Front End Systems for CMUT-on-CMOS Based Intracardiac Echocardiography (ICE) Catheters
F. Levent Degertekin
G.W. Woodruff School of Mechanical Engineering, Atlanta, GA, USA
School of Electrical & Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA
3-D imaging ICE catheters with large element counts present design challenges in achieving simultaneous data readout from all elements while significantly reducing cable count for a small catheter diameter. Large cable count is especially problematic for ICE catheters that would be used under magnetic resonance imaging (MRI) due to RF heating. In this talk, we give a brief overview of capacitive micromachined ultrasonic transducers (CMUTs) and CMUT-on-CMOS approach for integration of ultrasound imaging transducer arrays and electronics with examples from forward looking intravascular imaging. We then introduce an MRI compatible ICE system that we have been developing during the past two years. We describe the 1-D and 2-D CMUT arrays developed for this purpose and their characterization. We focus on the front-end electronics which uses area efficient real-time programmable on-chip transmit (TX) beamformer circuit to reduce the cable count on the TX side and analog 8:1 Time Division Multiplexing (TDM) and Direct Digital Demodulation (DDD) to reduce the cable count on the receive (RX) side . The 16 channel TX beamformer IC with 30V pulsers was fabricated in 0.18µm HV process occupying 3.22 x 0.9 mm2 area. Each of the pulsers have individual 8 bit registers to set delays up to 1.27µs in 5ns increments and a common 8 bit programmable counter is used to control the pulse width. The transmit beam can be reconfigured using a single additional coax cable in 0.7µs during RX allowing rapid phased array imaging. A TDM based RX Analog Front End (AFE) for 32 receiver channels containing amplifiers and multiplexers has been fabricated in the same process and operates from a 1.8V supply requiring 3.25x.55mm2 of IC area and consuming ~9mW (10% duty cycle) of power. The TDM multiplexers were designed for transducers with 7MHz center frequency and 80% fractional bandwidth, sampling each channel at 25MSPS. The output of the multiplexer runs at 200MSPS and is connected to a high speed ADC via 1m of 48AWG µ-coax cable with 0.18mm outer diameter – a realistic bandwidth limited channel. Both the TX beamformer and RX TDM ICs have been successfully tested. Characterization of the receiver using a tone signal shows less than -32dB inter-channel electrical crosstalk (Fig 1-a) and the demultiplexed time domain responses from elements of a 1-D CMUT array have been measured (Fig 1-b). Hydrophone beamplots of the TX IC driving 16 elements from the same CMUT array verify focused and steered modes of operation (Fig 1-c and d). These demonstrations indicate the feasibility of a single chip front end for challenging 3-D ICE imaging applications.
 Carpenter, Thomas M., et al. "Direct Digital Demultiplexing of Analog TDM Signals for Cable Reduction in Ultrasound Imaging Catheters," IEEE Trans. On Ultrasonics, Ferroelectrics, and Frequency Control, vol. 63, Issue. 8, pp. 1078 – 1085, August 2016.
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