2D Array Transducers Research Articles

Three-dimensional ultrasound imaging

This review is about the development of three-dimensional (3D) ultrasonic medical imaging, how it works, and where its future lies. It assumes knowledge of two-dimensional (2D) ultrasound, which is covered elsewhere in this issue. The three main ways in which 3D ultrasound may be acquired are described: the mechanically swept 3D probe, the 2D transducer array that can acquire intrinsically 3D data, and the freehand 3D ultrasound. This provides an appreciation of the constraints implicit in each of these approaches together with their strengths and weaknesses. Then some of the techniques that are used for processing the 3D data and the way this can lead to information of clinical value are discussed. A table is provided to show the range of clinical applications reported in the literature. Finally, the discussion relating to the technology and its clinical applications to explain why 3D ultrasound has been relatively slow to be adopted in routine clinics is drawn together and the issues that will govern its development in the future explored.

2D array based on fermat spiral

The main challenge faced by 3D ultrasonic imaging with 2D array transducer is the large number of elements required to achieve an acceptable level of quality in the images. Therefore, the optimization of the array layout to reduce the number of active elements in the aperture has been a research topic in the last years.Nowadays, CMUT array technology has made viable the production of 2D arrays with larger flexibility on elements size, shape and position. This is opening new options in 2D array design, allowing to revise as viable alternatives others layouts that had been studied in the past, like circular and Archimedes spiral layout.In this work the problem of designing an imaging system array with a diameter of 60λ and a limited number of elements using the Fermat spiral layout has been studied.This study has been done for two different numbers of electronic channels (N=128 and N=256). As summary, a general discussion of the results and the most interesting cases are presented.

New fabrication techniques for ring-array transducers for real-time 3D intravascular ultrasound.

We have previously described miniature 2D array transducers integrated into a Cook Medical, Inc. vena cava filter deployment device. While functional, the fabrication technique was very labor intensive and did not lend itself well to efficient fabrication of large numbers of devices. We developed two new fabrication methods that we believe can be used to efficiently manufacture these types of devices in greater than prototype numbers. One transducer consisted of 55 elements operating near 5 MHz. The interelement spacing is 0.20 mm. It was constructed on a flat piece of copper-clad polyimide and then wrapped around an 11 French catheter of a Cook Medical, Inc. inferior vena cava (IVC) filter deployment device. We used a braided wiring technology from Tyco Electronics Corp. to connect the elements to our real-time 3D ultrasound scanner. Typical measured transducer element bandwidth was 20% centered at 4.7 MHz and the 50 Omega round trip insertion loss was --82 dB. The mean of the nearest neighbor cross talk was -37.0 dB. The second method consisted of a 46-cm long single layer flex circuit from MicroConnex that terminates in an interconnect that plugs directly into our system cable. This transducer had 70 elements at 0.157 mm interelement spacing operating at 4.8 MHz. Typical measured transducer element bandwidth was 29% and the 50 Omega round trip insertion loss was -83 dB. The mean of the nearest neighbor cross talk was -33.0 dB.

2D array design based on Fermat spiral for ultrasound imaging

The main challenge faced by 3D ultrasonic imaging with 2D array transducers is the large number of elements required to achieve an acceptable level of quality in the images. Therefore, the optimisation of the array layout, in order to reduce the number of active elements in the aperture, has been a research topic in the last years. Nowadays, array technology has made viable the production of 2D arrays with larger flexibility on elements size, shape and position, allowing to study other configurations different to the classical matrix organisation, such as circular, archimedes spiral or polygonal layout between others. In this work, the problem of designing an imaging system array with large apertures and a very limited number of active elements ( N e = 128 and N e = 256 ) using the Fermat spiral layout has been studied. As summary, a general discussion about the most interesting cases is presented.

0340: 3D Ultrasound of the Upper Abdomen

The digital revolution has made three-dimensional (3D) ultrasonography a natural extension of 2D ultrasound scanning. This lecture reviews some of the principles and methods in 3D ultrasonography of the upper abdomen. At present, the process of making 3D images based on ultrasonography is commonly divided into 5 steps: Data acquisition, data digitization, data storage, data processing, and data display. Principally, data acquisition by 3D ultrasonography can be performed in 3 different ways; either by using a 2D probe attached to a motor which moves the probe in a computer-defined way or by a spatial localizing system connected to a 2D probe or by genuine, electronic 3D probes with the possibility of direct volume acquisition in real time. True volumetric 2D array transducers instantly generate a volume of ultrasound data, enabling dynamic 3D ultrasonography. A critical step in processing of 3D data is segmentation, which is the procedure where the object of interest is separated from the surrounding structures. Three fundamental approaches to segmentation have been utilised in 3D ultrasonography: Extraction by visual inspection and manual outlining of contours; semi-automatic separation using visualisation algorithms aided by operator interaction, and fully automatic computer segmentation. The final step in formation of 3D ultrasound images is to display the data so that the inherent voxel information is communicated accurately. Commonly, simple rotation of the object on the computer monitor provides some 3D effects. To further enhance the 3D outcome of the images, stereoglasses can be applied. Data processing and display requires specialized computer software to handle the ultrasound images. Ultrasound data contains a significant amount of noise and speckle and may exhibit boundary regions several pixels wide. It is important to keep in mind that the quality of the final 3D data display strongly depends on the resolution of the raw data. Transducer frequency and lateral resolution, frame rate of the scanner, accuracy of 3D probe, speed of scanning, methods of filtering and segmentation, are all factors that influences the final 3D image and subsequently accuracy in volume measurements.

Registration Strategies and Similarity Measures for Three-dimensional Ultrasound Mosaicing

The creation of two-dimensional (2D) ultrasound mosaics is becoming a common clinical practice with a high clinical value. The next step coming along with the increasing availability of 2D array transducers is the creation of three-dimensional mosaics. The correct alignment of multiple ultrasound images is, however, a complex task. In the literature of ultrasound registration, the alignment of two images has been often addressed, but not the alignment of multiple images. Therefore, we propose registration strategies for multiple image alignment and ultrasound specific similarity measures, which are able to cope with problems when aligning ultrasound images. In this study, we investigate the following strategies for multiple image alignment: pairwise registration with a successive Lie group normalization and simultaneous registration, which urges the usage of multivariate similarity measures. We propose alternative multivariate extensions for similarity measures based on a maximum likelihood framework. Moreover, we take the inherent contamination of ultrasound images by speckle patterns into consideration by using alternative noise models based on multiplicative Rayleigh distributed noise. This leads us to ultrasound-specific similarity measures. We compare the performances of pairwise and simultaneous registration approaches for the mosaicing scenario. Bivariate similarity measures are highly overlap-dependent, so that they rather favor the total overlap of the images than their correct alignment. Using ultrasound-specific bivariate measures leads to better results; however, a local optimum at the total overlap remains. In contrast, the derived multivariate similarity measures have a clear and single optimum at the correct alignment of the volumes. The experiments indicate that standard, pairwise registration techniques have problems by aligning multiple ultrasound images with partial overlap. We demonstrate that the usage of an ultrasound specific similarity measure leads to better results for pairwise registration. The highest robustness, however, can be achieved by using simultaneous registration with the developed multivariate similarity measures.

Correlation analysis of three-dimensional strain imaging using ultrasound two-dimensional array transducers

Two-dimensional (2D) transducer arrays represent a promising solution for implementing real time three-dimensional (3D) ultrasound elastography. 2D arrays enable electronic steering and focusing of ultrasound beams throughout a 3D volume along with improved slice thickness performance when compared to one-dimensional (1D) transducer arrays. Therefore, signal decorrelation caused by tissue motion in the elevational (out-of-plane) direction needs to be considered. In this paper, a closed form expression is derived for the correlation coefficient between pre- and postdeformation ultrasonic radio frequency signals. Signal decorrelation due to 3D motion of scatterers within the ultrasonic beam has been considered. Computer simulations are performed to corroborate the theoretical results. Strain images of a spherical inclusion phantom generated using 1D and 2D array transducers are obtained using a frequency domain simulation model. Quantitative image quality parameters, such as the signal-to-noise and contrast-to-noise ratios obtained using 1D, 2D, and 3D motion tracking algorithms, are compared to evaluate the performance with the 3D strain imaging system. The effect of the aperture size for 2D arrays and other factors that affect signal decorrelation are also discussed.

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.

Acoustic backscatter and effective scatterer size estimates using a 2D CMUT transducer

Compared to conventional piezoelectric transducers, new capacitive microfabricated ultrasonic transducer (CMUT) technology is expected to offer a broader bandwidth, higher resolution and advanced 3D/4D imaging inherent in a 2D array. For ultrasound scatterer size imaging, a broader frequency range provides more information on frequency-dependent backscatter, and therefore, generally more accurate size estimates. Elevational compounding, which can significantly reduce the large statistical fluctuations associated with parametric imaging, becomes readily available with a 2D array. In this work, we show phantom and in vivo breast tumor scatterer size image results using a prototype 2D CMUT transducer (9 MHz center frequency) attached to a clinical scanner. A uniform phantom with two 1 cm diameter spherical inclusions of slightly smaller scatterer size was submerged in oil and scanned by both the 2D CMUT and a conventional piezoelectric linear array transducer. The attenuation and scatterer sizes of the sample were estimated using a reference phantom method. RF correlation analysis was performed using the data acquired by both transducers. The 2D CMUT results indicate that at a 2 cm depth (near the transmit focus for both transducers) the correlation coefficient reduced to less than 1/e for 0.2 mm lateral or 0.25 mm elevational separation between acoustic scanlines. For the conventional array this level of decorrelation requires a 0.3 mm lateral or 0.75 mm elevational translation. Angular and/or elevational compounding is used to reduce the variance of scatterer size estimates. The 2D array transducer acquired RF signals from 140 planes over a 2.8 cm elevational direction. If no elevational compounding is used, the fractional standard deviation of the size estimates is about 12% of the mean size estimate for both the spherical inclusion and the background. Elevational compounding of 11 adjacent planes reduces it to 7% for both media. Using an experimentally estimated attenuation of 0.6 dB cm−1 MHz−1, scatterer size estimates for an in vivo breast tumor also demonstrate improvements using elevational compounding with data from the 2D CMUT transducer.

Simulation of ultrasound two-dimensional array transducers using a frequency domain model

Ultrasound imaging with two-dimensional (2D) arrays has garnered broad interest from scanner manufacturers and researchers for real time three-dimensional (3D) applications. Previously the authors described a frequency domain B-mode imaging model applicable for linear and phased array transducers. In this paper, the authors extend this model to incorporate 2D array transducers. Further approximations can be made based on the fact that the dimensions of the 2D array element are small. The model is compared with the widely used ultrasound simulation program FIELD II, which utilizes an approximate form of the time domain impulse response function. In a typical application, errors in simulated RF waveforms are less than 4% regardless of the steering angle for distances greater than 2 cm, yet computation times are on the order of 1/35 of those incurred using FIELD II. The 2D model takes into account the effects of frequency-dependent attenuation, backscattering, and dispersion. Modern beam-forming techniques such as apodization, dynamic aperture, dynamic receive focusing, and 3D beam steering can also be simulated.

TU‐C‐332‐05: Simulation of Ultrasound Two‐Dimensional Array Transducers Using a Frequency Domain Model

Purpose: Ultrasound imaging with two‐dimensional (2D) arrays has garnered broad interest from scanner manufacturers and researchers for real time three‐dimensional (3D) imaging. Previously we described a frequency domain B‐mode imaging model applicable for linear and phased array transducers. In this study, we extend this model to incorporate 2D array transducers. Method and Materials: The pressure field for a 64×64 square array with element dimension of 0.15 mm and center‐to‐center spacing of 0.2 mm was calculated by applying the paraxial approximation to solve the 2D Rayleigh integral. We assume a rigid baffle, no apodization, a 2.5 MHz center frequency, and a speed of sound of 1540 m/s. A single transmit focus at 30 mm and dynamic receive focus with an F‐number of 2 was utilized. The 2D array model is compared with the widely used ultrasound simulation program FIELD II, which utilizes an approximate form of the time domain impulse response function. Results: Discrepancies between waveforms computed using our model and FIELD II are less than 4%, regardless of the steering angle for distances greater than 2 cm, yet computation times are on the order of 1/35 of those using FIELD II. Modern beam‐forming techniques such as apodization, dynamic aperture, dynamic receive focusing and 3D beam steering can also be simulated. The simulated beam patterns and point spread function images allow evaluation of beam properties for specific transducer parameters. Simulations of B‐mode images provide vivid demonstrations of the ability of 2D arrays with specific imaging parameters to detect lesions of a given backscatter contrast and size. Conclusion: The frequency domain approach provides an effective and feasible tool to model transmitted and pulse‐echo fields as well as B‐mode images for 2D array transducers.

A Doppler ultrasound device for determining blood volume flow

A novel ultrasonic quantitative Doppler procedure has been developed which allows for the measurement of real-time volume flow in large blood vessels. It makes use of a 2D array transducer, which enables parallel sampling of a measuring slice placed in normal position to the sound beam. With this arrangement, volume flow can be computed without measuring the angle of incidence. Moreover, the 2D velocity distribution can be assessed within intervals of 10 to 30 ms.

Fabrication and evaluation of fully-sampled, two-dimensional transducer array for “Sonic Window” imaging system

A low-cost, fully-sampled, 3600 element 2D transducer array operating at 5 MHz and designed for use in a hand-held ultrasound system is described here. Four array configurations are presented – (1) array with both matching and pedestal backing layers, (2) array with a matching layer but no backing pedestal, (3) array with a backing pedestal but no matching layer, and (4) array with neither matching layer nor backing pedestal. Each array was characterized in terms of impedance measurements, pulse-echo response, and experimental beamprofile. Comparative finite element analysis simulations are also presented. Average estimated active element yield for the four arrays was 94%. The array with pedestal layer proved the most promising, providing a 26% bandwidth and a 1.7 dB improvement in sensitivity with respect to the array with neither pedestal nor matching layer. Although this bandwidth is acceptable for our specific application (C-scan imaging), reverberations within the substrate material remain a potential challenge. We are currently working to fabricate a custom PCB material to address this concern, and may also consider using a pre-compensated transmit waveform or matched digital filter approach to further reduce the effects of such reverberations.

Integration of 2D CMUT arrays with front-end electronics for volumetric ultrasound imaging

For three-dimensional (3D) ultrasound imaging, connecting elements of a two-dimensional (2D) transducer array to the imaging system's front-end electronics is a challenge because of the large number of array elements and the small element size. To compactly connect the transducer array with electronics, we flip-chip bond a 2D 16 x 16-element capacitive micromachined ultrasonic transducer (CMUT) array to a custom-designed integrated circuit (IC). Through-wafer interconnects are used to connect the CMUT elements on the top side of the array with flip-chip bond pads on the back side. The IC provides a 25-V pulser and a transimpedance preamplifier to each element of the array. For each of three characterized devices, the element yield is excellent (99 to 100% of the elements are functional). Center frequencies range from 2.6 MHz to 5.1 MHz. For pulse echo operation, the average - 6-dB fractional bandwidth is as high as 125%. Transmit pressures normalized to the face of the transducer are as high as 339 kPa and input-referred receiver noise is typically 1.2 to 2.1 mPa/pHz. The flip-chip bonded devices were used to acquire 3D synthetic aperture images of a wire-target phantom. Combining the transducer array and IC, as shown in this paper, allows for better utilization of large arrays, improves receive sensitivity, and may lead to new imaging techniques that depend on transducer arrays that are closely coupled to IC electronics.

Field Computation for Two-Dimensional Array Transducers with Limited Diffraction Array Beams

A method is developed for calculating fields produced with a two-dimensional (2D) array transducer. This method decomposes an arbitrary 2D aperture weighting function into a set of limited diffraction array beams. Using the analytical expressions of limited diffraction beams, arbitrary continuous wave (cw) or pulse wave (pw) fields of 2D arrays can be obtained with a simple superposition of these beams. In addition, this method can be simplified and applied to a 1D array transducer of a finite or infinite elevation height. For beams produced with axially symmetric aperture weighting functions, this method can be reduced to the Fourier-Bessel method studied previously where an annular array transducer can be used. The advantage of the method is that it is accurate and computationally efficient, especially in regions that are not far from the surface of the transducer (near field), where it is important for medical imaging. Both computer simulations and a synthetic array experiment are carried out to verify the method. Results (Bessel beam, focused Gaussian beam, X wave and asymmetric array beams) show that the method is accurate as compared to that using the Rayleigh-Sommerfeld diffraction formula and agrees well with the experiment.

Real-Time 3D Laparoscopic Ultrasonography

We have previously described 2D array ultrasound transducers operating up to 10 MHz for applications including real time 3D transthoracic imaging, real time volumetric intracardiac echocardiography (ICE), real time 3D intravascular ultrasound (IVUS) imaging, and real time 3D transesophageal echocardiography (TEE). We have recently built a pair of 2D array transducers for real time 3D laparoscopic ultrasonography (3D LUS). These transducers are intended to be placed down a trocar during minimally invasive surgery. The first is a forward viewing 5 MHz, 11 x 19 array with 198 operating elements. It was built on an 8 layer multilayer flex circuit. The interelement spacing is 0.20 mm yielding an aperture that is 2.2 mm x 3.8 mm. The O.D. of the completed transducer is 10.2 mm and includes a 2 mm tool port. The average measured center frequency is 4.5 MHz, and the -6 dB bandwidth ranges from 15% to 30%. The 50 omega insertion loss, including Gore MicroFlat cabling, is -81.2 dB. The second transducer is a 7 MHz, 36 x 36 array with 504 operating elements. It was built upon a 10 layer multilayer flex circuit. This transducer is in the forward viewing configuration and the interelement spacing is 0.18 mm. The total aperture size is 6.48 mm x 6.48 mm. The O.D. of the completed transducer is 11.4 mm. The average measured center frequency is 7.2 MHz, and the -6 dB bandwidth ranges from 18% to 33%. The 50 omega insertion loss is -79.5 dB, including Gore MicroFlat cable. Real-time in vivo 3D images of canine hearts have been made including an apical 4-chamber view from a substernal access with the first transducer to monitor cardiac function. In addition, we produced real time 3D rendered images of the right pulmonary veins from a right parastemal access with the second transducer, which would be valuable in the guidance of cardiac ablation catheters for treatment of atrial fibrillation.

3D computational method to study the focal laws of transducer arrays for NDE applications

Based on the impulse response and the discrete representation methods, a 3D computational method has been developed to calculate the optimal focal laws to focus the ultrasonic beams through interfaces of complex geometry, and the respective transmitted ultrasonic field generated by NDE transducer arrays. 1- and 2D array transducers are considered. Two different focusing techniques are used to obtain the time delays: the first travel time on each center of the array element, and the cross correlation between the simulated signals from neighboring array elements. Applying the time delays to the array, the transmitted field can be simulated using the same computational method. Several simulations were performed to present the ability of the computational method to focus through, for instance, curved and plane surfaces between two media (acrylic–steel). A comparison between the two focusing techniques is presented.

Stack based multidimensional ultrasonic transducer array

Multilayer piezoelectric fabrication methods are used to produce nD (e.g., 1.5D and 2D) ultrasonic transducer arrays. One embodiment concerns a 1.5 linear transducer in which impedance compensation is provided between the elemental transducers forming the array by varying the number of layers in each row of transducers. In another embodiment, a 2D transducer array is provided wherein elemental transducers are produced by dicing a conventional multilayer actuator to form equal sized elements which are embedded in a polymer matrix.

Uniform volumetric scanning ultrasonic diagnostic imaging system

An ultrasonic diagnostic imaging system and method are described for more uniformly scanning a volumetric region for 3D ultrasonic imaging. An image plane of steered scanning beams is rotated about an axis extending through the volumetric region with beams in the vicinity of the axis more widely separated than beams more remotely located from the axis. The nonuniform spacing of the scanning beams results in relatively less disparate sampling densities across the scanned volumetric region. The inventive method may be performed with both 1D and 2D array transducers.

Piezoelectric sectorial 2D array for 3D acoustical imaging

3D acoustical imaging for both NDT and Echography purposes needs of 2D array transducers. Among them, the sectorial design gives good performances maintaining enough resolution. A piezoelectric transducer array having a piezocomposite as substrate is designed, simulated, and tested following time and frequency measurement techniques. Simulations were performed with a multilayer scheme based on the KLM circuit. Time response, inter-element cross-coupling and acoustic aperture are measured.