NewsEtc., May, 1998. Steven Jacques, Oregon Medical Laser Center
A common use of medical lasers is to vaporize a tissue for the purpose of removing an obstruction or mass of unwanted tissue such a tumor or fibrous tissue. Some of the common lasers use for such a purpose are the continuous argon ion laser, Nd:YAG, and CO2 lasers. This article describes the basic dosimetry that predicts the rate of tissue removal by a continuous laser. This article does not consider tissue removal by pulsed lasers in the microsecond to nanosecond regime, which requires consideration of mechanical effects, or shorter sub-ns pulses which usually involve nonlinear absorption and plasma formation.
The basic idea is that tissue is mostly water and removing tissue is like boiling water. A gram of water requires a given amount of energy in order to be vaporized into gas. By calculating the amount of laser energy absorbed per unit area at the tissue surface, one can estimate the grams of water vaporized per unit time per unit area. It's all a matter of energy bookkeeping.
But the above basic idea is too simplistic. In general, more energy is required than predicted by the simple "boiling water" model.
The removal of tissue by vaporization is called "ablation". During ablation as tissue vaporizes, the position of the tissue surface, where tissue and air meet, moves deeper into the tissue. One is drilling a hole into the tissue. The velocity at which the hole becomes deeper is called the "ablation velocity".
Wet tissue strongly absorbs CO2 laser radiation within the first 12-15 µm of tissue (the absorption coefficient µa of water = 868 cm-1, 1/868 = 12 µm). The energy deposited is available for vaporization of water. The ablation of tissue by a continuous CO2 laser is approximated by a simple formula:
where
E = irradiance [W/cm2]
Q = heat of
vaporization of water [J/cm3]
f = the apparent efficiency of converting absorbed energy into ablation [dimensionless].
| KT Schomacker et al, Lasers Surgery and Medicine 10:74-84, 1990 ablation of skin | from Fig. 9., Mass loss (g) vs Total dose (J), 4.8-6.6 kJ/g f = 2520/4800 = 0.53 (using 680 µm dia spot) f = 2520/6600 = 0.38 (using 250 µm dia spot) |
| Coffelt DW et al., J Clin Periodontol 1997 Jan;24(1):1-7 ablation of bacteria from surface of teeth | (ablation threshold 11 J/cm2)(500 cm-1) = 5500 J/cm3 f = 2520/5500 = 0.46 |
| Arashiro DS et al, Int J Periodontics Restorative Dent. 1996 Oct; 16(5): 479-491 Ablation of skin | (5 pulses x 206 J/cm2/pulse)/(0.129 cm hole) = 7980 J/cm3 f = 2520/7980 = 0.32 |
For our example, let's assume f = 0.40.
So our 20-W CO2 laser would drill a 2-mm dia. hole at a rate of about 1.0 mm/s. That's pretty slow, unless it's YOUR skin and then it might seem plenty fast!
The ablation of tissue by a continuous Nd:YAG laser requires the formation of a carbonized tissue layer before rapid ablation occurs. At first, the Nd:YAG laser simply heats and dries the tissue until the combination of DRY and HOT causes tissue oxidation that yields a carbon layer. Once the carbon layer forms, the carbon layer strongly absorbs Nd:YAG laser radiation to drive rapid vaporization and ablation and creation of more carbon. The process is rather dynamic and chaotic with continuous cycling of a heating/dessication/carbonization/superheating/explosive vaporization/ablation cycle. On average, there is a net average carbon layer thickness and a net average velocity of ablation. The average velocity of ablation is approximated by a simple formula:
where
E = irradiance [W/cm2]
Q = heat of vaporization of water [J/cm3]
µa = absorption coefficient of carbonized tissue [cm-1]
d = thickness of carbon layer [cm]
k = an augmentation factor due to multiple passes of light through the carbon layer caused by light scattering and total internal reflection
f = the apparent efficiency of converting absorbed energy into ablation.
Q: Q = 2520 J/cm3.
The optical depth µad equals 0.2 at the NdYAG laser wavelength (1064 nm), based on the carbon layer formed during Nd:YAG laser ablation of chicken breast. NOTE: If the thickness d is 25 µm, a number reported by Verdaasdonk from microscopic examination of laser-carbonized chicken, then µa = 80 cm-1 at 1064 nm. The carbon layer thickness actually fluctuates during ablation as carbon is cyclically formed then ablated, but on average µad = 0.2.
k: k is the augmentation factor due to tissue scattering that accounts for multiple passes of photons through the carbon layer which also contributes to heating of the carbon layer. The delivered irradiance passes once through the carbonized layer to cause heating. However, diffuse reflectance backscattered from the tissue can also heat the carbon layer. Moreover, this reflected light arrives at the surface from all angles and on average passes through the carbon layer at 60° off the normal to the surface. Hence, the photons travel twice as far in passing through the carbon layer obliquely as they do when passing perpendicular to the tissue surface. The oblique passage augments heating by a factor of 2. There also is some total internal reflectance (ri = 0.50) at the air/tissue surface which redirects about half of the escaping light back into the tissue to again cross the carbon layer obliquely. A typical Nd:YAG diffuse reflectance may be about Rd = 0.49. The perpendicular incident transmission through the carbon layer is T1 = exp(-µad) = 0.82. The oblique diffuse transmission through the carbon layer is T2 = exp(-2µad) = 0.67. The factor k is calculated by considering the multiple passes through the carbon layer as photons are backscattered by the underlying tissue and internally reflected at the tissue surface:
f: The efficiency of converting energy absorbed by the carbon layer into ablation has been determined by experiments on Nd:YAG laser ablation of chicken breast from the grocery store. The process is less efficient than the f for CO2 laser ablation.
For our example, f = 0.13, which is lower than the CO2 laser efficiency.
So the 637-W/cm2 Nd:YAG laser would ablate tissue at the rate of 150 µm/s which is about 7-fold slower than the 637-W/cm2 CO2 laser. Keep in mind that the carbon layer is very thin and only laser energy absorbed by the carbon contributes to vaporization (µad k = 0.45). But even accounting for the partial heat deposition in the carbon layer, the efficiency is still less than the CO2 laser (f = 0.13 vs 0.40).
In summary, laser ablation of tissue is approximated by simple boiling of water due to energy deposited in the superficial tissue layer, modified by an efficiency factor f. 100% of the delivered CO2 laser radiation is absorbed by the tissue water near the surface and f = 40% of this absorbed energy is used for ablation. About 45% of the delivered Nd:YAG laser radiation is absorbed by a superficial carbon layer and only f = 13% of this absorbed energy is used for ablation. Other continuous lasers at wavelengths with low tissue absorption in the visible (argon ion laser, 488/514 nm) and near infrared (diode laser, 805 nm) also depend on a carbon layer to achieve ablation and should follow the rule of the Nd:YAG laser example but with a different factor µad.