Srinath Rajagopal

Acoustic pressure determined in water is a critical parameter for many applications of underwater acoustics (UW) and medical ultrasound (US). These areas cover diverse drivers, from the need to measure ocean acoustics in support of noise pollution regulation, to the acoustic output of diagnostic and therapeutic medical equipment to ensure safe clinical exposure levels. Within the metrology community, one of the lowest uncertainty primary realisations of the relevant unit, the acoustic Pascal, has been through optical interferometry. Here, an optical interferometer was used to probe picometre displacements of a thin, quasi-acoustically transparent, membrane suspended within acoustic fields to measure the acoustic Pascal at a specific spatial location. At the UK National Physical Laboratory, this SI unit is realised and disseminated to the user community through secondary calibration of user hydrophones. This manuscript describes a new realisation of the acoustic Pascal built around a commercial optical heterodyne-type vibrometer which allows limitations of the previous homodyne realisations to be overcome. These limitations included: sensitivity to external vibration, limited linear dynamic range and the need to individually calibrate multiple onboard electronics, including frequency response of the photodiodes. We demonstrate continuity in the realisation of the acoustic Pascal and hydrophone calibrations, with good agreement over a variety of hydrophone types. The new interferometer has shown good agreement with the previous optical primary standards, with free-field reciprocity, current secondary standard disseminations, and the key comparison reference values derived for hydrophones used in International Committee for Weights and Measures key comparisons spanning both UW and US frequency ranges. A novel aspect of the work is the improvement in hydrophone calibration uncertainties at high frequencies through the application of spatially broad acoustic fields using acoustic signals generated by finite amplitude distortion. Hydrophone calibration uncertainty budgets are presented demonstrating uncertainties which vary from 9.41% at 100 kHz to 14.03% at 60 MHz ( k = 2).

Accurate quantification of acoustic fields produced by biomedical ultrasound devices is essential for assessing thermal and mechanical bioeffects on the patient. Device characterization involves time-consuming measurements, which need to be performed under many operating conditions for complete risk evaluation. Also, there are difficulties in measuring high-intensity therapeutic fields, which can damage the measuring devices. In this study, measurement-based simulations of temperature rise induced by ultrasound absorption in a tissue mimicking material (TMM) under linear propagation conditions were undertaken. The objective was to demonstrate a methodology for quantifying the accuracy of measurement-based simulations (a case study) along with an assessment of the uncertainties associated with both experimental and simulation setups. The acoustic and thermal modelling was performed using a pseudospectral time-domain solver, k-Wave. (Note that the methodology is not specific to this solver.) Ultrasound heating was conducted in the TMM with source acoustic power levels ranging from 1.1 to 4.3 W for durations of 2 to 4 min. The peak temperature rises recorded at the acoustic focus from the embedded fine wire thermocouples ranged from 4.4 (0.6) to 16.4 (1.3) °C. The difference in simulated temperature rises across all heating conditions with respect to measurements ranged from –1.9% to 6.7%.

Although the physical properties of a structure, such as stiffness, can be determined using some statical tests, the identification of damping parameters requires a dynamic test. In general, both theoretical prediction and experimental identification of damping are quite difficult. There are many different techniques available for damping identification, and each method gives a different damping parameter. The dynamic indentation method, rheometry, atomic force microscopy, and resonant vibration tests are commonly used to identify the damping of materials, including soft materials. While the viscous damping ratio, loss factor, complex modulus, and viscosity are quite common to describe the damping of materials, there are also other parameters, such as the specific damping capacity, loss angle, half-power bandwidth, and logarithmic decrement, to describe the damping of various materials. Often, one of these parameters is measured, and the measured parameter needs to be converted into another damping parameter for comparison purposes. In this review, the theoretical derivations of different parameters for the description and quantification of damping and their relationships are presented. The expressions for both high damping and low damping are included and evaluated. This study is considered as the first comprehensive review article presenting the theoretical derivations of a large number of damping parameters and the relationships among many damping parameters, with a quantitative evaluation of accurate and approximate formulas. This paper could be a primary resource for damping research and teaching.

The physical properties of a structure such stiffness although can be determined by some statical tests, the identification of damping parameter requires a dynamic test. In general, both theoretical prediction and experimental identification of damping are quite difficult. There are many different techniques available for damping identification, and each method gives a different damping parameter. The dynamic indentation method, rheometry, atomic force microscopy, and resonant vibration tests are commonly used to identify the damping of materials, including soft materials. While the viscous damping ratio, loss factor, complex modulus and viscosity are quite common to describe damping of materials, there are also other parameters such as the specific damping capacity, loss angle, half-power bandwidth, and logarithmic decrement to describe damping of various materials. Often one of these parameters is measured in practical applications and the measured damping parameter needs to be converted into another damping parameter for comparison purposes. In this review, the theoretical derivations of different parameters for the description and quantification of damping and their relationships, and the methods for damping identification are presented. The expressions for both high damping and low damping are included and evaluated. This paper could be a primary resource for damping research and teaching.

Ultrasound Computed Tomography (UCT) shows significant promise for imaging the internal structures of soft-tissue organs such as the breast, in particular the distribution of the important biomarker speed of sound. Ultrasonic attenuation also represents a potentially highly informative biomarker relevant to disease classification, but it has been extremely difficult to reconstruct reliable images of this property due the highly aberrating nature of breast tissue coupled with the phase- sensitive nature of the conventional piezoelectric detectors deployed in UCT systems. This study reports on the essential properties of a large-area phase- insensitive detector, which exploits the pyroelectric effect within membranes of polyvinylidene fluoride and makes the detectors particularly appropriate for quantitative imaging of soft tissue ultrasonic attenuation. These properties include a broad directional response that is dependent on the frequency content of the incident acoustic radiation but can be flat (± 1 dB) out to an angle of incidence of at least ±60°, determined at 3.2 MHz. The thermal origin of the signals generated means that they can be influenced by extraneous background acoustic and vibration noise, and a differential sensor design is presented and modelled which is shown to significantly enhance signal-to-noise ratio for small signal retrieval whilst also contributing to the omnidirectionality. Finally, the high sensitivity of the developed detector and instrumentation combination is demonstrated which enables time-averaged acoustic powers of around several milliwatts to be measured, generated by estimated temperature increases immediately adjacent to the polyvinylidene fluoride membrane of a few mK.

Establishing tissue-mimicking biophotonic phantom materials that provide long-term stability are imperative to enable the comparison of biomedical imaging devices across vendors and institutions, support the development of internationally recognized standards, and assist the clinical translation of novel technologies. Here, a manufacturing process is presented that results in a stable, low-cost, tissue-mimicking copolymer-in-oil material for use in photoacoustic, optical, and ultrasound standardization efforts. The base material consists of mineral oil and a copolymer with defined Chemical Abstract Service (CAS) numbers. The protocol presented here yields a representative material with a speed of sound c(f) = 1,481 ± 0.4 m·s-1 at 5 MHz (corresponds to the speed of sound of water at 20 °C), acoustic attenuation α(f) = 6.1 ± 0.06 dB·cm-1 at 5 MHz, optical absorption µa(λ) = 0.05 ± 0.005 mm-1 at 800 nm, and optical scattering µs'(λ) = 1 ± 0.1 mm-1 at 800 nm. The material allows independent tuning of the acoustic and optical properties by respectively varying the polymer concentration or light scattering (titanium dioxide) and absorbing agents (oil-soluble dye). The fabrication of different phantom designs is displayed and the homogeneity of the resulting test objects is confirmed using photoacoustic imaging. Due to its facile, repeatable fabrication process and durability, as well as its biologically relevant properties, the material recipe has high promise in multimodal acoustic-optical standardization initiatives.