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This paper reports on the development of a fully integrated 4-channel analog front-end (AFE) transceiver designed for 1-D PMUT arrays operating in the 1-10 MHz range. The single channel consisted of a transceiver featuring a transmit (TX) pulser capable of delivering 100 mathbf{V}-{mathbf{pk}-mathbf{pk}}, 3-level pulses with independently programmable delay and duration followed by a T/R switch, and a receive (RX) chain comprising a programmable gain Low Noise voltage Amplifier (LNA) and a Buffer. Chip prototypes were fabricated using a 165nm BCD-SOI technology. Electrical and acoustic tests of the AFE were carried out in conjunction with a 2.5MHz, 80% fractional bandwidth, 64-element 1-D PMUT array fabricated using a sol-gel PZT thin film-based technology. Hydrophone and pulse-echo experiments were carried out in a water tank setup using a custom designed test board. Electrical characterization of the LNA reported a 0.65-40 MHz -3dB band, an equivalent input noise voltage density of 13 mathbf{nV}/surd Hz at 3 MHz at the maximum gain, and a power consumption of 5.4 mW/ch. The large signal TX pressure response of 4 adjacent PMUT array elements driven by 20Vpk, 3-cycle, 2MHz pulses was estimated at the transducer surface, resulting in a peak negative pressure of 543 kPa with a second harmonic distortion of -30 dB. Small signal TX pressure and pulse-echo impulse responses were processed to extract the transfer function of the RX chain, which is characterized by a peak sensitivity of 55 mV/ kPa at 2.3 MHz.

Capacitive Micromachined Ultrasonic Transducers (CMUTs) Collapse mode operation offers higher electro-mechanical coupling and transmit pressure compared to non-collapse mode. This paper proposes an Equivalent Circuit Modeling (ECM) approach capable of accurately predicting the large-signal CMUT behavior in both collapse and non-collapse modes. A lumped-parameter nonlinear Equivalent Circuit Model representing the CMUT as a two-port network is implemented in LTSpice. Electro-mechanical and acoustic transduction is described by coupled nonlinear differential equations, i.e., a time-varying capacitance equation relating voltage and current at the electrical port and a force equation balancing elastic, electrostatic, and radiation forces. The nonlinear parameters are computed by Finite Element Modeling (FEM) and implemented in LTSpice using arbitrary behavioral sources and look-up tables.

The characteristic depth dose deposition of ion beams, with a maximum at the end of their range (Bragg peak) allows for local treatment delivery, resulting in better sparing of the adjacent healthy tissues compared to other forms of external beam radiotherapy treatments. However, the optimal clinical exploitation of the favorable ion beam ballistic is hampered by uncertainties in the in vivo Bragg peak position. Ionoacoustics is based on the detection of thermoacoustic pressure waves induced by a properly pulsed ion beam (e.g., produced by modern compact accelerators) to image the irradiated volume. Co-registration between ionoacoustics and ultrasound imaging offers a promising opportunity to monitor the ion beam and patient anatomy during the treatment. Nevertheless, the detection of the ionoacoustic waves is challenging due to very low pressure amplitudes and frequencies (mPa/kHz) observed in clinical applications. We investigate contrast agents to enhance the acoustic emission. Ultrasound microbubbles are used to increase the ionoacoustic frequency around the microbubble resonance frequency. Moreover, India ink is investigated as a possible mean to enhance the signal amplitude by taking advantage of additional optical photon absorption along the ion beam and subsequent photoacoustic effect. We report amplitude increase of up to 200% of the ionoacoustic signal emission in the MHz frequency range by combining microbubbles and India ink contrast agents.

Spiral array transducers with a sparse 2-D aperture have demonstrated their potential in realizing 3-D ultrasound imaging with reduced data rates. Nevertheless, their feasibility in high-volume-rate imaging based on unfocused transmissions has yet to be established. From a metrology standpoint, it is essential to characterize the acoustic field of unfocused transmissions from spiral arrays not only to assess their safety but also to identify the root cause of imaging irregularities due to the array's sparse aperture. Here, we present a field profile analysis of unfocused transmissions from a density-tapered spiral array transducer (256 hexagonal elements, 220- [Formula: see text] element diameter, and 1-cm aperture diameter) through both simulations and hydrophone measurements. We investigated plane- and diverging-wave transmissions (five-cycle, 7.5-MHz pulses) from 0° to 10° steering for their beam intensity characteristics and wavefront arrival time profiles. Unfocused firings were also tested for B-mode imaging performance (ten compounded angles, -5° to 5° span). The array was found to produce unfocused transmissions with a peak negative pressure of 93.9 kPa at 2 cm depth. All transmissions steered up to 5° were free of secondary lobes within 12 dB of the main beam peak intensity. All wavefront arrival time profiles were found to closely match the expected profiles with maximum root-mean-squared errors of [Formula: see text] for plane wave (PW) and [Formula: see text] for diverging wave. The B-mode images showed good spatial resolution with a penetration depth of 22 mm in PW imaging. Overall, these results demonstrate that the density-tapered spiral array can facilitate unfocused transmissions below regulatory limits (mechanical index: 0.034; spatial-peak, pulse-average intensity: 0.298 W/cm2) and with suppressed secondary lobes while maintaining smooth wavefronts.

We assess the impact of the fabrication-related variability of the thickness of the effective gap layers on the transmission (TX) and reception (RX) performance of a reverse-fabricated CMUT array by means of finite element analysis (FEA). The extreme values of the thickness of the passivation layers and of the cavity height, obtained by interferometry measurements during in-line wafer inspection, are applied to the simulated design of a reverse-fabricated CMUT array of circular cells. The analysis follows a 2-by-2 factorial approach to evaluate the variability of the TX and RX sensitivity amplitude, phase, and bandwidth caused by the fabrication tolerances, and compute the main effects and interactions of the effective gap layers.

This work reports on the investigation of the element beam pattern of a 2D CMUT spiral array by finite element analysis, aiming at assessing the effect of the transducer integration with the ASIC on the fluid-coupled transmission performance. The array element acoustic directivity is computed for the clamped substrate condition and compared with that obtained by coupling the transducer with a Benzocyclobutene (BCB) layer, used as an underfill between the CMUT and the ASIC. Different packaging solutions are considered and their effect on the element directivity is compared. The CMUT spiral array element is experimentally characterized in water-coupled transmission operation using a hydrophone to validate the finite element simulation.

We analyze and optimize the transmission of ultrasonic waves through the back-etched cavities of broadband PMUT arrays designed for medical imaging and non-destructive testing applications. We use a simple waveguide model to determine the acoustic impedance matching condition and investigate the effectiveness of the matching approach through Finite Element Modeling (FEM). We show that both the characteristic impedance and the finite elasticity of the cavity-filling material may affect the transducer bandwidth. The theoretical results are validated through electro-acoustic measurements carried out using 2MHz, 12-element, 2-D PMUT arrays.

This paper presents an analog transceiver for PMUT-based portable ultrasound medical imaging probes, working in the 2-4 MHz frequency range. The transceiver, fabricated in a 160-nm BCD-SOI technology, delivers a three-level train of pulses with amplitude up to ±50 V to the ultrasound transducer and collects the back-scattered echoes with a receiver chain, consisting of a low-noise voltage amplifier with programmable gain and a buffer. The circuit, connected to a PMUT, achieves a RX sensitivity of 50 mV/kPa at minimum gain and an input-referred noise spectral density of 13\,\text{nV}/\sqrt{Hz} at 2.3 MHz, consuming 5.4 mW. The peak RX sensitivity, obtained with acoustic measurements in a water tank, is 50 mV/kPa at 2.3 MHz.

Objective. Image guidance and precise irradiation are fundamental to ensure the reliability of small animal oncology studies. Accurate positioning of the animal and the in-beam monitoring of the delivered radio-therapeutic treatment necessitate several imaging modalities. In the particular context of proton therapy with a pulsed beam, information on the delivered dose can be retrieved by monitoring the thermoacoustic waves resulting from the brief and local energy deposition induced by a proton beam (ionoacoustics). The objective of this work was to fabricate a multimodal phantom (x-ray, proton, ultrasound, and ionoacoustics) allowing for sufficient imaging contrast for all the modalities. Approach. The phantom anatomical parts were extracted from mouse computed tomography scans and printed using polylactic acid (organs) and a granite/polylactic acid composite (skeleton). The anatomical pieces were encapsulated in silicone rubber to ensure long term stability. The phantom was imaged using x-ray cone-beam computed tomography, proton radiography, ultrasound imaging, and monitoring of a 20 MeV pulsed proton beam using ionoacoustics. Main results. The anatomical parts could be visualized in all the imaging modalities validating the phantom capability to be used for multimodal imaging. Ultrasound images were simulated from the x-ray cone-beam computed tomography and co-registered with ultrasound images obtained before the phantom irradiation and low-resolution ultrasound images of the mouse phantom in the irradiation position, co-registered with ionoacoustic measurements. The latter confirmed the irradiation of a tumor surrogate for which the reconstructed range was found to be in reasonable agreement with the expectation. Significance. This study reports on a realistic small animal phantom which can be used to investigate ionoacoustic range (or dose) verification together with ultrasound, x-ray, and proton imaging. The co-registration between ionoacoustic reconstructions of the impinging proton beam and x-ray imaging is assessed for the first time in a pre-clinical scenario.

Non-standard scan modes, based on the transmission of plane and diverging waves, are the key to enable high frame rate and volumetric imaging modalities and are promising in the implementation of novel system architectures for ultra-integrated (high channel count) scanners. So far, capacitive micromachined ultrasonic transducer (CMUT) arrays have been benchmarked against standard piezoelectric transducers only in standard modalities. In this work, a 256-element CMUT probe and a state-of-the art single crystal composite piezoelectric probe, sharing the same array geometry and bandwidth, are compared in plane wave (PW) imaging mode. The CMUT probe provided, on average, improved signal power (+9 dB) and contrast (+3 dB) for all PW aperture angles. Also, compared to the piezoelectric probe, the CMUT one offered comparable axial resolution, but slightly worse (10%) lateral resolution.

This work presents a promising microfabrication technique employing the silicon-on-nothing (SON) process to form a $2\ \mu\mathrm{m}$ thick continuous monocrystalline silicon membrane over a vacuum cavity of $1\ \mu\mathrm{m}$ in depth. Utilizing the SON process, high fill-factor piezoelectric micromachined ultrasonic transducer (pMUT) arrays on an 8-inch silicon wafer with cavity widths ranging from $170\ \mu\mathrm{m}$ down to $38\ \mu\mathrm{m}$ have been demonstrated. Devices are designed with 15% scandium-doped aluminum nitride as the piezoelectric layer of the pMUT for both air-coupled and water-coupled applications. The air-coupled pMUTs show a peak displacement frequency from 0.8 to 1.6 MHz with a $Q$ -factor between 120 to 194. The water-coupled pMUT arrays show a transmit pressure measured by a needle hydrophone, in DI water at a distance of 20 mm, ranging between 0.4 to 6.9 kPa/V with peak frequency between 5 to 13.4 MHz and fractional bandwidth 56 to 36%, respectively. The piezoelectric-over-SON process proposed here has the potential to gain traction in low-cost and high-yield pMUT manufacturing.

Micromachined Ultrasonic Transducer (CMUT and PMUT) technologies are playing a fundamental role in the development of novel applications such as ultra-portable medical imaging systems. This paper presents the design, fabrication, characterization, and system integration of a 1-D PMUT array for low-frequency diagnostic imaging applications. The PMUT array was fabricated using a sol-gel PZT thin film-based MEMS technology from STMicroelectronics. The 1-D array was integrated into an ultrasound probe and acoustic characterization and imaging tests were carried out using the ULA-OP 256 open scanner. The two-way frequency response of the PMUT had a center frequency of 2.5 MHz and a -6dB fractional bandwidth of 81%, and peak transmit and receive sensitivities assessed at the transducer surface of 31 kPa/V and 3.2 mV/kPa, respectively. In vitro and in vivo scans of a tissue-mimicking phantom and of a carotid artery, respectively, were successfully carried out demonstrating the potentiality of this PMUT technology for medical imaging applications.

International audience; Although quantitative ultrasound techniques based on the parameterization of the backscatter coefficient (BSC) have been successfully applied to blood characterization, theoretical scattering models assume blood as an isotropic scattering medium. However, the red blood cell (RBC) aggregates form anisotropic structures such as rouleaux in healthy blood. Therefore, the present study aims to progress further in the understanding of ultrasonic scattering from anisotropic RBC aggregates. Computer simulations were performed to compute the frequency-dependent BSC for different aggregating configurations: polydisperse spherical aggregates, randomly oriented or perfectly aligned prolate-shaped aggregates, and complex configurations with partially aligned prolate-shaped aggregates. Two scattering models were examined: the isotropic effective medium theory combined with the local monodisperse approximation (EMTLMA) and the anisotropic formulation of the EMTLMA considering perfectly aligned structures. When considering aligned prolate-shaped aggregates, the anisotropic EMTLMA was found to be superior to the isotropic EMTLMA in estimating the scatterer volume distribution. When considering the complex aggregating configurations, both isotropic and anisotropic EMTLMA yielded similar scatterer volume distribution. The mean scatterer size was underestimated when considering randomly oriented prolate-shaped aggregates and complex configurations with partially aligned structures. These results contribute to the better interpretation of scatterer size estimates for characterizing anisotropic RBC aggregates in blood.

The measurement of hemodynamic and morphological parameters in carotid artery like the peak blood velocity, the wall distensibility, the blood volume flow, the pulsatility index, and others, is of high medical interest. These indexes are routinely acquired during ultrasound examinations of patients. Unfortunately, at present, there is no possibility of a mid- or long- term continuous monitoring of such parameters in a non-clinical environment, like it is, for example, for basic cardiac parameters acquired through an Holter monitor. A wearable ultrasound device capable of continuous and automatic monitoring of carotid parameters needs a suitable sensor and a method for the automatic localization of the vessel, which compensates for patient's movements. In this work we present a pilot study based on a 16-element Piezoelectric Micromachined Ultrasonic Transducers (PMUT) array. It is shown that the PMUT, connected to the ULA-OP research scanner, produced B-mode phased-images of a vessel phantom where the vessel position was automatically tracked. The vessel was correctly located with accuracy better than 1 mm in more than 90% of the images.

This paper presents a numerical reduced-order modeling (ROM) approach for complex multi-layered arrays of piezoelectric micromachined ultrasonic transducers (PMUTs). The numerical modeling technique adopted to generate an array of PMUTs consisting of a considerable number of transducers allows for a large reduction in computational cost without reducing accuracy. The modeling idea is based on coupling shell elements applied to the PMUT structural layers with 3D-solid elements applied to the piezoelectric layer. A set of eigenfrequency and frequency domain analyses are presented considering a single ROM of a PMUT performing in different ambients and the performing central frequencies are obtained for every considered scenario. A unique arrangement of 228 PMUTs is presented and tested for its ability to transmit and receive acoustic waves. The operating frequency band of the array and the level of interference and cross-talk among different PMUTs in the near field are estimated. Finally, the results from a preliminary experimental test performed to analyze the acoustic abilities of an 8 × 8 array of PMUTs are presented. A corresponding numerical model is created and the obtained results matched the experimental data, leading to a validation of the modeling technique proposed in this work. ispartof: MICROMACHINES vol:13 issue:6 ispartof: location:Switzerland status: published

Electrostatically actuated MEMS ultrasonic transducers, also known as Capacitive Micromachined Ultrasonic Transducers (CMUTs), offer better performance with respect to conventional piezoelectric transducers in terms of bandwidth and efficiency. The enhanced compatibility between MEMS and standard integrated circuit technologies enables the integration of the transducer and the front-end electronics, and is currently leveraged in the development of new-generation diagnostic systems for clinical, emergency and point-of-care uses. This paper reports on the development of an integrated 2-D CMUT array designed for volumetric ultrasound imaging. An acoustically optimized 3-D packaging technology has been applied to the design, fabrication and characterization of a Multi-Chip Module (MCM) obtained by 3-D interconnection of a 256-element CMUT spiral array and a 256-channel analog front-end Application Specific Integrated Circuit (ASIC) with integrated pulsers, low-noise receivers and a programmable TX beamformer.

This paper proposes the combined use of Finite Element Modeling (FEM) and Equivalent Circuit Modeling (ECM) to describe the nonlinear electromechanical and acoustic behavior of a CMUT in an integrated circuit simulation environment. CMUT ECM lumped parameters are computed by FEM simulations. The proposed method is implemented in LTSpice, and validated by comparing ECM and FEM static, small-signal harmonic, and large-signal transient simulation results. The model was then applied to the performance assessment of the CMUT coupled to a realistic integrated transceiver configuration by computing TX efficiency and noise figure. The proposed approach allows accurate large-signal analysis of CMUTs interfaced to ultrasound front-end circuits by considering non-ideal characteristics of the system, such as the pulser nonlinearities.

Polymer-based CMUT microfabrication approaches allow low-cost fabrication of flexible transducers. However, they are characterized by processing limitations mainly related to the low glass transition temperature of polymers, reducing the possible choice of materials that can be used for electrodes and in-cavity passivation layers, the latter having a major impact on transducer performance and reliability. In this paper, we experimentally evaluate the electrical properties of these two materials in terms of dielectric dispersion and losses, and high electric field response with respect to state-of-the-art materials.