Various shape bubbles were generated by changing holmium-yttrium-aluminum-garnet (Ho:YAG) laser irradiation parameters. Intensive pressure waves induced by their bubble collapse were measured. The Ho:YAG laser-induced bubble in water-containing liquid had been reported by many authors regarding its shape and generated collapse pressure. However, controllability of the bubble shape and generated collapse pressure with various irradiation parameters has been still unclear. In our experiments, we changed the core diameter of optical fiber (400μm or 600μm), laser pulsewidth (FWHM 100-300μs or 50-120μs, depends on laser output energy), and positions of the optical fiber tip in a sheath. The bubble shapes were observed with the time resolved flashlamp photography. The expansion and contraction rates of the bubble volume were determined by the obtained bubble shapes. The collapse pressure was measured with a small diameter (0.5mm) calibrated hydrophone. The long Ho:YAG laser pulse irradiation made long shape bubble so-called "pear shaped" bubble. This pear shaped bubble generated low collapse pressure comparing to the spherical shape bubble which was generated by the short pulsewidth. Using the constant laser pulse energy, we obtained large volume bubbles with high collapse pressure by the optical fiber of 600μm core diameter. When the optical fiber tip was located in the sheath, the bubble expanded to the lateral direction, and then the high collapse pressure was observed along the lateral direction. Therefore, we could arrange the bubble shape by changing the irradiation parameters. We discussed the proper bubble shape for various intra-vascular applications.
KEYWORDS: Temperature metrology, Arteries, In vitro testing, In vivo imaging, Injuries, Animal model studies, Tissues, Laser irradiation, Laser therapeutics, Metals
We have been proposed novel short-term (<10s) heating balloon using the combination of light-heat conversion mechanism and heated contrast medium irrigation in the balloon to improve dilatation characteristics of balloon angioplasty. Our new balloon angioplasty had suppressed intimal hyperplasia in rabbit model. We designed following experiments to understand the mechanism of suppression of intimal hyperplasia in our new thermal balloon angioplasty. We also aimed to obtain the suitable heating condition in our angioplasty to suppress intimal hyperplasia. We studied influence of the short-term heating on smooth muscle cells (SMCs) lethality in vitro. We investigated number of SMCs reduction in media in order to prevent intimal hyperplasia. We applied to our heating balloon dilatation to chronic rabbit model using normal iliac artery to study relation between heating condition and hyperplasia suppression. We estimated temperature history of the rabbit vascular wall by thermal conduction calculation. We related the estimated temperature history to the hyperplasia suppression effect in the chronic rabbit model. Finally, we obtained the relation between number of SMCs decreases and intimal hyperplasia suppression. We obtained that the short-term heating with 10s laser irradiation corresponding to estimated temperature of 50°C in the media and prevented intimal hyperplasia in the rabbit chronic model. In this case, we estimated about 30 percents of SMCs cellular lethality in media.
We proposed new method of blood removal using Ho:YAG laser(λ=2.1μm)-induced water-vapor bubble to prevent distal tissue ischemia during angioscopic imaging. We successfully demonstrated capability of this blood removal method using blood-filled pocine coronary artery ex vivo. We used laser irradiation conditions of 200mJ in pulse energy and 2Hz in repetition rate. Ho:YAG laser for blood removal and flash lamp lighting for endoscopic illumination were irradiated in the porcine coronary artery through individual fiber optics. Timing of the flash lamp lightning of 2μs in duration was arranged to illuminate the maximum blood removal space, i.e. the laser induced water-vapor bubble. We successfully obtained intra-lumen view via a thin angioscope using the laser blood removal without using saline injection. We studied to determine the optimum laser-induced bubble formation which indicated the minimum invasion against the blood vessel. The time resolved photography in vitro, transient pressure measurement in vitro, and acute historogical study on irradiated vessel wall in vivo were employed for this determination.
We designed the method for prevention of restenosis after balloon angioplasty using laser-induced bubble-collapse acoustic wave. This study was performed to evaluate the effect on smooth muscle cells (SMCs) by Ho:YAG laser (λ=2.10μm)-induced acoustic wave, in vitro and in vivo. The laser energy was delivered by a silica glass fiber into water. Sound pressure was measured with a hydrophone changing the laser energy. The laser-induced acoustic wave was loaded to SMCs in vitro. This acoustic effect on SMCs was measured by MTT assay. The acoustic wave loaded SMCs were controllably injured with the laser energy and laser shots. The balloon denudated rabbit aorta was used to evaluate in vivo effect. The laser-induced acoustic wave loaded aorta was extracted at 42 days after the laser irradiation, and was examined by Hematoxylin-Eosin staining. We found that the laser irradiation of 20 pulses with 60mJ/pulse prevented SMCs proliferation. We think the mechanism of this effect might be same as brachytherapy. We demonstrated the applicability of Ho:YAG laser-induced acoustic wave against vascular restenosis after balloon angioplasty.
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