Objective
The response of ultrasound contrast agents is sensitive to ambient pressure, especially via their scattered subharmonic signal, which makes them a promising candidate for non-invasive pressure measurements in vivo. This work aimed to understand the sensitivity to ambient pressure of subharmonic oscillations from single microbubbles.

Methods
The subharmonic oscillation amplitude of single microbubbles in response to varying ambient pressure was studied both experimentally and numerically. In experiment, approximately 2200 single microbubbles from a monodisperse population were measured at a driving frequency close to twice their resonance frequency.

Results
The results of the numerical simulations and experiments show that a pressure change leads to a small size change in the bubble that then changes the lipid packing density, and with that the stiffness of the bubble shell.

Conclusion
The dependency of subharmonic oscillation amplitude to changes in ambient pressure can be explained by a shift in the resonance frequency of the bubble as a function of ambient pressure. The subharmonic response increases with ambient pressure when the resonance frequency shifts toward half the driving frequency and decreases when the resonance frequency shifts away from half the driving frequency. These findings help to understand non-invasive pressure sensing through subharmonic ultrasound imaging.

This article presents a 4096-element ultrasound probe for high volume-rate (HVR) cardiovascular imaging. The probe consists of two application-specific integrated circuits (ASICs), each of which interfaces with a 2048-element monolithically-integrated capacitive micro-machined ultrasound transducer (CMUT) array. 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 2 × 2 delay-and-sum micro-beamforming (μBF) and 2× time-division multiplexing (TDM) to achieve an 8× receive (RX) channel-count reduction. 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 (PDM) waveforms. The TX BF drives element-level 65 V unipolar pulsers, which in turn drive the CMUT array. Both the TX BF and RX μBF are programmed with shift-registers (SRs) that can either be programmed in a row-column fashion for fast upload times, or daisy-chain fashion for a higher flexibility. The layout of the ASICs is matched to the 365-μm-pitch monolithically-integrated CMUT array. While operating, the RX and logic power consumption per element is 0.85 and 0.10 mW, respectively. TX power consumption is highly waveform dependent, but is nominally 0.34 mW. Compared to the prior art, the probe has the highest volume rate, and features among the largest imaging arrays (both in terms of element-count and aperture) with a high flexibility in defining the TX waveform. These properties make it a suitable option for applications requiring HVR imaging of a large region of interest.

IN [1], there is a mistake in the timing diagram shown in Fig. 6. Switches S 1-S 4 are skipping some of the samples and the rate at which they are operating implies a TDM rate of 10 MHz, whereas (as described in [1]) this should be 20 MHz. In the updated Fig. 6, S 1-S 4 have been updated and a minor change has been made to the timing shown for switches Q₁ and Q₂, such that the correct TDM rate is indicated and no sample provided to the S/H stage via N1-N4 is skipped in the diagram. (Figure presented).

Here we propose an ultrasound contrast-based imaging method that enables non-invasive quantitative assessment of ambient pressure changes inside the body (such as blood pressure). We subject the microbubbles in the contrast agent to two frequencies: A low-frequency (57 kHz) signal that dynamically manipulates the ambient pressure, and a series of high-frequency (4 MHz) pulses for exciting and imaging the bubble response. The imaging pulses exploit the ambient pressure sensitivity of the subharmonic microbubble response, while the low-frequency signal provided an intrinsic calibration for measurement of ambient pressure changes. We tested this approach in an in vitro setup and show that it can visualize and quantify ambient pressure differences with a sensitivity of 0.5 dB/kPa.

Ultrasound contrast agents, comprised of phospholipid-coated microbubbles, can be produced as monodisperse populations using a microfluidic flow-focusing device. However, microbubble coalescence remains a significant challenge. High production temperatures (e.g., 55 °C) can be used to suppress coalescence, but it complicates the microfluidic device design and is incompatible with targeting agents and drug conjugates. This study investigates the production of monodisperse microbubbles at room temperature with the addition of the amphiphilic surfactant Pluronic F68. Two 1,2-distearoyl-sn-glycero-3-phosphocholine (DSPC)-based phospholipid formulations were investigated: F1, containing 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine-N-[carbonyl-methoxypolyethylene glycol] (DPPE-PEG5000), and F2, which included both DPPE-PEG5000 and polyoxyethylene(40) stearate (PEG40-stearate). We characterized the size stability and acoustic behavior of monodisperse microbubbles produced with various Pluronic F68 concentrations. Adding 5-10 mol % Pluronic F68 was found to effectively suppress coalescence and facilitated the production of monodisperse microbubbles that remained shelf stable for at least 7 days. Acoustic attenuation measurements revealed a shell stiffness ranging from 0.78 to 0.93 N/m for these microbubbles. The 10 mol % Pluronic F68 addition (10PF) demonstrated superior monodispersity and was selected for further experiments. Upon dilution, the size and resonance frequencies of both F1-10PF and F2-10PF decreased over time, though F2-10PF showed better stability compared to F1-10PF for both metrics. Both F1-10PF and F2-10PF exhibited a stronger subharmonic scattering intensity than SonoVue (clinical approved microbubbles), which offers potential for blood pressure sensing. Our study shows that incorporating Pluronic F68 facilitates the production of monodisperse microbubbles at room temperature that are stable long-term and have excellent acoustical properties, with the F2-10PF formulation demonstrating better stability than the F1-10PF.

This article presents a pitch-matched transceiver application-specific integrated circuit (ASIC) for a wearable ultrasound device intended for transfontanelle ultrasonography, which includes element-level 20-V unipolar pulsers with transmit (TX) beamforming, and receive (RX) circuitry that combines eightfold multiplexing, four-channel micro-beamforming (?BF), and subgroup-level digitization to achieve an initial 32-fold channel-count reduction. The ?BF is based on passive boxcar integration, merged with a 10-bit 40 MS/s SAR ADC in the charge domain, thus obviating the need for explicit anti-alias filtering (AAF) and power-hungry ADC drivers. A compact and low-power reference generator employs an area-efficient MOS capacitor as a reservoir to quickly set a reference for the ADC in the charge domain. A low-power multi-level data link, based on 16-level pulse-amplitude modulation, concatenates the outputs of four ADCs, providing an overall 128-fold channel-count reduction. A prototype transceiver ASIC was fabricated in a 180-nm BCD technology, and interfaces with a 2-D PZT transducer array of 16 × 16 elements with a pitch of 125 ?m and a center frequency of 9 MHz. The ASIC consumes 1.83 mW/element. The data link achieves an aggregate 3.84 Gb/s data rate with 3.3 pJ/bit energy efficiency. The ASIC's functionality has been demonstrated through electrical, acoustic, and imaging experiments.

Objective: Described here is the development of an ultrasound matrix transducer prototype for high-frame-rate 3-D intra-cardiac echocardiography. Methods: The matrix array consists of 16 × 18 lead zirconate titanate elements with a pitch of 160 µm × 160 µm built on top of an application-specific integrated circuit that generates transmission signals and digitizes the received signals. To reduce the number of cables in the catheter to a feasible number, we implement subarray beamforming and digitization in receive and use a combination of time-division multiplexing and pulse amplitude modulation data transmission, achieving an 18-fold reduction. The proposed imaging scheme employs seven fan-shaped diverging transmit beams operating at a pulse repetition frequency of 7.7 kHz to obtain a high frame rate. The performance of the prototype is characterized, and its functionality is fully verified. Results: The transducer exhibits a transmit efficiency of 28 Pa/V at 5 cm per element and a bandwidth of 60% in transmission. In receive, a dynamic range of 80 dB is measured with a minimum detectable pressure of 10 Pa per element. The element yield of the prototype is 98%, indicating the efficacy of the manufacturing process. The transducer is capable of imaging at a frame rate of up to 1000 volumes/s and is intended to cover a volume of 70° × 70° × 10 cm. Conclusion: These advanced imaging capabilities have the potential to support complex interventional procedures and enable full-volumetric flow, tissue, and electromechanical wave tracking in the heart.

Objective: Post-operative brain injury in neonates may result from disturbed cerebral perfusion, but accurate peri-operative monitoring is lacking. High-frame-rate (HFR) cerebral ultrasound could visualize and quantify flow in all detectable vessels using spectral Doppler; however, automated quantification in small vessels is challenging because of low signal amplitude. We have developed an automatic envelope detection algorithm for HFR pulsed wave spectral Doppler signals, enabling neonatal brain quantitative parameter maps during and after surgery. Methods: HFR ultrasound data from high-risk neonatal surgeries were recorded with a custom HFR mode (frame rate = 1000 Hz) on a Zonare ZS3 system. A pulsed wave Doppler spectrogram was calculated for each pixel containing blood flow in the image, and spectral peak velocity was tracked using a max-likelihood estimation algorithm of signal and noise regions in the spectrogram, where the most likely cross-over point marks the blood flow velocity. The resulting peak systolic velocity (PSV), end-diastolic velocity (EDV) and resistivity index (RI) were compared with other detection schemes, manual tracking and RIs from regular pulsed wave Doppler measurements in 10 neonates. Results: Envelope detection was successful in both high- and low-quality arterial and venous flow spectrograms. Our technique had the lowest root mean square error for EDV, PSV and RI (0.46 cm/s, 0.53 cm/s and 0.15, respectively) when compared with manual tracking. There was good agreement between the clinical pulsed wave Doppler RI and HFR measurement with a mean difference of 0.07. Conclusion: The max-likelihood algorithm is a promising approach to accurate, automated cerebral blood flow monitoring with HFR imaging in neonates.

The accurate determination of the transfer function of ultrasound transducers is important for their design and operational performance. However, conventional methods for quantifying the transfer function, such as hydrophone measurements, radiation force balance, and pulse-echo measurements, are costly and complex due to specialized equipment required. In this study, we introduce a novel approach to estimate the transfer function of ultrasound transducers by measuring the acoustic streaming velocity generated by the transducer. We utilize an experimental setup consisting of a water tank with a millimeter scale, an ink-filled syringe, and a camera for recording the streaming phenomenon. Through streaming velocity measurements in the frequency range from 2 to 8 MHz, we determined the transfer function of an unfocused circular transducer with a center frequency of 5 MHz and a radius of 5.6 mm. We compared the performance of our method with hydrophone and pulse-echo measurements. At the center frequency, we measured a transmit efficiency of 1.9 kPa/V using the streaming approach, while hydrophone and pulse-echo measurements yielded transmit efficiencies of 2.1 kPa/V and 1.8 kPa/V, respectively. These findings demonstrate that the proposed method for estimating the transfer function of ultrasound transducers achieves a sufficient level of accuracy comparable to pulse-echo and hydrophone measurements.

This article presents an application-specific integrated circuit (ASIC) for catheter-based 3-D ultrasound imaging probes. The pitch-matched design implements a comprehensive architecture with high-voltage (HV) transmitters, analog front ends, hybrid beamforming analog-To-digital converters (ADCs), and data transmission to the imaging system. To reduce the number of cables in the catheter while maintaining a small footprint per element, transmission (TX) beamforming is realized on the chip with a combination of a shift register (SR) and a row/column (R/C) approach. To explore an additional cable-count reduction in the receiver part of the design, a channel with a combination of time-division multiplexing (TDM), subarray beamforming, and multi-level pulse amplitude modulation (PAM) data transmission is also included. This achieves an 18-fold cable-count reduction and minimizes the power consumption in the catheter by a load modulation (LM) cable driver. It is further explored how common-mode interference can limit beamforming gain and a strategy to reduce its impact with local regulators is discussed. The chip was fabricated in TSMC 0.18-m HV BCD technology and a 2-D PZT transducer matrix of 16 × 18 elements with a pitch of 160 m and a center frequency of 6 MHz was manufactured on the chip. The system can generate all required TX patterns at up to 30 V, provides quick settling after the TX phase, and has an reception (RX) power consumption of only 1.12 mW/element. The functionality and operation of up to 1000 volumes/s have been demonstrated in electrical and acoustic imaging experiments.

Drug transport from blood to extravascular tissue can locally be achieved by increasing the vascular permeability through ultrasound-activated microbubbles. However, the mechanism remains unknown, including whether short and long cycles of ultrasound induce the same onset rate, spatial distribution, and amount of vascular permeability increase. Accurate models are necessary for insights into the mechanism so a microvessel-on-a-chip is developed with a membrane-free extravascular space. Using these microvessels-on-a-chip, distinct differences between 2 MHz ultrasound treatments are shown with 10 or 1000 cycles. The onset rate is slower for 10 than 1000 cycles, while both cycle lengths increase the permeability in spot-wise patterns without affecting microvessel viability. Significantly less vascular permeability increase and sonoporation are induced for 10 versus 1000 cycles at 750 kPa (i.e., the highest studied peak negative acoustic pressure (PNP)). The PNP threshold for vascular permeability increases is 750 versus 550 kPa for 10 versus 1000 cycles, while this is 750 versus 220 kPa for sonoporation. Vascular permeability increases do not correlate with αvβ3-targeted microbubble behavior, while sonoporation correlates with αvβ3-targeted microbubble clustering. In conclusion, the further mechanistic unraveling of vascular permeability increase by ultrasound-activated microbubbles in a developed microvessel-on-a-chip model aids the safe and efficient development of microbubble-mediated drug transport.

Phospholipid-coated microbubbles with a uniform acoustic response are a promising avenue for functional ultrasound sensing. A uniform acoustic response requires both a monodisperse size distribution and uniform viscoelastic shell properties. Monodisperse microbubbles can be produced in a microfluidic flow focusing device. Here, we investigate whether such monodisperse microbubbles have uniform viscoelastic shell properties and thereby a uniform “mono-acoustic” response. To this end, we visualized phase separation of the DSPC and DPPE-PEG5000 lipid shell components and measured the resonance curves of nearly 2000 single and freely floating microbubbles using a high-frequency acoustic scattering technique. The results demonstrate inhomogeneous phase-separated shell microdomains across the monodisperse bubble population, which may explain the measured inhomogeneous viscoelastic shell properties. The shell viscosity varied over an order of magnitude and the resonance frequency by a factor of two indicating both a variation in shell elasticity and in initial surface tension despite the relatively narrow size distribution.

Ultrasound contrast-mediated medical imaging and therapy both rely on the dynamics of micron- and nanometer-sized ultrasound cavitation nuclei, such as phospholipid-coated microbubbles and phase-change droplets. Ultrasound cavitation nuclei respond non-linearly to ultrasound on a nanosecond time scale that necessitates the use of ultra-high-speed imaging to fully visualize these dynamics in detail. In this study, we developed an ultra-high-speed optical imaging system that can record up to 20 million frames per second (Mfps) by coupling two small-sized, commercially available, 10-Mfps cameras. The timing and reliability of the interleaved cameras needed to achieve 20 Mfps was validated using two synchronized light-emitting diode strobe lights. Once verified, ultrasound-activated microbubble responses were recorded and analyzed. A unique characteristic of this coupled system is its ability to be reconfigured to provide orthogonal observations at 10 Mfps. Acoustic droplet vaporization was imaged from two orthogonal views, by which the 3-D dynamics of the phase transition could be visualized. This optical imaging system provides the temporal resolution and experimental flexibility needed to further elucidate the dynamics of ultrasound cavitation nuclei to potentiate the clinical translation of ultrasound-mediated imaging and therapy developments.

Assessing the coronary circulation with contrast-enhanced echocardiography has high clinical relevance. However, it is not being routinely performed in clinical practice because the current clinical tools generally cannot provide adequate image quality. The contrast agent's visibility in the myocardium is generally poor, impaired by motion and nonlinear propagation artifacts. The established multipulse contrast schemes (MPCSs) and the more experimental singular value decomposition (SVD) filter also fall short to solve these issues. Here, we propose a scheme to process amplitude modulation/amplitude-modulated pulse inversion (AM/AMPI) echoes with higher order SVD (HOSVD) instead of conventionally summing the complementary pulses. The echoes from the complementary pulses form a separate dimension in the HOSVD algorithm. Then, removing the ranks in that dimension with dominant coherent signals coming from tissue scattering would provide the contrast detection. We performed both in vitro and in vivo experiments to assess the performance of our proposed method in comparison with the current standard methods. A flow phantom study shows that HOSVD on AM pulsing exceeds the contrast-to-background ratio (CBR) of conventional AM and an SVD filter by 10 and 14 dB, respectively. In vivo porcine heart results also demonstrate that, compared to AM, HOSVD improves CBR in open-chest acquisition (up to 19 dB) and contrast ratio (CR) in closed-chest acquisition (3 dB).

High frame rate 3D intracardiac echography (3D-ICE) might enable full-volumetric flow, tissue, and electro-mechanical wave tracking in the heart, thus supporting complex interventional procedures. Designing ICE transducers is very challenging due to the constraints imposed by the catheter's diameter on the transducer aperture and cable accommodation within the shaft. Current ICE probes only offer a 2D or limited 3D field of view at low frame rates. This work presents the design, fabrication, and characterization of an ICE prototype transducer aiming at high frame rate 3D imaging with a large field of view.

Understanding and controlling the ultrasound contrast agent (UCA)'s response to an applied ultrasound pressure field are crucial when investigating ultrasound imaging sequences and therapeutic applications. The magnitude and frequency of the applied ultrasonic pressure waves affect the oscillatory response of the UCA. Therefore, it is important to have an ultrasound compatible and optically transparent chamber in which the acoustic response of the UCA can be studied. The aim of our study was to determine the in situ ultrasound pressure amplitude in the ibidi μ -slide I Luer channel, an optically transparent chamber suitable for cell culture, including culture under flow, for all microchannel heights (200, 400, 600, and 800 μm). First, the in situ pressure field in the 800- μm high channel was experimentally characterized using Brandaris 128 ultrahigh-speed camera recordings of microbubbles (MBs) and a subsequent iterative processing method, upon insonification at 2 MHz, 45° incident angle, and 50-kPa peak negative pressure (PNP). Control studies in another cell culture chamber, the CLINIcell, were compared with the obtained results. The pressure amplitude was -3.7 dB with respect to the pressure field without the ibidi μ -slide. Second, using finite-element analysis, we determined the in situ pressure amplitude in the ibidi with the 800- μm channel (33.1 kPa), which was comparable to the experimental value (34 kPa). The simulations were extended to the other ibidi channel heights (200, 400, and 600 μm) with either 35° or 45° incident angle, and at 1 and 2 MHz. The predicted in situ ultrasound pressure fields were between -8.7 and -1.1 dB of the incident pressure field depending on the listed configurations of ibidi slides with different channel heights, applied ultrasound frequencies, and incident angles. In conclusion, the determined ultrasound in situ pressures demonstrate the acoustic compatibility of the ibidi μ -slide I Luer for different channel heights, thereby showing its potential for studying the acoustic behavior of UCAs for imaging and therapy.

Volumetric echocardiography can potentially give a more complete picture of cardiac dynamics than its two-dimensional (2D) counterpart. Current clinical volumetric imaging probes have relatively low frame rates, and often require ECG gating to stitch together an entire volume. This makes measuring fast dynamics of the heart as well as imaging patients with irregular heartbeats difficult. We have previously designed and manufactured 2D sparse arrays with elements seeded in a density-tapered spiral pattern for cardiac imaging. Using these prototypes, we demonstrate in this paper the first high-frame-rate volumetric closed-chest porcine cardiac as well as open-chest myocardial blood flow results. These preliminary results suggest the potential of performing high-frame-rate volumetric cardiac imaging using the sparse spiral arrays.

This paper presents a pitch-matched transceiver ASIC integrated with a 2-D transducer array for a wearable ultrasound device for transfontanelle ultrasonography. The ASIC combines 8-fold multiplexing, 4-channel micro-beamforming (μ BF) and sub-array-level digitization to achieve a 128-fold channel-count reduction. The μ BF is based on passive boxcar integration and interfaces with a 10-bit 40 MS/s SAR ADC in the charge domain, thus obviating the need for explicit anti-alias filtering and power-hungry ADC drivers. A compact and low-power reference generator employs an area-efficient MOS capacitor as a reservoir to quickly set a reference for the ADC in the charge domain. A low-power multi-level data link concatenates outputs of four ADCs, leading to an aggregate 3.84 Gb/s data rate. Per channel, the RX circuit consumes 2.06 mW and occupies 0.05 mm2.

Ultrasound-based shear wave elastography is a promising technique to non-invasively assess the dynamic stiffness variations of the heart. The technique is based on tracking the propagation of acoustically induced shear waves in the myocardium of which the propagation speed is linked to tissue stiffness. This measurement is repeated multiple times across the cardiac cycle to assess the natural variations in wave propagation speed. The interpretation of these measurements remains however complex, as factors such as loading and contractility affect wave propagation. We therefore applied transthoracic shear wave elastography in 13 pigs to investigate the dependencies of wave speed on pressure–volume derived indices of loading, myocardial stiffness, and contractility, while altering loading and inducing myocardial ischemia/reperfusion injury. Our results show that diastolic wave speed correlates to a pressure–volume derived index of operational myocardial stiffness (R = 0.75, p < 0.001), suggesting that both loading and intrinsic properties can affect diastolic wave speed. Additionally, the wave speed ratio, i.e. the ratio of systolic and diastolic speed, correlates to a pressure–volume derived index of contractility, i.e. preload-recruitable stroke work (R = 0.67, p < 0.001). Measuring wave speed ratio might thus provide a non-invasive index of contractility during ischemia/reperfusion injury.

Ultrasound imaging is an attractive imaging modality due to its low-cost and real-time feedback, although it often falls short in image quality compared to MRI and CT imaging. Conventional ultrasound image reconstruction, such as Delay-and-Sum beamforming, is derived from maximum-likelihood estimation. As such, no prior information is exploited in the image formation process, which limits potential image quality. Maximum-a-posteriori (MAP) beamforming aims to overcome this issue, but often relies on rough approximations of the underlying signal statistics. Deep learning based reconstruction methods have demonstrated impressive results over the past years, but often lack interpretability and require vast amounts of data.In this work we present a neural MAP beamforming technique, which efficiently combines deep learning in the MAP beamforming framework. We show that this model-based deep learning approach can achieve high-quality imaging, improving over the state-of-the-art, without compromising the real-time abilities of ultrasound imaging.