Ultrasound technologies such as high-intensity focused ultrasound and acoustic radiation force imaging require advanced or sophisticated transducer designs. Oftentimes, these designs have transducer requirements of wide power ranges, high sensitivity, and broad bandwidth. However, it would often times be desirable to use the same transducer for both. The objective of this proof-of-concept study is to demonstrate the feasibility of using active damping of air-backed, narrowband transducers to achieve broadband capability. Active damping is accomplished through the use of arbitrary waveform generators to cancel subsequent oscillations beyond the initial excitation. A modified 1-D KLM model written in Matlab is used to guide the design of the waveforms. Optimization of the waveforms is applied to the KLM model using minimization function that minimizes ripple. In the model, - 3 dB transmitted bandwidth increased from 10% to 44% for 1.5-cycle excitation and 11.4% to 63.8% for 1-cycle excitation. Comparable increases in bandwidth were also observed experimentally.
This paper presents a novel algorithm for reconstructing and visualizing ablated volumes using radiofrequency ultrasound echo data acquired with the electrode vibration elastography approach. The ablation needle is vibrated using an actuator to generate shear wave pulses that are tracked in the ultrasound image plane at different locations away from the needle. This data is used for reconstructing shear wave velocity maps for each imaging plane. A C-plane reconstruction algorithm is proposed which estimates shear wave velocity values on a collection of transverse planes that are perpendicular to the imaging planes. The algorithm utilizes shear wave velocity maps from different imaging planes that share a common axis of intersection. These C-planes can be used to generate a 3D visualization of the ablated region. Experimental validation of this approach was carried out using data from a tissue mimicking phantom. The shear wave velocity estimates were within 20% of those obtained from a clinical scanner, and a contrast of over 4 dB was obtained between the stiff and soft regions of the phantom.
Wall stress analysis of abdominal aortic aneurysms is a novel tool that has proven high potential to improve risk stratification of abdominal aortic aneurysms (AAAs). Wall stress analysis is based on computed tomography (CT) and magnetic resonance imaging, however, 3D ultrasound (US) has not been used yet. In this study, the feasibility of 3D US based wall stress analysis is investigated and compared to CT.Three-dimensional US and CT data were acquired in 15 patients (diameter 35 - 90 mm). US data were segmented manually and compared to automatically acquired CT geometries by calculating the similarity index (SI) and Hausdorff distance (HD). Wall stresses were simulated at p = 140 mmHg using a non-linear material model (Raghavan & Vorp).The SI of US vs CT was 0.75 - 0.91, with a median HD of 5 - 15 mm, with the higher values found at the proximal and distal sides of the AAA. Wall stresses were in accordance with literature and a good agreement was found between US and CT-based median stresses and inter-quartile stresses. Stress values of US were typically higher, the result of geometrical irregularities, caused by the manual segmentation of the US data. In future work, an automated segmentation approach is the essential point of improvement.
Clinical diagnostic ultrasound has been known as one of the safest imaging modalities available, yet very little is known about the cellular response to such acoustic conditions. With the increased interest in therapeutic ultrasound it is becoming ever more important to understand the effects of ultrasound on cells.In our work here we investigate the effect of clinical diagnostic ultrasound on several cell signalling proteins (p38 p-Thr180/p-Tyr182, ERK 1/2 p-Thr202/p-Tyr204 and p53 ac-Lys382) on leukaemia cells (MOLM-13) and monocytes.Our results show that leukaemia cells and monocytes react differently to ultrasound and microbubbles. A relatively small increase in p38 signalling was seen in the leukemic cells, and only at higher intensities in combination with microbubbles. In contrast the monocytes showed an increase in p38 signalling at all acoustic intensities with microbubbles and at the high acoustic intensity without microbubbles.Furthermore, the leukemic cells showed an overall increase in ERK1/2 signalling whereas the monocytes showed a decrease. These results indicate that the leukaemia cells are less sensitive to stress induced by ultrasound and microbubbles when compared to normal monocytes.In conclusion, our results show that clinical diagnostic ultrasound does have a measurable effect on intracellular signalling but may differ drastically between different cell types. This may affect the conditions necessary for therapeutic ultrasound.
To obtain high resolution ultrasound images, transducers able to operate at high frequencies are required. Optically generated ultrasound utilising the optoacoustic effect is a promising alternative to piezoelectric transducers to achieve this. To use optically generated ultrasound for imaging, a method to spatially steer and focus the acoustic pulses is desirable. In this paper, the use of binary amplitude holograms to focus broadband ultrasound pulses generated by a pulsed laser was investigated. This was done experimentally with patterned absorbers and in simulation. It was found that applying two or more laser pulses applied to a hologram at its design frequency was sufficient to form a focus. The position of this focus could be moved in 3-D by changing the hologram. Additional focal points to those designed for were also found in both the simulation and experimental data. These were generated by constructive interference of harmonics of the pulsing frequency. Simulations found that increasing hologram resolution and applying greater numbers of laser pulses to the hologram decreased the volume of the primary focal point, and that the volume of this focus decreased more rapidly with increasing laser pulse numbers at higher hologram resolutions.
A versatile silicon-based ultrasonic nebulizer that utilizes a twin-nozzle of multiple Fourier horns at 1-2 MHz drive frequencies has been realized to perform simultaneous aerosolization of cobinamide and magnesium thiosulfate drug solutions. The drive frequency of the individual nozzle for a desirable aerosol diameter was individually designed. Using the 2.0 MHz 4-Fourier horn twin-nozzle aerosols of the two drug solutions with mass median diameter (MMD) of 3.0±0.1μm and geometrical standard deviation (GSD) of 1.18±0.02 and total flow rate up to 400μL/min were produced.
Flexural transducers use the bending modes in a plate or membrane to produce sound in low acoustic impedance media. Traditionally, piezoelectrically actuated flexural transducers have been used to generate ultrasound with large amplitude for a relatively low excitation voltage. In this work, the use of electrodynamic forces generated by a current carrying coil is investigated, as an alternative method for generating ultrasound by flexural vibrations. Using a coil instead of a piezoelectric element makes the transducer easier to manufacture, and able to operate at high temperatures. The analytical theory of vibrating plates as well as finite element modelling was used to predict transducer behaviour, i.e. mode frequencies and shapes of the vibrating front face. Prototype transducers were made from aluminium with a pancake copper coil at the back for generation. A Polytec laser vibrometer was used to measure the front face displacement of these prototype transducers. The displacement measurements revealed a frequency spectrum with narrowband (~3 kHz full width half maximum) modal frequency peaks, and a dominant fundamental mode at ~50 kHz. The spectrum is in good agreement with calculated frequency values, and the experimental mode shapes are similar to those predicted by theory.
This paper presents the application of the one-bit time reversal technique to a longitudinal strain sensor. The setup consists of a pair of piezoelectric transducers bonded in the extremities of a strip of aluminum plate. When the plate is subjected to traction, time reversal focalization is performed, the mismatch between the impulse response at initial and strained levels causes loss in the focusing quality. The strain can be evaluated by measuring either the time of flight shift or the amplitude decrease in the focused signal. One-bit time reversal can simplify the electronic device to perform the proposed technique. In this work, the results using one-bit and normal time reversal implementation were compared. Experiments were performed using three different 2-2 piezocomposite transducers pairs at 500, 1000 and 2250 kHz. The longitudinal strain was applied up to 150 μ-strain using a strain gauge as a reference. The time reversal energy efficiency was used as a spectrum figure of merit and obeys the sensitivity behavior. The one-bit time reversal variation provided good focused signal for all experiments and no significant loss in focus quality. Moreover, every configuration showed a higher sensitivity than its normal time reversal version, at least 10% depending on the transducer. The one-bit technique reveals an important enhancement for the method; it holds the natural advantage of being simpler and the benefit of higher sensitivity.
Ultrasonic standing wave manipulation has many promising applications in cell biology, such as noncontact investigation of cell and tissue mechanics. In this paper, recent progress in developing a high frequency resonant chamber using a lithium niobate transducer is presented. This device is designed to sit on a petri dish or microscope slide, with the propagation direction parallel to the dish surface, in a configuration compatible with an optical microscope. It comprises a high frequency ultrasonic transducer with a low acoustic impedance transducer mounting, a polished reflector, and a set of precision spacers between the reflector and transducer. The prototype device demonstrates the feasibility of trapping microparticles with ultrasound radiation forces in multiple trapping sites, and the short wavelength reduces the separation of trapping sites to the same order as the cell dimensions. The basic design of device was validated with one dimensional modelling and finite element simulation. Experimental results of trapping 10 µm polystyrene beads correspond to simulated pressure distributions showing multiples of half-wavelength standing waves.
We developed new miniature 2D array ring transducers integrated into interventional device catheters such as deploy atrial septal occluders. One transducer consisted of 55 elements operating near 5 MHz. The interelement spacing is 0.20 mm. It was constructed on a flat piece of copper-clad polyimide and then wrapped around an 11 French catheter. We used a braided wiring technology from Tyco Electronics Corp to connect the elements to our real-time 3D ultrasound scanner. Typical measured transducer element bandwidth was 20% centered at 4.7 MHz. We built another 55 element transducer with a single matching layer. The matching layer is from EBL Products, Inc. and is the #107 matching layer, .19 mm thick. The matching layer plate was metallized on both sides with 500 Å of nickel and 1000 Å of gold prior to bonding to the PZT layer with Epotek 301 Epoxy. The resulting average bandwidth was 60% centered at 4.4 MHz.
The use of hollow support stems to reduce energy loss to the substrate while supporting all-polysilicon UHF micromechanical disk resonators has enabled quality factors as high as 56,061 at 329 MHz and 93,231 at 178 MHz - values now in the same range as previous disk resonators employing multiple materials with more complex fabrication processes. With a substantially smaller cross-sectional area compared with the full stems used by predecessors, the hollow stem of this work effectively squeezes the energy conduit between the disk structure and the substrate, thereby suppressing energy loss and maximizing Q for devices operating in radial-contour and whispering gallery modes. Measurements confirm Q enhancements of 2.6× for contour modes at 154 MHz and 2.9× for wine glass modes around 112 MHz over values previously achieved by full stem all-polysilicon disk resonators with identical dimensions. The measured results not only demonstrate an effective Q-enhancement method with minimal increase in fabrication complexity, but also provide insights into anchor loss mechanisms that have been largely responsible for limiting the Q's attainable by all-polysilicon capacitively-transduced MEMS resonators.
This paper presents a numerical method for calculating the frequency response of a CMUT with a large number of cells. In a multi-cell configuration, commonly found in CMUTs, each cell is affected by the acoustic loading from neighboring cells. Thus, for an accurate model of a multi-cell CMUT element it is better to consider the mutual acoustic impedance instead of the acoustic impedance of a single cell only. We calculate the velocity of every cell (plate movement) simultaneously, with the mutual impedance effects taken into account. The model predicts that the cells exhibit different frequency responses, based on their locations in the element. We used a laser interferometer to validate the model by measuring the displacement response of a CMUT immersed in vegetable oil. The device has 169 circular cells (single crystal silicon plates, 500 nm thick, 21 μm radii) placed in a hexagonal cell arrangement. The measurement results agree well with the numerical results. The computation time of our method is significantly shorter than finite element based calculations. Our model can be used for finding optimized cell configurations for CMUTs utilized in various applications such as medical imaging and therapeutic treatment.
In this work, we demonstrate 3-D photoacoustic imaging of optically absorbing targets embedded as deep as 5 cm inside a highly scattering background medium using a 2-D capacitive micromachined ultrasonic transducer (CMUT) array with a center frequency of 5.5 MHz. 3-D volumetric images and 2-D maximum intensity projection images are presented to show the objects imaged at different depths. Due to the close proximity of the CMUT to the integrated frontend circuits, the CMUT array imaging system has a low noise floor. This makes the CMUT a promising technology for deep tissue photoacoustic imaging.
A conventional capacitive micromachined ultrasonic transducer (CMUT) is composed of many cells connected in parallel. Since the plate in each CMUT cell is anchored at its perimeter, the average displacement is several times smaller than the displacement of an equivalent ideal piston transducer. In addition, the post areas, where the plates are anchored to, are non-active and, thus, do not contribute to the transduction. We propose a CMUT structure that resembles an ideal capacitive piston transducer, where the movable top plate only undergoes translation rather than deflection. Our proposed CMUT structure is composed of a rigid plate connected to a substrate using relatively long and narrow posts, providing the spring constant for the movement of the plate. Rather than the flexure of the plate as in a conventional CMUT, this device operates based on the compression of the compliant posts. For a capacitive transducer, a thin electrostatic gap is provided under the top plate. We used finite element analysis (FEA) to design and verify the structure's functionality. The simulation results show a fractional bandwidth of over 100% in immersion for all the designs. They also confirm that the average displacement of the top plate is above 90% of its peak displacement. We fabricated the first prototype based on this idea, which only requires a simple 3-mask fabrication process. In addition to 128 element 1-D arrays, we fabricated a variety of 240 μm × 240 μm, single-element transducers with different post configurations. We successfully measured the electrical input impedance of the fabricated devices and confirmed their resonant behavior in air. Further, we measured the acoustic pressure using a calibrated hydrophone at a known distance. Using this measurement, we calculated a peak-to-peak pressure of 1.5 MPa at the face of the transducer. Our results show that it is possible to fabricate CMUTs that exhibit ideal piston-like plate movement. Because - - of the substrate-embedded spring elements, the plate does not need to be operated in flexural mode, as in a conventional CMUT, resulting in a significantly improved fill-factor, and, thus, a more efficient device.
In 3-D ultrasound imaging where 2-D transducer arrays with more than hundreds of elements are used, sparse arrays can be used to reduce the number of active ultrasound channels. Under a restriction of desired number of active channels, we can maximize the image quality by optimally choosing the positions of active elements. Here we use the method of simulated annealing to find the optimal configuration of a 2-D sparse array. This algorithm tries to minimize the value of an objective function defined as the energy ratio between the nonfocal and focal regions in the point spread function (PSF). Optimal configurations were found for the cases of choosing 16, 20, 24, 28, and 32 transmit and receive elements from a 16×16-element rectangular transducer array. With only 32 transmit and 32 receive elements, we could achieve an energy ratio of 16%, compared to 6% of the full array, which is the gold standard utilizing all the 256 elements for both transmit and receive. Using Field II, we simulated imaging with the optimal sparse arrays, for off-axis targets as well as on-axis targets, and the resulting images were compared with those from some other configurations, such as full-transmit full-receive, full-transmit x-receive, x-transmit boundary-receive, and so on.
This paper proposes a compressed sensing method adapted to 3D ultrasound (US) imaging. Three undersampling patterns suited for 3D US imaging, together with a nonlinear conjugate gradient reconstruction algorithm of the US image k spaces, are investigated in vivo radio-frequency 3D US volumes. Reconstructions from 50% of the samples of the original 3D volume show little information loss in terms of normalized root mean squared errors.
Recently, a new standard has been proposed to augment the performance of future Global Positioning System (GPS) receivers, and is generally referred to as the Global Navigation Satellite System (GNSS) band. By also requiring differential signals from the output of the filter to the cell phone receiver, further performance improvements can be obtained. To accommodate the new GNSS specifications and to provide single-ended to differential (SE-DE) mode conversion, new filter designs are required. In this work, we describe a two-stage, SE-DE filter design for GNSS that is based on coupled resonator filter (CRF) technology. The design provides insertion loss better than -2.5 dB over a frequency range greater than 56 MHz. Out-of-band rejection is greater than 40 dB from DC to 6 GHz, and better than 60 dB below 2 GHz. As a stand-alone filter, excellent phase and amplitude balance are achieved without the need for external inductors. In a frontend module consisting of the filter and an Avago LNA, we have measured >;13 dB gain over the GNSS band. The module also shows excellent rejection and true differential output equivalent to surface acoustic wave filter solutions.
Real-time beam predictions are highly desirable for the patient-specific computations required in ultrasound therapy guidance and treatment planning. To address the longstanding issue of the computational burden associated with calculating the acoustic field in large volumes, we use graphics processing unit (GPU) computing to accelerate the computation of monochromatic pressure fields for therapeutic ultrasound arrays. In our strategy, we start with acceleration of field computations for single rectangular pistons, and then we explore fast calculations for arrays of rectangular pistons. For single-piston calculations, we employ the fast near-field method (FNM) to accurately and efficiently estimate the complex near-field wave patterns for rectangular pistons in homogeneous media. The FNM is compared with the Rayleigh-Sommerfeld method (RSM) for the number of abscissas required in the respective numerical integrations to achieve 1%, 0.1%, and 0.01% accuracy in the field calculations. Next, algorithms are described for accelerated computation of beam patterns for two different ultrasound transducer arrays: regular 1-D linear arrays and regular 2-D linear arrays. For the array types considered, the algorithm is split into two parts: 1) the computation of the field from one piston, and 2) the computation of a piston-array beam pattern based on a pre-computed field from one piston. It is shown that the process of calculating an array beam pattern is equivalent to the convolution of the single-piston field with the complex weights associated with an array of pistons. Our results show that the algorithms for computing monochromatic fields from linear and regularly spaced arrays can benefit greatly from GPU computing hardware, exceeding the performance of an expensive CPU by more than 100 times using an inexpensive GPU board. For a single rectangular piston, the FNM method facilitates volumetric computations with 0.01% accuracy at rates better than 30 ns per field point. Furthermore, we demonstrate array calculation speeds of up to 11.5 × 10 9 field-points per piston per second (0.087 ns per field point per piston) for a 512-piston linear array. Beam volumes containing 256 3 field points are calculated within 1 s for 1-D and 2-D arrays containing 512 and 20 2 pistons, respectively, thus facilitating future real-time thermal dose predictions.
We introduce a versatile fabrication process for direct wafer-bonded CMUTs. The objective is a flexible fabrication platform for single element transducers, 1D and 2D arrays, and reconfigurable arrays. The main process features are: A low number of litho masks (five for a fully populated 2D array); a simple fabrication sequence on standard MEMS tools without complicated wafer handling (carrier wafers); an improved device reliability; a wide design space in terms of operation frequency and geometric parameters (cell diameter, gap height, effective insulation layer thickness); and a continuous front face of the transducer (CMUT plate) that is connected to ground (shielding for good SNR and human safety in medical applications). All of this is achieved by connecting the hot electrodes individually through a thick buried oxide layer, i.e. from the handle layer of an SOI substrate to silicon electrodes located in each CMUT cell built in the device layer. Vertical insulation trenches are used to isolate these silicon electrodes from the rest of the substrate. Thus, the high electric field is only present where required - in the evacuated gap region of the device and not in the insulation layer of the post region. Array elements (1D and 2D) are simply defined be etching insulation trenches into the handle wafer of the SOI substrate.
The ability of ultrasound transducers to operate over a wide and varying pressure range is essential in applications such as ultrasonic flow metering (UFM) of flare gas. We propose a new operational mode for capacitive micromachined ultrasonic transducers (CMUTs), in which the plate is in permanent contact with the bottom of the cavity, even at zero DC bias and 1 atm pressure. Finite element analysis (FEA) software was used to investigate the performance of these CMUTs within the pressure range of 1 to 20 atm. First, we performed a static analysis to determine the plate deflection and, thus, the gap height. Further, from the static analysis, we obtained the static and free capacitances for calculating the coupling efficiency, and a modal analysis identified possible design geometries for frequencies lower than ~ 300 kHz. Our calculations show that conventionally operated CMUTs have huge changes in static operational point at different pressures, while our proposed mode exhibits an acceptable frequency range (73 - 340 kHz) over 1 - 20 atm pressure and an improved coupling efficiency at lower dc bias voltages. A donut shape partial electrode further allows us to tune the coupling efficiency, which translates into a better performance, especially at the higher pressure range. FEA shows that our proposed operation mode is a promising solution for flare gas metering applications.
