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“Studies of ultrashort laser pulse interaction with vascular and pigmented dermatologic lesions” |
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July 2004 - Aug 2006 | ||
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DESCRIPTION |
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During the time this grant was effective, we focused on the numerical and experimental assessment of important parameters that characterize the interaction of short and ultrashort laser pulses with tissues. As models to study vascular skin lesions, we used single and three-staked agar gel layers with the middle layer dyed with a known concentration of a red colorant to increase the absorption coefficient and thus mimic a blood vessel. These skin models were irradiated with single and multiple short and ultrashort laser pulses (USLP) at different fluences, focusing the beam in the bulk of the layer that mimics the blood vessel. A bubble formation phenomenon was identified (see Figure 1).
This phenomenon was characterized by well defined threshold fluence for permanent bubble formation and a fluence-dependent bubble diameter, which is larger for higher fluences. For laser fluences lower than such a threshold, we obtained laser-induced bubbles that diminished in size with time, this type of bubbles were also studied. It was found that when several laser pulses of fluence below threshold (for permanent bubble formation) were delivered to the sample, both types of bubbles (permanent and transient) could be created depending upon the number of pulses and its delivery rate to the sample. Furthermore, we found that when scattering was added to the skin models medium by mixing intralipid with agar to cast the upper agar gel layer, the fluence required to laser-induce a bubble is significantly larger as compared to the case of homogeneous clear non-scattering agar gel. Laser thermal effects were also studied numerically by solving the heat diffusion equation using a finite differences approach, and experimentally by inserting thermocouples (see Figure 2) to our skin models at specific locations close to the laser-irradiated spot, and measuring the temperature variation during and after laser irradiation. The appearance of bubbles is a clear indication of the thermo-mechanical interaction of short and USLP with tissue-like models. Studies of this phenomenon are of utmost importance for the understanding of laser-tissue interactions. Our studies have allowed us to correlate the extent of tissue damage with the intensity, pulse duration and repetition rates of long and USLP and have set the foundation for an innovative alternative to current laser therapy of vascular lesions and perhaps the only solution for the ablation of very small vasculature and other tissue chromophores, such as benign and malignant pigmented lesions, where pure laser-heating fails to fully remove vascular lesion. Figures 3 and 4 below show examples of the bubble formation experiments that we carried out throughout this project. The figure on the left shows the threshold fluence for bubble formation as a function of the linear absorption coefficient, for two tissue models (single and three layer); the figure on the right shows how a bubble shrinks as a function of time right after the last laser pulse has been applied to the sample.
Other Participants Gerardo Salvador Romo-Cárdenas Rodger Evans Alejandra Mina-Rosales Mr. Kevin Chu |
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