Laser Tissue Interaction

Laser-Tissue Interaction

The key to the success of soft tissue lasers is their ability to cut and coagulate the soft tissue at the same time. Laser light’s wavelength determines how it is absorbed by objects. Lasers of different wavelengths produce different effects on tissue.

For practical surgical lasers on the market today (diode, erbium and CO2 lasers), the laser light energy is transformed, through absorption, into the heat inside the tissue leading to elevated tissue temperatures that, in turn, can result in tissue ablation and coagulation. Such laser-tissue interaction is referred to as photo-thermal.

Comparing laser wavelengths

As illustrated with the photographs on this page, not all lasers are efficient at both cutting and coagulating the soft tissues. Some laser wavelengths (such as of erbium lasers) are great at cutting, but are not efficient at coagulating. Other laser wavelengths (such as of diode lasers) are highly efficient coagulators, but are poor scalpels. There are also lasers (such as a CO2 laser) that are efficient at both cutting and coagulating the soft tissue.

co2 laser incision and diode laser incisionThe CO2 laser easily incises soft tissue with instant coagulation, while the diode laser, of the same power and beam size, cannot do so.
co2 laser beamFigure 1 A CO2 laser resonator
What is a Laser?

A CO2 laser consists of the following interacting elements: an active medium confined between two mirrors: a total reflector and a partial reflector as shown in Figure 1.

The main difference between laser light and ordinary light, such as that produced by a light bulb, is directionality, i.e. it is highly collimated.  Such collimated, monochromatic and coherent beam of light is focusable to a very small focal spot (see Figure 2) that can be used for tissue incision, excision and ablation of soft biological tissue.

laser light vs ordinary lightFigure 2 Comparison of Laser Light to Ordinary Light
absorption depth and thermal relaxation time spectraFigure 3. Absorption (and estimated Near-IR attenuation) depth and Thermal Relaxation Time spectra of sub-epithelium/sub-epidermis soft-tissue with 75% water and 10% blood, derived from the soft tissue absorption and scattering data [1-3]. Logarithmic scale is in use. Note that the Near-IR parts of the spectra under 1,500 nm do not apply to other tissue types since other chromophores [1] (e.g. melanin in epithelium or epidermis or retina) and different light scattering properties [1, 4] may strongly modify the tissue’ absorption and attenuation depth spectra. At the same time, the mid-IR and IR parts of absorption depth spectrum is representative of all other soft tissue types due to negligently small light scattering with respect to strong absorption by water above 2,000 nm [1-4].
Photo-Thermal Laser-Tissue Interaction: Absorption and Thermal Relaxation Time

The key to understanding how the laser light ablates (cuts, incises, excises) and coagulates is through Absorption of laser light by the soft tissue. The wavelength-dependent Absorption Depth [mm] for the sub-epithelium connective oral soft tissue is illustrated in Figure 3; it is derived from absorption and reduced scattering coefficients of oral soft tissue dominant chromophores: water, hemoglobin and oxyhemoglobin. [1-4]

The cooling efficiency of the tissue irradiated by laser light is largely determined by tissue’s own thermal conductivity (or thermal diffusivity) to dissipate (or diffuse) the heat away from the irradiated tissue.

The rate of how fast the irradiated tissue diffuses the heat away is defined through the thermal diffusion time, or Thermal Relaxation Time (also presented in Figure 3) as TR = A2/K,[5] where A is optical absorption depth (also from Figure 3), and K is tissue’s thermal diffusivity.

Practical implications of Thermal Relaxation Time concept are important for appropriate application of laser energy. The most efficient heating of the irradiated tissue takes place when laser pulse duration t is much shorter than thermal relaxation TR. The most efficient tissue cooling takes place if time duration between laser pulses T is much greater than TR.

Photo-Thermal Laser-Tissue Interaction: Ablation and Coagulation/Hemostasis

Soft tissue photo-thermal ablation (or photovaporolysis) is a process of vaporization of intra- and extra-cellular water.

Figure 4 illustrates a laser beam irradiating the tissue surface from the left. Inside the tissue, the laser light intensity is exponentially attenuated: I = I0 Exp[-x/A], where A is absorption depth from Figure 3. When laser intensity I0 is greater than ablation threshold intensity, soft tissue ablation takes place in Ablation Zone 0<x<xa. Immediately below the Ablation Zone, there is a heat affected Coagulation Zone, where coagulation depth H = xc – xa, is defined by 60-100° C temperature range inside.

laser energy tissue absorptionFigure 4. Simplified graphical representation of pulsed (t ≤ TR) laser energy absorbed in the soft tissue in the absence of scattering.
Sub-epithelium/sub-epidermis soft tissue ablation threshold energy density spectrum. Logarithmic scale is in use. Figure 5. Sub-epithelium/sub-epidermis soft tissue ablation threshold energy density spectrum. Logarithmic scale is in use.
Photo-Thermal Ablation Energy Density Threshold

For a fixed laser beam diameter (or spot size), the volume of the tissue exposed to laser beam is proportional to the optical penetration (i.e. absorption or Near-IR attenuation as defined above) depth. The shorter the penetration depth – the less energy is required to ablate the tissue. The longer the optical penetration depth – the greater the volume of irradiated tissue and, therefore, more energy is required to ablate the tissue within the irradiated volume of tissue.

The ablation threshold energy density ETH spectrum for sub-epithelium connective oral soft tissue is illustrated in Figure 5.

Diode Laser Wavelengths

The diode laser wavelengths 800-1,100 nm are characterized by 1,000s times greater photo-thermal ablation threshold energy densities than Mid-IR and IR wavelengths because of much weaker Near-IR absorption by the soft tissue.

Erbium and Carbon Dioxide Laser Wavelengths

In sharp contrast to diode laser wavelengths, the erbium and the CO2 wavelengths are highly energy efficient at ablating the soft tissue photo-thermally with very low ablation thresholds due to extremely small volumes of irradiated tissue because of extremely short absorption depths.

Photo-Thermal Coagulation/Hemostasis Depth

Coagulation occurs as a denaturation of soft tissue proteins that occurs in the 60-100°C temperature range leading to a significant reduction in bleeding (and oozing of lymphatic liquids) on the margins of ablated tissue during laser ablation (and excision, incision) procedures.

The coagulation depth value H relative to the blood vessel diameter B is an important measure of coagulation efficiency; and is presented in Figure 6 for B = 21-40 micrometers (sub-epithelium oral gingival soft tissue), where absorption/attenuation depth A from Figure 3 is utilized to calculate H .

Erbium laser wavelengths

For H<<B (erbium laser wavelengths), optical absorption and coagulation depths are significantly smaller than blood vessel diameters; coagulation takes place on a relatively small spatial scale and cannot prevent bleeding from the blood vessels severed during tissue ablation. For this reason, the bleeding control is relatively low, when erbium lasers are used.

Diode laser wavelengths

For H>>B (diode laser wavelengths), optical absorption (Near-IR attenuation) and coagulation depths are significantly greater than blood vessel diameters; coagulation takes place over extended volumes – far away from ablation sites where no coagulation is required.

Carbon dioxide laser wavelengths

For H ≥ B (CO2 laser wavelengths), coagulation extends just deep enough into a severed blood vessel to stop the bleeding; the coagulation is more efficient then for the above two cases H<<B, and H>>B.

Coagulation depth can be extended with longer laser pulses that allow heat diffusion away from the irradiated tissue.

coagulation depth spectraFigure 6. Coagulation depth spectrum for ablation threshold conditions for sub-epithelium/sub-epidermis soft tissue. Logarithmic scales are in use.
optical absorption coefficient spectraFigure 7. Optical absorption coefficient spectra at different histologically relevant concentrations of Water, Hemoglobin (Hb), Oxyhemoglobin (HbO2) and Melanin. Logarithmic scales are in use.
Photo-Thermal Ablation and Coagulation Summary

Figure 7 is an illustration of vastly different absorption coefficients around 1,000 nm (sub 1 cm-1), around 3,000 nm (3,000-10,000 cm-1), and around 10,000 nm (400-800 cm-1) by the main chromophores in the sub-epithelium connective oral soft tissue:

CO2 laser wavelengths are a highly efficient and spatially accurate photo-thermal ablation tool with excellent coagulation efficiency (close match between coagulation depth and oral soft tissue blood capillary diameters). The CO2 laser is THE ONLY practical soft-tissue surgical laser, which uses the laser beam directly to both cut and coagulate the soft tissues.

Benefits of LightScalpel Laser Use

The LightScalpel CO2 laser is used in a broad range of clinical applications.  The laser can augment and even, in certain instances, replace, traditional instruments and methods. The LightScalpel CO2 laser is useful in procedures where:

  • Surface penetration is desired
  • Soft tissue is the target

The 10,600 nm CO2 laser is cleared by the FDA for soft tissue procedures only. The laser is an effective hemostatic tool for vascular tissue. When the CO2 laser is used for muscle dissection there is minimal heating or contraction of the muscle. This helps facilitate certain procedures and reduce post-surgical pain and edema.

The 10,600 nm CO2 laser is not cleared by the FDA for use on bone or in hard tissue procedures in the United States. Proper use of CO2 lasers such as the LightScalpel within the FDA-cleared indications for use may offer some of the following advantages over conventional treatment:

  • Improved access to some areas, compared to the scalpel
  • Reduced operative time in some procedures
  • Tissue sculpting ability
  • Easier removal of lesions without distortion of surrounding tissue
  • Less bleeding, often with less trauma
  • Less need for suturing
  • Reduced postoperative discomfort
  • Minimized edema

CO2 laser surgery wounds may initially take slightly longer to heal than scalpel incisions.  However, several studies indicate that there is no significant difference in healing of wounds made by the two modalities after two weeks.  Patients often report less postoperative pain with laser wounds.

Finally, the LightScalpel laser is more versatile than conventional surgical instruments because it can:

  • Incise, excise, or cut
  • Vaporize or ablate
  • Provide coagulation and hemostasis

co2 laser surgery photos

Want to learn more?

Have any questions about LightScalpel Lasers or CO2 Laser Tissue Interaction?

References
  1. Jacques SL. Optical properties of biological tissues: a review. Phys Med Biol 2013 Jun 7;58(11):R37-61.
  2. Shapshay SM. ed. Endoscopic laser Surgery Handbook, New York, NY: Marcel Dekker 1987:96-125.
  3. Wright CV, Fisher JC, ed. Laser surgery in gynecology: a clinical guide. Philadelphia, PA: Saunders 1993: 58-81.
  4. Cheong WF, Prahl SA, Welch AJ. A Review of the Optical Properties of Biological Tissues. IEEE J of Quant Electronics 1990; 26(12):2166-85.
  5. Vogel A, Venugopalan V. Mechanisms of pulsed laser ablation of biological tissues. Chem Rev 2003 Feb;103(2):577-644.
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