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It has been shown that light speckle fluctuations provide a means for noninvasive measurements of cerebral blood flow index (CBFi). While conventional Diffuse Correlation Spectroscopy (DCS) provides marginal brain sensitivity for CBFi in adult humans, new techniques have recently emerged to improve diffuse light throughput and thus, brain sensitivity. Here we further optimize one such approach, interferometric diffusing wave spectroscopy (iDWS), with respect to number of independent channels, camera duty cycle and full well capacity, incident power, noise and artifact mitigation, and data processing. We build the system on a cart and define conditions for stable operation. We show pulsatile CBFi monitoring at 4-4.5 cm source-collector separation in adults with moderate pigmentation (Fitzpatrick 4). We also report preliminary clinical measurements in the Neuro Intensive Care Unit (Neuro ICU). These results push the boundaries of iDWS CBFi monitoring performance beyond previous reports., Comment: 12 pages, 15 figures, 4 tables

Transcranial ultrasound applications require accurate simulations to predict intracranial acoustic pressure fields. The current gold standard typically consists of calculating a longitudinal ultrasound wave propagation using a fluid skull model, which is based on full head CT images for retrieving the skull's geometry and elastic constants. Although this approach has extensively been validated for deep brain targets and routinely used in transcranial ultrasound ablation procedures, its accuracy in shallow cortical regions remains unexplored. In this study, we explore the shear wave effects associated with transcranial focused ultrasound propagation, both numerically and experimentally. The intracranial acoustic pressure was measured at different incidence angles at the parietal and frontal regions in an ex vivo human skull. The fluid-like skull model was then compared to the solid model comprising both longitudinal and shear waves. The results consistently show a larger error and variability for both models when considering an oblique incidence, reaching a maximum of 125% mean deviation of the focal area when employing the fluid skull model. Statistical assessments further revealed that ignoring shear waves results in an average ~40% overestimation of the intracranial acoustic pressure and inability to obtain an accurate intracranial acoustic pressure distribution. Moreover, the solid model has a more stable performance, even when small variations in the skull-transducer relative position are introduced. Our results could contribute to the refinement of the transcranial ultrasound propagation modeling methods thus help improving the safety and outcome of transcranial ultrasound therapy in the cortical brain areas.

Multiple reflections between transducer and imaged object can naturally occur in ultrasound imaging and other acoustic sensing applications such as sonar. The repeated interaction of the emitted wavefront with the imaged object is traditionally regarded an undesired reverberation artifact, often misinterpreted as fictitious acoustic boundaries. Here we introduce high order reflection pulse-echo (HOPE)-ultrasound, a novel method that leverages high order reflections to improve on several aspects of conventional ultrasound imaging. HOPE is experimentally demonstrated to resolve sub-micrometer features by breaking through the Nyquist resolution limit. The major contrast enhancement of the high reflection orders allowed to reveal defects within materials invisible to conventional scanning acoustic microscopy. The technique is further shown to improve accuracy of frequency-dependent ultrasound attenuation measurements from biological tissues. HOPE ultrasound requires no additional hardware and is easy to implement, underscoring its potential to boost imaging performance in a broad range of biomedical imaging, nondestructive testing, and other acoustic sensing applications., Comment: 18 pages, 5 figures

Point-like broadband ultrasound detection can significantly increase the resolution of ultrasonography and optoacoustic (photoacoustic) imaging, yet current ultrasound detectors cannot be miniaturised sufficiently. Piezoelectric transducers lose sensitivity quadratically with size reduction, while optical micro-ring resonators and Fabry-P\'erot etalons fail to adequately confine light at dimensions smaller than $\sim$ 50 microns. Micromachining methods have been used to generate arrays of capacitive and piezoelectric transducers, but at bandwidths of only a few MHz and dimensions not smaller than 70 microns. Here we use the widely available silicon-on-insulator (SOI) platform to develop the worlds smallest ultrasound detector with a sub-micron sensing area of 220 $\times$ 500 nanometers. The SOI-based optical resonator design can provide per-area sensitivity that is $10^{4}$-fold higher than for micro-ring resonators and $10^{8}$-fold higher than for piezoelectric detectors. We also demonstrate ultra-wide bandwidth reaching 230 MHz and conduct the first imaging based on an SOI ultrasound detector. The technology showcased is suitable for manufacturing ultra-dense detector arrays ($\geq125$ $detectors\backslash mm^{2}$), which have the potential to revolutionise ultrasonography and optoacoustic imaging.

Human skull poses a significant barrier for the propagation of ultrasound waves. Development of methods enabling more efficient ultrasound transmission into and from the brain is therefore critical for the advancement of ultrasound-mediated transcranial imaging or actuation techniques. We report on the first observation of guided acoustic waves in the near-field of an ex vivo human skull specimen in the frequency range between 0.2 and 1.5 MHz. In contrast to what was previously observed for the guided wave propagation in thin rodent skulls, the guided wave observed in a higher frequency regime corresponds to a quasi-Rayleigh wave, mostly confined to the cortical bone layer. The newly discovered near-field properties of the human skull are expected to facilitate the development of more efficient diagnostic and therapeutic techniques based on transcranial ultrasound., Comment: 4 pages, 3 figures, 1 table

Ultrasound waves propagating in water or soft biological tissue are strongly reflected when encountering the skull, which limits the use of ultrasound-based techniques in transcranial imaging and therapeutic applications. Current knowledge on the acoustic properties of the cranial bone is restricted to far-field observations, leaving its near-field properties unexplored. We report on the existence of skull-guided acoustic waves, which was herein confirmed by near-field measurements of optoacoustically-induced responses in ex-vivo murine skulls immersed in water. Dispersion of the guided waves was found to reasonably agree with the prediction of a multilayered flat plate model. It is generally anticipated that our findings may facilitate and broaden the application of ultrasound-mediated techniques in brain diagnostics and therapy., Comment: 7 pages, 5 figures, appendix with 2 figures

Focusing light through turbid media presents a highly fascinating challenge in modern biophotonics. The unique capability of optoacoustics for high resolution imaging of light absorption contrast in deep tissues can provide a natural and efficient feedback to control light delivery in scattering medium. While basic feasibility of using optoacoustic readings as a feedback mechanism for wavefront shaping has been recently reported, the suggested approaches may require long acquisition times making them challenging to be translated into realistic tissue environments. In an attempt to significantly accelerate dynamic wavefront shaping capabilities, we present here a feedback-based approach using real-time three-dimensional optoacoustic imaging assisted with genetic-algorithm-based optimization. The new technique offers robust performance in the presence of noisy measurements and can simultaneously control the scattered wave field in an entire volumetric region., Comment: 4 pages, 3 figures

Bubbles display astonishing acoustical properties since they are able to absorb and scatter large amounts of energy coming from waves whose wavelengths are two orders of magnitude larger than the bubble size. Thus, as the interaction distance between bubbles is much larger than the bubble size, clouds of bubbles exhibit collective oscillations which can scatter acoustic waves three orders magnitude larger than the bubble size. Here we propose bubble based systems which resonate at frequencies that match the time scale relevant for seismogenic tsunami wave generation and may mitigate the devastating effects of tsunami waves. Based on a linear approximation, our na\"ive proposal may open new research paths towards the mitigation of tsunami waves generation., Comment: 5 pages, 5 figures

Recent experiments conducted in water at ultrasonic frequencies showed the possibility of overcoming the transmission loss provided by homogeneous plates at certain frequencies by drilling periodically distributed holes on it. In this letter, the feasibility of using slit arrays to increase the transmission loss at certain frequencies for airborne sound is studied. Numerical results predict a) very low transmission loss for a slit array in comparison with a homogeneous plate in air and b) the transmission loss of a slit array can overcome that of a homogeneous plate if the impedance mismatch is low enough., Comment: 3 pages, 3 figures

We discuss the differences and similarities in the interaction of scalar and vector wave-fields with particles and holes. Analytical results are provided for the transmission of isolated and arrayed small holes as well as surface modes in hole arrays for light, electrons, and sound. In contrast to the optical case, small-hole arrays in perforated perfect screens cannot produce acoustic or electronic surface-bound states. However, unlike electrons and light, sound is transmitted through individual holes approximately in proportion to their area, regardless their size. We discuss these issues with a systematic analysis that allows exploring both common properties and unique behavior in wave phenomena for different material realizations., Comment: 3 figures

We study sound transmission in perforated plates as a function of incident angle and conclude that it holds distinctive properties that make it unique and essentially different from optical transmission through perforated metallic plates. More precisely, we conclude the following: (a) similar to its optical counterpart, acoustic transmission minima respond to Wood anomalies in which the periodicity plays a central role; (b) in contrast to both the optical case and the acoustical case with slits, homogeneous-plate modes (Lamb and Scholte-Stoneley modes) are strongly coupled to lattice and Fabry-P\'erot resonances. This gives rise to unique transmission behavior, thus opening new perspectives for exotic wave phenomena., Comment: 4 figures. Submitted to Phys. Rev. Lett

We report extraordinary effects in the transmission of sound through periodically perforated plates, supported by both measurements and theory. In agreement with recent observations in slit arrays [M. H. Lu et al. Phys. Rev. Lett. 99, 174301 (2007)], nearly full transmission is observed at certain resonant frequencies, pointing out at similarities of the acoustic phenomena and their optical counterpart. However, acoustic screening well beyond that predicted by the mass law is achieved over a wide range of wavelengths in the vicinity of the period of the array, resulting in fundamentally unique behavior of the sound as compared to light. The randomness of the hole distribution and the impedance contrast between the fluid and the solid plate are found to play a crucial role., Comment: 5 pages and 4 figures. Submitted to Phys. Rev. Lett

The variable-pressure washer (VPW) is a proposed device that is so named because (1) it would play the role similar to that played by an ordinary washer, except that (2) the clamping pressure applied by it would vary with either circumferential or radial position. In a typical contemplated application, the radially varying clamping pressure would be used to obtain more nearly uniform compression on a pair of concentric seals (e.g., an O-ring or a gasket) in an assembly that experiences larger deformations normal to the sealing surface for locations around the outer diameter of the attachment flange when compared to locations around the inner diameter. The VPW (see figure) would include two interlocking channel rings pushed axially away from each other by compression spring-like components located at two or more radial positions. Each spring would have a different stiffness based on the radial location. Overlapping splits in each interlocking channel ring would allow for the non-uniform deformation in the rings. Each spring would be held in place by retaining cups attached to the inner flat surfaces of the channel rings. A plunger attached to one channel ring on the central axis would be captured in a plunger housing attached to the other channel ring: The capture of the plunger would hold the VPW together. When the VPW was clamped between two flat surfaces, the clamping force would be distributed unevenly across the face of the washer in the radial direction. The different stiffnesses of the springs would be chosen, in conjunction with other design parameters, to obtain a specified radial variation of clamping pressure in the presence of a specified clamping force.