John Fraser

Fabrication of capacitive micromachined ultrasonic transducers by micro-stereolithography

An ultrasonic transducer is formed by a plurality of cMUT cells, each comprising a charged diaphragm plate capacitively opposing an oppositely charged base plate. The cMUT cells can be fabricated by conventional semiconductor processes and hence integrated with ancillary transducer circuitry such as a bias charge regulator. The cMUT cells can also be fabricated by micro-stereolithography whereby the cells can be formed using a variety of polymers and other materials.

Piezoelectric micromachined ultrasonic transducers: modeling the influence of structural parameters on device performance

Piezoelectric micromachined ultrasonic transducers (pMUTs), a potential alternative for conventional one-dimensional phased array ultrasonic transducers, were investigated. We used a modeling approach to study the performance of lead zirconate titanate (PZT)-driven pMUTs for the frequency range of 2-10 MHz, optimized for maximum coupling coefficient, as a function of device design. Using original tools designed for the purpose, a comprehensive build-test finite element model was developed to predict and measure the device performance. In particular, the model estimates the device coupling coefficient and the acoustic impedance, besides the readily extractable resonance frequency and bandwidth. To validate the model, a prototype device was built and tested, showing good agreement between the model predictions and experimental results. Modeling results indicate that the coupling coefficient is significantly affected by silicon membrane, PZT, and top electrode thickness as well as the top electrode design. Results also indicate considerable flexibility in maximizing the coupling coefficient while maintaining the device acoustic impedance at a level matching that of the human body. The bandwidth proved to be superior to that of conventional transducers, reaching 102% in some cases.

Ultrasonic transducer probe with heat dissipating lens

A multiplane TEE probe is provided in which an aluminum sheet is embedded in the acoustic lens in front of the transducer to dissipate heat which accumulates in the lens. The embedded aluminum foil sheet in front of the transducer helps reduce such heat buildup by spreading out heat which develops at hot spots and conducting the heat trapped in the lens material to a heatsink mass behind the transducer and away from the patient contacting surface of the probe. A mylar sheet covers the front of the transducer. The inner surface of the mylar sheet is aluminized. This aluminum layer in the inner surface of the cover serves a like purpose.

Finite‐element method for determination of electromechanical coupling coefficient for piezoelectric and capacitive micromachined ultrasonic transducers

Research has been reported on ultrasonic transduction using capacitive micromachined ultrasonic transducers (cMUTs). These are thin membranes of diameters of order 100 μm suspended over a silicon substrate with a gap of order 1 μm. The devices become transducers when a bias voltage is applied, causing electrostatic forces to draw the membranes closer to the substrate. Operation is by applying an electrical signal to excite a vibration in the membrane, or an ultrasonic wave to excite an ac voltage. The cMUT literature includes equivalent circuits and studies on losses and spurious waves, and discussion of output, sensitivity, and bandwidth. Transducer engineers would like to see analogies to the properties of piezoelectric transducers, in order to compare the cMUTs to traditional designs. An extension of the definition of the piezoelectric coupling coefficient to an effective coupling coefficient for cMUT transducers is presented. A finite‐element method has been developed using the pzflex software package...

Phase aberration in Shear Wave Dispersion Ultrasound Vibrometry

Shearwave Dispersion Ultrasound Vibrometry (SDUV) is an acoustic radiation force based technique that measures tissue shear viscoelasticity by characterizing shear wave speed dispersion. An application of this technique is liver fibrosis staging. We previously reported findings from an animal study where shear modulus and viscosity reconstruction displayed larger variances for in vivo versus ex vivo cases. This study investigates two major causes of such increased variance, namely attenuation and phase aberration. Two sets of experiments were conducted using a custom phantom. In the first experiment, the phantom was imaged directly with varying pushing power by setting system transmit attenuation at different levels from 0 dB to 6 dB. The second set of experiments utilized different pieces of pork bellies as aberrators between the probe and the phantom, while maintaining the pushing power at 0 dB. For each data set, SDUV reconstruction algorithms yielded shear moduli within a region of interest (ROI) of 10 mm × 4 mm close to the pushing focus. The attenuation experiment showed that the variance in SDUV reconstruction results did not start to increase until the peak displacement dropped to 2.2 μm. On the other hand, insertion of an aberrator caused elevated variances even at a much higher peak displacement of 3.9 μm. The variances also swung greatly among different data sets with similar peak displacements. Moreover, thinner aberrators produced consistently better results even with lower peak displacements. All these observations indicate that phase aberration induced waveform distortion is more detrimental to SDUV than pure attenuation. It is beneficial to investigate phase aberration correction methods and apply them to improve SDUV performance.

Finite Element Modeling for Shear Wave Elastography

Shear wave elastography is an important imaging modality to evaluate tissue mechanical properties and supplement conventional ultrasound diagnostic imaging. A 3D finite element model has been created in PZFlex for simulating and understanding shear wave generation by the acoustic radiation force, and its propagation through different media. The simulation settings were based on a shear wave elastography prototype using a Philips iU22 scanner with a C5-1 curvilinear probe. The modeling process was divided into two steps. In the first step, the acoustic field of the ultrasound probe was calculated and the output acoustic radiation stress (ARS) result in the 3D volume was saved. In the second step, the ARS data was applied as a boundary condition to generate the shear wave. The shear wave displacement time profiles in the region of interest were recorded at the end of the second step. The simulation was performed for different media, including uniform tissues with various shear moduli and viscosities, as well as uniform tissue background with an embedded stiffer inclusion. Clear differences were observed on the shear wave displacement time profiles, as the displacement peak was attenuated and widened by the higher shear modulus and viscosity. The simulation results were also cross-checked with elasticity reconstruction algorithms based on wave equation (WE), Voigt model (VM) and time-to-peak (TTP) methods. For a medium similar to normal liver tissue with 2KPa shear modulus, all three reconstruction methods reported shear modulus approximately the same as input value when the viscosity was negligible (WE: 2.05KPa, VM: 2.06KPa, TTP: 2.12KPa). With increased viscosity in the medium (2KPa, 2PaS), TTP seemed to under-estimate shear modulus in the near-field (WE: 2.41KPa, VM: 1.98KPa & 2.11PaS, TTP: 1.38KPa). For a uniform medium with an embedded spherical inclusion, all three methods successfully detected the inclusion and reconstructed stiffness maps. The results suggested that the finite element modeling could provide valuable insight in simulating and understanding shear wave generation and propagation. It could also be an important tool to evaluate and analyze stiffness reconstruction algorithms for shear wave elastography.

Medical and industrial applications of ultrasonic imaging

Ultrasonic imaging has found widspread use in medical and industrial applications. Ultrasound in the frequency range of 1–10 MHz is used to image soft tissue in the body, a medium with many weak scatterers, acoustic impedance and velocity similar to water, and attenuation on the order of 0.8 dB/cm/MHz. Resolution is limited by depth of field and depth of penetration considerations, but typically is several times greatest than the diffraction limit. Imaging systems typically are operated in a pulse‐echo mode using either mechanically focused and scanned single element transducers or electronically focused and scanned transducer arrays. Most transducers are designed with octave bandwidths to achieve short pulses for range resolution and near 100% transduction efficiency for maximum sensitivity. Industrial applications are characterized by media of widely varying propagation modes, velocities, impedances, and attenuations. Consequently, industrial systems operate at frequencies ranging from 100 kHz to 1 GHz....