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.

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Medical ultrasound is a crucial imaging modality in modern healthcare. The advent of high-frame-rate imaging has broadened its applications, enabling the measurement of both high blood velocities over large fields-of-view, and slow-moving blood. High temporal resolution is essential, but accurate flow measurements also require high spatial resolution in all three orthogonal directions. This necessitates the use of matrix transducers, which face several challenges depending on the application.

For monitoring the brains of preterm-born babies, the main challenges include designing a application-specific integrated circuit (ASIC)-integrated matrix that meets clinical requirements and achieving the necessary data reduction to minimize the number of cables, ensuring that contact between mother and child remains unrestricted.

For atherosclerosis screening, challenges involve sensor-ASIC integration, developing a high-frame-rate data acquisition scheme for the ASIC-integrated matrix, including data processing for volumetric flow quantification, and addressing the high side-lobe level of a sparse matrix.
The objective of this thesis is to address these challenges and develop the necessary technology to enable the use of ultrasound matrix transducers for quantifying blood flow and detecting physiologically abnormal flow patterns and velocities in the two mentioned applications.@en

This article presents a low-power and small-area transceiver application-specific integrated circuit (ASIC) for 3-D trans-fontanelle ultrasonography. A novel micro-beamforming receiver architecture that employs current-mode summation and boxcar integration is used to realize delay-and-sum on an N -element sub-array using N× fewer capacitive memory elements than conventional micro-beamforming implementations, thus reducing the hardware overhead associated with the memory elements. The boxcar integration also obviates the need for explicit anti-aliasing filtering in the analog front end, thus further reducing die area. These features facilitate the use of micro-beamforming in smaller pitch applications, as demonstrated by a prototype transceiver ASIC employing micro-beamforming on sub-arrays of N=4 elements, targeting a wearable ultrasound device that monitors brain perfusion in preterm infants via the fontanel. To meet its strict spatial resolution requirements, a 10-MHz 100- μ m-pitch piezoelectric transducer array is employed, leading to a per-element die area > 2 × smaller than prior designs employing micro-beamforming.

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High frame rate three-dimensional (3D) ultrasound imaging would offer excellent possibilities for the accurate assessment of carotid artery diseases. This calls for a matrix transducer with a large aperture and a vast number of elements. Such a matrix transducer should be interfaced with an application-specific integrated circuit (ASIC) for channel reduction. However, the fabrication of such a transducer integrated with one very large ASIC is very challenging and expensive. In this study, we develop a prototype matrix transducer mounted on top of multiple identical ASICs in a tiled configuration. The matrix was designed to have 7680 piezoelectric elements with a pitch of 300 μm × 150 μm integrated with an array of 8 × 1 tiled ASICs. The performance of the prototype is characterized by a series of measurements. The transducer exhibits a uniform behavior with the majority of the elements working within the −6 dB sensitivity range. In transmit, the individual elements show a center frequency of 7.5 MHz, a −6 dB bandwidth of 45%, and a transmit efficiency of 30 Pa/V at 200 mm. In receive, the dynamic range is 81 dB, and the minimum detectable pressure is 60 Pa per element. To demonstrate the imaging capabilities, we acquired 3D images using a commercial wire phantom.

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Two-dimensional (2-D) arrays offer volumetric imaging capabilities without the need for probe translation or rotation. A sparse array with elements seeded in a tapering spiral pattern enables one-to-one connection to an ultrasound machine, thus allowing flexible transmission and reception strategies. To test the concept of sparse spiral array imaging, we have designed, realized, and characterized two prototype probes designed at 2.5-MHz low-frequency (LF) and 5-MHz high-frequency (HF) center frequencies. Both probes share the same electronic design, based on piezoelectric ceramics and rapid prototyping with printed circuit board substrates to wire the elements to external connectors. Different center frequencies were achieved by adjusting the piezoelectric layer thickness. The LF and HF prototype probes had 88% and 95% of working elements, producing peak pressures of 21 and 96 kPa/V when focused at 5 and 3 cm, respectively. The one-way -3-dB bandwidths were 26% and 32%. These results, together with experimental tests on tissue-mimicking phantoms, show that the probes are viable for volumetric imaging.

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The integration of 2D ultrasonic transducer arrays and pitch-matched ASICs has enabled the realization of various 3D ultrasound imaging devices in recent years [1]-[3]. As applications such as 3D intravascular ultrasonography, intra-cardiac echocardiography, and trans-fontanelle ultrasonography call for miniaturization and improved spatial resolution, higher-frequency transducers (>5MHz) with a correspondingly smaller array pitch (<150m) are needed. Such devices generally employ a large number of transducer elements, calling for channel-count reduction in the ASIC while meeting stringent restrictions on per-element power consumption and die area. Micro-beamforming (BF) is an effective way of reducing channel count by performing a delay-and-sum operation on the echo signals received within a sub-array [1]. However, prior BF implementations employ per-element capacitive memory to realize the delay [1], [2], making it increasingly difficult to apply BF in smaller-pitch arrays.

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Over the past decades, ultrasound imaging has made considerable progress based on the advancement of imaging systems as well as transducer technology. With the need for advanced transducer arrays with complex designs and technical requirements, there is also a need for suitable tools to characterize such transducers. However, despite the importance of acoustic characterization to assess the performance of novel transducer arrays, the characterization process of highly complex transducers might involve various manual steps, which are laborious, time-consuming, and subject to errors. These factors can hinder the full characterization of a prototype transducer, leading to an under-representation or inadequate evaluation. To come to an extensive, high-quality evaluation of a prototype transducer, the acoustic characterization of each transducer element is indispensable in both transmit and receive operations. In this paper, we propose a pipeline to automatically perform the acoustic characterization of a matrix transducer using a research imaging system. The performance of the pipeline is tested on a prototype matrix transducer consisting of 960 elements. The results show that the proposed pipeline is capable of performing the complete acoustic characterization of a high-element count transducer in a fast and convenient way.

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Over the past decades, real-time three-dimensional (3D) medical ultrasound has attracted much attention since it enables clinicians to diagnose more accurately. This calls for ultrasound matrix transducers with a large number of elements, which can be interfaced with an application-specific integrated circuit (ASIC) for data reduction. An important aspect of the design of such a transducer is the geometry of each element, since it affects the mode of vibration and, consequently, the efficiency of the transducer. In this paper, we experimentally investigate the effect of subdicing on a piezoelectric (PZT) transducer. We fabricate and acoustically characterize a prototype PZT matrix transducer built on top of ASICs. The prototype transducer contains subdiced and non-subdiced elements, whose performance can be directly compared under the same conditions. Measurement results show that subdiced elements have a better performance compared to non-subdiced ones. Subdicing increases the peak pressure by 25%, raises the bandwidth by 10% and reduces the ringing time by 25%.@en

This paper presents an ultrasound transceiver application-specific integrated circuit (ASIC) directly integrated with an array of 12 × 80 piezoelectric transducer elements to enable next-generation ultrasound probes for 3D carotid artery imaging. The ASIC, implemented in a 0.18 µm high-voltage Bipolar-CMOS-DMOS (HV BCD) process, adopted a programmable switch matrix that allowed selected transducer elements in each row to be connected to a transmit and receive channel of an imaging system. This made the probe operate like an electronically translatable linear array, allowing large-aperture matrix arrays to be interfaced with a manageable number of system channels. This paper presents a second-generation ASIC that employed an improved switch design to minimize clock feedthrough and charge-injection effects of high-voltage metal–oxide–semiconductor field-effect transistors (HV MOSFETs), which in the first-generation ASIC caused parasitic transmis-sions and associated imaging artifacts. The proposed switch controller, implemented with cascaded non-overlapping clock generators, generated control signals with improved timing to mitigate the effects of these non-idealities. Both simulation results and electrical measurements showed a 20 dB reduction of the switching artifacts. In addition, an acoustic pulse-echo measurement successfully demonstrated a 20 dB reduction of imaging artifacts.

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Current 3-D Vector Flow Imaging (VFI) techniques are typically limited by either low temporal resolution and/or high computational cost. In this work, we illustrate a 3-D high frame rate VFI method, which efficiently splits the 3-D (x, y, z) displacement estimation in two separate 2-D VFI estimations operated in the frequency domain. The echo-data received after the transmission of plane waves are beamformed and high-pass filtered before being VFI processed. The new method was experimentally tested by using the ULA-OP 256 research system connected to a 3 MHz, 256-element, 2-D spiral array. Phantom experiments were conducted in steady laminar flow conditions for different probe-to-flow angles. For the tested conditions, the mean relative bias was 11.5% with a standard deviation of 10.9%. The 2-step approach applied to the 2-D VFI technique makes the new 3-D VFI method accurate and computationally efficient.

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Ultrasound vector Doppler techniques for three-dimensional (3-D) blood velocity measurements are currently limited by low temporal resolution and high computational cost. In this paper, an efficient 3-D high-frame-rate vector Doppler method, which estimates the displacements in the frequency domain, is proposed. The novel method extends to 3-D an approach so far proposed for two-dimensional (2-D) velocity measurements by approximating the (x, y, z) displacement of a small volume through the displacements estimated for the 2-D regions parallel to the y and x directions, respectively. The new method was tested by simulation and experiments for a 3.7 MHz, 256-element, 2-D piezoelectric sparse spiral array. Simulations were also performed for an equivalent 7 MHz Capacitive Micromachined Ultrasonic Transducer spiral array. The results indicate performance (bias ± standard deviation: 6.5 ± 8.0) comparable to the performance obtained by using a linear array for 2-D velocity measurements. These results are particularly encouraging when considering that sparse arrays were used, which involve a lower signal-to-noise ratio and worse beam characteristics with respect to full 2-D arrays.

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Recently, we realized a prototype matrix transducer consisting of 48 rows of 80 elements on top of a tiled set of Application Specific Integrated Circuits (ASICs) implementing a row-level control connecting one transmit/receive channel to an arbitrary subset of elements per row. A fully sampled array data acquisition is implemented by a column-by-column (CBC) imaging scheme (80 transmit-receive shots) which achieves 250 volumes/second (V/s) at a pulse repetition frequency of 20 kHz. However, for several clinical applications such as carotid pulse wave imaging (CPWI), a volume rate of 1000 per second is needed. This allows only 20 transmit-receive shots per 3D image. In this study, we propose a shifting aperture scheme and investigate the effects of receive/transmit aperture size and aperture shifting step in the elevation direction. The row-level circuit is used to interconnect elements of a receive aperture in the elevation (row) direction. An angular weighting method is used to suppress the grating lobes caused by the enlargement of the effective elevation pitch of the array, as a result of element interconnection in the elevation direction. The effective aperture size, level of grating lobes, and resolution/sidelobes are used to select suitable reception/transmission parameters. Based on our assessment, the proposed imaging sequence is a full transmission (all 80 elements excited at the same time), a receive aperture size of 5 and an aperture shifting step of 3. Numerical results obtained at depths of 10, 15, and 20 mm show that, compared to the fully sampled array, the 1000 V/s is achieved at the expense of, on average, about two times wider point spread function and 4 dB higher clutter level. The resulting grating lobes were at -27 dB. The proposed imaging sequence can be used for carotid pulse wave imaging to generate an informative 3D arterial stiffness map, for cardiovascular disease assessment.

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Transducer arrays for 3D imaging are characterized by elements distributed over a 2D surface. The dimensions of each element are typically one half-wavelength in both x- and y-directions. Such small elements inherently have a high electrical impedance. When the elements are connected to a probe cable, the high cable capacitance decimates the delivered voltage and results in a poor Signal to Noise Ratio (SNR). This may have dramatic effects, especially in sparse arrays, where a small number of elements contributes to the beamformed signal. In this paper we demonstrate that the use of in-probe preamplifiers in a sparse PZT probe is valuable to significantly increase the SNR.

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An ultrasound scan generates a huge amount of data. To form an image this data has to be transferred to the imaging system. This is an issue for applications where the data transfer capacity is limited such as hand-held systems, wireless probes and miniaturized array probes. Two-stage beamforming methods can be used to significantly reduce the data transfer requirements. In the first stage, which is applied in-probe, the amount of data is reduced from channel to scanline data. In the imaging system the data is then beamformed to obtain images that are synthetically focused over the entire image. Currently two approaches exist for the second stage. The first approach is a time-of-flight approach called synthetic aperture sequential beamforming (SASB) that has been developed for both linear and phased arrays. SASB does however introduce artefacts in the image that can be reduced by tapering the first stage scan lines at the cost of lateral resolution. The second approach is based on the wave equation, but a computationally efficient method for phased arrays that is producing sector scan data is lacking. Here we propose an algorithm that uses the fast Hankel transform to obtain a fast algorithm. The imaging performance of this method is evaluated with simulations and experiments. Compared with PSASB, which is an adaption of SASB for phased arrays, our method requires a similar amount of operations to construct the entire image and there is no trade-off between resolution and artefacts. These results show the advantage of using the wave equation instead of a time-of-flight approach.

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To accurately investigate the state of the carotid artery by the local haemodynamics and motion of the plaque using ultrasound, high-frame rate volumetric imaging is necessary. We have specifically designed a matrix array for this purpose. In this proceeding we will focus on imaging a volumetric flow profile using this matrix. For this purpose, we extend a fast frequency domain vector flow imaging method to 3D and perform measurements on a flow phantom. The results indicate that it is feasible to estimate 3D velocity vectors on a 3D grid using our matrix transducer and the proposed algorithm.

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The huge amount of data that needs to be transferred between probe and imaging system becomes a major issue when the data transfer capacity is limited, e.g. in handheld systems, wireless probes and miniaturized probes. The amount of data can be significantly reduced by using two-stage beamforming. The first stage consists of a fixed focus algorithm that compresses channel data to scanline data. This can be done by integrated electronics in the handle. In the second stage the scanline data is further beamformed in the imaging system to obtain images that are synthetically focused at all depths. Here we present a wave equation two-stage beamforming method for phased array imaging that is computationally efficient and outperforms PSASB, a time-of-flight alternative, in terms of lateral resolution and contrast-to-noise ratio.@en

Matrix arrays for endoscopic and catheter-based applications are restricted to small physical apertures, which limits their lateral resolution. In addition, when aiming for high volume rate imaging and utilizing the recent methods of transmitting a single or few diverging waves (DW), lateral resolution further deteriorates. In this work, we propose a high frame rate transmission scheme which outperforms alternative methods in lateral resolution. To improve the lateral resolution and side-lobe level, we propose to transmit only on a sub-aperture on either side of the array and apply a specific weighting function to received data. Compared to single-DW imaging, the proposed Sub-aperture Virtually Extended Array reduces the PSF width and sidelobe level by 16% and 5dB respectively and provides a similar SNR at the cost of halving the frame rate.@en

Matrix ultrasound transducers for medical diagnostic purposes are commercially available for a decade. A typical matrix transducer contains 1000 + elements, with a trend towards more and smaller elements. This number renders direct connection of each individual element to an ultrasound machine impractical. Consequently, it is cumbersome to employ traditional focusing and beamforming approaches that are based on transmit and receive signals having an individual time delay for each element. To reduce cable count during receive, one approach is to apply sub-arrays that locally combine the element signals using programmable delay-and-sum hardware, resulting in reduction by a factor 10. In transmit, achieving cable count reduction while keeping focusing and steering capabilities turns problematic once it becomes impossible to locally equip each element with its own high voltage pulser. To overcome this bottleneck for decreasing element size, here we present a Fresnel-inspired hardware and beam forming approach that is based on transmit pulses consisting of several periods of an oscillating waveform. These will be derived from one oscillating high voltage signal by using local switching and timing hardware. To demonstrate the feasibilities of our approach, we will show beam profiles and images for a miniature matrix transducer that we are currently developing.@en