Bridging the 40-100 MHz Gap in Medical Ultrasound Arrays

DESCRIPTION (provided by applicant): The proposed research effort is aimed at bridging the frequency gap in broadband medical ultrasound transducer arrays between approximately 40 MHz and 100 MHz. The gap exists because of manufacturing limitations in the two processes used to construct transducer arrays. At low frequencies, mechanical techniques are employed and at frequencies above approximately 100 MHz microfabrication (i.e., MEMS) techniques are required. The proposed work entails the use of a new PZT thick film process that extends the reach of MEMS technology downward in frequency. The lower limit is currently not known, but it may be as low as 18 MHz. The thick film process is combined with a silicon molding process that greatly simplifies the production of ultrasound arrays. Large arrays can readily be formed, and this leads to improvements in the lateral resolution of phased arrays. A nominal cross-beam resolution of the order of 10-15 mu m is feasible near the focal point. Because 200-400 arrays can be placed on a single 4-inch silicon wafer, the cost of transducer production is small. The overall design is amenable to building CMOS circuitry directly on the substrate used to support the transducer. This will occur as part of the Phase 2 effort. The principal medical applications include imaging diagnostics for certain types of carcinoma (dermatological, mucosal oral and pharyngeal, gastrointestinal cancers amenable to endoscopic examination, and ocular tumors). The use of imaging ultrasound can also be extended to uterine cervical cancer. Ideally, one would image cells and tissue at two frequencies separated by about two octaves (e.g., 50 MHz and 200 MHz). This supports both deeper tissue penetration at lower radial resolution (50 MHz) and shallower penetration with greater radial resolution (200 MHz). Other applications include intravascular ultrasound (used to diagnose vascular pathologies and guide therapeutic measures), and the diagnosis of certain non-cancerous pathologies. In Phase 1, 32-element linear arrays will be constructed and tested to determine the feasibility of the proposed fabrication process. A complete ultrasound imaging system is expected to result from the Phase 2 effort.