Holographic Focused Ultrasound Hyperthermia System for Uniform Simultaneous Thermal Exposure of Multiple Tumor Spheroids.

Hyperthermia is currently used to treat cancer due to its ability to radio- and chemo-sensitize and to stimulate the immune response. While ultrasound is non-ionizing and can induce hyperthermia deep within the body non-invasively, achieving uniform and volumetric hyperthermia is challenging. This work presents a novel focused ultrasound hyperthermia system based on 3D-printed acoustic holograms combined with a high-intensity focused ultrasound (HIFU) transducer to produce a uniform iso-thermal dose in multiple targets. The system is designed with the aim of treating several 3D cell aggregates contained in an International Electrotechnical Commission (IEC) tissue-mimicking phantom with multiple wells, each holding a single tumor spheroid, with real-time temperature and thermal dose monitoring. System performance was validated using acoustic and thermal methods, ultimately yielding thermal doses in three wells that differed by less than 4%. The system was tested in vitro for delivery of thermal doses of 0-120 cumulative equivalent minutes at 43 °C (CEM43) to spheroids of U87-MG glioma cells. The effects of ultrasound-induced heating on the growth of these spheroids were compared with heating using a polymerase chain reaction (PCR) thermocycler. Results showed that exposing U87-MG spheroids to an ultrasound-induced thermal dose of 120 CEM43 shrank them by 15% and decreased their growth and metabolic activity more than seen in those exposed to a thermocycler-induced heating. This low-cost approach of modifying a HIFU transducer to deliver ultrasound hyperthermia opens new avenues for accurately controlling thermal dose delivery to complex therapeutic targets using tailored acoustic holograms. Spheroid data show that thermal and non-thermal mechanisms are implicated in the response of cancer cells to non-ablative ultrasound heating.

Spatial resolution and reconstructed size accuracy using advanced beamformers in linear array-based PAT systems.

Limitations associated with linear-array probes in photoacoustic tomography are partially compensated by using advanced beamformers that exploit the temporal and spatial coherence of the recorded signals, such as Delay Multiply and Sum (DMAS), Minimum Variance (MV) or coherence factor (CF), among others. However, their associated signal processing leads to an overestimation of the spatial resolution, as well as alterations in the reconstructed object size. Numerical and experimental results reported here support this hypothesis. First, we show that the Rayleigh criterion (RC) is the most suitable choice to characterize the spatial resolution instead of the Point Spread Function (PSF) when considering advanced beamformers. Then, we observe that several advanced beamformers fail to properly reconstruct target sizes slightly above the spatial resolution, underestimating their size. This work sheds light on the suitability of this type of beamformers combined with linear probes for determining sizes and morphology in photoacoustic images.

Numerical Study of Acoustic Holograms for Deep-Brain Targeting through the Temporal Bone Window.

Acoustic holograms can encode complex wavefronts to compensate the aberrations of a therapeutical ultrasound beam propagating through heterogeneous tissues such as the skull, and simultaneously, they can generate diffraction-limited acoustic images, that is, arbitrary shaped focal spots. In this work, we numerically study the performance of acoustic holograms focusing at the thalamic nuclei when the source is located at the temporal bone window. The temporal window is the thinnest area of the lateral skull and it is mainly hairless, so it is a desirable area through which to transmit ultrasonic waves to the deep brain. However, in targeting from this area the bilateral thalamic nuclei are not aligned with the elongated focal spots of conventional focused transducers, and in addition, skull aberrations can distort the focal spot. We found that by using patient-specific holographic lenses coupled to a single-element 650-kHz-frequency 65-mm-aperture source, the focal spot can be sharply adapted to the thalamic nuclei in a bilateral way while skull aberrations are mitigated. Furthermore, the performance of these holograms was studied under misalignment errors between the source and the skull, concluding that for misalignments up to 5°, acoustic images are correctly restored. This work paves the way to designing clinical applications of transcranial ultrasound such as blood-brain barrier opening for drug delivery or deep-brain neuromodulation using this low-cost and personalized technology, presenting desirable aspects for long-term treatments because the patient's head does not need to be shaved completely and skull heating is low.

Acoustic Holograms for Bilateral Blood-Brain Barrier Opening in a Mouse Model.

Transcranial focused ultrasound (FUS) in conjunction with circulating microbubbles injection is the sole non-invasive technique that temporally and locally opens the blood-brain barrier (BBB), allowing targeted drug delivery into the central nervous system (CNS). However, single-element FUS technologies do not allow the simultaneous targeting of several brain structures with high-resolution, and multi-element devices are required to compensate the aberrations introduced by the skull. In this work, we present the first preclinical application of acoustic holograms to perform a bilateral BBB opening in two mirrored regions in mice. The system consisted of a single-element focused transducer working at 1.68 MHz, coupled to a 3D-printed acoustic hologram designed to produce two symmetric foci in anesthetized mice in vivo and, simultaneously, compensate the aberrations of the wavefront caused by the skull bones. T1-weighed MR images showed gadolinium extravasation at two symmetric quasi-spherical focal spots. By encoding time-reversed fields, holograms are capable of focusing acoustic energy with a resolution near the diffraction limit at multiple spots inside the skull of small preclinical animals. This work demonstrates the feasibility of hologram-assisted BBB opening for low-cost and highly-localized targeted drug delivery in the CNS in symmetric regions of separate hemispheres.

Characterization of Viscoelastic Media Combining Ultrasound and Magnetic-Force Induced Vibrations on an Embedded Soft Magnetic Sphere.

We report a method to locally assess the complex shear modulus of a viscoelastic medium. The proposed approach is based on the application of a magnetic force to a millimeter-sized steel sphere embedded in the medium and the subsequent monitoring of its dynamical response. A coil is used to create a magnetic field inducing the displacement of the sphere located inside a gelatin phantom. Then, a phased-array system using 3 MHz ultrasound probe operating in pulse-echo mode is used to track the displacement of the sphere. Experiments were conducted on several samples and repeated as a function of phantom temperature. The dynamic response of the sphere measured experimentally is in good agreement with Kelvin-Voigt theory. Since the magnetic force is not affected by weak diamagnetic media, our proposal results in an accurate estimation of the force acting on the inclusion. Consequently, the estimated viscoelastic parameters show excellent robustness and the elastic modulus agrees with the measurements using a quasi-static indentation method, obtaining errors below 10% in the whole temperature range. The use of the macroscopic inclusion limits the direct application of this method in a biomedical context, but it provides a robust estimation of the elastic modulus that can be used for material characterization in industrial applications.

Spiral sound-diffusing metasurfaces based on holographic vortices.

In this work, we show that scattered acoustic vortices generated by metasurfaces with chiral symmetry present broadband unusual properties in the far-field. These metasurfaces are designed to encode the holographic field of an acoustical vortex, resulting in structures with spiral geometry. In the near field, phase dislocations with tuned topological charge emerge when the scattered waves interference destructively along the axis of the spiral metasurface. In the far field, metasurfaces based on holographic vortices inhibit specular reflections because all scattered waves also interfere destructively in the normal direction. In addition, the scattering function in the far field is unusually uniform because the reflected waves diverge spherically from the holographic focal point. In this way, by triggering vorticity, energy can be evenly reflected in all directions except to the normal. As a consequence, the designed metasurface presents a mean correlation-scattering coefficient of 0.99 (0.98 in experiments) and a mean normalized diffusion coefficient of 0.73 (0.76 in experiments) over a 4 octave frequency band. The singular features of the resulting metasurfaces with chiral geometry allow the simultaneous generation of broadband, diffuse and non-specular scattering. These three exceptional features make spiral metasurfaces extraordinary candidates for controlling acoustic scattering and generating diffuse sound reflections in several applications and branches of wave physics as underwater acoustics, biomedical ultrasound, particle manipulation devices or room acoustics.

Ultrasonic Monitoring of Dentin Demineralization.

Demineralization is a process of loss of minerals in the dental hard tissue that affects seriously the health of the patients, as it diminishes the tooth resistance, generating chewing problems by altering the occlusal structure, hypersensitivity, and pulpal problems. Demineralization can be produced by pathological processes as erosion or caries, or by surgical processes as etching. Due to the complexity of natural demineralization processes, it is mandatory to provide quantitative and standardized tests to allow their study in controlled laboratory conditions. Ultrasonic techniques are suitable for this purpose as they are nondestructive, quick, and provide localized mechanical information about the tissue, which is related with its degree of demineralization. In the present work, we evaluate the complete process of demineralization of the human dentin under controlled laboratory conditions using a pulse-echo ultrasonic technique. Up to 15 human dentin teeth have been demineralized with phosphoric acid at 10%. The time-of-flight measurements using the pulse-echo system allows to obtain the speed of sound in healthy (3415 m/s) and demineralized dentin tissue (1710 m/s), as well as to characterize the dynamical process of the acid penetration, which generates well-defined boundaries between two media (demineralized and mineralized dentin), showing very different mechanical properties. These boundaries advance in depth at an initial rate of [Formula: see text]/min, decelerating at -9.3 nm/min2 until the whole demineralization of the sample is achieved. In addition, the technique allows to measure the relevance of the demineralization produced by the acid residues inside the tooth once it has been removed from the acidic solution. Beyond the assessment of artificial demineralization lesions under laboratory conditions, as demonstrated in this article, the proposed technique opens new approaches to the assessment of demineralization caused by natural caries in vivo.

Generating Bessel beams with broad depth-of-field by using phase-only acoustic holograms.

We report zero-th and high-order acoustic Bessel beams with broad depth-of-field generated using acoustic holograms. While the transverse field distribution of Bessel beams generated using traditional passive methods is correctly described by a Bessel function, these methods present a common drawback: the axial distribution of the field is not constant, as required for ideal Bessel beams. In this work, we experimentally, numerically and theoretically report acoustic truncated Bessel beams of flat-intensity along their axis in the ultrasound regime using phase-only holograms. In particular, the beams present a uniform field distribution showing an elongated focal length of about 40 wavelengths, while the transverse width of the beam remains smaller than 0.7 wavelengths. The proposed acoustic holograms were compared with 3D-printed fraxicons, a blazed version of axicons. The performance of both phase-only holograms and fraxicons is studied and we found that both lenses produce Bessel beams in a wide range of frequencies. In addition, high-order Bessel beam were generated. We report first order Bessel beams that show a clear phase dislocation along their axis and a vortex with single topological charge. The proposed method may have potential applications in ultrasonic imaging, biomedical ultrasound and particle manipulation applications using passive lenses.

On the Evaluation of the Suitability of the Materials Used to 3D Print Holographic Acoustic Lenses to Correct Transcranial Focused Ultrasound Aberrations.

The correction of transcranial focused ultrasound aberrations is a relevant topic for enhancing various non-invasive medical treatments. Presently, the most widely accepted method to improve focusing is the emission through multi-element phased arrays; however, a new disruptive technology, based on 3D printed holographic acoustic lenses, has recently been proposed, overcoming the spatial limitations of phased arrays due to the submillimetric precision of the latest generation of 3D printers. This work aims to optimize this recent solution. Particularly, the preferred acoustic properties of the polymers used for printing the lenses are systematically analyzed, paying special attention to the effect of p-wave speed and its relationship to the achievable voxel size of 3D printers. Results from simulations and experiments clearly show that, given a particular voxel size, there are optimal ranges for lens thickness and p-wave speed, fairly independent of the emitted frequency, the transducer aperture, or the transducer-target distance.

Monitoring the Setting of Calcium Sulfate Bone-Graft Substitute Using Ultrasonic Backscattering.

We report a method to monitor the setting process of bone-graft substitutes (calcium sulfate) using ultrasonic backscattering techniques. Analyzing the backscattered fields using a pulse-echo technique, we show that it is possible to dynamically describe the acoustic properties of the material which are linked to its setting state. Several experiments were performed to control the setting process of calcium sulfate using a 3.5-MHz transducer. The variation of the apparent integrated backscatter (AIB) with time during the setting process is analyzed and compared with measurements of the speed of sound (SOS) and temperature of the sample. The correlation of SOS and AIB allows us to clearly identify two different states of the samples, liquid and solid, in addition to the transition period. Results show that using backscattering analysis, the setting state of the material can be estimated with a threshold of 15 dB. This ultrasonic technique is indeed the first step to develop real-time monitoring systems for time-varying complex media as those present in bone regeneration for dental implantology applications.

Rainbow-trapping absorbers: Broadband, perfect and asymmetric sound absorption by subwavelength panels for transmission problems.

Perfect, broadband and asymmetric sound absorption is theoretically, numerically and experimentally reported by using subwavelength thickness panels in a transmission problem. The panels are composed of a periodic array of varying crosssection waveguides, each of them being loaded by Helmholtz resonators (HRs) with graded dimensions. The low cut-off frequency of the absorption band is fixed by the resonance frequency of the deepest HR, that reduces drastically the transmission. The preceding HR is designed with a slightly higher resonance frequency with a geometry that allows the impedance matching to the surrounding medium. Therefore, reflection vanishes and the structure is critically coupled. This results in perfect sound absorption at a single frequency. We report perfect absorption at 300 Hz for a structure whose thickness is 40 times smaller than the wavelength. Moreover, this process is repeated by adding HRs to the waveguide, each of them with a higher resonance frequency than the preceding one. Using this frequency cascade effect, we report quasi-perfect sound absorption over almost two frequency octaves ranging from 300 to 1000 Hz for a panel composed of 9 resonators with a total thickness of 11 cm, i.e., 10 times smaller than the wavelength at 300 Hz.

Metadiffusers: Deep-subwavelength sound diffusers.

We present deep-subwavelength diffusing surfaces based on acoustic metamaterials, namely metadiffusers. These sound diffusers are rigidly backed slotted panels, with each slit being loaded by an array of Helmholtz resonators. Strong dispersion is produced in the slits and slow sound conditions are induced. Thus, the effective thickness of the panel is lengthened introducing its quarter wavelength resonance in the deep-subwavelength regime. By tuning the geometry of the metamaterial, the reflection coefficient of the panel can be tailored to obtain either a custom reflection phase, moderate or even perfect absorption. Using these concepts, we present ultra-thin diffusers where the geometry of the metadiffuser has been tuned to obtain surfaces with spatially dependent reflection coefficients having uniform magnitude Fourier transforms. Various designs are presented where, quadratic residue, primitive root and ternary sequence diffusers are mimicked by metadiffusers whose thickness are 1/46 to 1/20 times the design wavelength, i.e., between about a twentieth and a tenth of the thickness of traditional designs. Finally, a broadband metadiffuser panel of 3 cm thick was designed using optimization methods for frequencies ranging from 250 Hz to 2 kHz.

Dynamic nonlinear focal shift in amplitude modulated moderately focused acoustic beams.

The phenomenon of the displacement of the position of the pressure, intensity and acoustic radiation force maxima along the axis of focused acoustic beams under increasing driving amplitudes (nonlinear focal shift) is studied for the case of a moderately focused beam excited with continuous and 25kHz amplitude modulated signals, both in water and tissue. We prove that in amplitude modulated beams the linear and nonlinear propagation effects coexist in a semi-period of modulation, giving place to a complex dynamic behavior, where the singular points of the beam (peak pressure, rarefaction, intensity and acoustic radiation force) locate at different points on axis as a function of time. These entire phenomena are explained in terms of harmonic generation and absorption during the propagation in a lossy nonlinear medium both for a continuous and an amplitude modulated beam. One of the possible applications of the acoustic radiation force displacement is the generation of shear waves at different locations by using a focused mono-element transducer excited by an amplitude modulated signal.

Formation of high-order acoustic Bessel beams by spiral diffraction gratings.

The formation of high-order Bessel beams by a passive acoustic device consisting of an Archimedes' spiral diffraction grating is theoretically, numerically, and experimentally reported in this paper. These beams are propagation-invariant solutions of the Helmholtz equation and are characterized by an azimuthal variation of the phase along its annular spectrum producing an acoustic vortex in the near field. In our system, the scattering of plane acoustic waves by the spiral grating leads to the formation of the acoustic vortex with zero pressure on axis and the angular phase dislocations characterized by the spiral geometry. The order of the generated Bessel beam and, as a consequence, the size of the generated vortex can be fixed by the number of arms in the spiral diffraction grating. The obtained results allow for obtaining Bessel beams with controllable vorticity by a passive device, which has potential applications in low-cost acoustic tweezers and acoustic radiation force devices.

Macroscopic acousto-mechanical analogy of a microbubble.

Microbubbles, either in the form of free gas bubbles surrounded by a fluid or encapsulated bubbles used currently as contrast agents for medical echography, exhibit complex dynamics under specific acoustic excitations. Nonetheless, considering their micron size and the complexity of their interaction phenomenon with ultrasound waves, expensive and complex experiments and/or simulations are required for their analysis. The behavior of a microbubble along its equator can be linked to a system of coupled oscillators. In this study, the oscillatory behavior of a microbubble has been investigated through an acousto-mechanical analogy based on a ring-shaped chain of coupled pendula. Observation of parametric vibration modes of the pendula ring excited at frequencies between 1 and 5 Hz is presented. Simulations have been carried out and show mode mixing phenomena. The relevance of the analogy between a microbubble and the macroscopic acousto-mechanical setup is discussed and suggested as an alternative way to investigate the complexity of microbubble dynamics.

Nonlinear focal shift beyond the geometrical focus in moderately focused acoustic beams.

The phenomenon of the displacement of the position along the axis of the pressure, intensity, and radiation force maxima of focused acoustic beams under increasing driving voltages (nonlinear focal shift) is studied for the case of a moderately focused beam. The theoretical and experimental results show the existence of this shift along the axis when the initial pressure in the transducer increases until the acoustic field reaches the fully developed nonlinear regime of propagation. Experimental data show that at high amplitudes and for moderate focusing, the position of the on-axis pressure maximum and radiation force maximum can surpass the geometrical focal length. On the contrary, the on-axis pressure minimum approaches the transducer under increasing driving voltages, increasing the distance between the positive and negative peak pressure in the beam. These results are in agreement with numerical KZK model predictions and the existed data of other authors and can be explained according to the effect of self-refraction characteristic of the nonlinear regime of propagation.