Laser-induced chemistry for microelectronics.

Laser-controlled chemical reactions are being explored for use in all phases of the processing of semiconductor devices. Laser-induced chemical processing can produce submicrometer features without the aid of photolithography. Research is also providing new information on light-excited and light-enhanced interface reactions.

Preferential light absorption in atheromas in vitro. Implications for laser angioplasty.

Laser angioplasty, the in situ ablation of arterial obstructions with laser radiation, has been demonstrated in animal models and early clinical trials. A problem with this technique, however, is the possibility of thermal damage to adjacent or underlying normal tissues that also absorb the radiation. Using a spectrophotometer with an integrating sphere and a specially constructed tunable-dye laser-based spectrophotometer, we evaluated the transmittance and remittance of human cadaveric atheromas and adjacent normal aorta from 250 to 1,300 nm to identify wavebands where there is preferential light absorption by atheromas. Data were analyzed by both the Kubelka-Munk formalism and a Beer's law model. Both methods indicate that atheromas absorb more than normal aorta between 420 and 530 nm. At 470 nm the average Kubelka-Munk absorption coefficient of atheromas from 10 cadavers was 54 +/- 9 cm-1 compared with 26 +/- 6 cm-1 for normal aortic specimens from seven cadavers. Yellow chromophores responsible for the atheroma absorbance were extractable with xylenes. Thin-layer chromatography and absorption spectra identified the extracted chromophores as predominantly consisting of a mix of carotenoids, which are known constituents of atheromatous lesions. Preferential absorption of blue light by carotenoids in atheromas may permit selective ablation of atheromatous obstructions with appropriate pulses of laser radiation.

Epidermal growth factor receptor-targeted immunophotodiagnosis and photoimmunotherapy of oral precancer in vivo.

Immunophotodiagnosis uses a fluorescence-labeled monoclonal antibody (MAb) that recognizes a tumor-associated antigen to image the fluorescence emitted from the fluorophore-bound MAb that has localized in the tissue. It may be used to diagnose malignant or precancerous lesions, to delineate the margins for tumor resection, or as a feedback mechanism to assess response to treatment. In oral precancer, the epidermal growth factor receptor (EGFR) is overexpressed and could be used as a marker for early detection or as a target for therapy. The goal of this study was to test an anti-EGFR MAb (C225) coupled to either the near-infrared fluorescent dye N,N'-di-carboxypentyl-indodicarbocyanine-5,5'-disulfonic acid for detection or a photochemically active dye (chlorin(e6)) for therapy of early premalignancy in the hamster cheek pouch carcinogenesis model. Fluorescence levels in the carcinogen-treated tissue correlated with the histological stage of the lesions when the C225-N,N'-di-carboxypentyl-indodicarbocyanine-5,5'-disulfonic acid conjugate was used but did not do so with the irrelevant conjugates. Discrete areas of clinically normal mucosa with high fluorescence (hot spots) were subsequently shown by histology to contain dysplastic areas. The best contrast between normal and carcinogen-treated cheek pouches was found at 4-8 days after injection. To test the potential of immunophotodiagnosis as a feedback modality for therapeutic intervention, experiments were conducted with the same MAb conjugated to chlorin(e6) followed by illumination to reduce expression of the EGFR by a photodynamic effect. Subsequent immunophotodiagnosis showed that this treatment led to a significant reduction in fluorescence in the carcinogen-treated cheek pouch compared with nonilluminated areas. This difference between illuminated and dark areas was not seen in the normal cheek pouch. Taken together, the results demonstrate the potential for development of immunophotodiagnosis as a diagnostic tool and as a method of monitoring response to therapy and that the EGFR may be an appropriate target in head and neck cancer.

Pharmacokinetics of a fluorescent drug using laser-induced fluorescence.

Laser-induced fluorescence has been used to measure tissue levels of chloroaluminum sulfonated phthalocyanine in vivo in an implanted hamster cheek pouch carcinoma tumor model. The drug was excited at 610 nm via a pulsed nitrogen laser-pumped dye laser, and fluorescence intensity was monitored at 684 nm for up to 30 days after drug administration. Data were acquired noninvasively with high temporal and spatial resolution using the laser-induced fluorescence apparatus and were analyzed with a multicompartment pharmacokinetic model. In addition, our published data on a C6-BAG glioma rat brain tumor model were analyzed to illustrate the effect of different tumor models on the rates. The rates extracted from the pharmacokinetic model elucidate the mechanisms of drug uptake and retention in the cheek pouch and brain tumor models. The laser-induced fluorescence approach should lead to better drug dosimetry for photochemotherapy and allow quick characterization of the pharmacokinetics of new photosensitizers in tissue.

Semiconductor laser endophotocoagulation of the retina.

Laser endophotocoagulation is an important technique in vitreoretinal surgery. We performed successful retinal endophotocoagulation in the eyes of Dutch-belted rabbits, using high-power phased-array semiconductor lasers, emitting at 808 and 817 nm. The laser itself measured 25 X 30 X 21 mm, was air cooled, and was portable. At a treatment power of 100 mW, uniformly white photocoagulation lesions were seen at exposure durations of 0.2 to 1.0 s. Lesions were similar in appearance to argon laser photocoagulation lesions, as determined by ophthalmoscopy and fluorescein angiography. Thin-section histologic examination of acute lesions revealed injury localized to the outer retina; chorioretinal scar formation was noted ten days after photocoagulation. To our knowledge, this study is the first in which therapeutically useful lesions were produced using a diode laser and demonstrates the feasibility of using these highly efficient and compact laser sources for ophthalmic photocoagulation.

Spectroscopic measurement of diffuse reflectance for enhanced detection of bladder carcinoma.

OBJECTIVES: To assess the diagnostic potential of diffuse reflectance spectroscopy for the detection of bladder carcinoma during cystoscopy. Our hypothesis is that neovasculature in neoplastic (dysplastic and malignant) regions will lead to a blood absorption "signature" that is different from that of normal tissue. METHODS: Diffuse reflectance measurements have been performed in 14 patients undergoing mucosal biopsies or transurethral resection of a bladder tumor. A quartz optical fiber was advanced through the working channel of a cystoscope and placed in gentle contact with the bladder surface. A standard cystoscopy xenon light source was used for illumination and the reflectance spectra were recorded using an optical multichannel analyzer (OMA) system. From the spectra, the relative concentrations of hemoglobin (Hb), oxyhemoglobin (HbO2), oxygen saturation (HbO2%), and the total amount of blood (arbitrary units) were calculated to assess their usefulness in differentiating between neoplastic and benign bladder areas. RESULTS: The spectra of 26 bladder areas (9 malignant areas including 4 carcinomata in situ, 2 dysplastic lesions, and 15 benign areas) have been analyzed. Only the total amount of blood was a useful parameter for the differentiation between neoplastic and benign bladder areas. The sensitivity, specificity, and positive and negative predictive values of this method for neoplastic tissue were found to be 91%, 60%, 63%, and 90%, respectively. CONCLUSIONS: The measurement of diffuse reflectance is a fast, simple, and noninvasive method which allows in vivo determination of bladder blood perfusion. The total blood concentration was increased in neoplastic bladder tissue, making it a tool for tissue diagnosis. The relatively low specificity is a result of inflammatory areas also exhibiting an increased total blood concentration. This pilot study encourages further studies to assess the usefulness of reflectance measurements for enhanced detection of bladder cancer.

Improved osteoinduction of cortical bone allografts: a study of the effects of laser perforation and partial demineralization.

Massive cortical bone allografts have been found to incorporate slowly into host bone and thus are subject to complications such as nonunion, fatigue fracture, and infection. To better understand and improve the process of osteoinduction in these types of bone grafts, a new experimental model was developed with use of diaphyseal cortical bone grafts from rat tibiae that were prepared by partial demineralization and drilling of 0.33 mm diameter holes with a pulsed, 2.94 microns wavelength, erbium:yttrium-aluminum-garnet laser. Six types of grafts were analyzed: untreated (Type I), demineralized 25 microns deep (Type II), demineralized 150 microns deep (Type III), laser perforated (Type V), laser perforated and then demineralized 25 microns deep (Type V), and laser perforated and then demineralized 150 microns deep (Type VI). The graft was orthotopically transplanted in the tibia of an adult Sprague-Dawley rat and followed for as long as 4 months. Histologic evaluation at 1 and 4 months postoperatively with use of hematoxylin and eosin staining confirmed that there was new bone growth in Types II, III, V, and VI grafts. The amount of growth was estimated by comparing bone mineral density before implantation with values obtained after retrieval of the graft. These measurements were correlated to histomorphometric analysis of graft incorporation. The results show that the processes of partial demineralization (p < 0.000001) and laser perforation with partial demineralization (p < 0.000001) were both significant in enhancing bone growth in this model. New bone growth was significantly increased when the grafts were prepared with extensive demineralization (p < 0.015). This study demonstrates that osteogenesis in cortical bone grafts can be fostered through the process of partial demineralization and laser perforation. To the extent that minimal partial demineralization and laser perforation allow maintenance of structural integrity while altering the osteoinductive properties in such a way as to promote ingrowth of new bone, this experimental model represents an advance in understanding how osteogenesis in cortical bone grafts may be improved.

Effect of Er:YAG laser holes on osteoinduction in demineralized rat calvarial allografts.

Massive cortical autografts and allografts have been found to incorporate into host bone very slowly and thus are subject to complications such as fatigue fracture and infection. In order to understand and improve the process of osteogenesis in these types of bone grafts, a new experimental model was developed using bone discs from rat calvaria prepared by demineralization and drilling of 0.5 mm diameter holes with a pulsed, 2.94 microns wavelength Erbium:Yttrium-Aluminum-Garnet laser. Four types of bone discs were analyzed: untreated (Type I), demineralized (Type II), laser-ablated (Type III), and laser-ablated then demineralized (Type IV). The discs were transplanted into a subcutaneous site in adult Sprague-Dawley rats and followed for as long as 6 weeks. Histologic analysis of the discs at weekly intervals with use of hematoxylin and eosin staining confirmed the presence of new bone growth in Type-II and Type-IV discs. The amount of new bone growth in each disc was estimated by determining the mineral x-ray attenuation coefficient, which is proportional to mineral density, from digitized radiographs of the discs. The results showed that the processes of demineralization (P < 0.001) and laser ablation with demineralization (p < 0.05) were both significant in enhancing new bone growth in this model. This study demonstrated that osteoinduction can be fostered in cortical bone through the processes of demineralization and laser ablation. To the extent that laser ablation may allow maintenance of structural integrity while altering the surface geometry in such a way as to promote ingrowth of new bone, this experimental model represents an advance in understanding how osteogenesis in cortical bone grafts might be improved.

Laser-induced fluorescence: experimental intraoperative delineation of tumor resection margins.

The ability of laser-induced fluorescence spectroscopy to delineate tumor margins intraoperatively was studied using a rat intracerebral glioma model. A fluorescent dye, chloro-aluminum phthalocyanine tetrasulfonate (ClAlPcS4), was injected intravenously 24 hours before tumor resection. The animals underwent tumor resection under the operating microscope, guided by laser-induced fluorescence measurement in one group (Group 1) and visual assessment in the other (Group 2). The Group 1 rats had a significantly reduced volume of residual tumor following resection (0.5 +/- 0.2 cu mm vs. 13.7 +/- 4.0 cu mm, mean +/- standard error of the mean, p less than 0.02). Three of the nine animals in Group 1 were tumor-free at 2 weeks following resection, compared with none of the 10 rats in Group 2 (p less than 0.05). Interference from brain autofluorescence was minimized using spectrally resolved detection and the ClAlPcS4 dye, which has a 680-nm fluorescence peak significantly higher than the 470-nm autofluorescence peak of normal brain. Contrast ratios of up to 40:1 were found for glioma:normal brain fluorescence signals. Spatially-resolved spectra were acquired in approximately 5 seconds using a fiberoptic probe. This study demonstrates the ability of an intraoperative laser-induced fluorescence system to detect tumor margins that could not be identified with the operating microscope.

Lack of selectivity of protoporphyrin IX fluorescence for basal cell carcinoma after topical application of 5-aminolevulinic acid: implications for photodynamic treatment.

Clinical trials of topical ALA in photodynamic therapy (PDT) of basal cell carcinoma (BCC) show significant recurrence rates. Exogenous 5-aminolevulinic acid (ALA) is converted by intracellular enzymes to photoactive protoporphyrin IX (PpIX) in human tissues. PpIX generates cytotoxic singlet oxygen when irradiated with visible light in the 400-640 nm range. To evaluate variability and heterogeneity in PpIX production by tumors in such trials, and to assess the usefulness of PpIX for marking skin tumors, we measured PpIX fluorescence distribution in BCC after topical application of 20% ALA cream. ALA cream was applied under occlusion for periods ranging from 3 to 18 h (average 6.9 h, SD 4 h) to 16 BCCs. ALA conversion to PpIX in the BCCs was assessed by in vivo photography, ex vivo video fluorescence imaging, and fluorescence microscopy. External macroscopic PpIX fluorescence, as assessed by in vivo and ex vivo imaging, correlated with the clinical presence of BCC. Examination by a digital imaging fluorescence microscope revealed inter- and intratumor fluorescence variability and heterogeneity. PpIX fluorescence corresponding to full tomor thickness was found in six superficial and four nodular tumors, and partial-thickness fluorescence was observed in five nodular tumors, but no PpIX fluorescence was observed in some areas of superficial, nodular and infiltrating tumors. In a significant number of nodular and infiltrating BCCs, topical ALA appeared to provide little or no PpIX in deep tumor lobules. In addition, no selectivity for tumor tissue versus normal epidermis was seen. The grossly brighter external PpIX fluorescence over tumors may be due, therefore, to enhanced penetration through tumor-reactive stratum corneum and to the tumor thickness.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulsed CO2 laser ablation of tissue: effect of mechanical properties.

The ablation rate of guinea pig skin and bovine aorta, myocardium, and liver by a CO2 laser emitting 2-microseconds-long pulses was quantified. Ablation efficiency was found to be strongly dependent upon the ultimate tensile strength of the tissue; the ablation efficiency of liver is seven times that of skin. Gluteraldehyde cross linking of skin, which is known to greatly increase tissue stiffness but not significantly affect ultimate tensile strength, did not change the ablation rate. The water content of the tissues, which largely determines the optical and thermal properties, was measured and found to vary only slightly. The results demonstrate that tissue mechanical properties are important in the interpretation and modeling of pulsed laser ablation of tissue and that variations in these mechanical properties can lead to drastically different cutting rates for different tissues.

Pulsed CO2 laser tissue ablation: measurement of the ablation rate.

Ablation of guinea pig skin using a CO2 laser emitting 2-mu sec-long pulses has been quantified by measuring the mass of tissue removed as a function of incident fluence per pulse. The mass-loss curves show three distinct regimes in which water evaporation, explosive tissue removal, and laser-induced plasma formation dominate. The data are fit to two models that predict that the mass removed depends either linearly or logarithmically on fluence. Although the data are best fit by a linear dependence upon fluence, plasma formation at high fluences prohibited obtaining data over a wide enough fluence range to differentiate unambiguously between the two models. Ablation efficiency, ablation thresholds, and the optical penetration depth at 10.6 micron were obtained from the measurements.

Er:YAG laser ablation of tissue: measurement of ablation rates.

The ablation of both soft and hard tissue using the normal-spiking-mode Er:YAG laser has been quantified by measuring the number of pulses needed to perforate a measured thickness of tissue. Bone is readily ablated by 2.94 microns radiation; however, at per pulse fluences greater than 20 J/cm2, plasma formation decreases ablation efficiency. At low fluence, desiccation can prevent efficient ablation of bone. The ablation efficiency for aorta and skin is higher than for bone. The ablation efficiency, 540 micrograms/J, and the ablation depth per pulse, greater than 400 microns, for skin are too high to be readily explained by simple models of ablation and thus provide evidence for a more complex explosive removal process.

Atomic-transition lasers based on two-photon dissociation of metal-triiodide vapors.

Visible laser action on atomic transitions of Ga, In, Al, and Bi has been obtained by using an ArF excimer laser to photodissociate vapors of the corresponding metal triiodides. The excitation process involves sequential two-photon dissociation. Output energies of as much as 0.24 mJ at 417.2 nm and energy efficiencies of 2.9% have been obtained with Ga.

Optical imaging technology in minimally invasive surgery. Current status and future directions.

Optical engineering and imaging technology have played a major role in the evolving field of minimally invasive surgery (MIS) by making it possible to visualize the manipulation of tissue at remote internal sites. We assess and review the optical imaging technology used during a variety of MIS procedures from an engineering perspective. The field of MIS is evolving rapidly. Optic-based technologies have the potential to further improve diagnostic capabilities of MIS. Optical imaging technologies and instrument designs are discussed in relation to their current and future use in MIS procedures. Technical limitations in imaging technology are described, along with potential solutions. We review the current status and future role of optical imaging technology in MIS. In the future, synergistic benefits from engineering, imaging technology, and MIS are likely to improve diagnostic ability and patient care.

Measurement of the optical and thermal properties of biliary calculi using pulsed photothermal radiometry.

The optical absorption coefficients for biliary calculi are important in understanding the mechanism of laser-induced stone fragmentation. However, the heterogeneous composition of calculi and difficulties in producing optically thin samples prevent conventional spectrophotometric measurement techniques from being used. To overcome these limitations, we used a pulsed photothermal radiometry system to measure the optical absorption coefficients and thermal diffusivities of various biliary calculi. In the wavelength range examined (350-1,060 nm), there was strong optical absorption which was greater for pigment stones than for cholesterol stones. The data support the theory that the initiation of the plasma accompanying laser fragmentation of calculi is a thermal process.

Er:YAG laser ablation of tissue: effect of pulse duration and tissue type on thermal damage.

The thermal damage caused by 2.94-micron Er:YAG laser ablation of skin, cornea, aorta, and bone was quantified. The zone of residual thermal damage produced by normal-spiking-mode pulses (pulse duration approximately 200 microseconds) and Q-switched pulses (pulse duration approximately 90 ns) was compared. Normal-spiking-mode pulses typically leave 10-50 microns of collagen damage at the smooth wall of the incisions; however, at the highest fluences (approximately 80J/cm2) tears were produced in cornea and aorta and as much as 100 microns of damaged collagen is found at the incision edge. Q-switched pulses caused less thermal damage, typically 5-10 microns of damage in all tissues.

Co:MgF2 laser ablation of tissue: effect of wavelength on ablation threshold and thermal damage.

The wavelength dependence of the ablation threshold of a variety of tissues has been studied by using a tunable pulsed Co:MgF2 laser to determine how closely it tracks the optical absorption length of water. The Co:MgF2 laser was tuned between 1.81 and 2.14 microns, a wavelength region in which the absorption length varies by a decade. For soft tissues the ablation threshold tracks the optical absorption length; for bone there is little wavelength dependence, consistent with the low water content of bone. Thermal damage vs. wavelength was also studied for cornea and bone. Thermal damage to cornea has a weak wavelength dependence, while that to bone shows little wavelength dependence. Framing-camera pictures of the ablation of both cornea and liver show explosive removal of material, but differ as to the nature of the explosion.

Laser induced autofluorescence diagnosis of bladder cancer.

PURPOSE: We assessed the ability of laser induced autofluorescence to differentiate malignant from nonmalignant bladder lesions. MATERIALS AND METHODS: We studied 53 patients with bladder cancer undergoing mucosal biopsies or transurethral resection of a bladder tumor. A quartz optical fiber was advanced through the working channel of a cystoscope and placed in gentle contact with the bladder. Tissue fluorescence was excited by 337 nm. light pulses (nitrogen laser). One fiber was used for transmission of the excitation and emission (fluorescence) light. An optical multichannel analyzer system was used to record fluorescence spectra of the sites of interest. RESULTS: We analyzed the fluorescence spectra of 114 bladder areas (1 carcinoma in situ as well as 28 malignant, 35 inflammatory, 7 dysplastic, 1 squamous metaplastic and 42 normal areas). These lesions included 44 difficult to diagnose suspicious tumors (11 malignant and 33 nonmalignant). We developed an algorithm that used the I385:I455 nm. fluorescence ratio to distinguish malignant from nonmalignant lesions, including inflammatory areas. By analyzing the data on all 114 lesions, we noted the sensitivity, specificity, and positive and negative predictive values of this method for differentiating malignant from nonmalignant bladder lesions to be 97, 98, 93 and 99%, respectively. CONCLUSIONS: Under excitation with 337 nm. light a clear differentiation between malignant and nonmalignant bladder tissues can be made using the I385:I455 nm. autofluorescence ratio.

Microsecond-long flash photography of laser-induced ablation of biliary and urinary calculi.

High-speed flash photographs of laser-induced fragmentation of biliary and renal calculi under water were obtained using one-microsecond-long dye-laser pulses for both illumination and ablation. The photographs show the presence of a bubble with irregularities on the surface that suggest the early presence of debris or microbubbles. Fragmentation occurs before the bubble collapses, suggesting that fragmentation is due to laser-induced acoustic transients rather than to collapse of a laser-induced cavitation bubble.