CTO 괴사를 위한 HIFU의 음향 해석 및 전달 효율 연구

Abstract

Cardiovascular disease has been first cause of death in the world, and over 12 million a year has been died by this disease. It is caused by the deposition of materials which are made up the fat and the cholesterol in the artery. Blood flow is reduced because the internal area of the blood vessels is decreased by the precipitate. If these symptoms are persisted for a long time, the lumens of the artery are blocked by the precipitate. This lesion is called by the chronical total occlusion. The conventional cardiovascular treatment methods are difficult to see a therapeutic effect in the case of hardened CTO by the calcification of deposition materials. In addition, treatment is limited by size and space of blood vessel because it is progressed in the blood vessel. The lesion cell such as cancer can be selectively necrotized without affect on normal tissue, using the high intensity ultrasound that focused on one place from extracorporeal. In this study, CTO phantom was necrotized by the focused high-intensity ultrasound energy using HIFU system, and analyzed the acoustic characteristics in focal region. The agar gel is used in order to determine the transfer characteristics of the ultrasonic wave according to the couplant. The lower concentration of the agar gel makes it difficult to maintain the shape, and transmission efficiency of ultrasound is high. Acoustic characteristic according to each couplant was simulated using the COMSOL software. When using water as a couplant, maximum intensity and maximum pressure occurred on 63mm at the same geometric focal distance of the transducer. Position of the focal distance that maximum intensity and maximum pressure are occured, are different depending on property of each couplant. If couplants of two types are used, focal distance is reduced due to the refraction of the ultrasound at the interface. HIFU experimental devices was constructed in order to find the appropriate conditions for the necrosis of CTO. Chalk was used as a CTO phantom. The transmission efficiency of ultrasound is the highest when the couplant is water. The couplant of the agar gel and tissue phantom is less efficient than water, but it is sufficient to tunnel CTO phantom. Because the transfer characteristics of the ultrasound is changed in the focal region by the density difference of CTO phantom, the available frequency band for each density is chosen by the acoustic analysis. Rapid rising of temperature affects normal tissues during HIFU treatment. Tissue damage occurs within a few minutes at 50∼60℃, immediately at 60∼100℃. On-Off control is used in order to reduce the damage to normal tissue and to prevent continuous temperature rise. In case of continuous wave, maximum temperature was rised up to 100℃. On the other hand, maximum temperature was dropped to 50℃ by using On-Off control system.