The ablation velocity follows the rule:
v = (fµadk)E/Q
which is shown as the dashed line where fµadk = 0.06 and Q = 2520 [J/cm3]. Since µad = 0.20 and k = 2.27, f must equal 0.13.
A continuous Nd:YAG laser was used to ablate chicken breast tissue from the grocery store. The depth of ablation for a variety of laser exposures was measured to yield the ablation velocity v [cm/s].
Experiment 1: broad Gaussian beamA 90-W Nd:YAG laser was delivered through an optical fiber held distant from the tissue surface to yield a circular spot size that was roughly Gaussian in shape with a 1/e2 diameter of 5 mm. A flat-field circular beam of 5 mm diameter would yield a uniform irradiance of 90/(pi 0.252) = 458 [W/cm2]. The center of the Gaussian beam has twice the irradiance of such a flat-field beam, i.e., E = 916 [W/cm2]. |
Experiment 2: flat-field beamA 30-W Nd:YAG laser was focused into an optical fiber and the fiber output imaged by an objective lens to yield a flat-field beam of 1.4 mm diameter. The power was varied to yield a range of irradiances (E). |
In experiment 1, during the first 20 s of irradiation the tissue whitened due to coagulation but there was no ablation. At about 20 s, the surface finally began to dry and immediately a carbon layer formed. Thereafter, the absorption by the carbon layer enabled efficient laser energy deposition and ablation proceeded at a regular pace.
In experiment 2, a little India Ink was placed on the tissue surface to initiate energy deposition and elicit a carbon layer. The ink was immediately ablated and the ablation process proceeded based on a quasi-steady state layer of carbon created by the laser.
The following figure plots various ablation velocities as the laser irradiance E [W/cm3], was varied.
| | Ablation velocity (v) vs laser irradiance (E).
The ablation velocity follows the rule: v = (fµadk)E/Qwhich is shown as the dashed line where fµadk = 0.06 and Q = 2520 [J/cm3]. Since µad = 0.20 and k = 2.27, f must equal 0.13. |
In summary, the ablation of chicken breast tissue by the Nd:YAG laser relies on a carbon layer to absorb laser energy. The ablation velocity is less efficient than predicted by 100% efficient conversion of absorbed energy into evaporated water. Rather, the efficiency is only f = 0.13.
NewsEtc., May, 1998. Steven Jacques, Oregon Medical Laser Center