Ultrasound has become the most commonly performed medical imaging procedure in the world because it provides real-time imaging with high clinical value while being portable, non-ionizing and inexpensive. This course will provide an introductory survey of ultrasound imaging focused on the design, fabrication, and testing of medical ultrasound transducers. Starting from an overview of the basic types of phased-array transducers (linear, convex, sector), we will show how the probe’s design is derived from its target application. We will describe how engineering tools, like equivalent-circuit, finite-element, and acoustic field models, can be used to predict transducer performance accurately, and then to optimize the design. A discussion of the structure of an ultrasound probe will lead to a survey of the different types of materials used in probes and their critical properties. Typical fabrication processes will be reviewed and common problems in probe manufacturing will be summarized. Methods for evaluating completed transducers will be described. The course will include recent developments in probe technology, including single crystal piezoelectrics, cMUT transducers, catheters, 2D arrays, and electronics in probes, and will address some of the performance advantages and fabrication difficulties associated with them.
This course provides basic knowledge and understanding of capacitive micromachined ultrasonic transducers (CMUTs) and their applications.
After a short background discussion of previous implementations of capacitive ultrasonic transducers, we will provide all the information necessary for the successful design of a CMUT: The simple parallel plate capacitor transducer and its electrical equivalent circuit model will be explained in detail, including the derivation of all essential design equations, and the theoretical device performance limits. Developing an approximate analytical model that better represents the realizable membrane of a CMUT follows this. A motivation for a more sophisticated finite element model is given, and the key techniques of finite element analysis based CMUT designs are explained and demonstrated using brief examples. Next, we compare and contrast CMUTs that are designed for airborne and immersion applications. Only for immersed operation the periodic structure of a CMUT array needs to be considered to minimize parasitic cross-talk effects. Two acoustic cross-talk modelling techniques will be discussed for that purpose.
Then, the two main CMUT fabrication techniques, i.e. sacrificial release and direct wafer bonding, are explained and compared to each other. Next, we discuss device characterization that will cover electrical, mechanical and ultrasonic measurements: optical interferometer, electrical input impedance, output pressure; receive sensitivity, impulse response and dynamic range. As part of the electrical characterization we will discuss the design and implementation of low noise front-end electronics and the need for their integration in the vicinity of the CMUT.
Besides an overview of several CMUT applications, we conclude the course by giving two detailed design examples, one for an airborne device for chemical/biological sensing applications and one for medical imaging applications.
The organizers gratefully acknowledge the generous support provided by the following