Sparse 2-D array design for real time rectilinear volumetric imaging

Several sparse 2-D arrays for real time rectilinear volumetric imaging were investigated. All arrays consisted of 128x128=16,384 elements with lambda spacing operating at 5 MHz. Because of system limitations, not all of the elements could be used. From each array, 256 elements were used as transmitters, and 256 elements were used as receivers. These arrays were compared by computer simulation using Field II. For each array, beamplots for the on-axis case and an illustrative off-axis case were obtained. For the off-axis case, the effects of receive mode dynamic focusing were studied to maintain the beam perpendicular to the transducer face. Main lobe widths, side lobe heights, clutter floor levels, and pulse-echo sensitivities were quantified for each array. The sparse arrays, including a vernier periodic array, a random array, and a Mills cross array, were compared with a fully sampled array that served as the "gold standard". The Mills cross design showed the best overall performance under the current system constraints.     

A 32 x 32 element row-column addressed capacitive micromachined ultrasonic transducer

This paper presents characterization and initial imaging results of a 32 x 32 element two-dimensional capacitive micromachined ultrasonic transducer array. The devices are fabricated using a wafer bonding process in which both the insulation layer and the membrane are user-deposited silicon nitride. The transducers use a row-column addressing scheme to simplify the fabrication process and beamformer. By adjusting the number of rows and columns that are biased, the effective aperture of the transducer can be adjusted. This is significant because it permits imaging in the near-field of the transducer without the use of a lens. The effect on the transmit beam profile is demonstrated. The transducer has a center frequency of 5.9 MHz and a relative bandwidth of 110%. Images of horizontal and vertical wires are taken to demonstrate image resolution. A three-dimensional image of four pin heads is also demonstrated.     

2-D array for 3-D ultrasound imaging using synthetic aperture techniques

A two-dimensional (2-D) array of 256 X 256 = 65,536 elements, with total area 4 X 4 = 16 cm2, serves as a flexible platform for developing acquisition schemes for 3-D rectilinear ultrasound imaging at 10 MHz using synthetic aperture techniques. This innovative system combines a simplified interconnect scheme and synthetic aperture techniques with a 2-D array for 3-D imaging. A row-column addressing scheme is used to access different elements for different transmit events. This addressing scheme is achieved through a simple interconnect, consisting of one top, one bottom single-layer, flex circuits that, compared to multilayer flex circuits, are simpler to design, cheaper to manufacture, and thinner so their effect on the acoustic response is minimized. We present three designs that prioritize different design objectives: volume acquisiton time, resolution, and sensitivity, while maintaining acceptable figures for the other design objectives. For example, one design overlooks time-acquisition requirements, assumes good noise conditions, and optimizes for resolution, achieving -6 dB and -20 dB beamwidths of less than 0.2 and 0.5 mm, respectively, for an F/2 aperture. Another design can acquire an entire volume in 256 transmit events, with -6 dB and -20 dB beamwidths in the order of 0.4 and 0.8 mm, respectively.     

An integrated circuit with transmit beamforming flip-chip bonded to a 2-D CMUT array for 3-D ultrasound imaging

State-of-the-art 3-D medical ultrasound imaging requires transmitting and receiving ultrasound using a 2-D array of ultrasound transducers with hundreds or thousands of elements. A tight combination of the transducer array with integrated circuitry eliminates bulky cables connecting the elements of the transducer array to a separate system of electronics. Furthermore, preamplifiers located close to the array can lead to improved receive sensitivity. A combined IC and transducer array can lead to a portable, high-performance, and inexpensive 3-D ultrasound imaging system. This paper presents an IC flip-chip bonded to a 16 x 16-element capacitive micromachined ultrasonic transducer (CMUT) array for 3-D ultrasound imaging. The IC includes a transmit beamformer that generates 25-V unipolar pulses with programmable focusing delays to 224 of the 256 transducer elements. One-shot circuits allow adjustment of the pulse widths for different ultrasound transducer center frequencies. For receiving reflected ultrasound signals, the IC uses the 32-elements along the array diagonals. The IC provides each receiving element with a low-noise 25-MHz-bandwidth transimpedance amplifier. Using a field-programmable gate array (FPGA) clocked at 100 MHz to operate the IC, the IC generated properly timed transmit pulses with 5-ns accuracy. With the IC flip-chip bonded to a CMUT array, we show that the IC can produce steered and focused ultrasound beams. We present 2-D and 3-D images of a wire phantom and 2-D orthogonal cross-sectional images (Bscans) of a latex heart phantom.     

A miniaturized catheter 2-D array for real-time, 3-D intracardiac echocardiography

The design, fabrication, and characterization of a 112 channel, 5 MHz, two-dimensional (2-D) array transducer constructed on a six layer flexible polyimide interconnect circuit is described. The transducer was mounted in a 7 Fr (2.33 mm outside diameter) catheter for use in real-time intracardiac volumetric imaging. Two transducers were constructed: one with a single silver epoxy matching layer and the other without a matching layer. The center frequency and -6 dB fractional bandwidth of the transducer with a matching layer were 4.9 MHz and 31%, respectively. The 50 omega pitch-catch insertion loss was 80 dB, and the typical interelement crosstalk was -30 dB. The final element yield was greater than 97% for both transducers. The transducers were used to acquire real-time, 3-D images in an in vivo sheep model. We present in vivo images of cardiac anatomy obtained from within the coronary sinus, including the left and right atria, aorta, coronary arteries, and pulmonary veins. We also present images showing the manipulation of a separate electrophysiological catheter into the coronary sinus.     

Sparse 2-D arrays for 3-D phased array imaging--design methods

One of the most promising techniques for limiting complexity for real-time 3-D ultrasound systems is to use sparse 2-D layouts. For a given number of channels, optimization of performance is desirable to ensure high quality volume images. To find optimal layouts, several approaches have been followed with varying success. The most promising designs proposed are Vernier arrays, but also these suffer from high peaks in the sidelobe region compared with a dense array. In this work, we propose new methods based on the principles of suppression of grating lobes to form symmetric and non-symmetric regular sparse periodic and radially periodic designs. The proposed methods extend the concept of sparse periodic layouts by exploiting either an increased number of symmetry axes or radial symmetry. We also introduce two new strategies to form designs with nonoverlapping elements. The performance of the new layouts range from the performance of Vernier arrays to almost that of dense arrays. Our designs have simplicity in construction, flexibility in the number of active elements, and the possibility of trade off sidelobe peaks against sidelobe energy.     

A 32 x 32 capacitive micromachined ultrasonic transducer array manufactured in standard CMOS

As ultrasound imagers become increasingly portable and lower cost, breakthroughs in transducer technology will be needed to provide high-resolution, real-time 3-D imaging while maintaining the affordability needed for portable systems. This paper presents a 32 x 32 ultrasound array prototype, manufactured using a CMUT-in-CMOS approach whereby ultrasonic transducer elements and readout circuits are integrated on a single chip using a standard integrated circuit manufacturing process in a commercial CMOS foundry. Only blanket wet-etch and sealing steps are added to complete the MEMS devices after the CMOS process. This process typically yields better than 99% working elements per array, with less than ±1.5 dB variation in receive sensitivity among the 1024 individually addressable elements. The CMUT pulseecho frequency response is typically centered at 2.1 MHz with a -6 dB fractional bandwidth of 60%, and elements are arranged on a 250 μm hexagonal grid (less than half-wavelength pitch). Multiplexers and CMOS buffers within the array are used to make on-chip routing manageable, reduce the number of physical output leads, and drive the transducer cable. The array has been interfaced to a commercial imager as well as a set of custom transmit and receive electronics, and volumetric images of nylon fishing line targets have been produced.     

Simulation and experimental characterization of a 2-D capacitive micromachined ultrasonic transducer array element

In this letter, a 400-mumx400-mum 2-D capacitive micromachined ultrasonic transducer (cMUT) array element is experimentally characterised, and the results are found to be in good agreement with theoretical predictions. As a receiver, the transducer has a 1.8x10(-7) nm/ radical(Hz) displacement sensitivity, and, as a transmitter, it produces 16.4 kPa/V of output pressure at the transducer surface at 3 MHz. The transducer also has more than 100% fractional bandwidth around 3 MHz, which makes it suitable for ultrasound imaging.     

Harmonic 3-D echocardiography with a fast-rotating ultrasound transducer

Although the advantages of three-dimensional (3-D) echocardiography have been acknowledged, its application for routine diagnosis is still very limited. This is mainly due to the relatively long acquisition time. Only recently has this problem been addressed with the introduction of new real-time 3-D echo systems. This paper describes the design, characteristics, and capabilities of an alternative concept for rapid 3-D echocardiographic recordings. The presented fast-rotating ultrasound (FRU)-transducer is based on a 64-element phased array that rotates with a maximum speed of 8 Hz (480 rpm). The large bandwidth of the FRU-transducer makes it highly suitable for tissue and contrast harmonic imaging. The transducer presents itself as a conventional phased-array transducer; therefore, it is easily implemented on existing 2-D echo systems, without additional interfacing. The capabilities of the FRU-transducer are illustrated with in-vitro volume measurements, harmonic imaging in combination with a contrast agent, and a preliminary clinical study.     

Sparse 2-d arrays for 3-D phased array imaging--experimental validation

To be able to describe more precisely the behavior of a real-time 3-D ultrasound system with either a dense array or various sparse designs, experimental data from a 2-D fully connected array prototype with 50/spl times/50 elements have been collected. The data have been processed off line to form synthetic aperture 3-D volume images. Simulated and experimental results are compared and show good correlation. The performance of the best sparse designs, all thinned to more than 50%, offer performances comparable to a dense array.     

Real-time, 3-D ultrasound with multiple transducer arrays

Modifications were made to a commercial real-time, three-dimensional (3-D) ultrasound system for near simultaneous 3-D scanning with two matrix array transducers. As a first illustration, a transducer cable assembly was modified to incorporate two independent, 3-D intra-cardiac echo catheters, a 7 Fr (2.3 mm O.D.) side scanning catheter and a 14 Fr (4.7 mm O.D) forward viewing catheter with accessory port, each catheter using 85 channels operating at 5 MHz. For applications in treatment of atrial fibrillation, the goal is to place the sideviewing catheter within the coronary sinus to view the whole left atrium, including a pulmonary vein. Meanwhile, the forward-viewing catheter inserted within the left atrium is directed toward the ostium of a pulmonary vein for therapy using the integrated accessory port. Using preloaded, phasing data, the scanner switches between catheters automatically, at the push of a button, with a delay of about 1 second, so that the clinician can view the therapy catheter with the coronary sinus catheter and vice versa. Preliminary imaging studies in a tissue phantom and in vivo show that our system successfully guided the forward-viewing catheter toward a target while being imaged with the sideviewing catheter. The forward-viewing catheter then was activated to monitor the target while we mimicked therapy delivery. In the future, the system will switch between 3-D probes on a line-by-line basis and display both volumes simultaneously.     

4-D x 3-D ultrasound: real-time scan conversion, filtering, and display of displacement vectors with a motorized curvilinear transducer

Recent research in the field of elastography has sought to expand displacement tracking to three dimensions. Once the 3-D volumes of displacement data have been obtained, they must be scan converted so that further processing, such as inversion methods to obtain tissue elasticity, can take place in Cartesian coordinates. This paper details an efficient and geometrically accurate algorithm to scan convert 3-D volumes of displacement vectors obtained from a motorized sector transducer. The proposed algorithm utilizes the physical scan geometry to convert the 3-D volumes of displacement data to both Cartesian coordinates and Cartesian displacements. Spatially varying filters are also proposed to prevent aliasing while minimizing data loss. Validation of the system has shown the algorithm to be correct to floating point precision, and the 3-D scan conversion and filtering can be performed faster than the native rate of data acquisition for the motorized transducer.     

Prospective ECG-gated mouse cardiac imaging with a 34-MHz annular array transducer

Prospective imaging with electrocardiogram (ECG) and respiratory gating presents an imaging application that leverages the improved image quality of high-frequency (>20 MHz) annular arrays without the need for rapid mechanical motion. The limitation of prospective imaging is that the object being imaged must have a periodically stable motion. The present study investigated the implementation of prospective imaging with a 34 MHz annular-array scan system to image the mouse heart at high effective frame rates, >200 frames/s (fps). M-mode data for all transmit-to-receive pairs were acquired at a series of spatial locations using ECG and respiratory gating, and the data were then synthetically focused in postprocessing. The pulse-repetition frequency of the M-mode data determined the effective frame rate of the final B-mode image sequence. The hearts of adult mice were prospectively imaged and compared with retrospective data acquired with a commercial ultrasonic biomicroscope (UBM). The annulararray data were acquired at an effective frame rate of 500 fps spanning 0.5 s, and the UBM data were acquired at 1000 fps spanning 0.15 s. The resulting images showed that multiple heart cycles could be clearly resolved using prospective imaging and that synthetic focusing improved image resolution and SNR of the right ventricle, interventricular septum, posterior edge of the left ventricle (LV), and papillary muscles of the LV versus fixed-focused imaging and the retrospective imaging of the UBM machine.     

3-D ultrasound imaging using a forward-looking CMUT ring array for intravascular/intracardiac applications

Forward-viewing ring arrays can enable new applications in intravascular and intracardiac ultrasound. This work presents compelling, full-synthetic, phased-array volumetric images from a forward-viewing capacitive micromachined ultrasonic transducer (CMUT) ring array wire bonded to a custom integrated circuit front end. The CMUT ring array has a diameter of 2 mm and 64 elements each 100 microm x 100 microm in size. In conventional mode, echo signals received from a plane reflector at 5 mm had 70% fractional bandwidth around a center frequency of 8.3 MHz. In collapse mode, 69% fractional bandwidth is measured around 19 MHz. Measured signal-to-noise ratio (SNR) of the echo averaged 16 times was 29 dB for conventional operation and 35 dB for collapse mode. B-scans were generated of a target consisting of steel wires 0.3 mm in diameter to determine resolution performance. The 6 dB axial and lateral resolutions for the B-scan of the wire target are 189 microm and 0.112 radians for 8 MHz, and 78 microm and 0.051 radians for 19 MHz. A reduced firing set suitable for real-time, intravascular applications was generated and shown to produce acceptable images. Rendered three-dimensional (3-D) images of a Palmaz-Schatz stent also are shown, demonstrating that the imaging quality is sufficient for practical applications.     

Finite element simulation and modeling of 2-D arrays for 3-D ultrasonic imaging

Issues of modeling and design of 2-D arrays in three dimensions with finite element code are discussed. These ultrasonic arrays are used for real time dynamic imaging of the heart. Topics include optimization, sensitivity, and performance and methods to speed up the run times required for computer simulations of large three-dimensional models. Empirical results from 45×45 2-D arrays are also presented     

Front-end receiver electronics for a matrix transducer for 3-D transesophageal echocardiography

There is a clear clinical need for creating 3-D images of the heart. One promising technique is the use of transesophageal echocardiography (TEE). To enable 3-D TEE, we are developing a miniature ultrasound probe containing a matrix piezoelectric transducer with more than 2000 elements. Because a gastroscopic tube cannot accommodate the cables needed to connect all transducer elements directly to an imaging system, a major challenge is to locally reduce the number of channels, while maintaining a sufficient signal-to-noise ratio. This can be achieved by using front-end receiver electronics bonded to the transducers to provide appropriate signal conditioning in the tip of the probe. This paper presents the design of such electronics, realizing time-gain compensation (TGC) and micro-beamforming using simple, low-power circuits. Prototypes of TGC amplifiers and micro-beamforming cells have been fabricated in 0.35-μm CMOS technology. These prototype chips have been combined on a printed circuit board (PCB) to form an ultrasound-receiver system capable of reading and combining the signals of three transducer elements. Experimental results show that this design is a suitable candidate for 3-D TEE.     

1-3-2压电陶瓷/聚合物复合材料圆柱型换能器制备

改进的1-3(1-3-2)型压电复合材料由1-3压电复合材料与陶瓷基底沿陶瓷极化方向串联而成,其不仅具有与1-3复合材料相同的优点,而且克服了1-3复合材料遇热和受压容易变形等缺点。利用改进的1-3型压电复合材料制作的圆柱型水声换能器在直径为70mm时,其谐振频率为72kHz;发射电压响应为139dB;在20～60kHz频率范围内接受电压灵敏度为-212(起伏±4dB)。换能器水平方向具有基本均匀的指向性,垂直方向波束宽度为12°。     

Quantitative 3-d diagnostic ultrasound imaging using a modified transducer array and an automated image tracking technique

An approach for acquiring dimensionally accurate three-dimensional (3-D) ultrasound data from multiple 2-D image planes is presented. This is based on the use of a modified linear-phased array comprising a central imaging array that acquires multiple, essentially parallel, 2-D slices as the transducer is translated over the tissue of interest. Small, perpendicularly oriented, tracking arrays are integrally mounted on each end of the imaging transducer. As the transducer is translated in an elevational direction with respect to the central imaging array, the images obtained by the tracking arrays remain largely coplanar. The motion between successive tracking images is determined using a minimum sum of absolute difference (MSAD) image matching technique with subpixel matching resolution. An initial phantom scanning-based test of a prototype 8 MHz array indicates that linear dimensional accuracy of 4.6% (2 /spl sigma/) is achievable. This result compares favorably with those obtained using an assumed average velocity [31.5% (2 /spl sigma/) accuracy] and using an approach based on measuring image-to-image decorrelation [8.4% (2 /spl sigma/) accuracy]. The prototype array and imaging system were also tested in a clinical environment, and early results suggest that the approach has the potential to enable a low cost, rapid, screening method for detecting carotid artery stenosis. The average time for performing a screening test for carotid stenosis was reduced from an average of 45 minutes using 2-D duplex Doppler to 12 minutes using the new 3-D scanning approach.     

An approximated 3-D model of cylinder-shaped piezoceramic elements for transducer design

In this paper an approximated 3-D model of cylinder shaped piezoceramics is described. In the hypothesis of axial symmetry, the element vibration in the extensional and radial directions is described by two coupled differential wave equations. The model is obtained choosing, as solution of these equations, two orthogonal wave functions, each depending only on one axis, corresponding to the propagation direction. The mechanical boundary conditions are applied imposing continuity between the stresses and the external forces on the surfaces of the element in an integral way, while, as far as the electrical boundary condition is concerned, two possibilities are explored: to neglect the piezoelectric constant in the transverse direction and to impose an integral condition also for the electric field. Comparisons with experimental results show this last approach to give better results. The model predicts with sufficient accuracy only the first radial and the first thickness modes of the cylinder-shaped piezoceramic element of arbitrary aspect ratio; but, for these modes, it is able to compute all the relations between the input applied voltage and the output forces and velocities on every external surface. Because only these two modes are of relevance in the practical applications of piezoceramic elements as ultrasonic transducers, the model can be used as a simple and useful tool in transducer design and optimization. Experimental validations of the model are also shown in the work.