Element Ultrasound Transducer Research Articles

Deep Learning Enhances Multiparametric Dynamic Volumetric Photoacoustic Computed Tomography In Vivo (DL-PACT).

Photoacoustic computed tomography (PACT) has become a premier preclinical and clinical imaging modality. Although PACT's image quality can be dramatically improved with a large number of ultrasound (US) transducer elements and associated multiplexed data acquisition systems, the associated high system cost and/or slow temporal resolution are significant problems. Here, a deep learning-based approach is demonstrated that qualitatively and quantitively diminishes the limited-view artifacts that reduce image quality and improves the slow temporal resolution. This deep learning-enhanced multiparametric dynamic volumetric PACT approach, called DL-PACT, requires only a clustered subset of many US transducer elements on the conventional multiparametric PACT. Using DL-PACT, high-quality static structural and dynamic contrast-enhanced whole-body images as well as dynamic functional brain images of live animals and humans are successfully acquired, all in a relatively fast and cost-effective manner. It is believed that the strategy can significantly advance the use of PACT technology for preclinical and clinical applications such as neurology, cardiology, pharmacology, endocrinology, and oncology.

Image-Free Fast Ultrasound for Measurement of Local Pulse Wave Velocity: In Vitro Validation and In Vivo Feasibility.

Local pulse wave velocity (PWV), a metric of the target artery's stiffness, has been emerging in its clinical value and adoption. State-of-the-art ultrasound technologies used to evaluate local PWV based on pulse waves' features are sophisticated, non-real-time, and are not amenable for field and resource-constrained settings. In this work, we present an image-free ultrasound system to measure local PWV in real-time by employing a pair of ultrasound transducer elements. An in vitro study was performed on the arterial phantom to: 1) characterize the design aspects of the system and 2) validate its accuracy against beat-by-beat (invasive) local PWV measured by a reference dual-element catheter. Furthermore, a repeatability and reproducibility study on 33 subjects (21-52 years) investigated the in vivo measurement feasibility from the carotid artery. With the experimentally deduced optimal design (frame-rate =500 Hz, RF sampling rate =125 MHz, LPF cutoff =14 Hz, and order =4 ), the system yielded repeatable beat-to-beat measurements (variability =1.9 % and over 15 cycles) and achieved a high accuracy (root-mean-square-error =0.19 m/s and absolute-percentage-error =2.4 %) over a wide range of PWVs (2.7-11.4 m/s) from the phantom. Subsequently, on human subjects, the intra- and inter-operator PWV measurements were highly repeatable (intraclass correlation coefficient ). The system does not impose a demand for special processors with high-computational power while offering real-time feedback on acquisition and measurement quality and provides local PWV online. Future large population and animal studies are required to establish the device's clinical usability.

Classifying Muscle States with One-Dimensional Radio-Frequency Signals from Single Element Ultrasound Transducers.

The reliable assessment of muscle states, such as contracted muscles vs. non-contracted muscles or relaxed muscles vs. fatigue muscles, is crucial in many sports and rehabilitation scenarios, such as the assessment of therapeutic measures. The goal of this work was to deploy machine learning (ML) models based on one-dimensional (1-D) sonomyography (SMG) signals to facilitate low-cost and wearable ultrasound devices. One-dimensional SMG is a non-invasive technique using 1-D ultrasound radio-frequency signals to measure muscle states and has the advantage of being able to acquire information from deep soft tissue layers. To mimic real-life scenarios, we did not emphasize the acquisition of particularly distinct signals. The ML models exploited muscle contraction signals of eight volunteers and muscle fatigue signals of 21 volunteers. We evaluated them with different schemes on a variety of data types, such as unprocessed or processed raw signals and found that comparatively simple ML models, such as Support Vector Machines or Logistic Regression, yielded the best performance w.r.t. accuracy and evaluation time. We conclude that our framework for muscle contraction and muscle fatigue classifications is very well-suited to facilitate low-cost and wearable devices based on ML models using 1-D SMG.

Ultrasound Thermometry for HIFU-Therapy

Abstract High-Intensity Focused Ultrasound (HIFU) is an alternative tumour therapy with the ability for non-invasive thermal ablation of tissue. For a safe application, the heat deposition needs to be monitored over time, which is currently done with Magnetic Resonance Imaging. Ultrasound (US) based monitoring is a promising alternative, as it is less expensive and allows the use of a single device for both therapy and monitoring. In this work, a method for spatial and temporal US thermometry has been investigated based on simulation studies and in-vitro measurements. The chosen approach is based on the approximately linear dependence between temperature and speed of sound (SoS) in tissue for a given temperature range. By tracking the speckles of successive B-images, the possibility of detecting local changes in SoS and therefore in temperature is given. A speckle tracking algorithm was implemented for 2D and 3D US thermometry using a spatial compounding method to reduce artifacts. The algorithm was experimentally validated in an agar-based phantom and in porcine tissue for temperature rises up to △ 8°C. We used a focusing single element US transducer as therapeutic probe, a linear (/matrix array) transducer with 128 (/32∙32) elements for imaging and thermocouples for validation and calibration. In all experiments, both computational and in-vitro, we succeeded in monitoring the thermal induced SoS changes over time. The in-vitro measurements were in good agreement with the simulation results and the thermocouple measurements (rms temperature difference = 0.53 °C, rms correlation coefficient = 0. 96).

An adaptation of the ultrasound transducer element test for multi-row arrays.

A simple "paperclip test" for the function of individual elements in a diagnostic ultrasound transducer array is widely performed and has been adapted for phased arrays. The aim of this study was to adapt the test further for multi-row transducer arrays. An embossing tool was used in place of the usual paperclip or metal rod and was slowly moved along the transducer array, attempting to isolate the signal from each row in turn. Phased array transducers were operated in M-mode. Non-functioning elements were identified by a reduction in amplitude of the reverberation line. The test was repeated several times for each transducer, ensuring that all non-functioning elements were identified and looking for consistency of results. 28 phased arrays and 5 linear/curvi-linear arrays in clinical use and 1 phased array and 1 linear array already identified as faulty by electronic transducer testing, and not in clinical service, were available for testing. 8 of the clinical phased arrays were found to have 1 or more faulty elements; 3 had only minor defects and 5 were replaced under warranty or service contract. The linear/curvi-linear arrays showed no fault. The adapted test showed the failed elements in the known faulty phased array, except at the end of the array, but weak elements were not detected. The faulty linear array had a block of failed outer elements which was identified by the test. The adapted test is capable of detecting non-functioning elements in multi-row arrays, but weak elements were not detected.

Simulation of 28 Element Ultrasound Transducer for Non-Destructive Evaluation (NDE) Applications

Quantitative ultrasound based Non-Destructive Evaluation (NDE) is one of the leading test procedures used in testing of finished products in manufacturing industry. It can detect surface as well as subsurface flaws accurately. It is also employed in medical diagnostic for various purposes. The main objective of this paper is to simulate 28 element ultrasound transducer pair for through transmission testing procedure. The simulation is done using k-Wave toolbox (open source toolbox) and MATLAB. The simulation results show that the received signal strength and resolution is far more improved over single and dual element transducer probes available in the market. The simulation of wave propagation through three layered aluminum material is also shown in the results. Based on results, it can be concluded that 28 element transducer pair are the optimum solution for NDE evaluation challenges with reference to cost, size, and design complexity.

Optimization and Validation of Dual Element Ultrasound Transducers for Improved Pulse-Echo Measurements of Material Nonlinearity

In nonlinearity parameter ( $\beta $ ) measurements of solids, the pulse-echo method is more desirable than the through-transmission method. Dual element transducers consisting of an outer ring transmitter and an inner disk receiver have been found to be an efficient alternative to single element transducers because such configuration provides an improved second harmonic generation and dependence on the total correction in the pulse-echo mode. For accurate and reliable measurements of $\beta $ for relatively thin samples, further improvement is needed by optimizing the transmitter and receiver size of dual element transducers for a given sample thickness and frequency. The optimized transducer should provide the largest possible second harmonic reception and a total correction value close to one. In this paper, optimization of the dual element transducer is performed for 1 cm thick Al specimen at 5 MHz fundamental frequency, and the results are presented for the received second harmonic amplitude and the total correction. The optimized dual element transducer is fabricated and experimentally characterized to be applied to determine the absolute $\beta $ of Al specimens whose microstructure changes due to precipitation heat treatment. The experimental results validate the transducer optimized for pulse-echo $\beta $ measurements on 1 cm specimens, and provide a quantitative correlation between the $\beta $ and the microstructural change.

A real-time data-based scan conversion method for single element ultrasound transducers

The current work investigates the performance of a real-time scan conversion algorithm for generating a 2-D ultrasound image from a laterally scanned single-element ultrasound transducer, which has applications in point-of-care devices such as for skin imaging. The algorithm employs a fixed calibration curve to update a predefined image grid in real time. Simulations showed that the calibration curve (with a maximum of 1) is robust to changes in scatterer concentration (8.3×10-3 mean absolute error), signal to noise ratio (1.0×10-3 mean absolute error for −5 dB SNR), and can be accurately predicted from a small number (31) of point scatterers (6.9×10-3 mean absolute error). Good agreement was also found between the calibration curves obtained from simulated and experimental data (1.19×10-2 mean absolute error). The scan conversion algorithm was validated by evaluation of the position estimation errors on both simulations and experiments. Clinical images of skin lesions (N = 20) demonstrate the feasibility of the algorithm for real, non-homogeneous tissue. Use of a fixed calibration curve compared to an adaptive calibration curve gave similar accuracies in the scanning step size range of 150–350 μm (with an average overlap of the accuracy ranges of 92.94% for simulations and 42.83% for experiments), and a 350-fold improvement in computation time.

Design of patient-specific focused ultrasound arrays for non-invasive brain therapy with increased trans-skull transmission and steering range

The use of a phased array of ultrasound transducer elements to sonicate through the skull has opened the way for new treatments and the delivery of therapeutics beyond the blood-brain barrier. The limited steering range of current clinical devices, particularly at higher frequencies, limits the regions of the brain that are considered treatable by ultrasound. A new array design is introduced that allows for high levels of beam steering and increased transmission throughout the brain. These improvements are achieved using concave transducers normal to the outer-skull surface in a patient-specific configuration to target within the skull, so that the far-field of each beam is within the brain. It is shown that by using pulsed ultrasound waves timed to arrive in-phase at the desired target, sufficient levels of acoustic energy are delivered for blood-brain barrier opening throughout the brain.

Load‐insensitive active quasi‐circulator for continuous‐wave Doppler ultrasound applications

Circulators are often used in RF systems; however, their size, cost, and weight increase greatly at lower frequencies ( 19 dB for 20% of variation of the load impedance. The circuit was implemented and tested with an ultrasound probe in a water tank. The results show that the circuit was able to send and receive signals simultaneously from a single ultrasound transducer element.

Comparison of Deconvolution Filters for Photoacoustic Tomography.

In this work, we compare the merits of three temporal data deconvolution methods for use in the filtered backprojection algorithm for photoacoustic tomography (PAT). We evaluate the standard Fourier division technique, the Wiener deconvolution filter, and a Tikhonov L-2 norm regularized matrix inversion method. Our experiments were carried out on subjects of various appearances, namely a pencil lead, two man-made phantoms, an in vivo subcutaneous mouse tumor model, and a perfused and excised mouse brain. All subjects were scanned using an imaging system with a rotatable hemispherical bowl, into which 128 ultrasound transducer elements were embedded in a spiral pattern. We characterized the frequency response of each deconvolution method, compared the final image quality achieved by each deconvolution technique, and evaluated each method’s robustness to noise. The frequency response was quantified by measuring the accuracy with which each filter recovered the ideal flat frequency spectrum of an experimentally measured impulse response. Image quality under the various scenarios was quantified by computing noise versus resolution curves for a point source phantom, as well as the full width at half maximum (FWHM) and contrast-to-noise ratio (CNR) of selected image features such as dots and linear structures in additional imaging subjects. It was found that the Tikhonov filter yielded the most accurate balance of lower and higher frequency content (as measured by comparing the spectra of deconvolved impulse response signals to the ideal flat frequency spectrum), achieved a competitive image resolution and contrast-to-noise ratio, and yielded the greatest robustness to noise. While the Wiener filter achieved a similar image resolution, it tended to underrepresent the lower frequency content of the deconvolved signals, and hence of the reconstructed images after backprojection. In addition, its robustness to noise was poorer than that of the Tikhonov filter. The performance of the Fourier filter was found to be the poorest of all three methods, based on the reconstructed images’ lowest resolution (blurriest appearance), generally lowest contrast-to-noise ratio, and lowest robustness to noise. Overall, the Tikhonov filter was deemed to produce the most desirable image reconstructions.

Comparison of measurement of the augmentation index from ARTSENS and eTRACKING

Over past few years our group has been developing a fully automated and low-cost device, ARTSENS (ARTerial Stiffness Evaluation for Non-invasive Screening), to enable non-experts to measure arterial stiffness (AS). It uses a single element ultrasound transducer to obtain A-mode frames from a superficial artery such as the common carotid artery (CCA) and analyzes them to obtain the stiffness parameters of the vessel. We have earlier demonstrated that ARTSENS can accurately measure local arterial stiffness (LAS) and regional arterial stiffness (RAS) by tracing the distension waveforms of the CCA and the femoral artery. In this paper, we show that it is possible to estimate the augmentation index (AIx), a measure of the global arterial mechanics, from the distension waveforms obtained by ARTSENS. AIx measurements from ARTSENS are compared against the state-of-the-art Hitachi-Aloka eTRACKING system for 107 volunteers. Both devices show excellent agreement with a correlation coefficient (r) = 0.82 (p < 0.0001), which is comparable to similar studies reported in the literature. This development makes ARTSENS a unique device that can measure the three most widely used indices of arterial mechanics—LAS, RAS and the AIx.

Automatic Measurement of End-Diastolic Arterial Lumen Diameter in ARTSENS

Over past few years, we are developing a system for facilitating large scale screening of patients for cardiovascular risk—arterial stiffness evaluation for noninvasive screening (ARTSENS). ARTSENS is an image-free device that uses a single element ultrasound transducer to obtain noninvasive measurements of arterial stiffness (AS) in a fully automated manner. AS is directly proportional to end-diastolic lumen diameter (Dd). Multilayered structure of the arterial walls and indistinct characteristics of intima-lumen interface (ILI) makes it quite difficult to accurately estimate Dd in A-mode radio-frequency (RF) frames obtained from ARTSENS. In this paper, we propose a few methods based on fitting simple mathematical models to the echoes from arterial walls, followed by a novel method to fuse the information from curve fitting error and distension curve to arrive at an accurate measure of Dd. To bring down the curve fitting time and facilitate processing on low-end processors, a novel approach using the autocorrelation of echoes from opposite walls of the artery has been discussed. The methods were analyzed for their comparative accuracy against reference Dd obtained from 85 human volunteers using Hitachi-Aloka eTRACKING system. Dd from all reported methods show strong and statistically significant positive correlation with eTRACKING and mean error of less than 7% could be achieved. As expected, Dd from all methods show significant positive correlation with age.

A single element 3D ultrasound tomography system.

Over the past decade, substantial effort has been directed toward developing ultrasonic systems for medical imaging. With advances in computational power, previously theorized scanning methods such as ultrasound tomography can now be realized. In this paper, we present the design, error analysis, and initial backprojection images from a single element 3D ultrasound tomography system. The system enables volumetric pulse-echo or transmission imaging of distal limbs. The motivating clinical applications include: improving prosthetic fittings, monitoring bone density, and characterizing muscle health. The system is designed as a flexible mechanical platform for iterative development of algorithms targeting imaging of soft tissue and bone. The mechanical system independently controls movement of two single element ultrasound transducers in a cylindrical water tank. Each transducer can independently circle about the center of the tank as well as move vertically in depth. High resolution positioning feedback (~1μm) and control enables flexible positioning of the transmitter and the receiver around the cylindrical tank; exchangeable transducers enable algorithm testing with varying transducer frequencies and beam geometries. High speed data acquisition (DAQ) through a dedicated National Instrument PXI setup streams digitized data directly to the host PC. System positioning error has been quantified and is within limits for the imaging requirements of the motivating applications.

Ultrashort echo-time MRI as a substitute to CT for skull aberration correction in transcranial focused ultrasound: in vitro comparison on human calvaria

Clinical transcranial MR-guided focused ultrasound (TcMRgFUS) brain treatment systems compensate for skull-induced beam aberrations by adjusting the phase and amplitude of individual ultrasound transducer elements. These corrections are currently calculated based on a pre-acquired CT scan of the patient’s head. The purpose of the work presented here is to demonstrate the feasibility of using ultrashort echo-time (UTE) MRI instead of CT to calculate and apply aberration corrections on a clinical TcMRgFUS system.

An Imageless Ultrasound Device to Measure Local and Regional Arterial Stiffness.

Arterial stiffness (AS) has been shown to be an important marker for risk assessment of cardiovascular events. Local arterial stiffness (LAS) is conventionally measured by evaluating arterial distensibility at particular arterial sites through ultrasound imaging systems. Regional arterial stiffness (RAS) is generally obtained by evaluating carotid to femoral pulse wave velocity (cfPWV) through tonometric devices. RAS has a better prognostic value than LAS and cfPWV is considered as the gold standard of AS. Over the past few years our group has been developing ARTerial Stiffness Evaluation for Non-Invasive Screening (ARTSENS), an inexpensive and portable device to measure the LAS. It uses a single element ultrasound transducer to obtain A-Mode frames from the desired artery and is fully automated to enable a non-expert to perform measurements. In this work, we report an extension of ARTSENS to enable measurement of cfPWV that now makes it the only fully automatic device that can measure both LAS and RAS. In this paper, we provide a general review of the ARTSENS and compare it with other state-of-the-art AS measurement systems. cfPWV measurement using ARTSENS was cross-validated against SphygmoCor by successive measurements with both devices on 41 human subjects and excellent agreement between both devices was demonstrated (Coefficient of determination and, limits of agreement m/s). The inter-device correlation between ARTSENS and SphygmoCor was found to be better than other similar studies reported in the literature.

Carotid and Jugular Classification in ARTSENS.

Over past few years our group has been working on the development of a low-cost device, ARTSENS, for measurement of local arterial stiffness (AS) of the common carotid artery (CCA). This uses a single element ultrasound transducer to obtain A-mode frames from the CCA. It is designed to be fully automatic in its operation such that, a general medical practitioner can use the device without any prior knowledge of ultrasound modality. Placement of the probe over CCA and identification of echo positions corresponding to its two walls are critical steps in the process of measurement of AS. We had reported an algorithm to locate the CCA walls based on their characteristic motion. Unfortunately, in supine position, the internal jugular vein (IJV) expands in the carotid triangle and pulsates in a manner that confounds the existing algorithm and leads to wrong measurements of the AS. Jugular venous pulse (JVP), on its own right, is a very important physiological signal for diagnosis of morbidities of the right side of the heart and there is a lack of noninvasive methods for its accurate estimation. We integrated an ECG device to the existing hardware of ARTSENS and developed a method based on physiology of the vessels, which now enable us to segregate the CCA pulse (CCP) and the JVP. False identification rate is less than 4%. To retain the capabilities of ARTSENS to operate without ECG, we designed another method where the classification can be achieved without an ECG, albeit errors are a bit higher. These improvements enable ARTSENS to perform automatic measurement of AS even in the supine position and make it a unique and handy tool to perform JVP analysis.

Non-contact multi-particle annular patterning and manipulation with ultrasound microbeam

Multiparticle-trapping offers diverse opportunities and applications in biotechnology. It can be applied to creating various functional materials or organizing reactive particles. In this paper, we demonstrate that it is possible to trap and manipulate multi-particles in an annular pattern with a 24 MHz focused ring-type single element ultrasound transducer. Acoustic ring trap can be useful in undertaking biotropism studies due to an equal-distance condition from the center. Also, this ring trap could serve as a force shield to protect analysis area from other cells. The experimental results showed the capability of the proposed method as a multi-cell manipulator in formatting specific patterns of small cells like sperms.

Evaluation of the algorithm for automatic identification of the common carotid artery in ARTSENS

Arterial compliance (AC) is an indicator of the risk of cardiovascular diseases (CVDs) and it is generally estimated by B-mode ultrasound investigation. The number of sonologists in low- and middle-income countries is very disproportionate to the extent of CVD. To bridge this gap we are developing an image-free CVD risk screening tool–arterial stiffness evaluation for non-invasive screening (ARTSENS™) which can be operated with minimal training. ARTSENS uses a single element ultrasound transducer to investigate the wall dynamics of the common carotid artery (CCA) and subsequently measure the AC. Identification of the proximal and distal walls of the CCA, in the ultrasound frames, is an important step in the process of the measurement of AC. The image-free nature of ARTSENS creates some unique issues which necessitate the development of a new algorithm that can automatically identify the CCA from a sequence of A-mode radio-frequency (RF) frames. We have earlier presented the concept and preliminary results for an algorithm that employed clues from the relative positions and temporal motion of CCA walls, for identifying the CCA and finding the approximate wall positions. In this paper, we present the detailed algorithm and its extensive evaluation based on simulation and clinical studies. The algorithm identified the wall position correctly in more than 90% of all simulated datasets where the signal-to-noise ratio was greater than 3 dB. The algorithm was then tested extensively on RF data obtained from the CCA of 30 human volunteers, where it successfully located the arterial walls in more than 70% of all measurements. The algorithm could successfully reject frames where the CCA was not present thus assisting the operator to place the probe correctly in the image-free system, ARTSENS. It was demonstrated that the algorithm can be used in real-time with few trade-offs which do not affect the accuracy of CCA identification. A new method for depth range selection that leads to significant performance improvements has also been demonstrated.

Custom integrated circuit design for ultrasonic therapeutic CMUT array

This paper presents the design of a highly flexible and programmable transmit beam-former ASIC using a high voltage (HV) 0.35 μm CMOS technology to be flip-chip bonded to a 4 × 4 CMUT array for ultrasound therapeutic applications. However, proposed IC can be used as a transmitter circuitry in color Doppler 3D imaging applications. In our proposed chip, each CMUT element is provided by an 8-bit shift register, an 8-bit comparator, a one-shot circuit with adjustable pulse width, a programmable pulse train generator and a HV pulser circuit. The interface electronics can generate two types of outputs with programmable focusing delays to 16 ultrasound transducer elements in different modes: The first mode is for generating single pulses in which the one-shot circuits adjust the width of the pulses from a few nanoseconds to 650 ns with enough resolution for different operating frequencies of ultrasound transducer. The second one has been considered for generating controllable pulse trains with output frequencies in the range of 1---10 MHz. At final stage the amplitudes of single pulses or pulse trains are increased up to 45 V using HV pulsers. Using an external control system like FPGA, the pulser circuit fires correspondent CMUT element when its stored shift register value is identical with the value of global counter.