Biomedical subjects

Butrus T Khuri-Yakub

Publications and source records attributed to Butrus T Khuri-Yakub.

At least 19 recordsLinked to original sources

PubMed2006

Comparison of conventional and collapsed region operation of capacitive micromachined ultrasonic transducers.

We report experimental results from a comparative study on collapsed region and conventional region operation of capacitive micromachined ultrasonic transducers (CMUTs) fabricated with a wafer bonding technique. Using ultrasonic pulse-echo and pitch-catch measurements, we characterized single elements of 1-D CMUT arrays operating in oil. The experimental results from this study agreed with the simulation results: a CMUT operating in the collapsed region produced a higher maximum output pressure than a CMUT operated in the conventional region at 90% of its collapse voltage (3 kPa/V vs. 16.1 kPa/V at 2.3 MHz). While the pulse-echo fractional bandwidth (126%) was higher in the collapsed region operation than in the conventional operation (117%), the pulse-echo amplitude in collapsed region operation was 11 dB higher than in conventional region operation. Furthermore, within the range of tested bias voltages, the output pressure monotonously increased with increased bias during collapsed region operation. It was also found that in the conventional mode, short AC pulses (larger than the collapse voltage) could be applied without collapsing the membranes. Finally, while no significant difference was observed in reflectivity of the CMUT face between the two regions of operation, hysteretic behavior of the devices was identified in the collapsed region operation.

Journal Article↗

PubMed2006

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.

Echocardiography↗

PubMed2006

Experimental characterization of collapse-mode CMUT operation.

This paper reports on the experimental characterization of collapse-mode operation of capacitive micromachined ultrasonic transducers (CMUTs). CMUTs are conventionally operated by applying a direct current (DC) bias voltage less than the collapse voltage of the membrane, so that the membrane is deflected toward the bottom electrode. In the conventional regime, there is no contact between the membrane and the substrate; the maximum alternating current (AC) displacement occurs at the center of the membrane. In collapse-mode operation, the DC bias voltage is first increased beyond the collapse voltage, then reduced without releasing the collapsed membrane. In collapse-mode operation, the center of the membrane is always in contact with the substrate. In the case of a circular membrane, the maximum AC displacement occurs along the ring formed between the center and the edge of the membrane. The experimental characterization presented in this paper includes impedance measurements in air, pulse-echo experiments in immersion, and one-way optical displacement measurements in immersion for both conventional and collapse-mode operations. A 205-microm x 205-microm 2-D CMUT array element composed of circular silicon nitride membranes is used in the experiments. In pulse-echo experiments, a custom integrated circuit (IC) comprising a pulse driver, a transmit/receive switch, a wideband low-noise preamplifier, and a line driver is used. By reducing the parasitic capacitance, the use of a custom IC enables pulse-echo measurements at high frequencies with a very small transducer. By comparing frequency response and efficiency of the transducer in conventional and collapse regimes, experimental results show that a collapsed membrane can be used to generate and detect ultrasound more efficiently than a membrane operated in the conventional mode. Furthermore, the center frequency of the collapsed membrane can be changed by varying the applied DC voltage. In this study, the center frequency of a collapsed transducer in immersion is shown to vary from 20 MHz to 28 MHz with applied DC bias; the same transducer operates at 10 MHz in the conventional mode. In conventional mode, the maximum peak-to-peak pressure is 370 kPa on the transducer surface for a 40-ns, 25-V unipolar pulse excitation. In collapse mode, a 25-ns, 25-V unipolar pulse generates 590 kPa pressure at the surface of the transducer.

Journal Article↗

PubMed2005

Coherent-array imaging using phased subarrays. Part I: basic principles.

The front-end hardware complexity of a coherent array imaging system scales with the number of active array elements that are simultaneously used for transmission or reception of signals. Different imaging methods use different numbers of active channels and data collection strategies. Conventional full phased array (FPA) imaging produces the best image quality using all elements for both transmission and reception, and it has high front-end hardware complexity. In contrast, classical synthetic aperture (CSA) imaging only transmits on and receives from a single element at a time, minimizing the hardware complexity but achieving poor image quality. We propose a new coherent array imaging method--phased subarray (PSA) imaging--that performs partial transmit and receive beam-forming using a subset of adjacent elements at each firing step. This method reduces the number of active channels to the number of subarray elements; these channels are multiplexed across the full array and a reduced number of beams are acquired from each subarray. The low-resolution subarray images are laterally upsampled, interpolated, weighted, and coherently summed to form the final high-resolution PSA image. The PSA imaging reduces the complexity of the front-end hardware while achieving image quality approaching that of FPA imaging.

Computer Simulation↗

PubMed2005

Coherent array imaging using phased subarrays. Part II: simulations and experimental results.

The basic principles and theory of phased subarray (PSA) imaging imaging provides the flexibility of reducing the number of front-end hardware channels between that of classical synthetic aperture (CSA) imaging--which uses only one element per firing event--and full-phased array (FPA) imaging-which uses all elements for each firing. The performance of PSA generally ranges between that obtained by CSA and FPA using the same array, and depends on the amount of hardware complexity reduction. For the work described in this paper, we performed FPA, CSA, and PSA imaging of a resolution phantom using both simulated and experimental data from a 3-MHz, 3.2-cm, 128-element capacitive micromachined ultrasound transducer (CMUT) array. The simulated system point responses in the spatial and frequency domains are presented as a means of studying the effects of signal bandwidth, reconstruction filter size, and subsampling rate on the PSA system performance. The PSA and FPA sector-scanned images were reconstructed using the wideband experimental data with 80% fractional bandwidth, with seven 32-element subarrays used for PSA imaging. The measurements on the experimental sector images indicate that, at the transmit focal zone, the PSA method provides a 10% improvement in the 6-dB lateral resolution, and the axial point resolution of PSA imaging is identical to that of FPA imaging. The signal-to-noise ratio (SNR) of PSA image was 58.3 dB, 4.9 dB below that of the FPA image, and the contrast-to-noise ratio (CNR) is reduced by 10%. The simulated and experimental test results presented in this paper validate theoretical expectations and illustrate the flexibility of PSA imaging as a way to exchange SNR and frame rate for simplified front-end hardware.

Computer Simulation↗

PubMed2005

Capacitive micromachined ultrasonic transducer design for high power transmission.

Capacitive micromachined ultrasonic transducers (cMUTs) were developed to meet the demands of the ultrasonic industry. To achieve maximum efficiency, the conventional operation of the cMUT requires a bias voltage close to the collapse voltage. Total acoustic output pressure is limited by the efficiency of the cMUT and the maximum-allowed pulse voltage on the membrane. In this paper, we propose the collapse-snapback operation of the cMUT: the membrane is collapsed onto the substrate in the collapsing cycle, and released in the snapback cycle. The collapse-snapback operation overcomes the above-mentioned limitations of the conventional operation. The collapse-snapback operation utilizes a larger range of membrane deflection profiles (both collapsed and released profiles) and generates higher acoustic output pressures. The static finite element calculations were performed to design cMUTs with specific collapse and snapback voltages by changing the electrode parameters (radius (re), position (de), and thickness (te)). These designs were refined for optimum average displacement per cycle. An electrode radius greater than 60% of the membrane radius significantly improved the displacement per volt. Moderately thick membranes (te approximately 0.2 microm) were preferred, as thicker membranes reduced the displacement per volt. Under proper bias conditions, the collapse-snapback operation, designed for high-power transmission, allowed the application of pulse voltages larger than the difference of collapse and snapback voltages. Dynamic finite element calculations of an infinite cMUT array on the substrate loaded with acoustic fluid medium were performed to determine the dynamic response of the cMUT. Commercially available FEM packages ANSYS and LS-DYNA were used for static and dynamic calculations, respectively. The cMUTs were fabricated for optimal performance in the collapse-snapback operation. The transmit experiments were performed on a 2-D cMUT array using a calibrated hydrophone. Taking into account the attenuation and diffraction losses, the pressure on the cMUT surface was extracted. The cMUT generated 0.47 MPa (6 kPa/V) and 1.04 MPa (11 kPa/V) in the conventional and collapse-snapback operations, respectively. Therefore, collapse-snapback operation of the cMUTs was superior for high-power transmission.

Journal Article↗

PubMed2005

A solution to the charging problems in capacitive micromachined ultrasonic transducers.

We report on a capacitive micromachined ultrasonic transducer (CMUT) featuring isolation posts (PostCMUT) as a solution to the charging problems caused by device fabrication and operation. This design improves the device reliability. The PostCMUTs were fabricated using a newly developed process based on the wafer-bonding technique. Paired tests showed the superior reliability characteristics of the PostCMUT design compared to those of conventional CMUT designs. No deleterious effect of the new design was seen in preliminary ultrasonic tests or in process yield. PostCMUTs, a design that serves as a solution to the aforementioned reliability problem, constitutes a major contribution to CMUT commercialization.

Letter↗

PubMed2005

Finite-element analysis of capacitive micromachined ultrasonic transducers.

In this paper, we present the results of finite-element analysis performed to investigate capacitive micromachined ultrasonic transducers (CMUTs). Both three-dimensional (3-D) and 2-D models were developed using a commercially available finite-element modeling (FEM) software. Depending on the dimensionality of the model, the membranes were constructed using plane or shell elements. The electrostatic gap was modeled using many parallel plate transducers. An axisymmetric model for a single membrane was built; the electrical input impedance of the device then was calculated in vacuum to investigate series and parallel resonant frequencies, where the input impedance has a minimum and a maximum, respectively. A method for decomposing the membrane capacitance into parasitic and active parts was demonstrated, and it was shown that the parallel resonant frequency shifted down with increased biased voltage. Calculations then were performed for immersion transducers. Acoustic wave propagation was simulated in the immersion medium, using appropriate elements in a 3-D model. Absorbing boundaries were implemented to avoid the reflections at the end of the medium mesh. One row of an array element, modeled with appropriate boundary conditions, was used to calculate the output pressure. The results were compared with a simpler model: a single membrane in immersion, with symmetry boundary conditions on the sidewalls that cause the calculations to reflect the properties of an infinitely large array. A 2-D model then was developed to demonstrate the effect of membrane dimensions on the output pressure and bandwidth. Our calculations revealed that the small signal transmit pressure was inversely proportional to the square root of gap height. We also compared FEM results with analytical and experimental results.

Computer Simulation↗

PubMed2005

Capacitive micromachined ultrasonic transducers: fabrication technology.

Capacitive micromachined ultrasonic transducer (cMUT) technology is a prime candidate for next generation imaging systems. Medical and underwater imaging and the nondestructive evaluation (NDE) societies have expressed growing interest in cMUTs over the years. Capacitive micromachined ultrasonic transducer technology is expected to make a strong impact on imaging technologies, especially volumetric imaging, and to appear in commercial products in the near future. This paper focuses on fabrication technologies for cMUTs and reviews and compares variations in the production processes. We have developed two main approaches to the fabrication of cMUTs: the sacrificial release process and the recently introduced wafer-bonding method. This paper gives a thorough review of the sacrificial release processes, and it describes the new wafer-bonding method in detail. Process variations are compared qualitatively and quantitatively whenever possible. Through these comparisons, it was concluded that wafer-bonded cMUT technology was superior in terms of process control, yield, and uniformity. Because the number of steps and consequent process time were reduced (from six-mask process to four-mask process), turn-around time was improved significantly.

Biotechnology↗

PubMed2005

Dynamic analysis of capacitive micromachined ultrasonic transducers.

Electrostatic transducers are usually operated under a DC bias below their collapse voltage. The same scheme has been adopted for capacitive micromachined ultrasonic transducers (cMUTs). DC bias deflects the cMUT membranes toward the substrate, so that their centers are free to move during both receive and transmit operations. In this paper, we present time-domain, finite element calculations for cMUTs using LS-DYNA, a commercially available finite element package. In addition to this DC bias mode, other new cMUT operations (collapse and collapse-snapback) have recently been demonstrated. Because cMUT membranes make contact with the substrate in these new operations, modeling of these cMUTs should include contact analysis. Our model was a cMUT transducer consisting of many hexagonal membranes; because it was symmetrical, we modeled only one-sixth of a hexagonal cell loaded with a fluid medium. The finite element results for both conventional and collapse modes were compared to measurements made by an optical interferometer; a good match was observed. Thus, the model is useful for designing cMUTs that operate in regimes where membranes make contact with the substrate.

Computer Simulation↗

PubMed2004

Ultrasonic mixing in microfluidic channels using integrated transducers.

This paper presents a microfluidic mixer that uses acoustic stirring created by ultrasonic waves. The ultrasound is introduced into the channel by integrated piezoelectric transducers. The transducers are made of a zinc oxide thin film, which is deposited on the bottom surface of a quartz substrate. The poly(dimethylsiloxane) channel is aligned to the transducers on the top surface of the substrate. The transducers are designed for operation around 450 MHz. The main mechanism of the mixing is the acoustic stirring of the fluid perpendicular to the flow direction. The radiation pressure that is generated by the transducer causes the stirring inside the microfluidic channel. The performance of the mixer is characterized by mixing phenolphthalein solution and sodium hydroxide dissolved in ethyl alcohol. Flow rates on the order of 1-100 microL/min are used. The transducers are driven by 1.2 V(rms) sinusoidal voltages at 450 MHz.

Microfluidics↗

PubMed2004

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.

Electric Capacitance↗

PubMed2003

Calculation and measurement of electromechanical coupling coefficient of capacitive micromachined ultrasonic transducers.

The electromechanical coupling coefficient is an important figure of merit of ultrasonic transducers. The transducer bandwidth is determined by the electromechanical coupling efficiency. The coupling coefficient is, by definition, the ratio of delivered mechanical energy to the stored total energy in the transducer. In this paper, we present the calculation and measurement of coupling coefficient for capacitive micromachined ultrasonic transducers (CMUTs). The finite element method (FEM) is used for our calculations, and the FEM results are compared with the analytical results obtained with parallel plate approximation. The effect of series and parallel capacitances in the CMUT also is investigated. The FEM calculations of the CMUT indicate that the electromechanical coupling coefficient is independent of any series capacitance that may exist in the structure. The series capacitance, however, alters the collapse voltage of the membrane. The parallel parasitic capacitance that may exist in a CMUT or is external to the transducer reduces the coupling coefficient at a given bias voltage. At the collapse, regardless of the parasitics, the coupling coefficient reaches unity. Our experimental measurements confirm a coupling coefficient of 0.85 before collapse, and measurements are in agreement with theory.

Computer Simulation↗

PubMed2003

A new regime for operating capacitive micromachined ultrasonic transducers.

We report on a new operation regime for capacitive micromachined ultrasonic transducers (cMUTs). Traditionally, cMUTs are operated at a bias voltage lower than the collapse voltage of their membranes. In the new proposed operation regime, first the cMUT is biased past the collapse voltage. Second, the bias voltage applied to the collapsed membrane is reduced without releasing the membrane. Third, the cMUT is excited with an ac signal at the bias point, keeping the total applied voltage between the collapse and snapback voltages. In this operation regime, the center of the membrane is always in contact with the substrate. Our finite element methods (FEM) calculations reveal that a cMUT operating in this new regime, between collapse and snapback voltages, possesses a coupling efficiency (k(T)2) higher than a cMUT operating in the conventional regime below its collapse voltage. This paper compares the simulation results of the coupling efficiencies of cMUTs operating in conventional and new operation regimes.

Journal Article↗

PubMed2003

Capacitive micromachined ultrasonic Lamb wave transducers using rectangular membranes.

This paper details the theory, fabrication, and characterization of a new Lamb wave device. Built using capacitive micromachined ultrasonic transducers (CMUTs), the structure described uses rectangular membranes to excite and receive Lamb waves on a silicon substrate. An equivalent circuit model for the transducer is proposed that produces results, which match well with those observed by experiment. During the derivation of this model, emphasis is placed on the resistance presented to the transducer membranes by the Lamb wave modes. Finite element analysis performed in this effort shows that the dominant propagating mode in the device is the lowest order antisymmetric flexural wave (A0). Furthermore, most of the power that couples into the Lamb wave is due to energy in the vibrating membrane that is transferred to the substrate through the supporting posts of the device. The manufacturing process of the structure, which relies solely on fundamental IC-fabrication techniques, is also discussed. The resulting device has an 18-microm-thick substrate that is almost entirely made up of crystalline silicon and operates at a frequency of 2.1 MHz. The characterization of this device includes S-parameter and laser vibrometer measurements as well as delay-line transmission data. The insertion loss, as determined by both S-parameter and delay-line transmission measurements, is 20 dB at 2.1 MHz. When configured as a delay-line oscillator, the device functions well as a sensor with sensitivity to changes in the mass loading of its substrate.

Journal Article↗

PubMed2003

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.

Algorithms↗

PubMed2002

Piezoelectrically actuated flextensional micromachined ultrasound transducers.

This paper presents novel micromachined two-dimensional array piezoelectrically actuated flextensional transducers that can be used to generate sound in air or water. Micromachining techniques to fabricate these devices are also presented. Individual unimorph array elements consist of a thin piezoelectric annular disk and a thin, fully clamped, circular plate. We manufacture the transducer in two-dimensional arrays using planar silicon micromachining and demonstrate ultrasound transmission in air at 2.85 MHz with 0.15 microm/V peak displacement. The devices have a range of operating resonance frequencies starting from 450 kHz up to 4.5 MHz. Such an array could be combined with on-board driving and addressing circuitry for different applications.

Journal Article↗

PubMed2002

Finite element analysis of underwater capacitor micromachined ultrasonic transducers.

A simple electro-mechanical equivalent circuit model is used to predict the behavior of capacitive micromachined ultrasonic transducers (cMUT). Most often, cMUTs are made in silicon and glass plates that are in the 0.5 mm to 1 mm range in thickness. The equivalent circuit model of the cMUT lacks important features such as coupling to the substrate and the ability to predict cross-talk between elements of an array of transducers. To overcome these deficiencies, a flnite element model of the cMUT is constructed using the commercial code ANSYS. Calculation results of the complex load impedance seen by single capacitor cells are presented, then followed by a calculation of the plane wave real load impedance seen by a parallel combination of many cells that are used to make a transducer. Cross-talk between 1-D array elements is found to be due to two main sources: coupling through a Stoneley wave propagating at the transducer-water interface and coupling through Lamb waves propagating in the substrate. To reduce the cross-talk level, the effect of structural variations of the substrate are investigated, which includes a change of its thickness and etched trenches or polymer walls between array elements.

Artifacts↗