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Download as PDFAnalog Front-end Design for Portable Ultrasound Systems
The Capacitive Micromachined Ultrasound Transducer (CMUT) is an alternative to traditional piezoelectric transducers. The CMUT technology provides an opportunity for highly integrated ultrasound-imaging system solutions because of its CMOS compatibility and ease of large array fabrication [1] .
This project aims to provide a highly flexible platform for 3D ultrasound imaging. Figure 1 presents the system architecture. The CMUT device is flip-chip bonded to the supporting electronic circuits, which eliminates the cables. As a result, the channel count of the imaging system is increased and the capacitive loading due to cables is greatly reduced. The transmitters in the system are reconfigurable to implement Tx Beamforming; the analog front-end receivers and the DSP perform various Rx Beamforming algorithms from the received echo waveforms. We successfully implemented a 2D ultrasound imaging system based on a 1D transducer and corresponding electronics as the first step. Currently a 3D ultrasound-imaging system using 2D transducers is investigated.
In the finished 1D IC fabricated in 0.18µm CMOS process, we implemented 4 channels of Tx and Rx analog front-end circuits. Each channel includes an LNA, a VGA and a high voltage pulser. Acoustic measurements are carried out to demonstrate the improved energy efficiency of the 3-level pulser in Tx. A mechanical 3D translation stage is set up, which mounts a hydrophone to probe ultrasound pressure in the medium. The pressure field measured is referred back to the surface of the CMUT transducer, so that the total emitted acoustic power can be calculated. The Tx efficiency is measured by dividing the acoustic power by the total transmission power. Different pulse shapes lead to different Tx efficiency results. The efficiency improvement of 3-level pulse shape over the traditional 2-level pulse shape is measured to be approximately 50%, which is in agreement with the theory (shown in Figure 2).
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- Figure 1: The architecture of a 3D ultrasound imaging system with CMUT devices.
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- Figure 2: The measured Tx efficiency improvement of 3-level pulse shape over the traditional 2-level pulse shape.
- O. Oralkan, “Acoustical imaging using capacitive micromachined ultrasonic transducer arrays: Devices, circuits, and systems,” Ph.D. dissertation, Stanford, Palo Alto, 2004. [↩]