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Key words: lasers, optical fibers, prostate
It has been estimated that at least one-third of all men in the U.S. will eventually seek surgical relief of the symptoms produced by benign prostatic hyperplasia (BPH). While CDRH has received a number of applications for the use of lasers in the treatment of BPH, the distribution of the laser radiation within the prostate tissue is not well documented. OST scientists are investigating the performance of optical fibers which are used to deliver Nd:YAG laser radiation (at a wavelength of 1.06 um) to the prostate in order to answer these questions and apply the results to review guidelines.
This procedure, referred to as transurethral laser prostatectomy, typically involves advancing the sidefiring fiber through a cystoscope into the urethra and coagulating and/or ablating the prostate with the laser radiation under visual observation. Published reports of fiber failure during clinical exposures and questions regarding the performance of different fiber designs prompted an OST study of the optical and thermal performance of the sidefiring fibers used in transurethral laser prostatectomy.
The optical performance is being evaluated by determining the location and volume of laser fluence emitted from the sidefiring fiber into an optical phantom. The thermal performance is being evaluated by determining the amount of material that is coagulated in a protein-based phantom. The thermal performance will also be simulated by first-order algorithms which will predict the distribution of tissue temperatures.
Optical phantoms and clear distilled water were used to help measure the distribution of light from sidefiring fibers. These phantoms consist of a scattering medium and a diluent. The optical properties of the phantom were chosen to simulate those of canine prostate. This allows a comparison of these laboratory results to those involving sidefiring fibers and dogs, as well as providing a distribution of emitted light over a physically measurable volume. Typical light distributions for Nd:YAG laser radiation are shown in figure 31 for a water and an optical phantom environments. The distributions in water are very similar to a recently published study appearing in the British Journal of Urology.
In order to determine the thermal performance of these devices, OST scientists investigated a phantom which both distributes light similarly to prostate tissue and coagulates. This phantom consisted of a coagulating protein, egg albumin, and Interlipid, a clinical nutrient which also scatters light. Early measurements indicate that this coagulating phantom will have optical properties which are similar to those of the optical phantom, but scatter light in a more forward direction, which is similar to that of actual tissue.
An algorithm for determining the temperature distribution in phantom material for an isotropic optical point source has been developed in collaboration with scientists at the Oregon Medical Laser Center. This algorithm uses the optical and thermal properties of the medium and the power of the laser beam, along with its exposure's duration to compute the resulting spatial temperature distribution. This algorithm can be calculated on an office personal computer. It is hoped that the algorithm will predict the coagulation volume that is obtained in laboratory phantoms. [ProA, PreME]
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Key words: cataract, contact/fundus lens, retinal photocoagulation
OST scientists are collaborating with ophthalmologists at the Bethesda National Naval Medical Center to evaluate the risk of producing a cataract in a patient's lens during retinal laser photocoagulation. This procedure is commonly used to prevent or limit blindness from diabetic retinopathy and retinopathy of prematurity. The "laser contact lens," also known as a fundus lens, creates a focus in the back of the eye that is used by the ophthalmologist for viewing the retina and for directing the laser beam. Several different designs of these lenses are available, and some newer designs have a wider field-of-view angle than older designs. There is a potential problem with these wide angle lenses, in that high power density levels can be produced in the patient's natural lens. If the power density reaches a sufficient level to overheat the proteins in the natural lens, a cataract will be formed.
Various optical designs of laser contact lenses were evaluated with different laser beam parameters. Computer modeling and image analysis of photographs taken during simulated treatments were completed. The photographs were taken of a model eye with a side observation window that was designed in OST. This model eye was filled with a scattering medium to allow visualization of the laser beam inside the model. Both the analysis of the computer models and photographs showed that at least two of the designs were more likely to produce high power density levels in the patient's lens.
The ultimate goal of this study is to create awareness in the ophthalmic community of the potential for this kind of unintended injury. An ophthalmologist so warned may choose to avoid certain combinations of fundus lenses and treatment beam parameters. Warning information on the device labels may be appropriate. [ProA, PostMS]
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Key words: hollow fibers, FEL lasers
OST scientists are currently investigating the performance of hollow reflective waveguides because of their potential to be optimized for transmission at many of the wavelengths of interest for laser surgical procedures. Free electron lasers (FEL) are useful for this testing since they are tunable over a wide range of wavelengths. These devices currently are generating output radiation in the mid and far IR and soon will be configured for UV and x-ray wavelengths as well. The tunability of the FEL laser makes it useful for investigating new laser wavelengths for use in cutting and coagulating tissues. One potentially useful new wavelength is 6.45 µm which coincides with a peak absorption wavelength of the amide II band of protein.
A variety of hollow reflective waveguides have been studied with three FELs at 6.45 µm. These waveguides have successfully transmitted pulses with energies up to 80 mJ without damage. OST measured 50% transmission per 1 meter of length at 10 Mwatts peak power, and about 70% for 1 Mwatt peak power pulses. Bending loss and output emission patterns were measured. Spatial homogenation of mode patterns due to multiple reflections have been seen. Work is continuing on the measurement of other wavelengths and on coatings to allow high transmission with wavelengths from the mid-infrared through the x-ray region. [PreME, ProA]
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Key words: silicone, Raman spectroscopy, partial least squares
In April 1992, the FDA restricted the use of silicone breast implants, citing lack of adequate information on health risks associated with such implants. To date, about 2 million women have received polydimethylsiloxane (silicone) - containing implants. Thus, a means for noninvasive determination of implant integrity continues to be a goal of the medical community. The potential sensitivity of optical detection could aid in early detection of leaks.
While methods for quantification of particular components of optically dilute samples are well known, these cannot typically be directly applied to tissue-like systems. The effects of scattering and absorption typically superimpose nonlinearly, making spectral interpretation difficult without sophisticated analytical techniques. Preliminary research in fluorescence and FT-IR spectroscopy indicates that the method of Partial Least Squares (PLS) accurately predicts concentrations in tissue-like systems. An attractive feature of this method is that it requires very little a priori information about sample composition. In short, PLS can be used to accurately determine PDMS concentrations from turbid sample spectra and does not require a priori information if a training set with known concentrations is available.
The work to date on this project represents a preliminary application of Raman spectroscopy in conjunction with the chemometric method of PLS to predict silicone in homogenous turbid samples. The chemometric technique is applied to Raman spectra to develop an empirical, linear model relating sample spectra to polydimethysiloxane (PDMS) concentration. The composition of the phantoms was chosen to span the range typical for human tissue with minimal intersample correlations between chromophore concentrations. PLS, performed via cross-validation, was able to predict silicone concentrations in good agreement with true values. The detection limit obtained for this preliminary investigation is on par with that of magnetic resonance spectroscopy. The data acquisition time for this Raman-based method is 200 seconds, which compares favorably with the 17-hour acquisition required for magnetic resonance spectroscopy to obtain a similar sensitivity. Prediction of PDMS concentration for this sample set is depicted figure 33. The arrow represents the MRS results noted above. The combination of Raman spectroscopy and chemometrics shows promise as a tool for quantification of silicone concentrations from turbid samples. [ProA, PostMS]
