First, I would like to thank the dissertation committee for their support and assistance in completing this research. Additionally, I would like to thank Dave Piston for his work in supporting my research.
Introduction and Motivation
Consideration of these processes leads to an understanding of the crucial role of tissue ejection in the ablation process [6]. It is particularly important to facilitate the clinical use of the alternative non-FEL sources as they become available.
Specific Aims
The effect of the pulse structure on the ablation process was studied by changing the original micropulse structure of the Mark-III FEL through the use of a pulse stretcher. This manuscript examines the current state of the fiber delivery technology available for the delivery of mid-infrared light.
The output from the probe laser source (both) was coupled to a 600 µm multimode fiber with a length of ~1000 meters (3M Specialty Fibers Power Core Optical Fiber). The ablation of the liquid was performed using Perfluorodecalin (which has a high boiling point), PF-5060 (which has a low boiling point) and water at 6.45 µm wavelength with FEL.
BACKGROUND
Significance
Traditionally, the free electron laser (FEL) has been the only laser source cable that delivers 6.45 µm laser radiation. It is the aim of this research to facilitate/enable the transition from the FEL to an alternative source capable of delivering 6.45 µm laser radiation.
Tissue Interactions at 6.45 µm
This wavelength of light has efficient ablation with minimal collateral damage, likely due to tissue denaturation due to amide-II bond disruption [1]. Two models that describe infrared laser ablation include the optical breakdown model and the thermal confinement model.
Effect of Pulse Duration/ Pulse Structure on Ablation
- Vanderbilt Mark-III Free Electron Laser
- Pulse-stretched FEL
- Optical Parametric Oscillators
- Strontium Vapor Laser
Another attempt to analyze the effects of pulse structure versus wavelength at 6.45 µm involves using the FEL with a different pulse structure. This device uses the FEL's wide natural frequency bandwidth (50-100 nm) to its advantage.
Ablation
- Phase Diagrams
- Surface Vaporization
- Normal Boiling
- Phase Explosions
Ablation is often driven by a phase transition from the liquid to the vapor state, and thus. At the spinodal temperature, the liquid phase disappears and the superheated liquid becomes unstable, and the transition to the vapor phase via spinodal decomposition is spontaneous.
Precise Tissue Ablation
Ablation with the FEL
The FEL is used in dermatological research for the application of skin resurfacing by Ellis et al. To date, the FEL has been used to perform eight human surgeries and there are still many ongoing research projects to provide new surgical applications focused on this unique wavelength due to efficient ablation with minimal collateral damage.
Thermomechanical Response of Tissue to Pulsed Irradiation
- Thermal Confinement
- Stress Confinement
Thermal confinement is achieved when the ratio of the laser pulse duration to the thermal diffusion time is slightly less than 1. Voltage confinement is achieved when the ratio of the laser pulse duration to the voltage propagation time, τstr., is less than 1.
Ablation Plume Dynamics
In soft tissue ablation, material ejection continues for a longer time after the end of the laser pulse. A possible driving force for the continuation of the ablation process after the end of the laser pulse is the heat retained in the tissue.
Ablation Metrics
- Ablation Threshold
- Ablation Efficiency
For steady-state ablation processes, the ablation efficiency increases monotonically and asymptotically approaches one over the ablation heat for large radiation exposures. In contrast, for blow-off processes, ablation efficiency reaches a maximum at moderate radiation exposures, where energy is wasted by "overheating" the superficial layers of the tissue sample [6].
Mid-infrared Beam Delivery
A direct correlation between ablation efficiency and ultimate tissue tensile strength was observed [22]. This requires the removal or displacement of water or saline to allow delivery of mid-infrared light.
Significance
The existence of optical fibers that can deliver mid-infrared light for minimally invasive surgical procedures is critical to the advancement of this technology for broader clinical use. These studies have demonstrated that the delivery of mid-infrared light for minimally invasive surgical applications is feasible.
Joos, K.M., et al., Free Electron Laser (FEL) laser-tissue interaction with human cornea and optic nerve. Joos, K.M., et al., Optical nerve sheath fenestration with a new wavelength produced by the free electron laser (FEL).
Abstract
Introduction
An analysis of the effect of pulse structure at 6.45 µm involves using the FEL with a different pulse structure. Keck Free Electron Laser Center that can "stretch" the picosecond micropulse of the FEL into a longer pulse.
Methods
- The FEL Pulse Stretcher
- Ablation Threshold Determination
- Ablated Crater Depth Measurement
Losses in the pulse expander are due to grating efficiency and atmospheric losses. To measure the ablation threshold, we chose two different samples; water and mouse dermis.
Results and Discussion
- Ablation Threshold Determination
- Ablated Crater Depth Measurement
Greater distribution of the data is also expected due to the inherent biological variability of the mouse dermis. Thermal confinement is achieved when the ratio between the laser pulse duration and the thermal diffusion time is equal.
Conclusions
With the micropulse structure removed, the intensity of the macropulse can be varied to investigate the effect of average power without worrying about peak power. The tissue effects of the micropulse train remain to be seen and are addressed in chapter 4.
This study focuses on the ablation mechanism at 6.1 and 6.45 µm with careful emphasis on the role of the FEL pulse structure. The output of the oscillator is then sent to a regenerative amplifier (Spectra Physics Spitfire), which in turn is pumped by a pulsed Nd:YLF at 527 nm at 8 mJ per pulse (Spectra Physics Merlin).
THE EFFECT OF FREE ELECTRON LASER PULSE STRUCTURE ON THE
Abstract
The first was a careful analysis of the ablation plume dynamics seen for a 1 ps micropulse compared to a 200 ps micropulse as seen by brightfield analysis. While small differences are seen in the size, but not the dynamics of ablation, due to the bright field imaging; given the 200-fold decrease in peak energy, the differences are negligible.
Introduction
Since most semiconductor or gas lasers have a much simpler pulse structure (often seemingly continuous for several tens of nanoseconds), comparing FELs with more traditional lasers, even if they operate at the same wavelength, is quite complicated. Analysis of the effect of the pulse structure at 6.1 and 6.45 µm involves the use of an FEL with a different pulse structure using a pulse stretcher as described in Chapter 3.
Methods
- Brightfield Imaging
- Histological Analysis
- Canine Cornea
- Mouse Dermis
- Mass Spectroscopic Protein Analysis
The timing of the probe laser was varied relative to the start of the pump-laser pulse. Panel B shows the orientation of the two hemispheres of the cornea once they are embedded in paraffin.
Results and Discussion
- Brightfield Imaging
- Histological Analysis
- Canine Cornea
- Mouse Dermis
- Mass Spectrometry
The zone of thermal damage is clearly seen as a discoloration of the stroma surrounding the ablated crater. It is clear that there is no effect on the extent or type of thermal damage associated with the micropulse structure of the FEL.
Conclusions
The results of the mass spectrometric analysis show that there is no change to the chemical structure of the protein (cytochrome C), either dry protein (data not shown), or protein mixed with deuterium oxide as determined by experts in mass spectrometry. In addition, we showed that the high peak intensity of the native FEL micropulse had no effect on the chemical structure of ablated proteins.
A small difference in cloud size is seen between FEL and OPO due to the small difference in site size. A variable resistor and switch were then added to the timing circuit of the laser driver.
COMPARISON OF ZnGeP 2 OPTICAL PARAMETRIC OSCILLATOR AND
Abstract
The time scale of the cratering, ejection, and collapse phases occurs on a faster scale (5–10 times) for the OPO compared to the FEL. This research has shown that a ZGP-OPO laser may prove to be a viable alternative to the FEL for clinical applications.
Introduction
The results of this research indicate that the micropulse structure of the FEL is not important for the process of soft tissue ablation. This paper focuses on the comparison of the Mark-III FEL with the ZGP-OPO at and 6.73 µm (similar water absorption compared to 6.45 µm without protein absorption) in wavelength with a similar spot size.
Methods
- Mark-III Free Electron Laser
- Er:YAG pumped ZnGeP 2 Optical Parametric Oscillator
- Ablation Threshold Determination
- Ablated Crater Depth Measurements
- Brightfield Imaging
This spot size was required due to the energy limitations of ZGP-OPO to achieve the threefold ablation threshold for water. The hydration state of the sample was carefully maintained to ensure the accuracy of this method.
Results and Discussion
- Ablation Threshold Determination
- Ablated Crater Depth Measurements
- Brightfield Imaging
Furthermore, there is no significant difference between the 500-pulse data point for any of the three OPO wavelengths. Almost all crater depths were found to be significant (P<0.01) for OPO compared to FEL.
Conclusions
The increased rate of ablation was visualized via the bright-field imaging analysis (pump probe). The time scale of the cratering, ejection, and collapse phases occurs on a faster scale (5-10 times) for the OPO compared to the FEL.
For comparison, the laser parameters and stage speed were changed. The first goal was to increase the percentage of 6.45 µm light in the total output of the laser.
CONCLUSIONS AND FUTURE DIRECTIONS
Summary
The effect of the unique micropulse structure of the FEL is not significant with respect to the threshold radiation exposure for ablation at 6.1 and 6.45 µm. Although there is a significant difference due to the micropulse structure of the FEL in terms of ablation efficiency, it is quite small given the 200-fold reduction in peak intensity.
Future Directions
The results of the soft tissue ablation on bovine muscle can be seen in figure A.4. A standard pump-probe imaging technique was used to image the ablation process at the distal end of the fiber [ 32 ]. Under these conditions, no ablation of the target tissue was observed at any time after the laser pulse (data not shown).
DELIVERY OF MID-INFRARED (6-7 µM) LASER RADIATION IN A
