II. BACKGROUND
3.4: Results and Discussion
3.4.2: Ablated Crater Depth Measurement
this depth, very little material is removed, making the detection of the event increasingly difficult. As a result, the detection threshold would tend to be seen as higher than the actual value for λ=6.1 µm when compared to λ=6.45 µm.
A trend can also be seen for increasing micropulse durations. As the micropulse duration is increased for both wavelengths, the measured ablation threshold is reduced.
There is a significant difference (P<0.01) for the threshold at 100 and 200 ps when compared with the 1 ps (native) micropulse duration. The only data point that was not significant was the 200 ps data point on mouse dermis at 6.1 µm and is considered to be an outlier. While the trend of decreasing threshold for increasing micropulse durations is consistent for both wavelengths, the absolute amount of change is quite small (<50% in all cases) given the large decrease in peak intensity due to the stretching of the micropulse. The data shows a 1.5 time reduction in threshold given a 200 fold reduction in peak energy. This suggests that any difference in micropulse duration with respect to the ablation threshold is insignificant.
0 200 400 600 800 1000 1200
0 100 200 300 400 500 600
Number of Pulses
Depth (um)
1 ps 100 ps 200 ps
Figure 3.6 A. Crater depth (µm) versus the number of pulses (macropulse) delivered to 90% w/w gelatin at 6.1 µm. The number of pulses was varied between 5, 10, 25, 50, 100, and 500. The micropulse length was varied between 1, 100, and 200 ps. Each data point represents an average of 5 craters. The error bars represent the standard deviation of the 5 craters at each data point. The 100 and 500 pulse craters at 200 ps were found to be significantly deeper (P<0.01) when compared with the 1 ps pulse length.
0 50 100 150 200 250 300 350 400 450
0 100 200 300 400 500 600
Number of Pulses
Depth (um)
1 ps 100 ps 200 ps
Figure 3.6 B. Crater depth (µm) versus the number of pulses (macropulse) delivered to mouse dermis at 6.1 µm. Each data point represents an average of 5 craters (10 craters at 200 ps). The 50 pulse craters at 100 ps and the 25, 50, 100, and 500 pulse craters at 200 ps were found to be significantly shallower (P<0.01) when compared with the 1 ps pulse length.
ablation efficiency by fifty percent when compared to the gelatin data. This reduction is expected due to the increased structural integrity when compared to gelatin. An increased spread of the data is also expected due to the inherent biological variability of the mouse dermis. The opposite trend is also seen when compared to the gelatin data.
The 1 ps pulse leads to the deepest craters, with the 200 ps pulse duration leading to the shallowest craters. No statistically significant difference was seen between the 100 and 200 ps pulses due to the large spread in the mouse dermis data. The 50 pulse craters at 100 ps and the 25, 50, 100, and 500 pulse craters at 200 ps were found to be significant (P<0.01) when compared with the 1 ps pulse length. Figures 3.7a and 3.7b represent the results of the same experiment at 6.45 µm in wavelength. Once again, the stretched pulses were responsible for creating deeper craters in gelatin, but shallower craters in mouse dermis. The 10 and 25 pulse craters at 100 ps and the 5, 10, 25, and 500 pulse craters at 200 ps were found to be significant (P<0.01) when compared with the 1 ps pulse length in gelatin. No significance was seen given the criterion for P<0.01 in the mouse dermis data at 6.45 µm. While the same trends were seen at both 6.1 and 6.45 µm in gelatin and mouse dermis, the absolute differences seen are not large given the spread in data and the large reduction in peak intensity.
In the absence of photochemical processes, the energy absorbed by the tissue in response to pulsed laser irradiation is entirely converted to heat. Once the energy is absorbed, it is subject to spatial redistribution by thermal diffusion. Spatially confined microsurgical effects can be achieved by the use of laser exposures that are shorter than the characteristic thermal diffusion time of the heated volume[5]. Thermal confinement is attained when the ratio of the laser pulse duration to the thermal diffusion time is
0 100 200 300 400 500 600 700 800 900 1000
0 100 200 300 400 500 600
Number of Pulses
Depth (um)
1 ps 100 ps 200 ps
Figure 3.7 A. Crater depth (µm) versus the number of pulses (macropulse) delivered to 90% w/w gelatin at 6.45 µm. The number of pulses was varied between 5, 10, 25, 50, 100, and 500. The micropulse length was varied between 1, 100, and 200 ps. Each data point represents an average of fifteen craters (10 craters at 100 ps). The error bars represent the standard deviation of the 15 craters at each data point. The 10 and 25 pulse craters at 100 ps and the 5, 10, 25, and 500 pulse craters at 200 ps were found to be significantly deeper (P<0.01) when compared with the 1 ps pulse length.
0 50 100 150 200 250 300 350
0 100 200 300 400 500 600
Number of Pulses
Depth (um)
1 ps 100 ps 200 ps
Figure 3.7 B. Crater depth (µm) versus the number of pulses (macropulse) delivered to mouse dermis at 6.45 µm. Each data point represents an average of 5 craters. The error bars represent the standard deviation of the 5 craters at each data point. There was no significant difference (P<0.01) for the 100 and 200 ps data when compared with the 1 ps pulse length.
somewhat less than 1. If the laser radiation is thermally confined in a small volume of the tissue, the possibility exists that ablation can occur before it would be expected in the same volume of water [5, 13]. The rapid heating of tissue by pulsed laser radiation also leads to the generation and propagation of thermoelastic stresses as the heated tissue volume reconfigures to its new equilibrium state. Stress confinement is achieved when the ratio of the laser pulse duration to the stress propagation time is less than 1. Under these conditions, thermoelastic tensile stresses can be generated leading to a reduction in both the ablation threshold and the ablation enthalpy [14]. Changing the micropulse duration from 1 to 200 picoseconds has not caused a change in either the thermal confinement or stress confinement of the target; therefore, we would not expect to see any large changes in the ablation metrics studied.