Hydrophone Measurements for Biomedical Ultrasound Applications: A Review

Toward widespread use of virtual trials in medical imaging innovation and regulatory science

The rapid advancement in the field of medical imaging presents a challenge in keeping up to date with the necessary objective evaluations and optimizations for safe and effective use in clinical settings. These evaluations are traditionally done using clinical imaging trials, which while effective, pose several limitations including high costs, ethical considerations for repetitive experiments, time constraints, and lack of ground truth. To tackle these issues, virtual trials (aka in silico trials) have emerged as a promising alternative, using computational models of human subjects and imaging devices, and observer models/analysis to carry out experiments. To facilitate the widespread use of virtual trials within the medical imaging research community, a major need is to establish a common consensus framework that all can use. Based on the ongoing efforts of an AAPM Task Group (TG387), this article provides a comprehensive overview of the requirements for establishing virtual imaging trial frameworks, paving the way toward their widespread use within the medical imaging research community. These requirements include credibility, reproducibility, and accessibility. Credibility assessment involves verification, validation, uncertainty quantification, and sensitivity analysis, ensuring the accuracy and realism of computational models. A proper credibility assessment requires a clear context of use and the questions that the study is intended to objectively answer. For reproducibility and accessibility, this article highlights the need for detailed documentation, user-friendly software packages, and standard input/output formats. Challenges in data and software sharing, including proprietary data and inconsistent file formats, are discussed. Recommended solutions to enhance accessibility include containerized environments and data-sharing hubs, along with following standards such as CDISC (Clinical Data Interchange Standards Consortium). By addressing challenges associated with credibility, reproducibility, and accessibility, virtual imaging trials can be positioned as a powerful and inclusive resource, advancing medical imaging innovation and regulatory science.

Triggerable Patches for Medical Applications

Medical patches have garnered increasing attention in recent decades for several diagnostic and therapeutic applications. Advancements in material science, manufacturing technologies, and bioengineering have significantly widened their functionalities, rendering them highly versatile platforms for wearable and implantable applications. Of particular interest are triggerable patches designed for drug delivery and tissue regeneration purposes, whose action can be controlled by an external signal. Stimuli-responsive patches are particularly appealing as they may enable a high level of temporal and spatial control over the therapy, allowing high therapeutic precision and the possibility to adjust the treatment according to specific clinical and personal needs. This review aims to provide a comprehensive overview of the existing extensive literature on triggerable patches, emphasizing their potential for diverse applications and highlighting the strengths and weaknesses of different triggering stimuli. Additionally, the current open challenges related to the design and use of efficient triggerable patches, such as tuning their mechanical and adhesive properties, ensuring an acceptable trade-off between smartness and biocompatibility, endowing them with portability and autonomy, accurately controlling their responsiveness to the triggering stimulus and maximizing their therapeutic efficacy, are reviewed.

Masked Modeling-Based Ultrasound Image Classification via Self-Supervised Learning

Recently, deep learning-based methods have emerged as the preferred approach for ultrasound data analysis. However, these methods often require large-scale annotated datasets for training deep models, which are not readily available in practical scenarios. Additionally, the presence of speckle noise and other imaging artifacts can introduce numerous hard examples for ultrasound data classification. In this paper, drawing inspiration from self-supervised learning techniques, we present a pre-training method based on mask modeling specifically designed for ultrasound data. Our study investigates three different mask modeling strategies: random masking, vertical masking, and horizontal masking. By employing these strategies, our pre-training approach aims to predict the masked portion of the ultrasound images. Notably, our method does not rely on externally labeled data, allowing us to extract representative features without the need for human annotation. Consequently, we can leverage unlabeled datasets for pre-training. Furthermore, to address the challenges posed by hard samples in ultrasound data, we propose a novel hard sample mining strategy. To evaluate the effectiveness of our proposed method, we conduct experiments on two datasets. The experimental results demonstrate that our approach outperforms other state-of-the-art methods in ultrasound image classification. This indicates the superiority of our pre-training method and its ability to extract discriminative features from ultrasound data, even in the presence of hard examples.

ITRUSST Consensus on Standardised Reporting for Transcranial Ultrasound Stimulation

As transcranial ultrasound stimulation (TUS) advances as a precise, non-invasive neuromodulatory method, there is a need for consistent reporting standards to enable comparison and reproducibility across studies. To this end, the International Transcranial Ultrasonic Stimulation Safety and Standards Consortium (ITRUSST) formed a subcommittee of experts across several domains to review and suggest standardised reporting parameters for low intensity TUS, resulting in the guide presented here. The scope of the guide is limited to reporting the ultrasound aspects of a study. The guide and supplementary material provide a simple checklist covering the reporting of: (1) the transducer and drive system, (2) the drive system settings, (3) the free field acoustic parameters, (4) the pulse timing parameters, (5) in situ estimates of exposure parameters in the brain, and (6) intensity parameters. Detailed explanations for each of the parameters, including discussions on assumptions, measurements, and calculations, are also provided.

A Miniature Ultrasound Source for Neural Modulation

This work describes a unique ultrasound (US) exposure system designed to create very localized ( [Formula: see text]) sound fields at operating frequencies that are currently being used for preclinical US neuromodulation. This system can expose small clusters of neuronal tissue, such as cell cultures or intact brain structures in target animal models, opening up opportunities to examine possible mechanisms of action. We modified a dental descaler and drove it at a resonance frequency of 96 kHz, well above its nominal operating point of 28 kHz. A ceramic microtip from an ultrasonic wire bonder was attached to the end of the applicator, creating a 100- [Formula: see text] point source. The device was calibrated with a polyvinylidene difluoride (PVDF) membrane hydrophone, in a novel, air-backed, configuration. The experimental results were confirmed by simulation using a monopole model. The results show a consistent decaying sound field from the tip, well-suited to neural stimulation. The system was tested on an existing neurological model, Drosophila melanogaster, which has not previously been used for US neuromodulation experiments. The results show brain-directed US stimulation induces or suppresses motor actions, demonstrated through synchronized tracking of fly limb movements. These results provide the basis for ongoing and future studies of US interaction with neuronal tissue, both at the level of single neurons and intact organisms.

Use of transcranial low-intensity focused ultrasound for targeted delivery of stem cell-derived exosomes to the brain

The blood-brain barrier (BBB) presents a significant challenge for targeted drug delivery. A proposed method to improve drug delivery across the BBB is focused ultrasound (fUS), which delivers ultrasound waves to a targeted location in the brain and is hypothesized to open the BBB. Furthermore, stem cell-derived exosomes have been suggested as a possible anti-inflammatory molecule that may have neural benefits, if able to pass the BBB. In the present study, transcranial low-intensity focused ultrasound (LIFU), without the use of intravenous microbubbles, was assessed for both (1) its ability to influence the BBB, as well as (2) its ability to increase the localization of intravenously administered small molecules to a specific region in the brain. In vivo rat studies were conducted with a rodent-customized 2 MHz LIFU probe (peak pressure = 1.5 MPa), and injection of labeled stem cell-derived exosomes. The results suggested that LIFU (without microbubbles) did not appear to open the BBB after exposure times of 20, 40, or 60 min; instead, there appeared to be an increase in transcytosis of the dextran tracer. Furthermore, the imaging results of the exosome study showed an increase in exosome localization in the right hippocampus following 60 min of targeted LIFU.

Mechanistic and synergistic aspects of ultrasonics and hydrodynamic cavitation for food processing

Compared with traditional methods, cavitation-based processing technology has received extensive attention for its low energy consumption and high processing efficiency. The cavitation phenomenon releases high energy due to the generation and collapse of bubbles, which improves the efficiency of various food processing. This review details the cavitation mechanism of ultrasonic cavitation (UC) and hydrodynamic cavitation (HC), factors affecting cavitation, the application of cavitation technology in food processing, and the application of cavitation technology in the extraction of various natural ingredients. The safety and nutrition of food processed by cavitation technology and future research directions are also discussed. The mechanism of UC refers to longitudinal displacement of the particles of the medium induced by ultrasonic waves causing a series of alternating compression and rarefaction of particles, whereas HC occurs when liquid enters a narrow section and undergoes large pressure differentials, both of which can trigger the generation, growth, and collapse of microbubbles. Cavitation could be applied in microbial inactivation, and drying and freezing processing. In addition, cavitation bubbles can have mechanical and thermal effects on plant cells. In general, cavitation technology is a new sustainable, green, and innovative technology with broad application prospects and capabilities.

Nominal Versus Actual Spatial Resolution: Comparison of Directivity and Frequency-Dependent Effective Sensitive Element Size for Membrane, Needle, Capsule, and Fiber-Optic Hydrophones

Frequency-dependent effective sensitive element radius [Formula: see text] is a key parameter for elucidating physical mechanisms of hydrophone operation. In addition, it is essential to know [Formula: see text] to correct for hydrophone output voltage reduction due to spatial averaging across the hydrophone sensitive element surface. At low frequencies, [Formula: see text] is greater than geometrical sensitive element radius a_g . Consequently, at low frequencies, investigators can overrate their hydrophone spatial resolution. Empirical models for [Formula: see text] for membrane, needle, and fiber-optic hydrophones have been obtained previously. In this article, an empirical model for [Formula: see text] for capsule hydrophones is presented, so that models are now available for the four most common hydrophone types used in biomedical ultrasound. The [Formula: see text] value was estimated from directivity measurements (over the range from 1 to 20 MHz) for five capsule hydrophones (three with [Formula: see text] and two with [Formula: see text]). The results suggest that capsule hydrophones behave according to a "rigid piston" model for k a _g ≥ 0.7 ( k = 2π /wavelength). Comparing the four hydrophone types, the low-frequency discrepancy between [Formula: see text] and a_g was found to be greatest for membrane hydrophones, followed by capsule hydrophones, and smallest for needle and fiber-optic hydrophones. Empirical models for [Formula: see text] are helpful for choosing an appropriate hydrophone for an experiment and for correcting for spatial averaging (over the sensitive element surface) in pressure and beamwidth measurements. When reporting hydrophone-based pressure measurements, investigators should specify [Formula: see text] at the center frequency (which may be estimated from the models presented here) in addition to a_g .

Dissemination of the Acoustic Pascal: The Role and Experiences of a National Metrology Institute

Hydrophones are pivotal measurement devices ensuring medical ultrasound acoustic exposures comply with the relevant national and international safety criteria. These devices have enabled the spatial and temporal distribution of key safety parameters to be determined in an objective and standardized way. Generally based on piezoelectric principles of operation, to convert generated voltage waveforms to acoustic pressure, they require calibration in terms of receive sensitivity, expressed in units of [Formula: see text]Pa-1. Reliable hydrophone calibration with associated uncertainties plays a key role in underpinning a measurement framework that ensures exposure measurements are comparable and traceable to internationally agreed units, irrespective of where they are carried out globally. For well over three decades, the U.K. National Physical Laboratory (NPL) has provided calibrations to the user community covering the frequency range 0.1-60 MHz, traceable to a primary realization of the acoustic pascal through optical interferometry. Typical uncertainties for sensitivity are 6%-22% (for a coverage factor k = 2), degrading with frequency. The article specifically focuses on the dissemination of the acoustic pascal through NPL's calibration services that are based on a comparison with secondary standard hydrophones previously calibrated using the NPL primary standard. The work demonstrates the stability of the employed dissemination protocols by presenting representative calibration histories on a selection of commercially available hydrophones. Results reaffirm the guidance provided within international standards for regular calibration of a hydrophone in order to underpin measurement confidence. The process by which internationally agreed realizations of the acoustic pascal are compared and validated through key comparisons (KCs) is also described.