the incident laser is perpendicular to the tissue. Sometimes the reflection is enough to produce a high intensity laser beam bounce causing damage to unintended targets. This can occur in eye operations in particular.

Light scattering occurs inside the tissues due to variations in particle size and the tissue’s refractive index. Scattering of light causes radiation of a larger area than intended and reduces the extent of penetration into the tissue because so much light is scattered forwards. Generally, the higher the light wave length the less the scatter and the more the penetration into the tissue. The exceptions are laser wave lengths above 1300nm, which hardly penetrate the skin due to high absorption by the tissue water. Scattering of laser light is caused mostly by the skin dermal collagens.

Absorption of laser by the targeted tissues is the goal of all clinicians because it is absorption of laser photons which has an effect on the tissues – reflectance or scattering of the laser light does not have an effect. The parts of the tissues that absorb the photons are called “chromaphores”. Common chromaphores which are targeted by lasers include melanin, hemoglobin and water. Laser energy absorbed by chromaphores is converted into heat energy.

Selective photothermolysis

By this procedure, damage to tissues surrounding the target chromaphores is avoided. To achieve this, the wave length should be absorbed only by the chromaphore. Just as importantly, the pulse duration should be less than the thermal relaxation time of the chromaphore. Thermal relaxation time is the time taken by the target to cool to half the peak temperature after stoppage of the laser treatment. Lastly, the energy delivered by the laser should be high enough to destroy the chromaphore within the pulse duration.

Laser parameters and tissue interactions

Beam features: An important feature of most laser beams used on the skin is the distribution of intensity across the beam cross-section. The intensity is highest at the centre of the beam and reduces towards the periphery. This makes it necessary to treat tissue with some overlap of the laser beam in order to deliver energy uniformly to the tissue.

Spot size: The energy and power density of the laser beam depends on the size of the spot, being inversely proportional to the square of the radius of the spot size. Therefore, halving the beam radius increases the two parameters by a factor of 4. Spot size has important clinical implications. A small spot size causes more scattering resulting in less power density in the tissue, as compared with a larger spot size. A spot size of 7mm to 10mm is needed to reach targets at the mid-dermal or deeper levels of the skin. Depth of penetration does not increase beyond a spot size of 10 to 12mm.

Pulse duration: Laser light can be delivered continuously or in a pulse mode. Continuous delivery carries higher risk of damage to untargeted tissues. In most forms of laser treatment, pulsed light is used. The pulse duration ranges from very short nanoseconds to long milliseconds depending on the thermal relaxation time of the target. The thermal relaxation time is generally proportional to the size of the target.

Surface cooling: Surface cooling can improve laser penetration, without injury to intervening tissues, when the target is located deep inside the skin. There are three ways of cooling the surface – pre-cooling, parallel-cooling and post-cooling.

Pre-cooling is done by cooling the epidermis just before the laser treatment, usually with a cryogen spray. Parallel-cooling is done at the same time as the laser treatment, usually by a water cooled sapphire laser tip. Parallel-cooling is preferred in long pulse duration treatments. Post-cooling is done after laser treatment with ice. Post-cooling reduces pain and edema, but has no effect on thermal injury.