Medical Ultrasound Applications Research Articles

A 15-V Bidirectional Ultrasound Interface Analog Front-End IC for Medical Imaging Using Standard CMOS Technology

A high-voltage (HV) interface analog front-end (AFE) integrated circuit (IC) for medical ultrasound imaging applications using 0.18-μm standard CMOS process is presented. The proposed AFE IC includes a HV pulser in the transmit path that safely generates up to 15-Vpp of unipolar pulses at 2.6 MHz, a HV switch for isolation between transmitter and receiver frontend parts, and a 95.1-dBΩ low-power transimpedance preamplifier with 12-MHz bandwidth and 3.5-pA/√Hz input referred noise in the receive path operating at a low 1.1-V supply voltage. Both the pulser and the switch utilize dynamically-gate-biased stacked 3.3-V transistors to enable HV operation without compromising device reliability. The implemented single-channel AFE IC prototype intended for interfacing capacitive micromachined ultrasound transducer consumes 0.15 mm2 of core die area, making it feasible to be applied for various multi-array medical ultrasound imaging systems.

Tribonucleation of bubbles.

We report on the nucleation of bubbles on solids that are gently rubbed against each other in a liquid. The phenomenon is found to depend strongly on the material and roughness of the solid surfaces. For a given surface, temperature, and gas content, a trail of growing bubbles is observed if the rubbing force and velocity exceed a certain threshold. Direct observation through a transparent solid shows that each bubble in the trail results from the early coalescence of several microscopic bubbles, themselves detaching from microscopic gas pockets forming between the solids. From a detailed study of the wear tracks, with atomic force and scanning electron microscopy imaging, we conclude that these microscopic gas pockets originate from a local fracturing of the surface asperities, possibly enhanced by chemical reactions at the freshly created surfaces. Our findings will be useful either for preventing undesired bubble formation or, on the contrary, for "writing with bubbles," i.e., creating controlled patterns of microscopic bubbles.

Ultrasonic Imaging Transceiver Design for CMUT: A Three-Level 30-Vpp Pulse-Shaping Pulser With Improved Efficiency and a Noise-Optimized Receiver

This paper demonstrates a four-channel transceiver chip for medical ultrasonic imaging, interfacing to the capacitive micromachined ultrasonic transducers (CMUTs). The high-voltage transmitter (Tx) uses a three-level pulse-shaping technique with charge recycling to improve the power efficiency. The design requires minimum off-chip components and is scalable for more channels. The receiver is implemented with a transimpedance amplifier (TIA) topology and is optimized for tradeoffs between noise, bandwidth, and power dissipation. The test chip is characterized with both acoustic and electrical measurements. Comparing the three-level pulser against traditional two-level pulsers, the measured Tx efficiency shows 56%, 50%, and 43% more acoustic power delivery with the same total power dissipation at 2.5, 3.3, and 5.0 MHz, respectively. The CMUT receiver achieves the lowest noise efficiency factor compared with that of the literature (2.1 compared to a previously reported lowest of 3.6, in units of mPA ·√(mW/Hz). In addition, the transceiver chip is tested as a complete system for medical ultrasound imaging applications, in experiments including Tx beamformation, pulse-echo channel response characterization, and ultrasonic Doppler flow rate detection.

Gapped sampled spectrum Doppler estimation.

Duplex and triplex transmit patterns that involve gaps in the spectrum Doppler samples allow pulse repetition frequency increases and/or frame rate increases that cannot be flexibly achieved by conventional uniformly sampled transmit schemes. We make two claims in this paper. First, previously reported nonparametric gapped sampled spectrum estimators are technically feasible for handling the duplex and triplex transmit patterns found in common medical ultrasound applications. Second, such estimators that coherently average within an axial/temporal 2-D window have superior SNR compared with their incoherent counterparts. Moreover, this fact extends to previously reported fully sampled incoherent estimators, which can be improved by using their coherent version. We verify the methods by steady-state flow phantom experiments and in vivo examples of the left clavicular artery and the ascending aorta. For the flow phantom experiments, we use the three quantitative metrics of SNR, root mean square error, and zero frequency peak full-width at half-maximum to evaluate robustness and resolution. Results indicate that through proper parameters, periodically gapped estimators can produce results similar to their fully sampled counterparts. Fourier synthesis of the spectral estimates produces the fully sampled time-domain audio signal, and we give stereo audio examples for the clavicular artery.

A low-complexity adaptive beamformer for ultrasound imaging using structured covariance matrix

In recent years, adaptive beamforming methods have been successfully applied to medical ultrasound imaging, resulting in simultaneous improvement in imaging resolution and contrast. These improvements have been achieved at the expense of higher computational complexity, with respect to the conventional non-adaptive delay-and-sum (DAS) beamformer, in which computational complexity is proportional to the number of elements, O(M). The computational overhead results from the covariance matrix inversion needed for computation of the adaptive weights, the complexity of which is cubic with the subarray size, O(L(3)). This is a computationally intensive procedure, which makes the implementation of adaptive beamformers less attractive in spite of their advantages. Considering that, in medical ultrasound applications, most of the energy is scattered from angles close to the steering angle, assuming spatial stationarity is a good approximation, allowing us to assume the Toeplitz structure for the estimated covariance matrix. Based on this idea, in this paper, we have applied the Toeplitz structure to the spatially smoothed covariance matrix by averaging the entries along all subdiagonals. Because the inverse of the resulting Toeplitz covariance matrix can be computed in O(L(2)) operations, this technique results in a greatly reduced computational complexity. By using simulated and experimental RF data-point targets as well as cyst phantoms-we show that the proposed low-complexity adaptive beamformer significantly outperforms the DAS and its performance is comparable to that of the minimum variance beamformer, with reduced computational complexity.

Optimization of the Multi-element Synthetic Transmit Aperture Method for Medical Ultrasound Imaging Applications

The paper presents the optimization problem for the multi-element synthetic transmit aperture method (MSTA) in ultrasound imaging applications. The optimal choice of the transmit aperture size is made as a trade-off between the lateral resolution, penetration depth and the frame rate. Results of the analysis obtained by a developed optimization algorithm are presented. The maximum penetration depth and lateral resolution at given depths are chosen as optimization criteria. The results of numerical experiments carried out in MATLABr using synthetic aperture data of point reflectors obtained by the FIELD II simulation program are presented. The visualization of experimental synthetic aperture data of a tissue mimicking phantom and in vitro measurements of the beef liver performed using the SonixTOUCH Research system are also shown.

Design and implementation of a smartphone-based portable ultrasound pulsed-wave doppler device for blood flow measurement

Blood flow measurement using Doppler ultrasound has become a useful tool for diagnosing cardiovascular diseases and as a physiological monitor. Recently, pocket-sized ultrasound scanners have been introduced for portable diagnosis. The present paper reports the implementation of a portable ultrasound pulsed-wave (PW) Doppler flowmeter using a smartphone. A 10-MHz ultrasonic surface transducer was designed for the dynamic monitoring of blood flow velocity. The directional baseband Doppler shift signals were obtained using a portable analog circuit system. After hardware processing, the Doppler signals were fed directly to a smartphone for Doppler spectrogram analysis and display in real time. To the best of our knowledge, this is the first report of the use of this system for medical ultrasound Doppler signal processing. A Couette flow phantom, consisting of two parallel disks with a 2-mm gap, was used to evaluate and calibrate the device. Doppler spectrograms of porcine blood flow were measured using this stand-alone portable device under the pulsatile condition. Subsequently, in vivo portable system verification was performed by measuring the arterial blood flow of a rat and comparing the results with the measurement from a commercial ultrasound duplex scanner. All of the results demonstrated the potential for using a smartphone as a novel embedded system for portable medical ultrasound applications.

Development and Application of Piezoelectric Materials for Ultrasound Generation and Detection

The piezoelectric effect and its converse are the primary means used in biomedical ultrasound for converting acoustical energy into electrical energy and vice versa. Piezoelectricity has found many bioengineering applications ranging from ultrasound imaging and therapeutics, to piezoelectric surgery and microelectromechanical systems, and to biomedical implants with associated energy harvesting. Because of its fundamental importance to the proper functioning of most medical ultrasound systems, it is important to gain a general understanding of the effect, the history of its development and from this, an appreciation of its limitations and advantages in the generation and detection of ultrasound. This article describes the historical evolvement associated with its use in relation to most medical ultrasound applications and is intended to serve as an introduction for non-expert readers.

A full wave method for modeling nonlinear acoustic wave fields in media with inhomogeneous wave speed, attenuation, and parameter of nonlinearity

The iterative nonlinear contrast source (INCS) method is a numerical method that has originally been developed for modeling transient nonlinear acoustic wave fields in homogeneous (i.e., spatially independent) lossless or lossy media. Starting from the Westervelt equation, the INCS method considers the nonlinear term of the latter as a nonlinear contrast source in an otherwise linear wave equation and iteratively solves the corresponding nonlinear integral equation using a Neumann scheme. By adding an attenuative contrast source, the method has recently been extended to deal with spatially dependent attenuation, and a spatially dependent parameter of nonlinearity has also been introduced. This presentation is about the introduction of an additional contrast source that account for a spatially dependent wave speed. In this case, convergence problems arise with the Neumann scheme. This problem is solved by replacing the Neumann scheme with a conjugate gradient scheme in which the error functional is based on the complete nonlinear integral equation. Results show that this approach yields very accurate results for the nonlinear wave field (tested up to the fifth harmonic) in media with a spatially dependent wave speed, attenuation, and parameter of nonlinearity, as encountered in realistic medical ultrasound applications.

Application of vortex identification to acoustic streaming flows.

Acoustic streaming has an important role in a variety of industrial and medical ultrasound applications. Enhanced mixing and chemical processing can be achieved with the addition of ultrasound. Coherent structures in streaming flows, such as vortices, contribute to the overall transport and convective heat transfer. Their unsteady development is not well understood and thus the subject of on-going investigations. Previous studies measuring both acoustic heating and streaming have used thermocouples and been limited to specific spatial locations within the flow field. In this study, simultaneous velocity and temperature measurements are made using synchronized particle imaging velocimetry (PIV) and infrared thermography in a model sono-chemical reactor. This experimental system is water-filled acrylic tank with a 20 kHz Langevin acoustic horn mounted on the side. With overlapping fields of view for an IR camera and the PIV camera, the spatial-temporal evolution of the acoustic streaming and heating at the liquid surface is investigated. A vortex pair adjacent to the horn is identified with respect to its swirling strength and unsteady flow regimes are quantified in terms of the Lagrangian coherent structures (LCS) methodology. The swirling strength reveals the location of the vortex cores and LCS shows the evolving boundaries of the vortices.

The Rise of Second Harmonics in Forced Oscillation of Gas Bubbles in Thixotropic Fluids

In the present work, nonlinear acoustic oscillation of a single spherical gas bubble immersed in an unbounded thixotropic fluid is studied numerically. Using Moore's structural model as the constitutive equation of the liquid surrounding the bubble, the general Rayleigh-Plesset integro-differential equation, which governs bubble dynamics, is solved numerically using the Gauss-Laguerre Quadrature (GLQ) method. It is shown that at sufficiently high pressure amplitudes, typical of medical ultrasound applications, a second harmonics may be observed in the bubble's response.

Optimization of a 100 MHz fiber optic hydrophone for medical ultrasound applications using nanometer scale gold coating.

This work describes a novel numerical model developed to optimize the design of a broadband, spatial averaging free fiber optic hydrophone probe (FOHP) for characterization of ultrasound fields in the frequency range 1–100 MHz. The design included minimization of the active sensor dimensions, so its cross-section is comparable with the half acoustic wavelength at the highest frequency of interest and improvement of voltage-to-pressure sensitivity of the probe measured in V/Pa. Initial simulations based on the assumption that bulk refractive index of sputtered material could be used indicated that a thin film gold coating (1–35 nm) could indeed enhance the voltage sensitivity of the FOHP by 16–30 dB; however, a follow up analysis revealed that determination of the coating thickness influence on the FOHP performance would require an introduction of the complex (as opposed to bulk) index of refraction of the sputtered film. The input parameters to the model, their selection criteria, and implementation of the coupled acousto-optic interaction of gold layer will be discussed. The measured prototypes produced unprecedented voltage sensitivity between −234 and −245 dB re 1 V per uPa or 2 and 560 mV/MPa, respectively. [The authors wish to acknowledge support of the NIH R01EB007117 grant.]

12b 50 MS/s 0.18 [micro sign]m CMOS ADC with highly linear input variable gain amplifier

A 12b 50 MS/s 0.18 µm CMOS ADC with a highly linear variable gain amplifier (VGA) for medical ultrasound and CCD image sensor applications is presented. The proposed four-step pipeline ADC optimises power and chip area at target specifications while the front-end VGA, based on a conventional approximated log function, employs a merged capacitor switching scheme to improve the VGA gain linearity. The proposed input VGA shows a linearity error less than 0.013 dB in a gain range from −3 to 0 dB by a 0.2 dB step. The measured prototype ADC with an active die area of 1.09 mm2 shows a maximum SNDR and SFDR of 62.6 and 73.1 dB, respectively, and consumes 28.1 mW at 1.8 V and 50 MS/s.

Reliable design of digitally programmable gain amplifier for ultrasound diagnostic applications

This paper presents a reliable design of programmable gain amplifier suitable for medical ultrasound applications that has distinguishing features. First, it has a precise digitally programmable voltage gain which is set by a binary code without a need to adjust any external resistor. Second, it has a wide bandwidth independent of adjusted gain. Third, it has single element control of its gain and this element has no impact on the amplifier bandwidth which results in a flexible gain setting. Fourth, it has very low output noise voltage spectral density (less than 10 nV/√Hz for frequencies higher than 1 MHz). Fifth, it has fully symmetrical differential inputs that provide cancellation of CFA input offset and common-mode noise. Last, its gain and bandwidth are optimised for proper use in ultrasound diagnostic applications.

Comparison of the properties of tonpilz transducers fabricated with ⟨001⟩ fiber-textured lead magnesium niobate-lead titanate ceramic and single crystals

Tonpilz transducers are fabricated from 001 fiber-textured 0.72Pb(Mg(1/3)Nb(2/3))O(3)-0.28PbTiO(3) (PMN-28PT) ceramics, obtained by the templated grain growth process, and PMN-28PT ceramic and Bridgman grown single crystals of the same composition. In-water characterization of single element transducers shows higher source levels, higher in-water coupling, and more usable bandwidth for the 81 vol % textured PMN-28PT device than for the ceramic PMN-28PT element. The 81 vol % textured PMN-28PT tonpilz element measured under large signals shows linearity in sound pressure levels up to 0.23 MV/m drive field but undergoes a phase transition due to a lowered transition temperature from the SrTiO(3) template particles. Although the textured ceramic performs well in this application, it could be further improved with compositional tailoring to raise the transition temperature and better processing to improve the texture quality. With these improvements textured piezoelectric ceramics will be viable options for medical ultrasound, actuators, and sonar applications because of their ease of processing, compositional homogeneity, and potentially lower cost than single crystal.

3D estimation of soft biological tissue deformation from radio-frequency ultrasound volume acquisitions

The current research and development of 2D (matrix-shaped) transducer arrays to acquire 3D ultrasound data sets provides new insights into medical ultrasound applications and in particular into elastography. Until very recently, tissue strain estimation techniques commonly used in elastography were mainly 1D or 2D methods. In this paper, a 3D technique estimating biological soft tissue deformation under load from ultrasound radiofrequency volume acquisitions is introduced. This method locally computes axial strains, while considering lateral and elevational motions. Optimal deformation parameters are estimated as those maximizing a similarity criterion, defined as the normalized correlation coefficient, between an initial region and its deformed version, when the latter is compensated for according to these parameters. The performance of our algorithm was assessed with numerical data reproducing the configuration of breast cancer, as well as a physical phantom mimicking a pressure ulcer. Simulation results show that the estimated strain fields are very close to the theoretical values, perfectly discriminating between the harder lesion and the surrounding medium. Experimental strain images of the physical phantom demonstrated the different structures of the medium, even though they are not all detectable on the ultrasound scans. Finally, both simulated and experimental results demonstrate the ability of our algorithm to provide good-quality elastograms, even in the conditions of significant out-of-plane motion.

Advanced Single Crystal Piezoelectric Transducers for Naval Sonar and Medical Ultrasound Applications

Single crystal piezoelectrics have received much attention due to their very high piezoelectric and electromechanical coupling coefficients. Naval sonar and medical ultrasound transducers, fabricated from this material, exhibit unprecedented increases in bandwidth, source level, and sensitivity compared to ceramic based devices. In this paper we report many new developments in the growth and manufacture of single crystals. These include increased diameter (> 3) crystal boules, improved compositional uniformity, increased thermal stability, and minimized surface damage. Manufacturing improvements are being applied to a broad range of crystal transducer applications. For navy sonar systems, crystals are resulting in very high bandwidths (> 100%) for transducer sizes that are 1/2 the resonating length and 1/4 the weight of ceramic counterparts with equivalent (or often lower) source level. Manufacturing improvements such as high tensile strength, achieved by precision polishing crystal surfaces, are enablin...

Embedded System for Real-Time Digital Processing of Medical Ultrasound Doppler Signals

Ultrasound (US) Doppler systems are routinely used for the diagnosis of cardiovascular diseases. Depending on the application, either single tone bursts or more complex waveforms are periodically transmitted throughout a piezoelectric transducer towards the region of interest. Extraction of Doppler information from echoes backscattered from moving blood cells typically involves coherent demodulation and matched filtering of the received signal, followed by a suitable processing module. In this paper, we present an embedded Doppler US system which has been designed as open research platform, programmable according to a variety of strategies in both transmission and reception. By suitably sharing the processing tasks between a state-of-the-art FGPA and a DSP, the system can be used in several medical US applications. As reference examples, the detection of microemboli in cerebral circulation and the measurement of wall _distension_ in carotid arteries are finally presented.

Lead-free piezoelectric ceramic transducer in the donor-doped K<sub>1/2</sub>Na<sub>1/2</sub>NbO<sub>3</sub> solid solution system

Lead-based piezoelectric ceramics are suitable materials for noninvasive applications of ultrasound in medicine. However, the embedded therapeutic and diagnostic procedures require the use of lead-free piezoelectric materials as active elements in transducers. With this goal in mind, we investigated the substitution of Ba(2+) cations in a lead-free piezoelectric system of K(1/2)Na(1/2)NbO(3)-LiTaO(3)-LiSbO(3) (KNN-LT-LS) with perovskite structure. The Ba(2+) was added to the system as an A-site dopant in the range of 0-2 mol% in increments of 0.5 mol%. The addition of Ba(2+) improved the piezoelectric charge coefficient, d(33), and longitudinal coupling coefficient, k(33). The composition with 1 mol% Ba2+ had 36% and 58% higher d(33) and k(33), respectively, than the undoped composition. It appeared that the addition of Ba(2+) induced "soft" characteristics in this lead-free piezoelectric system. This was verified by the increase of remnant polarization along with the decline of coercive field. The Ba(2+) behaved as a grain growth inhibitor and caused a drastic reduction in polarization level (approximately 60%) when the grain size became smaller than approximately 1.5 microm. Incorporation of Ba(2+) up to 1.5 mol% increased the bulk resistivity of the KNN-LT-LS system and then reduced it drastically at higher dopant concentrations. The electron-hole compensation model fit well with the results obtained in this study and verified the A-site substitution of donor-doped barium. KNN-LT-LS ceramics with 0 mol% and 1 mol% Ba(2+) were used to fabricate single-element ultrasonic transducers resonating at 5.5 MHz. The -6 dB fractional bandwidth and -20 dB pulse length of the probe made of doped ceramic were 50% and 1.68 micros, respectively. This indicated that this system could be considered as a candidate for invasive and/or embedded medical ultrasound applications.

Fully nonlinear model for spherical bubble oscillations between rigid parallel plates

Many medical ultrasound applications involve the interaction of intense acoustic waves with bubbles confined by blood vessels or tissue. As a step toward understanding such systems, a bubble confined between two parallel rigid plates was modeled using the image method. The model equation is of Rayleigh‐Plesset form, including compressibility of the liquid and accounting for boundary conditions imposed by the plates [Cui et al., Proc. 17th ISNA, edited by Atchley et al. (AIP, New York, 2006), pp. 229–232]. An analytical solution for the second harmonic, and fully nonlinear solutions in the quadratic approximation, were reported previously. Here, a hybrid time‐frequency algorithm is presented for solving the complete nonlinear model equation. In particular, the effect of the plates is most conveniently dealt with in the frequency domain, whereas the nonlinearity due to the gas law is best handled in the time domain. The procedure was verified for moderate drive amplitudes by comparison with a spectral algorithm for solving the model equation in the quadratic approximation [Cui et al., J. Acoust. Soc. Am. 118, 1942 (2005)]. Results obtained from the hybrid algorithm will be presented that illustrate the effect of plate separation for different amplitudes and frequencies of acoustic excitation.