Laser Physics and Tissue Interaction
Surgical lasers produce a range of wavelengths (Figure 12.1) with varying tissue interactions, the understanding of which is required to predict the laser's effect upon tissue. Many surgical wavelengths are invisible. Interactions are determined by the degree to which the tissue absorbs the particular wavelength of laser energy (Figure 12.2). The more a tissue absorbs laser energy, the less it penetrates into the tissue, thereby concentrating the surface effect. Whereas deeper penetration allows controlled coagulation (denaturation of protein) of a larger volume of tissue but may put associated deeper structures at risk of being injured. Complete lack of absorption of a wavelength by a tissue allows complete passage affecting only a deeper tissue. Interaction between laser light and a tissue that preferentially absorbs that wavelength apart from surrounding tissue, allowing selective coagulation/necrosis of that tissue, characterizes the principle of selective photothermolysis [3–7].
The laser tissue effect is due to optical and thermal interactions [8]. Optical interaction is the result of true absorption of electromagnetic energy and usually results in a thermal effect once absorbed by tissue. Depending upon the amount, heat may “boil” the cytosol thereby vaporizing the tissue into a smoke plume or simply denature tissue proteins. When the optical interaction does not achieve the desired effect (e.g. Nd:YAG/GAA diode lasers on pale surfaces), the irradiation is sometimes “artificially” converted into heat by the delivery device before applying it to the tissue, thereby causing the energy to be absorbed at the tip of the fiber, producing heat and a profound surface effect on the tissue while minimizing penetration to deeper structures. Means to transform irradiation into heat include blackening the tip of the bare quartz fiber or using a sapphire tip on a gas‐cooled quartz fiber [2].
Figure 12.1 Wavelengths of surgical lasers. Wavelengths in common veterinary use are in gray. The surgical lasers are generally not in the visible range.
Figure 12.2 Tissue absorption common surgical laser wavelengths. The visible spectrum is shown on the horizontal axis. The near‐infrared GAA Diode and Nd:YAG lasers are highly absorbed by dark pigment. However, note the increased absorption of the GAA Diode laser on the water curve compared to the Nd:YAG laser. The Ho:YAG and CO2 lasers are both highly absorbed by water.
Lasers are rated by power, the rate at which they can deliver energy. Power is expressed in watts (W) (1 W = 1 joule/second). Energy is measured in joules or calories (1 joule = 0.24 calories). The total amount of energy delivered per unit area is fluence expressed in joules/cm2, which depends upon time of exposure as well as power density. Power density (PD) (W/cm2) is a critically important value that expresses the amount of energy delivered per unit area of tissue. Similar to water at a constant flow through a hose, laser energy delivered through a wider aperture will have a less profound effect than the same amount of laser energy delivered through a narrower aperture (Figure 12.3). Power density is varied by adjusting the output of the laser, by varying spot size of the laser beam on the tissue (Figure 12.4), by changing the distance from the delivery device to the tissue (Figure 12.5), or by changing the delivery device. Power density varies with the square of the spot size and is calculated by the following formula where s = spot size in mm, and W = power setting of the laser [2].
Figure 12.3 Power density profoundly affects rate of tissue effect and collateral heating of tissue. Both water hoses transmit identical flows of water. The wider aperture of delivery in the top image produces no mechanical effect on the flower, whereas the narrower aperture in the lower image produces a jet of water that can disrupt the flower.
Source: Kenneth E. Sullins.
Figure 12.4 Power density decreases with the square of the increase in spot size, which in turn increases with distance from the surface. The beams depicted are all CO 2 laser beams from machines set to 50 W. The power densities shown below each demonstrate the profound reduction in tissue effect by increasing spot size. Moving the handpiece away from the tissue increases spot size and decreases power density.
Delivering identical power density values over different periods of time produces different results. If an acceptable full‐thickness skin incision could be created with a 10‐W laser beam delivered as a 0.4‐mm spot size advanced along the skin for 5 sec just penetrating the skin completely, doubling the rate of advancement (total time halved), the incision would be shallower because the total laser energy (fluence) has been halved. Conversely, if the original time were doubled (advancement slowed), the depth of the incision would increase beyond the skin and damaging collateral heating of adjacent skin would increase. Furthermore, varying spot size or increasing distance from tissue dramatically changes power density. Using a single power setting, the power density (tissue effect) can regress from focal incision/ablation (vaporization) to coagulation to negligible by simply moving the delivery device away from the tissue surface. This is described further below under Carbon Dioxide Laser [2].
Figure 12.5 CO2 laser handpiece with focusing lens. The stylus indicates the point of maximum focus (power density) for incision. Slight increase of distance widens the spot size and tissue can still be vaporized. More distance from the tissue further increases the spot size and reduces
