A 2-D Ultrasound Transducer With Front-End ASIC and Low Cable Count for 3-D Forward-Looking Intravascular Imaging: Performance and Characterization

Variable Curvature Flexible Transducer for Abdominal Expandable Imaging

Expanding the imaging field of view (FOV) of medical ultrasound transducers will more effectively detect pathological behaviors of tissues or organs. Conventional rigid transducers can be realized by increasing the number of array elements or the curvature; however, the imaging aperture is fixed by the original size and shape during the manufacturing process. This article presented a 128-element, 3-MHz flexible curvature abdominal array (FCAA) with the goal of dynamically expanding the FOV within a 120° range. The piezoelectric stack was divided into small pitches through a double-cut process, and two-component viscoelastic substrates (TCVSs) were filled between adjacent array elements to generate tensile and compressive stresses during decomposition deformation. A 3-D-printed push-pull device provides sufficient mechanical support, resulting in a conformal minimum curvature radius of 46 mm. The innovative rigid-flexible composite backing layer was used to balance mechanical flexibility and high bandwidth (BW) of -6 dB to 67.6%. The results showed that the axial and lateral resolutions of the commercial phantom line target are 0.35 and 0.77 mm, respectively, and the axial and lateral resolutions of FOV 120° are 0.36 and 1.02 mm, respectively. The imaging performance of FCAA was verified by B-model imaging of the kidneys, intestines, uterus, and bladder of volunteers with a different body mass index (BMI). In addition, the 5-mm renal artery phantom verified the Doppler imaging function of FCAA. All the results demonstrated that FCPA has great potential clinical value in abdominal ultrasound and gynecological examination.

2D array imaging system for mechanically-steered, forward-viewing ultrasound guidewire

Approximately 4 million people with peripheral artery disease (PAD) present with critical limb ischemia each year, requiring urgent revascularization to avoid loss of limb. Minimally-invasive (i.e. endovascular) revascularization is preferable due to increased recovery time and increased risk of complications associated with open surgery. However, 40% of people with PAD also have chronic total occlusions (CTOs), resulting in > 20% of revascularization procedures failing when CTOs are present. A steerable robotic guidewire with integrated forward-viewing imaging capabilities would allow the guidewire to navigate through tortuous vasculature and facilitate crossing CTOs in revascularization procedures that currently fail due to inability to route the guidewire. The robotic steering capabilities of the guidewire can be leveraged for 3D synthetic aperture imaging with a simplified, low element count, forward-viewing 2D array on the tip of the mechanically-steered guidewire. Images can then be formed using a hybrid beamforming approach, with focal delays calculated for each element on the tip of the guidewire and for each physical location to which the robotically-steered guidewire is steered. Unlike synthetic aperture imaging with a steerable guidewire having only a single element transducer, an array with even a small number of elements can allow estimation of blood flow and physiological motion in vivo. A miniature, low element count 2D array transducer with 9 total elements (3 × 3) having total dimensions of 1.5 mm × 1.5 mm was designed to operate at 17 MHz. A proof-of-concept 2D array transducer was fabricated and characterized acoustically. The developed array was then used to image a wire target, a peripheral stent, and an ex vivo porcine iliac artery. Images were formed using the described synthetic aperture beamforming strategy. Acoustic characterization showed a mean resonance frequency of 17.6 MHz and a -6 dB bandwidth of 35%. Lateral and axial resolution were 0.271 mm and 0.122 mm, respectively, and an increase in SNR of 4.8 dB was observed for the 2D array relative to the single element case. The first 2D array imaging system utilizing both mechanical and electronic steering for guidewire-based imaging was developed and demonstrated. A 2D array imaging system operating on the tip of the mechanically-steered guidewire provides improved frame rate and increases field of view relative to a single element transducer. Finally, 2D array and single element imaging were compared for introduced motion errors, with the 2D array providing a 46.1% increase in SNR, and 58.5% and 17.3% improvement in lateral and axial resolution, respectively, relative to single element guidewire imaging.

Four-dimensional computational ultrasound imaging of brain hemodynamics

Four-dimensional ultrasound imaging of complex biological systems such as the brain is technically challenging because of the spatiotemporal sampling requirements. We present computational ultrasound imaging (cUSi), an imaging method that uses complex ultrasound fields that can be generated with simple hardware and a physical wave prediction model to alleviate the sampling constraints. cUSi allows for high-resolution four-dimensional imaging of brain hemodynamics in awake and anesthetized mice.

Supervised segmentation for guiding peripheral revascularization with forward-viewing, robotically steered ultrasound guidewire

BACKGROUND

Approximately 500 000 patients present with critical limb ischemia (CLI) each year in the U.S., requiring revascularization to avoid amputation. While peripheral arteries can be revascularized via minimally invasive procedures, 25% of cases with chronic total occlusions are unsuccessful due to inability to route the guidewire beyond the proximal occlusion. Improvements to guidewire navigation would lead to limb salvage in a greater number of patients.

PURPOSE

Integrating ultrasound imaging into the guidewire could enable direct visualization of routes for guidewire advancement. In order to navigate a robotically-steerable guidewire with integrated imaging beyond a chronic occlusion proximal to the symptomatic lesion for revascularization, acquired ultrasound images must be segmented to visualize the path for guidewire advancement.

METHODS

The first approach for automated segmentation of viable paths through occlusions in peripheral arteries is demonstrated in simulations and experimentally-acquired data with a forward-viewing, robotically-steered guidewire imaging system. B-mode ultrasound images formed via synthetic aperture focusing (SAF) were segmented using a supervised approach (U-net architecture). A total of 2500 simulated images were used to train the classifier to distinguish the vessel wall and occlusion from viable paths for guidewire advancement. First, the size of the synthetic aperture resulting in the highest classification performance was determined in simulations (90 test images) and compared with traditional classifiers (global thresholding, local adaptive thresholding, and hierarchical classification). Next, classification performance as a function of the diameter of the remaining lumen (0.5 to 1.5 mm) in the partially-occluded artery was tested using both simulated (60 test images at each of 7 diameters) and experimental data sets. Experimental test data sets were acquired in four 3D-printed phantoms from human anatomy and six ex vivo porcine arteries. Accuracy of classifying the path through the artery was evaluated using microcomputed tomography of phantoms and ex vivo arteries as a ground truth for comparison.

RESULTS

An aperture size of 3.8 mm resulted in the best-performing classification based on sensitivity and Jaccard index, with a significant increase in Jaccard index (p < 0.05) as aperture diameter increased. In comparing the performance of the supervised classifier and traditional classification strategies with simulated test data, sensitivity and F1 score for U-net were 0.95 ± 0.02 and 0.96 ± 0.01, respectively, compared to 0.83 ± 0.03 and 0.41 ± 0.13 for the best-performing conventional approach, hierarchical classification. In simulated test images, sensitivity (p < 0.05) and Jaccard index both increased with increasing artery diameter (p < 0.05). Classification of images acquired in artery phantoms with remaining lumen diameters ≥ 0.75 mm resulted in accuracies > 90%, while mean accuracy decreased to 82% when artery diameter decreased to 0.5 mm. For testing in ex vivo arteries, average binary accuracy, F1 score, Jaccard index, and sensitivity each exceeded 0.9.

CONCLUSIONS

Segmentation of ultrasound images of partially-occluded peripheral arteries acquired with a forward-viewing, robotically-steered guidewire system was demonstrated for the first-time using representation learning. This could represent a fast, accurate approach for guiding peripheral revascularization.

High-Voltage Bias-Switching Electronics for Volumetric Imaging Using Electrostrictive Row-Column Arrays

Top orthogonal to bottom electrode (TOBE) arrays, also known as row-column arrays, hold great promise for fast high-quality volumetric imaging. Bias-voltage-sensitive TOBE arrays based on electrostrictive relaxors or micromachined ultrasound transducers can enable readout from every element of the array using only row and column addressing. However, these transducers require fast bias-switching electronics which are not part of a conventional ultrasound system and are nontrivial. Here we report on the first modular bias-switching electronics enabling transmit, receive, and biasing on every row and every column of TOBE arrays, supporting up to 1024 channels. We demonstrate the performance of these arrays by connection to a transducer testing interface board (IB) and demonstrate 3-D structural imaging of tissue and 3-D power Doppler imaging of phantoms with real-time B-scan imaging and reconstruction rates. Our developed electronics enable interfacing of bias-switchable TOBE arrays to channel-domain ultrasound platforms with software-defined reconstruction for next-generation 3-D imaging at unprecedented scales and imaging rates.

A Tiled Ultrasound Matrix Transducer for Volumetric Imaging of the Carotid Artery

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.

Design, Implementation, and Medical Applications of 2-D Ultrasound Sparse Arrays

An ultrasound sparse array consists of a sparse distribution of elements over a 2-D aperture. Such an array is typically characterized by a limited number of elements, which in most cases is compatible with the channel number of the available scanners. Sparse arrays represent an attractive alternative to full 2-D arrays that may require the control of thousands of elements through expensive application-specific integrated circuits (ASICs). However, their massive use is hindered by two main drawbacks: the possible beam profile deterioration, which may worsen the image contrast, and the limited signal-to-noise ratio (SNR), which may result too low for some applications. This article reviews the work done for three decades on 2-D ultrasound sparse arrays for medical applications. First, random, optimized, and deterministic design methods are reviewed together with their main influencing factors. Then, experimental 2-D sparse array implementations based on piezoelectric and capacitive micromachined ultrasonic transducer (CMUT) technologies are presented. Sample applications to 3-D (Doppler) imaging, super-resolution imaging, photo-acoustic imaging, and therapy are reported. The final sections discuss the main shortcomings associated with the use of sparse arrays, the related countermeasures, and the next steps envisaged in the development of innovative arrays.

Fine Pitch Flexible Printed Circuit Board Patterning for Miniaturized Endoscopic MicroUltrasound Arrays

Microultrasound (micro-US) has become an invaluable tool for preclinical research and in emerging applications in clinical diagnosis and treatment guidance. Several such applications can benefit from arrays with a small footprint and endoscopic form factor. However, critical challenges arise in making electrical connections to array elements in such spatial constraints. In this work, we describe a method to pattern a high-density flexible circuit cabling on a copper-on polyimide film, using laser ablation of a polymer resist and wet etching, and then demonstrate a connection to a micro-US array. We investigate laser ablation process parameters and evaluate the ability to consistently pattern continuous copper traces. A minimum 30- [Formula: see text] pitch was achieved with 5- [Formula: see text]-wide electrode lines, and continuity of a 5-m-long trace is demonstrated. A flexible circuit with 30-mm-long traces with 30- [Formula: see text] line and 30- [Formula: see text] space before fan-out was fabricated to connect in an interleaved manner to a 32-element array with 30- [Formula: see text] element pitch. Metal deposition and laser ablation were used to connect and pattern the element electrodes to the copper traces of the flexible circuit. Electrical and acoustic measurements show good yield and consistent impedance across channels. Element pulse-echo tests demonstrated device functionality; the two-way pulse had 43-MHz center frequency and 40% fractional bandwidth (-6 dB). The proposed manufacturing methods facilitate the prototyping and fabrication of flexible endoscopic or small-footprint micro-US devices.

Sparse 2-D PZT-on-PCB Arrays With Density Tapering

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.

Stent with Piezoelectric Transducers for High Spatial Resolution Ultrasound Neuromodulation- a Finite Element Analysis

Deep brain stimulation is currently the only technique used in the clinical setting to modulate the neural activity of deep brain nuclei. Recently, low-intensity transcranial focused ultrasound (LIFU) has been shown to reversibly modulate brain activity through a transcranial pathway. Transcranial LIFU requires a low-frequency ultrasound of around 0.5 MHz due to skull attenuation, thus providing poor axial and lateral resolution. This paper proposes a new conceptual device that would use a stent to place a high-frequency ultrasound array within the brain vasculature to achieve high axial and lateral spatial resolution. The first part of this work identified the most commonly treated deep brain nuclei and examined the human brain vasculature for stent placement. Next, a finite element analysis was carried out using a piezoelectric array that follows the blood vessels curvature, and its ability to focus ultrasound waves in clinically relevant brain nuclei was evaluated. The analytical solution provided promising results for deep brain stimulation via a stent with ultrasound transducers for high spatial resolution neuromodulation.

Requirements and Hardware Limitations of High-Frame-Rate 3-D Ultrasound Imaging Systems

The spread of high frame rate and 3-D imaging techniques has raised pressing requirements for ultrasound systems. In particular, the processing power and data transfer rate requirements may be so demanding to hinder the real-time (RT) implementation of such techniques. This paper first analyzes the general requirements involved in RT ultrasound systems. Then, it identifies the main bottlenecks in the receiving section of a specific RT scanner, the ULA-OP 256, which is one of the most powerful available open scanners and may therefore be assumed as a reference. This case study has evidenced that the “star” topology, used to digitally interconnect the system’s boards, may easily saturate the data transfer bandwidth, thus impacting the achievable frame/volume rates in RT. The architecture of the digital scanner was exploited to tackle the bottlenecks by enabling a new “ring“ communication topology. Experimental 2-D and 3-D high-frame-rate imaging tests were conducted to evaluate the frame rates achievable with both interconnection modalities. It is shown that the ring topology enables up to 4400 frames/s and 510 volumes/s, with mean increments of +230% (up to +620%) compared to the star topology.

Echo Signal Receiving and Data Conversion Integrated Circuits for Portable High-Frequency Ultrasonic Imaging System

Ultrasonic imaging has become a very promising technology, and it has been widely applied in biomedicine, geology, and other fields due to its advantages of safety, nondamaging, and real time. Especially, the portable high-frequency (>20 MHz) ultrasonic imaging system (UIS) has been generally used in biomedical detection and diagnosis. In the complex actual environment, the effect of integrated circuits (ICs) on the performance of portable high-frequency UIS is obvious. In the echo signal transmission link, the analog front end (AFE) and the analog-to-digital converter (ADC) are the two most critical modules, where AFE is used to receive and preprocess the analog ultrasonic echo signals and ADC to convert the analog signals from the AFE output to digital. The structure and performance of the ICs integrated into terminal equipment and in-probe for the portable high-frequency UIS are introduced and discussed. Some typical commercial ICs are also summarized. Based on the requirements and challenges of portable high-frequency UIS, the future development directions of ICs mainly include high integration, ultralow power consumption, high speed, and high precision, which can provide valuable reference and advice for the design of AFE and ADC for portable high-frequency UIS.

Dual-Resonance (16/32 MHz) Piezoelectric Transducer With a Single Electrical Connection for Forward-Viewing Robotic Guidewire

Peripheral artery disease (PAD) affects more than 200 million people globally. Minimally invasive endovascular procedures can provide relief and salvage limbs while reducing injury rates and recovery times. Unfortunately, when a calcified chronic total occlusion is encountered, ~25% of endovascular procedures fail due to the inability to advance a guidewire using the view provided by fluoroscopy. To enable a sub-millimeter, robotically steerable guidewire to cross these occlusions, a novel single-element, dual-band transducer is developed that provides simultaneous multifrequency, forward-viewing imaging with high penetration depth and high spatial resolution while requiring only a single electrical connection. The design, fabrication, and acoustic characterization of this device are described, and proof-of-concept imaging is demonstrated in an ex vivo porcine artery after integration with a robotically steered guidewire. Measured center frequencies of the developed transducer were 16 and 32 MHz, with -6 dB fractional bandwidths of 73% and 23%, respectively. When imaging a 0.2-mm wire target at a depth of 5 mm, measured -6 dB target widths were 0.498 ± 0.02 and 0.268 ± 0.01 mm for images formed at 16 and 32 MHz, respectively. Measured SNR values were 33.3 and 21.3 dB, respectively. The 3-D images of the ex vivo artery demonstrate high penetration for visualizing vessel morphology at 16 MHz and ability to resolve small features close to the transducer at 32 MHz. Using images acquired simultaneously at both frequencies as part of an integrated forward-viewing, guidewire-based imaging system, an interventionalist could visualize the best path for advancing the guidewire to improve outcomes for patients with PAD.

High Frame Rate Volumetric Imaging of Microbubbles Using a Sparse Array and Spatial Coherence Beamforming

Volumetric ultrasound imaging of blood flow with microbubbles enables a more complete visualization of the microvasculature. Sparse arrays are ideal candidates to perform volumetric imaging at reduced manufacturing complexity and cable count. However, due to the small number of transducer elements, sparse arrays often come with high clutter levels, especially when wide beams are transmitted to increase the frame rate. In this study, we demonstrate with a prototype sparse array probe and a diverging wave transmission strategy, that a uniform transmission field can be achieved. With the implementation of a spatial coherence beamformer, the background clutter signal can be effectively suppressed, leading to a signal to background ratio improvement of 25 dB. With this approach, we demonstrate the volumetric visualization of single microbubbles in a tissue-mimicking phantom as well as vasculature mapping in a live chicken embryo chorioallantoic membrane.

Multielement Ring Array Based on Minute Size PMUTs for High Acoustic Pressure and Tunable Focus Depth

This paper presents a multielement annular ring ultrasound transducer formed by individual high-frequency PMUTs (17.5 MHz in air and 8.7 MHz in liquid) intended for high-precision axial focalization and high-performance ultrasound imaging. The prototype has five independent multielement rings fabricated by a monolithic process over CMOS, allowing for a very compact and robust design. Crosstalk between rings is under 56 dB, which guarantees an efficient beam focusing on a range between 1.4 mm and 67 µm. The presented PMUT-on-CMOS annular array with an overall diameter down to 669 µm achieves an output pressure in liquid of 4.84 kPa/V/mm2 at 1.5 mm away from the array when the five channels are excited together, which is the largest reported for PMUTs. Pulse-echo experiments towards high-resolution imaging are demonstrated using the central ring as a receiver. With an equivalent diameter of 149 µm, this central ring provides high receiving sensitivity, 441.6 nV/Pa, higher than that of commercial hydrophones with equivalent size. A 1D ultrasound image using two channels is demonstrated, with maximum received signals of 7 mVpp when a nonintegrated amplifier is used, demonstrating the ultrasound imaging capabilities.

A Robotically Steerable Guidewire With Forward-Viewing Ultrasound: Development of Technology for Minimally-Invasive Imaging

OBJECTIVE

The current standard of care for peripheral chronic total occlusions involves the manual routing of a guidewire under fluoroscopy. Despite significant improvements in recent decades, navigation remains clinically challenging with high rates of procedural failure and iatrogenic injury. To address this challenge, we present a proof-of-concept robotic guidewire system with forward-viewing ultrasound imaging to allow visualization and maneuverability through complex vasculature.

METHODS

A 0.035" guidewire-specific ultrasound transducer with matching layer and acoustic backing was designed, fabricated, and characterized. The effect of guidewire motion on signal decorrelation was assessed with simulations and experimentally, driving the development of a synthetic aperture beamforming approach to form images as the transducer is steered on the robotic guidewire. System performance was evaluated by imaging wire targets in water. Finally, proof-of-concept was demonstrated by imaging an ex vivo artery.

RESULTS

The designed custom transducer was fabricated with a center frequency of 15.7 MHz, 45.4% fractional bandwidth, and 31 dB SNR. In imaging 20 μm wire targets at a depth of 6 mm, the lateral -6 dB target width was 0.25 ± 0.03 mm. The 3D artery reconstruction allowed visualization of vessel wall structure and lumen.

CONCLUSION

Initial proof-of-concept for an ultrasound transducer-tipped steerable guidewire including 3D image formation without an additional sensor to determine guidewire position was demonstrated for a sub-mm system with an integrated ultrasound transducer and a robotically-steered guidewire.

SIGNIFICANCE

The developed forward-viewing, robotically-steered guidewire may enable navigation through occluded vascular regions that cannot be crossed with current methods.

Architecture for an Ultrasound Advanced Open Platform With an Arbitrary Number of Independent Channels

Ultrasound open platforms are programmable and flexible tools for the development and test of novel methods. In most cases, they embed the electronics for the independent control of (maximum) 256 probe elements. However, a higher number of channels is needed for the control of 2-D array probes. This paper presents a system architecture that, through the hardware and software synchronization of multiple ULA-OP 256 scanners, may implement advanced open platforms with an arbitrary number of channels. The proposed solution needs a single personal computer, maintains real-time features, and preserves portability. A prototype demonstrator, composed of two ULA-OP 256 scanners connected to 512 elements of a matrix array, was implemented and tested according to different channel configurations. Experiments performed under MATLAB control confirmed that by doubling the number of elements (from 256 to 512) the signal-to-noise and contrast ratios improve by 9 dB and 3 dB, respectively. Furthermore, as a full 512-channel scanner, the demonstrator can produce real-time B-mode images at 18 Hz, high enough for probe positioning during acquisitions. Also, the demonstrator permitted the implementation of a new high frame rate, bi-plane, triplex modality. All probe elements are excited to simultaneously produce two planar, perpendicular diverging waves. Each scanner independently processes the echoes received by the 256 connected elements to beamform 1300 frames per second. For each insonified plane, good quality morphological (B-mode), qualitative (color flow-), and quantitative (spectral-) Doppler images are finally shown in real-time by a dedicated interface.

A 30-MHz, 3-D Imaging, Forward-Looking Miniature Endoscope Based on a 128-Element Relaxor Array

This work describes the design, fabrication, and characterization of a 128-element crossed electrode array in a miniature endoscopic form factor for real-time 3-D imaging. Crossed electrode arrays address some of the key challenges surrounding probe fabrication for 3-D ultrasound imaging by reducing the number of elements required (2N compared with N²). However, there remain practical challenges in packaging a high-frequency crossed electrode array into an endoscopic form factor. A process has been developed that uses a thinly diced strip of flex circuit to bring the back-side connections to common bond surface, which allows the final size of the endoscope to measure only [Formula: see text] mm. An electrostrictive ceramic composite design was developed for the crossed electrode array. A laser dicing system was used to cut the 1-3 composite as well as etch the array electrode pattern. A single quarter wavelength Parylene matching layer made was vacuum deposited to finish the array. The electrical impedance magnitude of array elements on resonance was measured to be 49 Ω with a phase angle of -55.5°. The finished array elements produced pulses with -6-dB two-way bandwidth of 60% with a 34-MHz center frequency. The average measured electrical crosstalk on the nearest neighboring element and next to nearest neighboring element was -37 and -29 dB, respectively. One- and two-way pulse measurements were completed to confirm the pulse polarity and fast switching speed. Preliminary 3-D images were generated of a wire phantom using the previously described simultaneous azimuth and Fresnel elevation (SAFE) compounding imaging technique.

Capacitive micromachined ultrasound transducers for intravascular ultrasound imaging

Intravascular ultrasound (IVUS) is a burgeoning imaging technology that provides vital information for the diagnosis of coronary arterial diseases. A significant constituent that enables the IVUS system to attain high-resolution images is the ultrasound transducer, which acts as both a transmitter that sends acoustic waves and a detector that receives the returning signals. Being the most mature form of ultrasound transducer available in the market, piezoelectric transducers have dominated the field of biomedical imaging. However, there are some drawbacks associated with using the traditional piezoelectric ultrasound transducers such as difficulties in the fabrication of high-density arrays, which would aid in the acceleration of the imaging speed and alleviate motion artifact. The advent of microelectromechanical system (MEMS) technology has brought about the development of micromachined ultrasound transducers that would help to address this issue. Apart from the advantage of being able to be fabricated into arrays with lesser complications, the image quality of IVUS can be further enhanced with the easy integration of micromachined ultrasound transducers with complementary metal-oxide-semiconductor (CMOS). This would aid in the mitigation of parasitic capacitance, thereby improving the signal-to-noise. Currently, there are two commonly investigated micromachined ultrasound transducers, piezoelectric micromachined ultrasound transducers (PMUTs) and capacitive micromachined ultrasound transducers (CMUTs). Currently, PMUTs face a significant challenge where the fabricated PMUTs do not function as per their design. Thus, CMUTs with different array configurations have been developed for IVUS. In this paper, the different ultrasound transducers, including conventional-piezoelectric transducers, PMUTs and CMUTs, are reviewed, and a summary of the recent progress of CMUTs for IVUS is presented.

Miniaturized 0.13-μm CMOS Front-End Analog for AlN PMUT Arrays

This paper presents an analog front-end transceiver for an ultrasound imaging system based on a high-voltage (HV) transmitter, a low-noise front-end amplifier (RX), and a complementary-metal-oxide-semiconductor, aluminum nitride, piezoelectric micromachined ultrasonic transducer (CMOS-AlN-PMUT). The system was designed using the 0.13-μm Silterra CMOS process and the MEMS-on-CMOS platform, which allowed for the implementation of an AlN PMUT on top of the CMOS-integrated circuit. The HV transmitter drives a column of six 80-μm-square PMUTs excited with 32 V in order to generate enough acoustic pressure at a 2.1-mm axial distance. On the reception side, another six 80-μm-square PMUT columns convert the received echo into an electric charge that is amplified by the receiver front-end amplifier. A comparative analysis between a voltage front-end amplifier (VA) based on capacitive integration and a charge-sensitive front-end amplifier (CSA) is presented. Electrical and acoustic experiments successfully demonstrated the functionality of the designed low-power analog front-end circuitry, which outperformed a state-of-the art front-end application-specific integrated circuit (ASIC) in terms of power consumption, noise performance, and area.

Impact of Bit Errors in Digitized RF Data on Ultrasound Image Quality

This article quantitatively analyzes the impact of bit errors in digitized RF data on ultrasound image quality. The quality of B-mode images in both linear array and phased array imaging is evaluated by means of three objective image quality metrics: peak signal-to-noise ratio, structural similarity index, and contrast-to-noise ratio, when bit errors are introduced to the RF data with different bit-error rates (BERs). The effectiveness of coding schemes for forward error detection and correction to improve the image quality is also studied. The results show that ultrasound imaging is inherently resilient to high BER. The image quality suffers unnoticeable degradation for BER lower than 1E-6. Simple 1-bit parity coding with 9% added redundancy helps to retain similar image quality for BER up to 1E-4, and Hamming coding with 33.3% added redundancy allows the BER to increase to 1E-3. These results can serve as a guideline in the datalink design for ultrasound probes with in-probe receive digitization. With much more relaxed BER requirements than in typical datalinks, the design can be optimized by allowing fewer cables with higher data rate per cable or lower power consumption with the same cable count.

Multi-Channel Signal-Generator ASIC for Acoustic Holograms

A complementary metal-oxide-semiconductor (CMOS) application-specific integrated circuit (ASIC) has been developed to generate arbitrary, dynamic phase patterns for acoustic hologram applications. An experimental prototype has been fabricated to demonstrate phase shaping. It comprises a cascadable 1 ×9 array of identical, independently controlled signal generators implemented in a 0.35- [Formula: see text] minimum-feature-size process. It can individually control the phase of a square wave on each of the nine output pads. The footprint of the integrated circuit is [Formula: see text]. A 128-MHz clock frequency is used to produce outputs at 8 MHz with a phase resolution of 16 levels (4 bits) per channel. A 6 ×6 air-coupled matrix array ultrasonic transducer was built and driven by four ASICs, with the help of commercial buffer amplifiers, for the application demonstration. Acoustic pressure mapping and particle manipulation were performed. In addition, a 2 ×2 array piezoelectric micromachined ultrasonic transducer (PMUT) was connected and driven by four output channels of a single ASIC, demonstrating the flexibility of the ASIC to work with different transducers and the potential for direct integration of CMOS and PMUTs.