We experimentally demonstrate that the acoustic transients propagating as a result Free-Electron Laser (FEL) ablation
in brain tissue exhibit a strong FEL wavelength dependence. These acoustic transients were measured with a time-resolved,
polarization quadrature laser interferometer. The transients are multiphased, with displacements of tens of
microns and durations of tens of milliseconds. We calculated the Fourier transforms, power spectra, and pressure
transients based on these displacement data sets. For 3.0 μm irradiation, the bandwidth of the Fourier components
extends to ~20 kHz, while for 6.45 μm irradiation the bandwidth of the Fourier components extend to ~8 kHz. For the
3.0 μm irradiation, the power spectra indicate acoustic energy propagates in the bandwidth up to ~12 kHz, with
structure in the 1-4 kHz range. For the 6.45 μm radiation, the mechanical power spectra indicate the acoustic energy
propagates in the bandwidth up to ~7 kHz, with structure throughout. The pressure transients resulting from 3.0 μm
irradiation have a leading phase with a faster onset, shorter duration, and more than ten times the peak pressure
compared to that observed in pressure transients resulting from 6.45 μm irradiation. For 3.0 μm irradiation, the
observed pressure transients have peak pressures in the MPa range and durations of ~1 ms, while for 6.45 μm
irradiation the pressure transients have peak pressures in the 0.1 MPa range and durations of about ~3 ms.
Representative examples of applications research based on Free-Electron Lasers are reviewed. Research highlights include: observation of absolute negative conductance in semiconductor superlattices using a terahertz Free-Electron Laser at the University of California, Santa Barbara; infrared photon echoes as a technique in nonlinear spectroscopy to investigate vibrational dynamics in liquids and glasses using an infrared Free-Electron Laser at Stanford University; attributing the 20.1 μm stellar spectral feature to titanium carbide clusters using an infrared Free-Electron Laser in The Netherlands; human laser neurosurgery and ophthalmic laser surgery using an infrared Free-Electron Laser at Vanderbilt University; imaging of nanoscale island dynamics during thin film growth using the ultraviolet Free-Electron Laser at Duke University; and nuclear resonant fluorescence measurements for parity assignments in 138Ba using the high intensity gamma ray source at Duke University.
The temperature alterations in the absorption coefficients of water in cartilage and cornea under laser radiation of an IR Free Electron Laser (FEL) were studied for several wavelengths near 2.9 and 6.1 μm water absorption bands using a pulsed photo-thermal radiometer (PPTR). A computation algorithm has been modified to take into account the real IR absorption spectra of the tissue and the spectral sensitivity of the IR detector used. The IR absorption spectra of cartilage and cornea have been also measured by the FTIR spectrometer. It is shown that the values of absorption obtained ising PPTR differ from that obtained by the spectrometer. The limitation and possible errors of two techniques used for have been discussed.
We present in some detail a theoretical model that provides a dynamical account for the experimentally observed ablative properties of an FEL tuned near 6.45 microns. The model is based on thermal diffusion and chemical kinetics in a system of alternating layers of protein and saline as heated by an infrared Mark-III FEL. We compare exposure at 3.0 microns, where water is the sole absorber, to that at 6.45 microns, where both protein and water absorb. The picosecond pulses of the Mark-III superpulse are treated as a train of impulses. We consider the onset of both the helix-coil transition and chemical bond breaking in terms of the thermal, chemical, and mechanical properties of the system as well as laser wavelength and pulse structure.
We describe the commissioning of a novel two-color beamline at the Duke Free Electron Laser Laboratory, designed to perform time-resolved FTIR spectroscopy in a pump-probe scheme with sub-nanosecond resolution to measure dynamical processes with durations as long as ten nanoseconds. The UV pump pulses are produced by the tunable (193 to 700 nm) output of the OK-4 Storage-Ring FEL. The broadband, infrared probe pulses are generated as synchrotron radiation in a bending magnet downstream of the OK-4 wiggler. The repetition rate of the light source (2.79 MHz) is ideal for operating the interferometer in the rapid-scan, asynchronous sampling mode.
We have investigated the experimental consequences of two picosecond infrared lasers, both tuned to 6.45micrometers and focused on ocular tissue. The exposure conditions were comparable, other than pulse repetition rate, where an optical parametric oscillator/amplifier laser (OPA) system operates at a kilohertz and the Mark-III FEL at 3 gigahertz. In both cases, the peak intensity was near 2x1014 W/m2 and the total delivered energy was approximately 125 mJ. The Mark-III consistently ablates tissue, while the OPA fails to ablate or to damage corneal tissue. In particular, there is no experimental evidence for protein denaturation due to OPA irradiation. WE account for these observations in terms of a theoretical model based on thermal diffusion and threshold conditions for superheating and chemical kinetics. We comment on the relevance of tissue geometry.
The Duke FEL Laboratory is a national and international users facility. We describe the current light source capabilities in the infrared, visible, ultraviolet, and Gamma rays. Plans are summarized for the development of two novel beamlines, one for UV-resonant Raman spectroscopy and the other an essentially jitter-free UV-pump, IR-probe `two- color' source with rapid-scan FTIR time-resolved detection of the broadband infrared. Current applications research is summarized, with a more detailed description of research in corneal wound healing.
The effect of temperature dependent shift of water absorption band, known for pure water, has been examined, for the first time, for tissue water, using the IR Free Electron Laser radiation. Cooling kinetics of cartilage and cornea irradiated was measured with a fluorimeter. We have modified the computation algorithm to calculate the optical properties from these measurements more precisely. Temperature dependence of the absorption coefficient of tissue water is studied, for both sides of water absorption bands at 3.0 and 6.1 micrometers . It is shown that cooling kinetics for samples irradiated with small laser intensity is the same, for both wavelengths of each pair: 6.2 and 6.0; 6.35 and 5.92; 3.22 and 2.81; 3.15 and 2.87 micrometers .
The development of the capability for sub-nanosecond time- resolved infrared spectroscopy, combining the broad spectral bandwidths and other well-established advantages of Fourier transform interferometry with the high power, high repetition rate and wide tunability of an electron storage ring-based UV free-electron laser pump, along with the broadband, pulsed, featureless IR continuum of synchrotron radiation from the same storage ring as a probe, is discussed. The capabilities of the system compared to other alternatives for fast, time-resolved infrared spectroscopy are discussed.
We consider the denaturation process as an alteration in ordered organization of tissue structure and study the threshold and kinetics of laser-induced denaturation in cartilage and cornea undergoing irradiation from a free electron laser (FEL) in the wavelength range 2.2 - 8.5 (mu) . Light-scattering by cartilage samples was measured in real- time during FEL irradiation using a 630-nm diode laser and a diode array with time resolution of 10 ms. We found that denaturation threshold is slightly lower than that for cartilage, and both depend on laser wavelength. A strong inverse correlation between denaturation thresholds and the absorption spectrum of the tissue is observed. Only for the wavelength region near the 3 (mu) water absorption band was the denaturation threshold not inversely proportional to the absorption coefficient. We believe this was because the radiation penetration depth was very small in this high- absorption region, so tissue denaturation occurred only in a layer too thin to produce significant light scattering. ATR spectra of 2.4 mm thick cartilage samples was measured before and after irradiation at 6.0 and 2.2 (mu) . At 6.0 (mu) , where the absorption is high, the spectrum of the irradiated (front) surface showed changes, while the spectrum of the back surface was identical to that before irradiation. This difference results from dramatic denaturation (with chemical bond breaking) at the front surface due to laser heating in a small absorption depth. For 2.2 (mu) irradiation, where the absorption is small, the spectra of the front and back of the irradiated sample were unchanged from before irradiation, while light scattering alteration shown the denaturation process began, for laser fluences above the denaturation threshold. This indicates that the absorption is too small to produce deep denaturation of the tissue with dramatic alteration of structure. Thus, we have shown that light scattering is useful for measuring denaturation thresholds and kinetics for biotissues except where the initial absorptivity is very high.
The Free Electron Loss (FEL) at Vanderbilt University is tunable form 2 μm to 9 μm in the mid-IR spectrum, which is capable of controlling predicted laser-tissue interaction by selecting a specific wavelength. However, delivery of this laser into the internal portion of the eye is difficult because of strong water absorption at this spectrum range and the high peak power of the FEL. We used a metallic coated hollow waveguide with a 530 μm inner diameter and 1 meter in length, and delivered the FEL beam to an autoclaved surgical probe. The prove tip was an 18 gauge canula with a mini CaF2 window fixed in front of it to protect the waveguide from contacting water. Human and animal cadaver eyes were used to perform an open sky retinal cutting procedure. The system was able to deliver 60 percent of FEL energy to the intraocular tissues. Up to 6 X 105w peak power was reached without damage to the waveguide or the surgical probe at the spectrum range of 2.94 μm to 7.7 μm. In conclusion, the hollow waveguide is suitable for delivering the IR FEL for intraocular microsurgical procedures.
The purpose of this study is to investigate the healing process in cultured human cornea after infrared Free Electron Laser ablation. Fresh human cadaver cornea was ablated using the Free Electron Laser at the amide II band peak (6.45 micrometers ). The cornea was then cultured in an incubator for 18 days. Haze development within the ablated area was monitored during culture. Histologic sections of the cornea showed complete re-epithelialization of the lased area, and ablation of the underlying Bowman's layer and stroma. The endothelium appeared unaffected. Cultured human corneas may provide useful information regarding the healing process following laser ablation.
A free electron laser (FEL) may be tuned to novel wavelengths to explore laser-tissue interactions for development or improvement of laser surgical procedures. This study investigated the effect of selected infrared wavelengths upon human cornea and optic nerve tissues. Human cadaver eyes were placed in 10% dextran solution to normalize corneal thickness, and solution was injected intraocularly to achieve a physiologic intraocular pressure. The corneas and optic nerves were lased with the 6.0 micrometer amide I band, 6.1 micrometer water absorbency peak, 6.45 micrometer amide II band, and 7.7 micrometer. The Vanderbilt FEL produces 5 microsecond long macropulses at 10 Hz with each macropulse consisting of 1 ps micropulses at 3 GHz. Histologic examination of the corneal tissue showed the least amount of collateral damage (10 - 20 micrometers) with the 6.0 micrometer amide I band, while marked shrinkage occurred with the 7.7 micrometer wavelength. For optic nerve tissue, the least amount of collateral damage (0 micrometer visible) occurred at 6.1 micrometer water absorbency peak and 6.45 micrometer amide II band, while the most damage (30 - 50 micrometers) was observed with the 7.7 micrometer wavelength. We conclude that different tissues may have different optimal wavelengths for surgical laser procedures.
Absorption spectra of 0.1 - 0.2 mm thick, dehydrated sections of human teeth were measured in the transmission mode with a Bruker FT-IR spectrometer from 2.5 - 20 micrometers . Absorption peaks for amide I, II and III, carbonate and phosphate were identified. Craters were ablated in dentin and enamel using a tunable FEL at 6.45 micrometers at various fluences. Pulse duration: 3 microsecond(s) ; spot size (Gaussian, FWHM): 300 micrometers ; repetition rate: 10 Hz. Crater depth and width were measured from digitized optical images. Ablation rates were computed from crater depth and volume data. Selected specimens were examined with scanning electron microscopy to determine ablation surface characteristics. Depth of thermal damage and dentinal tubule morphology were estimated from SEM examination of fractures through ablation sites. Functions describing crater depth vs. number of pulses (quadratic function) were not the same as crater volume vs. number of pulses (linear function). Crater depth decreases with successive pulses, concurrently, the crater width increases. Thus, each pulse removes approximately a constant volume. Material was observed to flow through the dentinal tubules during and after ablation. Patent tubules on crater walls and floor were observed with SEM. Ablation rates in dentin were approximately 3X those in enamel at 6.45 micrometers . Ablation rates and surface characteristics varied across wavelengths from 5.8 to 8.0 micrometers .
A theoretical model is presented to account for the experimental observation that infrared tissue ablation is optimized by the use of wavelengths near the amide II band of proteins. The model recognizes the partitioned absorption of IR photons between protein and water due to overlapping spectral features along with the dynamics of biopolymers, the loss of mechanical integrity in proteins, and the explosive role played by the vaporization of water. The theoretical foundation for this model can be found in previous accounts of thermal confinement, multicomponent models, and selective photothermolysis.
Glenn Edwards, Regan Logan, Michael Copeland, Lou Reinisch, Jeffrey Davidson, J. Johnson, Robert Maciunas, Marcus Mendenhall, Robert Ossoff, Jerri Tribble, Jay Werkhaven, Denis O'Day
The Vanderbilt free-electron laser has been operational for several years. This extended collaboration has been investigating outstanding problems in biological physics and medical physics with several research goals in mind. Our most fundamental goal is to improve the understanding of intermolecular and intramolecular vibrational energy transfer mechanisms in biopolymers. Our approach is to pursue both experimental and theoretical research addressing vibrational energy transfer in biological physics. The remaining goals can be summarized as the application of our fundamental advancements in polymer physics to molecular biology and to clinical and surgical medicine.
Theoretical considerations of thermal lens effect due to linear and nonlinear optical absorption is presented. Based on this model, Z-scan technique, especially two-color Z-scan due can be used to detect very low level of impurities or defects in optical materials. Depending upon the optical cross section of the particular species being probed, two-color Z-scan can detect impurities, for example, the OH groups in fused silica at sub-ppm level by weight or better.
The vibrational dynamics of O-H groups in fused silica have been examined by a time- resolved pump-probe technique using the Vanderbilt Free Electron Laser (FEL). We consider two effects, local heating and transient thermal lensing, which can influence measured T1 values in one color pump-probe measurements. The dependence of these two effects on both the micropulse spacing and the total number of micropulses delivered to the sample are analyzed in detail for the O-H/SiO2 system. The results indicate that transient thermal lensing can significantly influence the measured probe signal. The local heating may cause thermally induced changes in the ground state population of the absorber, thereby complicating the analysis of the relaxation dynamics.
Tunable, pulsed radiation sources in the ultraviolet, visible, and infrared wavelength ranges offer novel opportunities for investigating laser-induced biomedical effects. Free-electron lasers (FELs) deliver continuously tunable, pulsed radiation in the infrared, providing the capability to selectively target radiation into the vibrational modes of water or other biopolymers. Experimental techniques for measuring the absorption spectra of biological samples are described. These spectra indicate wavelengths that potentially serve as the basis for laser-induced biomedical effects. Some practical considerations for infrared, visible, and UV spectroscopy of biological samples are summarized, and the connection between biomedical research and more fundamental investigations of vibrational energy transfer are emphasized.
Free-electron lasers (FELs) provide tunable, pulsed radiation in the infrared. Using the FEL as a pump beam, we are investigating the mechanisms for energy transfer between localized vibrational modes and between vibrational modes and lattice or phonon modes. Either a laser-Raman system or a Fourier transform infrared (FTIR) spectrometer will serve as the probe beam, with the attribute of placing the burden of detection on two conventional spectroscopic techniques that circumvent the limited response of infrared detectors. More specifically, the Raman effect inelastically shifts an exciting laser line, typically a visible frequency, by the energy of the vibrational mode; however, the shifted Raman lines also lie in the visible, allowing for detection with highly efficient visible detectors. With regards to FTIR spectroscopy, the multiplex advantage yields a distinct benefit for infrared detector response.
Our group is investigating intramolecular and intermolecular energy transfer processes in both biopolymers and more traditional materials. For example, alkali halides contain a number of defect types that effectively transfer energy in an intermolecular process. Similarly, the functioning of biopolymers depends on efficient intramolecular energy transfer. Understanding these mechanisms will enhance our ability to modify biopolymers and materials with applications to biology, medecine, and materials science.
The newly commissioned Vanderbilt Free Electron Laser Center for Biomedical and Materials Research is a multidisciplinary users facility intended as an international resource. It provides extremely intense, continuously tunable, pulsed radiation in the mid-infrared (2-10 j.tm). Projects already underway include the linear and nonlinear interaction of laser radiation with optical materials, semiconductors, and mammalian tissue, the spectroscopy of species adsorbed on surfaces, measurement of vibrational energy transfer in DNA and RNA, the dynamics of proteins in cell membranes, the biomodulation of wound healing by lasers, image-guided stereotactic neurosurgery, and the use of monochromatic X-rays in medical imaging and therapy. The purpose of this article is to introduce the machine to the user community and to describe some of the new experimental opportunities that it makes possible. Details of several research projects are presented.
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