Fabio Quaglia

A volumetric CMUT-based ultrasound imaging system simulator with integrated reception and μ-beamforming electronics models

In modern ultrasound imaging devices, two-dimensional probes and electronic scanning allow volumetric imaging of anatomical structures. When dealing with the design of such complex 3-D ultrasound (US) systems, as the number of transducers and channels dramatically increases, new challenges concerning the integration of electronics and the implementation of smart micro-beamforming strategies arise. Hence, the possibility to predict the behavior of the whole system is mandatory. In this paper, we propose and describe an advanced simulation tool for ultrasound system modeling and simulation, which conjugates the US propagation and scattering, signal transduction, electronic signal conditioning, and beamforming in a single environment. In particular, we present the architecture and model of an existing 16-channel integrated receiver, which includes an amplification and micro-beamforming stage, and validate it by comparison with circuit simulations. The developed model is then used in conjunction with the transducer and US field models to perform a system simulation, aimed at estimating the performance of an example 3-D US imaging system that uses a capacitive micromachined ultrasonic transducer (CMUT) 2-D phased-array coupled to the modeled reception front-end. Results of point spread function (PSF) calculations, as well as synthetic imaging of a virtual phantom, show that this tool is actually able to model the complete US image reconstruction process, and that it could be used to quickly provide valuable system-level feedback for an optimized tuning of electronic design parameters.

A CMUT transceiver front-end with 100-V TX driver and 1-mW low-noise capacitive feedback RX amplifier in BCD-SOI technology.

A CMUT front-end for high performance portable ultrasound medical imaging is proposed. Designed for CMUTs operating in the 1-15 MHz range, it comprises a 100-V TX driver, a high voltage T/R switch and an ultra-low-power RX amplifier. The impact of the large parasitic capacitances of high-voltage devices, responsible for SNR degradation, is minimized by careful design and dedicated circuit techniques while the RX amplifier exploits capacitive feedback to achieve a better noise-power performance than commonly adopted trans-resistance topologies. Electrical characterization as well as pulse-echo measurements are shown. The transceiver proves state-of-the-art noise performance and very high dynamic range with power dissipation of the RX amplifier of 1 mW only.

A 90V pp 720MHz GBW linear power amplifier for ultrasound imaging transmitters in BCD6-SOI

Transducer drivers for ultrasound imaging are required to have high output power and operation frequency into the MHz range. Furthermore, in harmonic imaging, a well-established method resulting in enhanced contrast, higher harmonic echoes (usually the second) of the fundamental transmitted frequency, either generated by reflection from micro-bubbles or on propagation, are selectively detected and used for imaging [1]. As a consequence, the second harmonic of the transmitted signal is suppressed as much as possible and the received signal is derived solely from the nonlinear behavior of the body. Pulsers are usually adopted having high efficiency, being simple and lending themselves to harmonic imaging for example by means of out-of-phase pulse transmission techniques [1]. Output signals with arbitrary shape allow better performances, enabling apodization profiles with high resolution, beams with low harmonic content and instantaneous changes in transmit energy between pulses [2]. However, the implementation of arbitrary waveform generators in commercial solutions is presently limited to high-end systems because of manufacturing costs, power dissipation and space constraints. To this extent, the availability of an adequate IC technology such as BCD, handling high dynamic range signals at high frequency and capable of multi-watt level driving, opens up to the investigation of integrated amplifier topologies [3]. Advantages in terms of EM interference reduction, reliability improvement, space and cost would derive. Achieving adequate gain-bandwidth products is nonetheless challenging considering the limited fT of high voltage devices.

Second-harmonic reduction in CMUTs using unipolar pulsers

In this paper, a CMUT linearization method based on the predistortion of unipolar excitation pulses is proposed. The predistortion is achieved by acting on the amplitude, duty cycle, and slope of the rising and falling edges of the trapezoidal excitation pulses. The pulse-shaping parameters are computed using an open-loop charge control algorithm that uses electrical impedance data for the identification of the nonlinear input-output characteristic of the CMUT. A 0-200V unipolar pulser, provided with a digital control logic for the trapezoidal pulse-shaping, was designed and fabricated using a BCD-SOI technology available at STMicroelectronics. The pulser was connected to a 10MHz CMUT linear array developed for medical imaging applications. Transmit pressure measurements were performed proving a second-harmonic reduction of more than 20 dB, resulting in a final second-harmonic distortion lower than -30 dB, suitable for typical nonlinear imaging operation.

Analysis and Design of a High Voltage Integrated Class-B Amplifier for Ultra-Sound Transducers

Pulsers are usually adopted in ultra-sound applications due to their high efficiency. On the other hand, linear amplifiers would enable apodization profiles with high resolution, beams with low harmonic content and instantaneous changes in transmit energy between pulses. Their use is rather limited when relying on a discrete technology approach, due to high manufacturing costs and space occupation. In view of an increased level of integration, the availability of a BCD technology, capable of watts level driving at high frequency, encourages investigation of linear amplifiers, the topic of this work. Advantages in terms of reliability, space and cost would derive. The proposed amplifier, closed in a feedback loop, uses a high gm low-voltage transconductor driving a high voltage trans-impedance stage operating in class-B. Device parameters vary with signal amplitude making circuit analysis involved. Techniques to analyze large signal frequency response, distortion and stability are proposed in this paper leading to useful design insights. The amplifier has been realized in a BCD6-SOI technology and, from experimental results, it shows the following performances: 90Vpk-pk output swing with better than 60% power efficiency, 720 MHz GBW, and HD2 lower than -35 dB when driving a load made of a 100 Ω resistor shunted with a 150 pF capacitor, emulating the ultrasound transducer. Quiescent power dissipation is 37 mW only.

An ultra-low-power fully integrated ultrasound imaging CMUT transceiver featuring a high-voltage unipolar pulser and a low-noise charge amplifier

In this paper, we report on the development of a fully-integrated, low-power analog frontend circuit, consisting in a high-voltage unipolar pulser and a low-noise charge amplifier, specifically designed for 1D CMUT arrays operating in the 1-15 MHz range. The proposed circuit comprises a high-voltage unipolar pulser, a T/R switch, and a low-noise charge amplifier (LNA), which were carefully co-designed in order to minimize the overall parasitic capacitance and power consumption. The high-voltage pulser allows generating up to 100-V unipolar pulses. The T/R switch was designed to achieve sufficient isolation of the LNA during transmission together with a low parasitic resistance in on state. The LNA is based on a capacitive feedback topology providing sufficient bandwidth and better noise-power performance than commonly adopted trans-resistance topologies. Chip prototypes were fabricated using a BCD-SOI technology available at STMicroelectronics. Experimental characterization results are achieved in conjunction with a 10 MHz CMUT linear array developed for medical imaging applications.

High-voltage integrated Class-B amplifier for ultrasound transducers

High-voltage linear amplifiers driving transducers for ultrasound imaging enable apodization profiles with high resolution, beams with low harmonic content and instantaneous changes in transmit energy between pulses. In view of an increased level of integration, the availability of a BCD technology, capable of watts level driving at high frequency, encourages investigation of linear amplifiers, the topic of this work. A feedback amplifier with high voltage - high efficiency Class B output stage is proposed. Device parameters vary significantly with signal amplitude making circuit analysis more involved. We propose the use of describing functions to estimate the large-signal frequency response. Analytical results are in good agreement with simulations and provide useful design insights. From measurements, a test-chip in BCD-6 SOI displays 40.9dB gain, 90Vpk-pk output voltage with a minimum 3dB cut-off frequency of 5.5MHz. Quiescent power dissipation is 37mW only.

Simulating ultrasound fields for 2D phased-array probes design optimization

Nowadays, ultrasound diagnostic imaging is one of the non-invasive techniques mostly used in the clinical practice. Recent advances in this field have brought to the development of small and portable systems. New bidimensional probes consisting of 2D phased arrays, allow to obtain real-time 3D representations of moving organs and blood vessels anatomy. Being the complexity of such 4D ultrasound imaging systems significantly increased, new challenges concerning electronics integration arise for designers. In this paper a software simulator is described, which has been developed in order to model ultrasound wave generation, pressure field distribution and echoes reception, with the aim to become a useful tool for optimizing the probe design. The paper mainly focuses on linear ultrasound field modeling; preliminary results on non-linear interactions with contrast agents are also here introduced.

A Comparative Analysis of CMUT Receiving Architectures for the Design Optimization of Integrated Transceiver Front Ends

A formal comparison between fundamental RX amplifier configurations for capacitive micromachined ultrasonic transducers (CMUTs) is proposed in this paper. The impact on both RX and the pulse-echo frequency response and on the output SNR is thoroughly analyzed and discussed. It is shown that the resistive-feedback amplifier yields a bandpass RX frequency response, while both open-loop voltage and capacitive-feedback amplifiers exhibit a low-pass frequency response. For a given power dissipation, it is formally proved that a capacitive-feedback amplifier provides a remarkable SNR improvement against the commonly adopted resistive feedback stage, achieved at the expense of a reduced pulse-echo center frequency, making its use convenient in low-frequency and midfrequency ultrasound imaging applications. The advantage mostly comes from a much lower noise contributed by the active devices, especially with low-

An ultrasound system simulation tool for advanced front-end electronics design

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