Design, Prototyping and Characterization of a CMUT-on-ASIC Probe for High-Volume-Rate 3-D Abdominal Ultrasound Imaging

Abstract

Accurately estimating the risk of Abdominal Aortic Aneurysm (AAA) rupture is key to improving the treatment of patients suffering from this disease. While conventional risk assessment is based on the geometrical properties of the vessel, acquired via ultrasound imaging, it is expected that acquiring data on more of the vessel's properties will lead to an improved prediction of the risk of rupture, and thus improved treatment of patients. These properties include 3-dimensional (3-D) blood flow through the vessel and its 3-D elasticity, which require an ultrasound probe consisting of thousands of transducer elements capable of imaging a large volume at very high volume-rates. To address this challenge, this dissertation presents the design, fabrication and characterization of a 4096-element ultrasound probe for high-volume-rate (HVR) cardiovascular imaging.

The probe consists of two custom-designed application-specific integrated circuits (ASICs), each of which interfaces with a 2048-element transducer array, which in turn can consist of bulk-fabricated piezo-electric transducers, or monolithically integrated capacitive micro-machined ultrasound transducers (CMUTs). The probe can image a 60◦×60◦×10-cm volume at 2000 volumes/s, the highest volume-rate with in-probe channel-count reduction reported to date. It uses a 2×2 delay-and-sum micro-beamformer (µBF) and 2× time-division multiplexing (TDM) to achieve an 8× receive (RX) channel count reduction, and is the first to scale this combination of techniques to an array of thousands of elements. Equalization, trained using a pseudorandom bit-sequence generated on the chip, reduces TDM-induced crosstalk by 10 dB, enabling power-efficient scaling of the cable drivers. The ASICs also implement a novel transmit (TX) beamformer (BF) that operates as a programmable digital pipeline, which enables steering of arbitrary pulse-density modulated waveforms. The TX BF drives element-level 65 V unipolar pulsers, which in turn drive the transducer elements. Both the TX BF and RX µBF are programmed with shift-registers that can either be programmed in a row-column fashion for fast upload.

As the ASICs in the probe can accommodate multiple transducer technologies, two variants of the probe were developed and acoustically validated to compare the performance of a CMUT and bulk PZT transducer array, demonstrating the potential of using the probe as a prototyping platform for further research activities, enabling validation of ultrasound arrays of up to thousands of elements. The CMUT variant probe was also used to characterize and compare the performance of multiple imaging schemes for the intended application. The scheme for imaging a 60◦×60◦×10-cm volume at 2000 volumes/s with half of the array achieves a median resolution of 4.0◦x1.9◦x660 µm, which accurately matches simulation results. Using this imaging scheme, Doppler images were reconstructed without aliasing artefacts of a blood-mimicking fluid flowing through a flow phantom with an average velocity up to 400 mm/s, which represents the peak velocity of blood in AAAs. Overall, the dissertation demonstrates that the developed system is a promising solution for ultrasound research and the improved treatment of AAAs.