Ultrasound bandwidth enhancement through pulse compression using a CMUT probe

The capacitive micromachined ultrasound transducer (CMUT) is a new ultrasound (US) probe manufactured by state-of-the-art cutting-edge semi-conductor micromachined electro-mechanical systems (MEMS) technology. To demonstrate the peculiar characteristics of each probe and the limitations that should be improved. This study was performed from March to April 2018. The only inclusion criterion was the presence of disease, so all patients with musculoskeletal, skin, and subcutaneous pathology were included. A total of 66 patients entered this study. The exams of each patient, with both probes, were evaluated retrospectively and independently by three radiologists. Panoramicity of the images, the definition of superficial structures (<2 cm of depth), the definition of deep structures (>2 cm), and Doppler signal were assessed. A 5-point scale was used for each parameter. A total of 89 pathologies were detected. The mean of score for 4G-CMUT was higher than L64 for the panoramicity of the images and the definition of the deep structures. Instead, the mean score for L64 was higher than for 4G-CMUT in the evaluation of superficial structures and Doppler signal. A statistically significant difference was found (P < 0.05). CMUT is a breakthrough in US technology. It allows the use of a single probe for different US examinations. The musculoskeletal, skin, and subcutaneous US can be evaluated with a piezoelectric linear transducer or CMUT. In the present study, the overall diagnostic performance was similar. Improvements in CMUT will provide even more dynamic and flexible imaging capabilities by a transducer, with a wider bandwidth.

Plane wave ultrasound imaging using compressive sensing and minimum variance beamforming

The possibility to perform an early and repeatable assessment of imaging performance is fundamental in the design and development process of new ultrasound (US) probes. Particularly, a more realistic analysis with application-specific imaging targets can be extremely valuable to assess the expected performance of US probes in their potential clinical field of application. The experimental protocol presented in this work was purposely designed to provide an application-specific assessment procedure for newly-developed US probe prototypes based on Capacitive Micromachined Ultrasonic Transducer (CMUT) technology in relation to brain imaging. The protocol combines the use of a bovine brain fixed in formalin as the imaging target, which ensures both realism and repeatability of the described procedures, and of neuronavigation techniques borrowed from neurosurgery. The US probe is in fact connected to a motion tracking system which acquires position data and enables the superposition of US images to reference Magnetic Resonance (MR) images of the brain. This provides a means for human experts to perform a visual qualitative assessment of the US probe imaging performance and to compare acquisitions made with different probes. Moreover, the protocol relies on the use of a complete and open research and development system for US image acquisition, i.e. the Ultrasound Advanced Open Platform (ULA-OP) scanner. The manuscript describes in detail the instruments and procedures involved in the protocol, in particular forthe calibration, image acquisition and registration of US and MR images. The obtained results prove the effectiveness of the overall protocol presented, which is entirely open (within the limits of the instrumentation involved), repeatable, and covers the entire set of acquisition and processing activities for US images.

Capacitive Micromachined Ultrasonic Transducers (CMUTs), introduced about few decades ago, have been shown to be a good alternative to conventional piezoelectric transducers in various aspects, such as sensitivity, transduction efficiency, and bandwidth. It is used in a wide range of applications including Non destructive testing applications, medical imaging, flow metering, micro/nanoelectronics and industrial cleaning etc. The advent of MEMS technology has lead to intensive study of micro machined ultrasonic transducers. They are similar to other capacitance transducers as they employ a vibrating membrane to send and receive ultrasound in air and in water. The advantage of hexagonal Capacitance micro machined ultrasonic transducers (CMUTs) is that they can be operated in the high temperature range and the transducer array can be fabricated at a lower cost. Presented in this paper is a study on some aspects of the design of a modified CMUT, which the authors believe would lead to a CMUT with many advantages, including less loading effect and higher packing density. The focus of discussion in the paper is FEM simulation of a single cell hexagonal CMUT using polysilicon as membrane material. Also discussed is the simulation study of the critical parameters which influence the operation of the CMUTs. The simulations were carried out using MATLAB and IntelliSuite 8.0. The study indicates that in the range of study, the best performance is achieved at the natural resonance frequency, which is very desirable.

Capacitive micromachined ultrasonic transducers (CMUTs) have a nonlinear relationship between the applied voltage and the emitted signal, which is detrimental to conventional contrast enhanced ultrasound (CEUS) techniques. Instead, a three-pulse amplitude modulation (AM) sequence has been proposed, which is not adversely affected by the nonlinearly emitted harmonics. In this paper, this is shown theoretically, and the performance of the sequence is verified using a 4.8 MHz linear capacitive micromachined ultrasonic transducer (CMUT) array, and a comparable lead zirconate titanate (PZT) array, across 6-60 V applied alternating current (AC) voltage. CEUS images of the contrast agent SonoVue flowing through a 3D printed hydrogel phantom showed an average enhancement in contrast-to-tissue ratio (CTR) between B-mode and CEUS images of 49.9 and 37.4 dB for the PZT array and CMUT, respectively. Furthermore, hydrophone recordings of the emitted signals showed that the nonlinear emissions from the CMUT did not significantly degrade the cancellation in the compounded AM signal, leaving an average of 2% of the emitted power between 26 and 60 V of AC. Thus, it is demonstrated that CMUTs are capable of CEUS imaging independent of the applied excitation voltage when using a three-pulse AM sequence.

A novel pulse compression technique is developed that improves the axial resolution of an ultrasonic imaging system and provides a boost in the echo signal-to-noise ratio (eSNR). The new technique, called the resolution enhancement compression (REC) technique, was validated with simulations and experimental measurements. Image quality was examined in terms of three metrics: the eSNR, the bandwidth, and the axial resolution through the modulation transfer function (MTF). Simulations were conducted with a weakly-focused, single-element ultrasound source with a center frequency of 2.25 MHz. Experimental measurements were carried out with a single-element transducer (f/3) with a center frequency of 2.25 MHz from a planar reflector and wire targets. In simulations, axial resolution of the ultrasonic imaging system was almost doubled using the REC technique (0.29 mm) versus conventional pulsing techniques (0.60 mm). The -3 dB pulse/echo bandwidth was more than doubled from 48% to 97%, and maximum range sidelobes were -40 dB. Experimental measurements revealed an improvement in axial resolution using the REC technique (0.31 mm) versus conventional pulsing (0.44 mm). The -3 dB pulse/echo bandwidth was doubled from 56% to 113%, and maximum range sidelobes were observed at -45 dB. In addition, a significant gain in eSNR (9 to 16.2 dB) was achieved.

Ultrafast Plane Wave Imaging Using Tensor Completion-Based Minimum Variance Algorithm.

We present measurement results of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode over a wide pressure range (1 - 8 atm). The CMUT plates are in contact with the bottom of the cavities due to atmospheric pressure, even without any dc bias voltage. The electrical input impedance at various dc bias voltages are measured at elevated pressure to characterize individual devices. The open and short circuit resonant frequencies are extracted from the impedance data, and the acoustic performance of pairs of devices is evaluated by performing pitch-catch measurements. A frequency matching method is proposed and used to determine the optimal dc bias voltages for the transmitting and receiving CMUTs individually. Our electrical impedance results show good agreement with the finite element model results (modal and harmonic analysis performed with ANSYS) over the entire pressure range. Moreover, the pitch-catch measurement results validate the proposed frequency matching method for an optimal biasing scheme, and a received signal with good signal-to-noise ratio of 45 dB was observed at a pressure of 7 atm. In conclusion, the behavior of CMUTs in permanent contact mode can be predicted well with our FEA, and they are indeed a promising solution in providing ultrasonic transducers that can operate over a wide pressure range.

We report the development of air-coupled ultrasound transducers for noncontact nondestructive evaluation applications. Air-coupled ultrasound is an attractive inspection technique for materials or structures that are not suitable for contact or immersion ultrasound inspections. However, due to large acoustic impedance mismatch between air and materials in the system, tremendous acoustic power losses exist. To overcome this problem, the efficiency of the ultrasound transducers needs to be improved. We have developed large-gap capacitive micromachined ultrasound transducers (CMUTs) that have high transduction efficiency in air. The CMUT has a patterned dielectric insulation layer that supports large acoustic output but eliminates dielectric breakdown or charging failures. Acoustic tests showed that the CMUTs had significantly higher transduction efficiency than the state-of-the-art commercial transducers. This indicates improvement of air-coupled CMUTs is achievable and may lead to wide use of CMUTs for air inspections.

Intravascular ultrasound (IVUS) is a burgeoning imaging technology that provides vital information for the diagnosis of coronary arterial diseases. A significant constituent that enables the IVUS system to attain high-resolution images is the ultrasound transducer, which acts as both a transmitter that sends acoustic waves and a detector that receives the returning signals. Being the most mature form of ultrasound transducer available in the market, piezoelectric transducers have dominated the field of biomedical imaging. However, there are some drawbacks associated with using the traditional piezoelectric ultrasound transducers such as difficulties in the fabrication of high-density arrays, which would aid in the acceleration of the imaging speed and alleviate motion artifact. The advent of microelectromechanical system (MEMS) technology has brought about the development of micromachined ultrasound transducers that would help to address this issue. Apart from the advantage of being able to be fabricated into arrays with lesser complications, the image quality of IVUS can be further enhanced with the easy integration of micromachined ultrasound transducers with complementary metal-oxide-semiconductor (CMOS). This would aid in the mitigation of parasitic capacitance, thereby improving the signal-to-noise. Currently, there are two commonly investigated micromachined ultrasound transducers, piezoelectric micromachined ultrasound transducers (PMUTs) and capacitive micromachined ultrasound transducers (CMUTs). Currently, PMUTs face a significant challenge where the fabricated PMUTs do not function as per their design. Thus, CMUTs with different array configurations have been developed for IVUS. In this paper, the different ultrasound transducers, including conventional-piezoelectric transducers, PMUTs and CMUTs, are reviewed, and a summary of the recent progress of CMUTs for IVUS is presented.

We present the design and fabrication of 1D capacitive micromachined ultrasonic transducer (CMUT) arrays optimized for imaging using multiple modalities. A new variation of the fabrication process based on a thick buried oxide layer is used to build these CMUTs. In our process, the via connections for each cell's electrode with the handle layer are made from the front side. This enables fabricating CMUT cells with smaller sizes, and higher resonant frequencies. Initial characterization results agree well with our design simulations.

In this paper, we present design guideline and simulation results of capacitive micromachined ultrasonic transducer (CMUT) embedded field effect transistor (FET) for high frequency mode operation. The CMUT-FET consists of a mechanical CMUT part and an electrical sensing FET part. The device has a characteristic of a shared gate dielectric between a CMUT and an FET; hence, the gate capacitance change in the CMUT part can be reflected to the FET current as pressure changes. With an FET's current amplifying characteristic, the CMUT-FET makes it possible to achieve higher sensitivity and high resonant frequency with smaller element pitch and smaller active cell area than existing CMUTs. Combination of a 3-D finite element analysis (FEA) model of the CMUT part and a commercial technology computer aided design (TCAD) simulation of the FET device was used for inspection of the CMUT-FET device performance. In the 3-D FEA simulation, we focused on investigating pull-in voltage and electrical potential of a rectangular CMUT plate which have 20MHz resonant frequency in immersion with different dimensions. The FET part simulation was performed with variation of gate oxide thickness, source/drain doping concentration, channel doping concentration, substrate doping concentration. We combined data and inspected overall device performance with pressure variation at specific gate voltage and optimized the factor above-mentioned to get high sensitivity. Simulation results show that the optimized model has the collapse voltage of 13.4V and the pressure sensitivity of 11.34µA/Pa at gate voltage of 11V.

It is well known that acoustic emission (AE) signals, generated by external impacts or damages such as crack initiation, mainly propagate in the form of Lamb waves in plate-like structures. In this work, MEMS-based resonant capacitive micro-machined ultrasonic transducers (CMUTs), which are designed for sensing out-of-plane displacements, have been verified by finite element method (FEM) modeling and theoretical analysis for their feasibility of detecting low-order Lamb waves (A0 and S0). First, combining the propagation theory of Lamb waves and the “spring-mass-damper” model of CMUTs, the out-of-plane sensing mechanism has been explained, together with the analytical expression of sensitivity. Then, simulations based on FEM have been carried out to show that the designed CMUTs are sensitive to out-of-plane displacements, while extremely insensitive to in-plane displacements. Meanwhile, a transient analysis has found the potential abilities of CMUTs for sensing A0 and S0 lamb waves. Besides, the sensing characteristics of CMUTs have also been investigated, including the influence of squeezed-film damping, the amplitude of the input signal, the cell number, and cell space. Finally, the ball drop impact is simulated to show the potential of identifying the location of the AE source by CMUTs. Our studies reveal the out-of-plane sensing behaviors of CMUTs for Lamb waves and may have the potential in promoting the miniaturization and integration of AE sensors.

Capacitive micromachined ultrasonic transducers (CMUTs) have prospective clinical applications in ultrasound diagnostics and therapy. We have modeled, fabricated, and characterized CMUT arrays suitable for intravascular and gastrointestinal use. The elements of the CMUT arrays are connected from the back side of the array by through-silicon-vias (TSVs) with a pitch of 25 μm. In the reported work, the presence of TSVs does not reduce the fill factor of the array. Two different membrane designs were fabricated, resulting in a resonance frequency in air of 35.6 and 41.0 MHz, respectively. The measurements were in good agreement with the simulated performance. The realized devices are promising for the fabrication of a 3D-ready and bio-compatible device, suitable for in vivo ultrasound applications.

Synthetic aperture focusing based on planar waves also known as coherent plane wave compounding (CPWC) was introduced to reduce the diffraction spreading effect of ultrasound at depths (Song et al. US Patent 6736780, 2004). CPWC has been widely used in shear wave elasticity imaging due to its capability of ultrafast frame rate (Montaldo, et al. IEEE TUFFC, vol. 56, 2009). However, CPWC based on unfocused (plane waves) or defocused Tx beams has an inherent limitation in imaging wider field of view (FOV) due to the use of sampling angle in transmitting steered ultrasound wave. The sampling angle causes an unwanted side-lobe artifact if the sampling interval of angle is not small enough to cancel the artifact.