서강대학교

초록/요약

For high-intensity focused ultrasound (HIFU) transducers, it is desirable to make an F-number as small as possible to achieve tight focusing. Also, a large aperture is preferable to generate ultrasound energy high enough to induce tissue coagulation. Satisfying the both conditions at the same time is demanding, in particular, for some applications such as cosmetic surgery for skin and treatment of skin cancers. This thesis proposes a double-focusing technique to make an F-number unity or less while keeping an aperture size as large as possible. In the proposed method, the aperture of an ultrasound transducer is spherically shaped for initial focusing and an acoustic lens is used to finally focus ultrasound on a target depth. By doing so, it is possible to build a focal depth at the desired location without reducing aperture. In accordance with the proposed method, we designed two double-focusing ultrasound transducers; the one has a concave acoustic lens and another one is with a convex acoustic lens. To verify the performance, we fabricated a double-focused ultrasound transducer with concave acoustic lens, of which the F-number has a unity: aperture size of 10 mm in diameter and focal depth of 10 mm. This causes ultrasound energy to be focused at the depth of 3.5 mm from the skin surface because the maximum lens thickness is 6.5 mm. The experimental results demonstrated that the fabricated double-focused transducer had the focal depth reduced from 15 mm after the initial press-focusing to 9.35 mm after the final lens-focusing, and thus the F-number changed from 1.5 to 0.935. Based on the results, we concluded that the proposed double-focusing method is suitable to decrease the F-number while maintaining a large aperture size.