MTL Annual Report

Most current ultrasound imaging systems use piezoelectric materials for the ultrasound transducer. The recent development of micro-electromechanical systems (MEMS) allowed fabrication of capacitive micromachined ultrasound transducers (CMUTs).  A CMUT is a micromachined capacitor whose value changes according to the DC bias voltage or external pressure due to the physical deformation of the top plate by electrostatic force or external pressure. The major advantages of this transducer technology are the potential for integration with supporting electronic circuits, ease of fabrication, higher resolution due to small transducer size, and improved bandwidth and sensitivity ^[1].

Figure 1

This project focuses on the front-end design of portable ultrasound systems using CMUTs. Figure 1 presents a conceptual block diagram of the system. Implementing an ADC at each channel input makes possible digital beam-forming in the receive (Rx) path, which enhances ultrasound image quality. To implement as many ADCs as the number of transducer channels, each ADC must consume as little power as possible, and each should be implemented in a small area. Considering the required performance, zero-crossing-based (ZCB) pipelined ADC is a suitable architecture ^[2].  For the first part of this project, a 150-MHz 12-bit ZCB pipelined ADC is designed. The highly digital implementation characteristic of the zero-crossing detection technique enables energy-efficient operation and voltage scaling. Supply-voltage scaling based on the required sampling frequency and resolution provides constant energy efficiency over a wide range of sampling frequencies and resolutions.

Recently, a few 2D imaging systems using CMUT as ultrasound transducers have been reported, but they do not use real-time imaging [1]. The digital image processing block will be considered in the system level for real-time imaging.  After completing the 2D ultrasound image system using a 1D transducer, we will examine the feasibility of the 3D ultrasound image system using 2D transducers.

1. O. Oralkan, “Acoustical Imaging Using Capacitive Micromachined Ultrasonic Transducer Arrays: Devices, Circuits, and Systems,” Ph.D. thesis, Stanford University, 2004. [↩]
2. L. Brooks and H.-S. Lee, “A Zero-Crossing-Based 8b 200MS/s Pipelined ADC,” Solid-State Circuits Conference, 2007, ISSCC 2007. Digest of Technical Papers. IEEE International, pp. 460-615, 2007. [↩]