Low-Attenuation Acoustic Silicone Lens for Medical Ultrasonic Array Probes

Medical ultrasonic diagnostic apparatus have been used as a noninvasivemethod for diagnosing the human body since the 1970s. Figure 8.1 shows various types of commercialized medical pulse-echo ultrasound probes: (1) cardiac probe, (2) abdomen probe, and (3) high-frequency linear probe for diagnostic applications.

The medical ultrasonic array probe consists of four basic materials: backing, piezoelectric, acoustic matching layers, and acoustic lens. A piezoelectric material such as lead zirconate titanate (PZT) or relaxor single crystals is the only active material used for the transducer, which transmits and receives ultrasound. On the other hand, acoustic matching layers are installed on the transducer in order to raise the transmission efficiency of the ultrasound, because the acoustic impedance (Z), which is expressed as sound velocity (c) × density(ρ), of PZT ceramic and that of human tissue are mismatched otherwise;i.e., Z_PZT = 35×10⁶ kg/m²s vs. Ztissue = 1.55×10⁶ kg/m² s. In general, the ultrasound is re-flected 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.