Volumetric real-time imaging using a CMUT ring array

A ring array provides a very suitable geometry for forward-looking volumetric intracardiac and intravascular ultrasound imaging. We fabricated an annular 64-element capacitive micromachined ultrasonic transducer (CMUT) array featuring a 10-MHz operating frequency and a 1.27-mm outer radius. A custom software suite was developed to run on a PC-based imaging system for real-time imaging using this device. This paper presents simulated and experimental imaging results for the described CMUT ring array. Three different imaging methods--flash, classic phased array (CPA), and synthetic phased array (SPA)--were used in the study. For SPA imaging, two techniques to improve the image quality--Hadamard coding and aperture weighting--were also applied. The results show that SPA with Hadamard coding and aperture weighting is a good option for ring-array imaging. Compared with CPA, it achieves better image resolution and comparable signal-to-noise ratio at a much faster image acquisition rate. Using this method, a fast frame rate of up to 463 volumes per second is achievable if limited only by the ultrasound time of flight; with the described system we reconstructed three cross-sectional images in real-time at 10 frames per second, which was limited by the computation time in synthetic beamforming.     

A photoacoustic imager with light illumination through an infrared-transparent silicon CMUT array

A novel hardware design and preliminary experimental results for photoacoustic imaging are reported in this paper. This imaging system makes use of an infrared-transparent capacitive micromachined ultrasonic transducer (CMUT) chip for ultrasound reception and illuminates the image target through the CMUT array. The cascaded arrangement between the light source and transducer array allows for a more compact imager head and results in more uniform illumination. Taking advantage of the low optical absorption coefficient of silicon in the near infrared spectrum as well as the broad acoustic bandwidth that CMUTs provide, an infrared-transparent CMUT array has been developed for ultrasound reception. The center frequency of the polysilicon-membrane CMUT devices used in this photoacoustic system is 3.5 MHz, with a fractional bandwidth of 118% in reception mode. The silicon substrate of the CMUT array has been thinned to 100 μm and an antireflection dielectric layer is coated on the back side to improve the infrared-transmission rate. Initial results show that the transmission rate of a 1.06-μm Nd:Yag laser through this CMUT chip is 12%. This transmission rate can be improved if the thickness of silicon substrate and the thin-film dielectrics in the CMUT structure are properly tailored. Imaging of a metal wire phantom using this cascaded photoacoustic imager is demonstrated.     

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.     

Speckle reduction in pulse-echo ultrasonic imaging using a two-dimensional receiving array

An experimental pulse-echo imager was developed for the purpose of reducing speckle in ultrasonic images. The system utilized a 64-element spherically focused segmented annuli array receiver with a common transmitter. Compounded images were formed using subapertures of varying size, shape, and overlap, and the speckle and resolution characteristics of the final images were observed. A pointlike scatterer was imaged to determine the resolution, point spread function, and sensitivity of the system along with a new measure called the resolution cell size. The response of the system was also simulated for comparisons. It was found that lateral resolution, and resolution cell sizes only gradually increased with a decrease in subaperture size and system sensitivity was not greatly diminished. Incoherent summation of signals from small groups of elements decreased the speckle noise by a factor of four while maintaining enough resolution to improve the image quality as measured by the CSR/d by a factor of almost two.     

Volumetric ultrasound imaging using 2-D CMUT arrays

Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as a candidate to overcome the difficulties in the realization of 2-D arrays for real-time 3-D imaging. In this paper, we present the first volumetric images obtained using a 2-D CMUT array. We have fabricated a 128 x 128-element 2-D CMUT array with through-wafer via interconnects and a 420-microm element pitch. As an experimental prototype, a 32 x 64-element portion of the 128 x 128-element array was diced and flip-chip bonded onto a glass fanout chip. This chip provides individual leads from a central 16 x 16-element portion of the array to surrounding bondpads. An 8 x 16-element portion of the array was used in the experiments along with a 128-channel data acquisition system. For imaging phantoms, we used a 2.37-mm diameter steel sphere located 10 mm from the array center and two 12-mm-thick Plexiglas plates located 20 mm and 60 mm from the array. A 4 x 4 group of elements in the middle of the 8 x 16-element array was used in transmit, and the remaining elements were used to receive the echo signals. The echo signal obtained from the spherical target presented a frequency spectrum centered at 4.37 MHz with a 100% fractional bandwidth, whereas the frequency spectrum for the echo signal from the parallel plate phantom was centered at 3.44 MHz with a 91% fractional bandwidth. The images were reconstructed by using RF beamforming and synthetic phased array approaches and visualized by surface rendering and multiplanar slicing techniques. The image of the spherical target has been used to approximate the point spread function of the system and is compared with theoretical expectations. This study experimentally demonstrates that 2-D CMUT arrays can be fabricated with high yield using silicon IC-fabrication processes, individual electrical connections can be provided using through-wafer vias, and flip-chip bonding can be used to integrate these dense 2-D arrays with electronic circuits for practical 3-D imaging applications.     

Intravascular photoacoustic imaging using an IVUS imaging catheter

Catheter-based imaging of atherosclerosis with high resolution, albeit invasive, is extremely important for screening and characterization of vulnerable plaques. Currently, there is a need for an imaging technique capable of providing comprehensive morphological and functional information of plaques. In this paper, we present an intravascular photoacoustic imaging technique to characterize vulnerable plaques by using optical absorption contrast between normal tissue and atherosclerotic lesions. Specifically, we investigate the feasibility of obtaining intravascular photoacoustic (IVPA) images using a high-frequency intravascular ultrasound (IVUS) imaging catheter. Indeed, the combination of IVPA imaging with clinically available IVUS imaging may provide desired functional and morphological assessment of the plaque. The imaging studies were performed with tissue-mimicking arterial vessel phantoms and excised samples of rabbit artery. The results of our study suggest that catheter-based intravascular photoacoustic imaging is possible, and the combination of IVPA with IVUS has the potential to detect and differentiate atherosclerosis based on both the structure and composition of the plaque.     

Forward-viewing CMUT arrays for medical imaging

This paper reports the design and testing of forward-viewing annular arrays fabricated using capacitive micromachined ultrasonic transducer (CMUT) technology. Recent research studies have shown that CMUTs have broad frequency bandwidth and high-transduction efficiency. One- and two-dimensional CMUT arrays of various sizes already have been fabricated, and their viability for medical imaging applications has been demonstrated. We fabricated 64-element, forward-viewing annular arrays using the standard CMUT fabrication process and carried out experiments to measure the operating frequency, bandwidth, and transmit/receive efficiency of the array elements. The annular array elements, designed for imaging applications in the 20 MHz range, had a resonance frequency of 13.5 MHz in air. The immersion pulse-echo data collected from a plane reflector showed that the devices operate in the 5-26 MHz range with a fractional bandwidth of 135%. The output pressure at the surface of the transducer was measured to be 24 kPa/V. These values translate into a dynamic range of 131.5 dB for 1-V excitation in 1-Hz bandwidth with a commercial low noise receiving circuitry. The designed, forward-viewing annular CMUT array is suitable for mounting on the front surface of a cylindrical catheter probe and can provide Doppler information for measurement of blood flow and guiding information for navigation through blood vessels in intravascular ultrasound imaging.     

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.     

Safety profile of two-dimensional Pd nanosheets for photothermal therapy and photoacoustic imaging

Two-dimensional (2D) nanosheets have emerged as an important class of nanomaterial with great potential in the field of biomedicines,particularly in cancer theranostics.However,owing to the lack of effective methods that synthesize uniform 2D nanomaterials with controlled size,systematic evaluation of size-dependent bio-behaviors of 2D nanomaterials is rarely reported.To the best of our knowledge,we are the first to report a systematic evaluation of the influence of size of 2D nanomaterials on their bio-behaviors.2D Pd nanosheets with diameters ranging from 5 to 80 nm were synthesized and tested in cell and animal models to assess their size-dependent bioapplication,biodistribution,elimination,toxicity,and genomic gene expression profiles.Our results showed size significantly influences the biological behaviors of Pd nanosheets,including their photothermal and photoacoustic effects,pharmacokinetics,and toxicity.Compared to larger-sized Pd nanosheets,smaller-sized Pd nanosheets exhibited more advanced photoacoustic imaging and photothermal effects upon ultralow laser irradiation.Moreover,in vivo results indicated that 5-nm Pd nanosheets escape from the reticuloendothelial system with a longer blood half-life and can be cleared by renal excretion,while Pd nanosheets with larger sizes mainly accumulate in the liver and spleen.The 30-nm Pd nanosheets exhibited the highest tumor accumulation.Although Pd nanosheets did not cause any appreciable toxicity at the cellular level,we observed slight lipid accumulation in the liver and inflammation in the spleen.Genomic gene expression analysis showed that 80-nm Pd nanosheets interacted with more cellular components and affected more biological processes in the liver,as compared to 5-nm Pd nanosheets.We believe this work will provide valuable information and insights into the clinical application of 2D Pd nanosheets as nanomedicines.     

A two-dimensional hydrophone array using piezoelectric PVDF

A new development in ultrasound measurement technology-a two-dimensional hydrophone array-is presented. Using the established technology of PVDF ultrasound sensors, this novel hydrophone array has 64 circular elements (each of 0.2 mm diameter) arranged in an 8/spl times/8 pattern with 1 mm spacing between element centers. Construction of the hydrophone array is discussed, and results of a series of measurements to characterize its performance are presented. Hydrophone elements exhibit a flat frequency response (/spl plusmn/0.8 dB over the frequency range 1-25 MHz) and have an effective radius that appears to have minimal frequency dependency (within /spl plusmn/8% of mean value) and is within 8% of the nominal sensor radius. The hydrophone array performance also is assessed in terms of dynamic range, interelement variation, and acquisition time. Applications of this new technology (such as mapping of transducer fields, assessment of diagnostic ultrasound fields, and use in point source-array receiver nondestructive testing) are discussed.     

Real-time volume imaging using a crossed electrode array

This paper describes a unique crossed electrode array for real-time volume ultrasound imaging. By placing orthogonal linear array electrode patterns on the opposite sides of a hemispherically shaped composite transducer substrate, a 2D array can be fabricated using a small fraction of the elements required for a traditional 2D array. The performance of the array is investigated using a computer simulation of the radiation pattern. We show that by using a 288-element crossed electrode pattern it is possible to collect large field of view volume images (60deg times 60degsector) at real-time frame rates (>20 volume images/s), with image contrast and resolution comparable to what can be obtained using a conventional 128-element linear phased array.     

Design of a front-end integrated circuit for 3D acoustic imaging using 2D CMUT arrays

Integration of front-end electronics with 2D capacitive micromachined ultrasonic transducer (CMUT) arrays has been a challenging issue due to the small element size and large channel count. We present design and verification of a front-end drive-readout integrated circuit for 3D ultrasonic imaging using 2D CMUT arrays. The circuit cell dedicated to a single CMUT array element consists of a high-voltage pulser and a low-noise readout amplifier. To analyze the circuit cell together with the CMUT element, we developed an electrical CMUT model with parameters derived through finite element analysis, and performed both the pre- and postlayout verification. An experimental chip consisting of 4 X 4 array of the designed circuit cells, each cell occupying a 200 X 200 microm2 area, was formed for the initial test studies and scheduled for fabrication in 0.8 microm, 50 V CMOS technology. The designed circuit is suitable for integration with CMUT arrays through flip-chip bonding and the CMUT-on-CMOS process.     

Three-dimensional imaging using a new synthetic aperture focusingtechnique

The drawbacks of the multifrequency automatic focusing technique (MF-AFT) have been eased considerably when the scanned-frequency signals are transmitted simultaneously in a pulse. While the focusing capabilities of the MF-AFT are maintained, substantial reduction in data processing is obtained when the new procedure is used. In addition, the derived pulse can be tailored to further enhance the quality of the image produced by this new procedure. Simulation results are included to demonstrate the focusing capabilities of the procedure for both two- and three-dimensional objects     

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.     

Multi-wavelength raman imaging using a small-diameter image guide with a dimension-reduction imaging array

A commercially available fiber-optic Raman probe was modified for high-resolution spectral Raman imaging using a 350 microm diameter optical fiber image guide coupled to a dimension-reduction imaging array (DRIA). The DRIA comprised 672 optical fibers, arranged as a square array (21 x 32 fibers) on one end and a linear array (672 x 1 fibers) on the other. An imaging spectrograph was used with the DRIA to acquire multi-wavelength Raman images from -250 to 1800 cm(-1) at a spectral resolution of approximately 5 cm(-1). The utility of this technique for in situ and remote Raman imaging is demonstrated by monitoring the polymerization of a model polymer, dibromostyrene (DBS), while simultaneously measuring the Raman Stokes/ anti-Stokes ratio as a function of sample heating time, over a sample area of approximately 4 x 1.6 mm.     

Label-free protein recognition two-dimensional array using nanomechanical sensors

We demonstrate two-dimensional multiplexed real-time, label-free antibody-antigen binding assays by optically detecting nanoscale motions of two-dimensional arrays of microcantilever beams. Prostate specific antigen (PSA) was assayed using antibodies covalently bound to one surface of the cantilevers by two different surface chemistries, while the nonreaction surfaces were passivated by poly(ethylene glycol)-silane. PSA as low as 1 ng/mL was detected while 2 mg/microl of bovine serum albumin induced only negligible deflection on the cantilevers.     

Three-dimensional imaging using a frequency-domain synthetic aperture focusing technique

A frequency-domain method for implementing the synthetic aperture focusing technique is developed and demonstrated using computer simulation. As presented, the method is well suited to reconstructing ultrasonic reflectivity over a volumetric region of space using measurements made over an adjacent two-dimensional aperture. Extensive use is made of both one- and two-dimensional Fourier transformations to perform the temporal and spatial correlation required by the technique, making the method well suited to general-purpose computing hardware. Results are presented demonstrating both the lateral and axial resolution achieved by the method. The effect of limiting the reconstruction bandwidth is also demonstrated.     

Three-dimensional Dammann array

We demonstrate a scheme that can produce a three-dimensional (3D) focus spot array in a 3D lattice structure, called a 3D Dammann array, in focal region of an objective. This 3D Dammann array is generated by using two separate micro-optical elements, a Dammann zone plate (DZP) that produces a series of coaxial focus spots and a conventional two-dimensional (2D) Dammann grating (DG). A simple, fast, and clear method is presented to design this binary pure-phase (0,π) DZP in vectorial Debye theory regime. Based on this kind of DZP, one can always obtain a 3D Dammann array both for low and high numerical aperture (NA) focusing objectives. For experimental demonstration, an arrangement combining a DZP, a 2D DG, and a pair of opposing lenses is proposed to generate a 5×5×5 Dammann array in focal region of an objective with NA=0.127 and another 6×6×7 Dammann array for an objective of NA=0.66. It is shown that this arrangement makes it possible to achieve 3D Dammann arrays with micrometer-sized focus spots and focus spacings of tens of micrometers for various practical applications, such as 3D parallel micro- and nanomachining, 3D simultaneous optical manipulation, 3D optical data storage, and multifocal fluorescence microscope, etc.