An endoscopie imaging system based on a two-dimensional CMUT array: real-time imaging results

Real-time catheter-based ultrasound imaging tools are needed for diagnosis and image-guided procedures. The continued development of these tools is partially limited by the difficulty of fabricating two-dimensional array geometries of piezoelectric transducers. Using capacitive micromachined ultrasonic transducer (CMUT) technology, transducer arrays with widely varying geometries, high frequencies, and wide bandwidths can be fabricated. A volumetric ultrasound imaging system based on a two-dimensional, 16×l6-element, CMUT array is presented. Transducer arrays with operating frequencies ranging from 3 MHz to 7.5 MHz were fabricated for this system. The transducer array including DC bias pads measures 4 mm by 4.7 mm. The transducer elements are connected to flip-chip bond pads on the array back side with 400-μm long through-wafer interconnects. The array is flip-chip bonded to a custom-designed integrated circuit (IC) that comprises the front-end electronics. Integrating the front-end electronics with the transducer array reduces the effects of cable capacitance on the transducer's performance and provides a compact means of connecting to the transducer elements. The front-end IC provides a 27-V pulser and 10-MHz bandwidth amplifier for each element of the array. An FPGA-based data acquisition system is used for control and data acquisition. Output pressure of 230 kPa was measured for the integrated device. A receive sensitivity of 125 mV/kPa was measured at the output of the amplifier. Amplifier output noise at 5 Mhz is 112 nV/√Hz. Volumetric images of a wire phantom and vessel phantom are presented. Volumetric data for a wire phantom was acquired in real-time at 30 frames per second.Publisher's Versio

FINITE ELEMENT ANALYSIS OF CMUTs: CONVENTIONAL VS. COLLAPSE OPERATION MODES

Collapse mode has been proposed to improve Capacitive Micromachined Ultrasonic Transducer (CMUT) performance in terms of output pressure and receive sensitivity. The focus of this study is to compare the performance of optimized designs for conventional and collapse mode operations using finite element analysis (FEA). For this purpose, we have developed a 2D finite element model for output pressure calculation of CMUTs using commercially available FEA software (ANSYS 10.0). The model is composed of a membrane, a fluid waveguide and an electrical port. The membrane and the fluid part are constructed using axisymmetric plane elements. The electrical port is added to the model by using transducer elements that are already available in ANSYS. Through transient analysis non-linear calculations are performed and average output pressure over the membrane surface is calculated for a given electrical excitation that is applied to the electrical port. First, the effect of various transducer parameters such as radius, thickness, bias voltage, are investigated for conventional mode. Then, the performance of membranes operating in different modes are compared. The parameters of the membranes are selected such that their center frequencies and the collapse voltages match for a fair comparison of transducer performance. Our calculations show that the membrane operating in conventional mode required 100 Vpp AC amplitude for 2 MPa peak-to-peak output pressure whereas 38 Vpp AC excitation generated the same output pressure for collapse mode membrane. In addition, we have compared the second harmonic levels generated by the both designs. For 2MPa peak-to-peak output, the second harmonic amplitude is found to be 13.5 dB below the fundamental for conventional mode design. However, the second harmonic amplitude is -12.0 dB for the collapse mode design

5F-4 Acoustic Crosstalk Reduction Method for CMUT Arrays

This paper reports on the finite element analysis (FEA) of crosstalk in capacitive micromachined ultrasonic transducer (CMUT) arrays. Finite element calculations using a commercial package (LS-DYNA) were performed for an immersed I-D CMUT array operating in the conventional and collapsed modes. LS-DYNA was used to model the crosstalk in CMUT arrays under specific voltage bias and excitation conditions, and such a modeling is well worth the effort for special-purpose CMUT arrays for ultrasound applications such as medical imaging and high intensity focused ultrasound (HIFU) treatment. Compared to the existing finite element analysis (FEA) in literature, our FEA is distinguished by having all 5 main features together: First, the explicit, time domain solver of LS-DYNA enables the modeling of the actual CMUT array in detail, i.e. all 41 array elements are modeled. Second, user-defined subroutines provide an efficient electrostatic-structural coupling method. Third, the robust contact capability offers the CMUT modeling in collapsed operation. Fourth, a fast method to bias the CMUT array in conventional and collapsed modes is implemented. Fifth, the FEA results are verified with interferometer measurements. Our finite element calculations show that the main crosstalk mechanism is the dispersive guided modes propagating in the fluid-solid interface. Conventional operation has a crosstalk level of -23 dB and the guided modes are not present above the cut-off frequency of 4 MHz. Most importantly, the crosstalk wave has a center frequency of 2.3 MHz with a narrow bandwidth although the transmitter element has a center frequency of 5.8 MHz with more than 130% fractional bandwidth. Crosstalk level is improved to -39 dB in collapsed operation, and the cut-off frequency becomes 10 MHz because of the contact between the membrane and the substrate. The Lamb wave modes have a crosstalk level around -43 dB in both operation modes. These finite element results show excellent agreement with the interferometer measurements of the fabricated CMUT array. Using our verified FEA, we implemented a powerful method for the first time to reduce the crosstalk by impeding the propagation of the guided interface waves. This method is based on the acoustic band gap resulting from the periodic CMUT membranes on the fluid-solid interface. The crosstalk was effectively reduced by 10 dB down to -33 dB in the conventional operation without loss of acoustic pressure of the transmitter element. This method can be easily introduced into the fabrication of I-D and 2-D CMUT arrays to achieve superior crosstal

High-Efficiency Output Pressure Performance Using Capacitive Micromachined Ultrasonic Transducers with Substrate-Embedded Springs

Capacitive micromachined ultrasonic transducers (CMUTs) with substrate-embedded springs offer highly efficient output pressure performance over conventional CMUTs, owing to their nonflexural parallel plate movement. The embedded silicon springs support thick Si piston plates, creating a large nonflexural average volume displacement efficiency in the operating frequency range from 1–3 MHz. Static and dynamic volume displacements of the nonflexural parallel plates were examined using white light interferometry and laser Doppler vibrometry. In addition, an output pressure measurement in immersion was performed using a hydrophone. The device showed a maximum transmission efficiency of 21 kPa/V, and an average volume displacement efficiency of 1.1 nm/V at 1.85 MHz with a low DC bias voltage of 55 V. The device element outperformed the lead zirconate titanate (PZT) ceramic HD3203, in the maximum transmission efficiency or the average volume displacement efficiency by 1.35 times. Furthermore, its average volume displacement efficiency reached almost 80% of the ideal state-of-the-art single-crystal relaxor ferroelectric materials PMN-0.33PT. Additionally, we confirmed that high-efficiency output pressure could be generated from the CMUT device, by quantitatively comparing the hydrophone measurement of a commercial PZT transducer