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The effects of fine metal oxide particles, particularly those of high-density elements (7.7 to 9.7 x 10(3) kg/m3), on the acoustic properties of silicone rubber have been investigated in order to develop an acoustic lens with a low acoustic attenuation. Silicone rubber doped with Yb2O3 powder having nanoparticle size of 16 nm showed a lower acoustic attenuation than silicone rubber doped with powders of CeO2, Bi2O3, Lu2O3 and HfO2. The silicone rubber doped with Yb2O3 powder showed a sound speed of 0.88 km/s, an acoustic impedance of 1.35 x 10(6) kg/m2s, an acoustic attenuation of 0.93 dB/mmMHz, and a Shore A hardness of 55 at 37 degrees C. Although typical silicone rubber doped with SiO2 (2.6 x 10(3) kg/m3) shows a sound speed of about 1.00 km/s, heavy metal oxide particles decreased the sound velocities to lower than 0.93 km/s. Therefore, an acoustic lens of silicone rubber doped with Yb2O3 powder provides increased sensitivity because it realizes a thinner acoustic lens than is conventionally used due to its low sound speed. Moreover, it has an advantage in that a focus point is not changed when the acoustic lens is pressed to a human body due to its reasonable hardness.

Effects of heavy density (rho = 9.2 x 10(3) kg/m(3)) Yb(2)O(3) fine dopant (16 nm in diameter) on the acoustic properties of a high-temperature-vulcanization (HTV) silicone rubber have been investigated, to develop a new acoustic lens material with a low acoustic attenuation (alpha) for the medical array probe application. The HTV silicone rubber has advantages in that it shows a lower alpha than that of a room-temperature-vulcanization (RTV) silicone rubber and it can be mixed by applying shear stress, using roll-milling equipment. Roll-milling time dependence of the HTV silicone rubber indicates that the alpha is closely affected by the dispersion of nanopowders in the rubber matrix. The 8 vol% Yb(2)O(3)-doped HTV silicone rubber mixed for 30 min showed the lowest alpha of 0.73 dB/mm MHz with an acoustic impedance [AI = sound speed (c) x density (rho)] of 1.43 x 10(6) kg/m(2)s at 37 degrees C. Moreover, simulation results reveal that a 5 MHz linear probe using the HTV silicone rubber doped with Yb(2)O(3) powder showed relative sensitivity around 2.6 to 3.0 dB higher than a probe using RTV silicone rubber doped with Yb(2)O(3) powder or SiO2-doped conventional silicone rubber for the ultrasonic medical application.

35 × 10 6 kg/m 2 sv s.Ztissue = 1.55 × 10 6 kg/m 2 s. In general, the ultrasound is reflected at the boundary with different Z. Moreover, a convex-shaped acoustic lens is attached on top of the acoustic matching layers to focus the ultrasound beam and to ensure good contact to the human body. Specifically, the acoustic lens is used for focusing of the short axis; the elevation dimension determines the effective slice thickness of the image plane. The ultrasound transmission is attenuated in acoustic matching layers, acoustic lens, and human tissue. The acoustic attenuation (α) of the acoustic lens is very important because it is usually thicker than the matching layers. Moreover, α becomes large with increasing frequency; the α of the acoustic lens has a great effect on the sensitivity of a medical ultrasonic probe as well. Silicone rubber has been the predominant acoustic lens material for external focusing since 1974 (Iinuma and Hashiguchi 1974, 1978) because it is safe to use

Effects of fine nano-powder ceramic powders on the acoustic properties of silicone rubber have been investigated, in order to develop an acoustic lens material with a low sound velocity (c) and a low acoustic attenuation (alpha). Silicone rubber doped with Yb2O3 powders having nanopowder size of 16 nm showed lower alpha than silicone rubber doped with other powders. The silicone rubber doped with Yb2O3 powder showed a density of 1.54 times 103 kg/m3, c=919 m/s, an acoustic impedance Z= 1.42 times 106 kg/m2s, alpha = 1.08 dB/mm/MHz at 25degC. The low c and alpha of the lens material contributes to realize a low loss acoustic lens for medical array probes for diagnostic application.