Capacitive Micromachined Ultrasonic Transducers for 3-D Imaging

Ultrasound imaging is a widely used technique for medical diagnostics. For the last 30 years 3-D ultrasound imaging has received increasing interest, as it offers several advantages compared to conventional 2-D imaging. Two-dimensional images are dependent on both position and scan angle, making some imaging planes inaccessible due to the anatomy of the human body. Volumetric imaging does not have the same drawback, as any plane is available from the volume data. It also offers accurate estimation of the size of organs, cysts, and tumors without relying on assumptions and the operator skills needed when using 2-D imaging estimations. However, 3-D ultrasound probes are far more complex than conventional probes, resulting in expensive equipment that impairs the low-cost advantage of ultrasound, and thus limits it more widespread use.
The objective of this thesis is to develop and demonstrate a transducer technology that can produce real-time volumetric images, but without the complexity and cost of available 3-D ultrasound systems. Focus has been on row–column-addressed arrays, offering volumetric imaging with a greatly reduced amount of electrical connections. This reduces data processing requirements and manufacturing cost. To manufacture such arrays, capacitive micromachined ultrasonic transducer (CMUT) technology was chosen as a platform because it offers a high degree of flexibility and interesting properties such as a large bandwidth.
A theoretical treatment of CMUTs is presented, including investigations using an analytic multilayered plate model, and finite element analysis of full arrays. Three different microfabrication processes were investigated to produce stable and reliable transducers. Based on the developed techniques, a 62+62-element row–column-addressed array was then fabricated and assembled into a fully functioning hand-held probe. The transducer and imaging performance was evaluated in relation to a similar piezoelectric probe. Acoustic lens materials were developed to create another row–column-addressed probe with a diverging compound lens. The lens spreads the acoustic energy enabling a curvilinear field-of-view, which is required for abdominal and cardiac imaging applications.
The results show that the row–column technology is a realistic alternative to matrix probes for volumetric imaging, and especially as a low cost alternative. This can contribute to a more widespread use of volumetric ultrasound imaging and to the development of
new clinical applications benefiting both patients and the society