Abstracts:Though it has been recognized that the sensitivity of hydrophones depends on the temperature of the water they are used in, the amount of specific data that is available is very limited. This is possible because the measurements are technically challenging, laborious, and time-consuming. A broadband primary hydrophone calibration setup was extended to implement stable calibration conditions at different temperature settings. Four hydrophones of different types commonly used in ultrasound exposimetry were then characterized in the range from 17 °C to 29 °C, and average change rates of the sensitivity with temperature were determined for different ultrasonic frequencies. Two different membrane hydrophones showed an increase in sensitivity with increasing temperature in the range from 0.55% to 1.10% per 1 °C temperature rise within their bandwidths. The results for a capsule-type and a needle-type hydrophone were different in the sense that a decreasing sensitivity with increasing temperature was also observed. For the capsule-type hydrophone, a small increase was observed up to 15 MHz and a decrease for higher frequencies. The needle-type hydrophone provided a decrease at all frequencies, and the results were more noisy. Data, as determined in this study, can be applied to correct acoustic output measurements of medical ultrasonic equipment if the water temperature of the hydrophone application differs from that during calibration. Alternatively, it may suffice in some applications to consider a sensitivity change with temperature within the uncertainty estimation of the exposure measurement, in particular, if the temperature difference is limited to 1 °C or 2 °C.

Abstracts:This article proposes a double metal dots interdigital transducer (IDT) configuration for suppression of spurious responses in temperature-compensated surface acoustic wave (SAW) resonators employing the SiO2/LiNbO3 structure. The proposed IDT configuration includes primary metal dots at the edges of the IDT aperture region, and in addition, secondary metal dots placed near the busbars with the addition of short dummy electrodes. This double metal dots IDT configuration enables us to effectively suppress two kinds of spurious resonances: the transverse resonances that are common and the ones that are predominantly excited in the gap regions between IDT finger edges and busbars. The impact of geometric parameters on suppressing spurious responses is studied using periodic 3-D finite element simulations. Then, it is shown both theoretically and experimentally that the proposed mechanism can effectively suppress almost all spurious responses without affecting the critical performance metrics such as the quality factor and electromechanical coupling coefficient.

Abstracts:Holographic acoustic tweezers have various biomedical applications due to their ability to flexibly and rapidly synthesize acoustic fields for manipulating single or multiple particles. Existing multiparticle manipulation techniques are usually realized by precisely designing the incident wave’s phase distribution to synthesize a complex and steady-state acoustic field containing multiple acoustic trapping beams. However, interference effects between multiple beams tend to produce artifacts that trap particles in unwanted positions, limiting accuracy, and the number of manipulated particles. In addition, those techniques can only holistically manipulate multiple particles, namely, lacking parallel working ability. In this study, we proposed a time-sharing acoustic tweezer method to achieve the manipulation of multiple particles by rapidly switching individual trapping beams, minimizing interference artifacts. We applied this method to a 256-element phased-array acoustic tweezer system with designed ultrasonic pulse sequences to synthesize a single focused, twin trap, and vortex beam, enabling the pseudo-parallel manipulation of multiple particles in 3-D space at a beam switching frequency of ≥10 kHz. The experiments on polydimethylsiloxane particles ranging from micrometers to millimeters in diameter demonstrated that up to 96 particles can be successfully trapped and assembled into a 2-D lattice. Different numbers of particles were also patterned into dynamic contours, such as sinusoidal vibration (ten particles) and butterfly flapping (24 particles). In addition, the trapped multiple particles can also be rotated around their respective orbits. The proposed technique improved the number of objects dynamically manipulated in a parallel manner, advancing holographic acoustic tweezers and their applications.

Abstracts:Mapping wave field in space has many applications such as optimizing design of radio antennas, improving and developing ultrasound transducers, and planning and monitoring the treatment of tumors using high-intensity focused ultrasound (HIFU). Currently, there are methods that can map wave fields remotely or locally. However, there are limitations to these methods. For example, when mapping the wave fields remotely, the spatial resolution is limited due to a poor diffraction-limited resolution of the receiver, especially when the f-number of the receiver is large. To map the wave fields locally, the receiver is either subject to damage in hazardous environments (corrosive media, high temperature, high wave intensity, and so on) or difficult to be placed inside an object. To address these limitations, in this article, the point spread function (PSF)-modulation super-resolution imaging method was applied to map pulse ultrasound wave fields remotely at a high spatial resolution, overcoming the diffraction limit of a focused receiver. For example, to map a pulse ultrasound field of a full-width-at-half-maximum (FWHM) beamwidth of 1.24 mm at the focal distance of a transmitter, the FWHM beamwidths of the super-resolution mapping of the pulse wave field with a spherical glass modulator of 0.7 mm diameter at two receiver angles (0° and 45°) were about 1.13 and 1.22 mm, respectively, which were close to the theoretical value of 1.24 mm and were much smaller than the diffraction-limited resolution (1.81 mm) of the receiver. Without using the super-resolution method to remotely map the same pulse wave field, the FWHM beamwidth was about 2.06 mm. For comparison, the FWHM beamwidth obtained with a broadband (1–20 MHz) and 0.6-mm-diameter polyvinylidene fluoride (PVDF) needle hydrophone was about 1.41 mm. In addition to the focused pulse ultrasound wave field, a pulse Bessel beam near the transducer surface was mapped remotely with the super-resolution method, which revealed high spatial frequency components of the beam.

Abstracts:In this study, we design and implement pulses [1.67 MHz, 20–1000 cycles, 0.8–2.5 MPa, and 5–100 ms pulse repetition time (PRT)] suitable for microbubble cavitation treatments with a phased array of a clinical ultrasound scanner. A range of acoustic parameters was evaluated in a tissue-mimicking phantom with suspended Sonazoid microbubbles. Hydrophone measurements were used to optimize the transmit beamforming. A passive cavitation detection (PCD) system was designed to measure the microbubble scattered signals over a 1 s exposure. Postprocessing of the scattered signals evaluated frequency content to extract broadband energy and calculate the inertial cavitation dose (ICD). ICD was maximized at 1000 cycles (maximum pulse length), 5 ms (fastest firing rate), and 2.5 MPa peak negative pressure (PNP) (maximum pressure). Inertial cavitation was only sustained for about three pulses (out of hundreds fired) occurring within the first 100 ms of treatment. Temporal analysis of the first 1000-cycle pulse revealed that broadband energy is sustained for the entire pulse. We also demonstrate that while inertial cavitation is possible with clinically available pulse wave Doppler settings, ICD can be significantly increased using the new conditions suggested in this work. We have delivered successful image-guided cavitation treatment after modifying a clinical scanner and monitored the cavitation dose with a PCD system on a gel phantom with suspended microbubbles. We plan to apply this technique in vivo in animal tumor models next. This work demonstrates the first implementation of long, high-pressure pulses on a clinical scanner that users can optimize for cavitation treatments.

Abstracts:Quantitative ultrasound (QUS) holds promise in enhancing diagnostic accuracy. For attenuation imaging, the regularized spectral log difference (RSLD) can generate accurate local attenuation maps. However, the performance of the method degrades when significant changes in backscatter amplitude occur. Variations in the technique were introduced involving a weighted approach to backscatter regularization, which, however, is not effective when changes in both attenuation and backscatter are present. This study introduces a novel approach that incorporates an L1-norm for backscatter regularization and spatially varying weights for both fidelity and regularization terms. The weights are calculated from an initial estimation of backscatter changes. Comparative analyses with simulated, phantom, and clinical data were performed. When changes in backscatter and attenuation occur, the proposed approach reduced the lowest root mean square error by up to 73%. It also improved the contrast-to-noise ratio (CNR) by a factor of 4.4 on average compared with previously available methods, considering the simulated and phantom data. In vivo results from healthy livers, thyroid nodules, and a breast tumor further confirm its effectiveness. In the liver, it is shown to be effective at reducing artifacts of attenuation images. In thyroid and breast tumors, the method demonstrated an enhanced CNR and better consistency of the attenuation measurements with the posterior acoustic enhancement. Overall, this approach offers promise for enhancing ultrasound attenuation imaging by helping differentiate tissue characteristics that may indicate pathology.

Abstracts:Ultrafast plane-wave (PW) ultrasound imaging is a versatile tool that has become increasingly relevant for blood flow imaging using speckle tracking but suffers from a low signal-to-noise ratio (SNR). Cascaded dual-polarity wave (CDW) imaging can improve the SNR by transmitting pulse trains, which are subsequently decoded to recover the imaging resolution. However, the current decoding method (in the time domain) requires a set of two acquisitions, which introduces motion artifacts that result in incorrect speckle tracking at high flow velocities. Here, we evaluate an inverse filtering approach that uses frequency-domain decoding to decode acquisitions independently. Experiments using a disk phantom show that frequency-domain decoding of a four-pulse train achieves an SNR gain of up to 4.2 dB, versus 5.9 dB for conventional decoding. The benefit of frequency-domain decoding for flow quantification is assessed through experiments performed with a rotating disk phantom and a parabolic flow, and through matching linear simulations. Both CDW methods improve the tracking accuracy compared to single PW imaging. Time-domain decoding outperforms frequency-domain decoding in low SNR conditions and low velocities ( $\leq 0.25$ m/s), as a result of the higher SNR gain. In contrast, frequency-domain decoding outperforms time-domain decoding for high peak velocities in imaging of the rotating disk (1 m/s) and of the parabolic flow (2 m/s), when significant scatterer motion between acquisitions causes imperfect time-domain decoding. Its ability to decode individual acquisitions makes the used frequency-domain decoding of CDW (F-CDW) a promising approach to improve the SNR and thereby the accuracy of flow quantification at high velocities.

Abstracts:Locally disturbed blood flow patterns are known to create an atherogenic environment, particularly in the presence of other cardiovascular risk factors. Given the geometry of a healthy carotid artery, complex flow patterns are expected to be present. This study aims to characterize (complex) blood flow patterns and estimate flow-derived parameters in the carotid bifurcation of healthy subjects. Ultrasound-based velocity vector imaging (US-VVI) was acquired in the carotid bifurcation of 20 healthy subjects. Hemodynamic parameters, including temporal velocity profile, vector complexity (VC), vortex presence, and wall shear stress (WSS), were derived and compared between two age groups (20–30 and 65–75 years). Lower velocities and higher VC values were observed in the older age group for all timepoints. The highest presence of vortices was observed during the systolic deceleration, which was more exposed in younger subjects (5 out of 10) compared to older subjects (3 out of 9). A quick build-up and consequent resolving of the vortices was reflected by the relatively short vortex duration, with a vortex presence of 11.4% (7.9–15.6) and 13.1% (5.9–18.6) as a percentage of the cardiac cycle in younger and older subjects, respectively. Larger WSS estimates, represented as median along the complete vessel wall, were found in the younger subjects at all timestamps, except at systolic deceleration. In conclusion, the presence of complex flow patterns was confirmed in healthy subjects and multiple flow-derived hemodynamic parameters were evaluated in two age groups, providing an insight into age-related differences in hemodynamics. Aging seemed to result in higher vector complexities, whereas the presence of recirculating flow is less in older subjects.

Abstracts:Expanding the imaging field of view (FOV) of medical ultrasound transducers will more effectively detect pathological behaviors of tissues or organs. Conventional rigid transducers can be realized by increasing the number of array elements or the curvature; however, the imaging aperture is fixed by the original size and shape during the manufacturing process. This article presented a 128-element, 3-MHz flexible curvature abdominal array (FCAA) with the goal of dynamically expanding the FOV within a 120° range. The piezoelectric stack was divided into small pitches through a double-cut process, and two-component viscoelastic substrates (TCVSs) were filled between adjacent array elements to generate tensile and compressive stresses during decomposition deformation. A 3-D-printed push-pull device provides sufficient mechanical support, resulting in a conformal minimum curvature radius of 46 mm. The innovative rigid-flexible composite backing layer was used to balance mechanical flexibility and high bandwidth (BW) of −6 dB to 67.6%. The results showed that the axial and lateral resolutions of the commercial phantom line target are 0.35 and 0.77 mm, respectively, and the axial and lateral resolutions of FOV 120° are 0.36 and 1.02 mm, respectively. The imaging performance of FCAA was verified by B-model imaging of the kidneys, intestines, uterus, and bladder of volunteers with a different body mass index (BMI). In addition, the 5-mm renal artery phantom verified the Doppler imaging function of FCAA. All the results demonstrated that FCPA has great potential clinical value in abdominal ultrasound and gynecological examination.