A 150-MHz-Bandwidth Membrane Hydrophone for Acoustic Field Characterization

To measure the beam parameters of intravascular ultrasound imaging transducers with operating center frequencies exceeding 20 MHz and beamwidths below 200 m, a hydrophone with a spot diameter less than 50 m and a bandwidth greater than 150 MHz is required. The hydrophone described in this article is a step towards meeting these requirements. Diagnostic ultrasound imaging is used routinely in a growing number of medical applications. Hewlett-Packard manufactures a range of ultrasound imaging systems that use digital beamformers for cardiology and multipurpose imaging and mechanical beamformers for cardiology, general-purpose, and intravascular imaging. At the heart of an ultrasound system is a transducer, an electroacoustic device that converts electrical signals into a focused mechanical wave and reconverts reflected mechanical echoes from organs and tissue for subsequent real-time image construction. The transducer is a resonant device that has a bandpass filter frequency response. Currently available transducers have a center frequency in the range of 2 to 30 MHz. A phased array of individual transducers, typically consisting of 30 to 300 elements, is electronically focused and steered to provide a beam with the dimensions desired for a selected medical application. Current trends in imaging are to the following higher-frequency applications: Intravascular imaging for plaque detection in blood vessels such as the coronary arteries Contrast-agent-assisted harmonic frequency imaging to view blood flow and perfusion in the hear