Abstract.High intensity transcranial focused ultrasound is an FDA approved treatment for essential tremor, while low-intensity applications such as neurostimulation and opening the blood brain barrier are under active research. Simulations of transcranial ultrasound propagation are used both for focusing through the skull, and predicting intracranial fields. Maps of the skull acoustic properties are necessary for accurate simulations, and can be derived from medical images using a variety of methods. The skull maps range from segmented, homogeneous models, to fully heterogeneous models derived from medical image intensity. In the present work, the impact of uncertainties in the skull properties is examined using a model of transcranial propagation from a single element focused transducer. The impact of changes in bone layer geometry and the sound speed, density, and acoustic absorption values is quantified through a numerical sensitivity analysis. Sound speed is shown to be the most influential acoustic property, and must be defined with less than 4% error to obtain acceptable accuracy in simulated focus pressure, position, and volume. Changes in the skull thickness of as little as 0.1 mm can cause an error in peak intracranial pressure of greater than 5%, while smoothing with a 1 mm 3 kernel to imitate the effect of obtaining skull maps from low resolution images causes an increase of over 50% in peak pressure. The numerical results are confirmed experimentally through comparison with sonications made through 3D printed and resin cast skull bone phantoms.

Accurately representing the behavior of acoustic sources is an important part of ultrasound simulation. This is particularly challenging in ultrasound therapy where multielement arrays are often used. Typically, sources are defined as a boundary condition over a 2-D plane within the computational model. However, this approach can become difficult to apply to arrays with multiple elements distributed over a nonplanar surface. In this paper, a grid-based discrete source model for single- and multielement bowl-shaped transducers is developed to model the source geometry explicitly within a regular Cartesian grid. For each element, the source model is defined as a symmetric, simply connected surface with a single grid point thickness. Simulations using the source model with the open-source k-Wave toolbox are validated using the Rayleigh integral, O'Neil solution, and experimental measurements of a focused bowl transducer under both quasi-continuous wave and pulsed excitations. Close agreement is shown between the discrete bowl model and the axial pressure predicted by the O'Neil solution for a uniform curved radiator, even at very low grid resolutions. Excellent agreement is also shown between the discrete bowl model and experimental measurements. To accurately reproduce the near-field pressure measured experimentally, it is necessary to derive the drive signal at each grid point of the bowl model directly using holography. However, good agreement is also obtained in the focal region using uniformly radiating monopole sources distributed over the bowl surface. This allows the response of multielement transducers to be modeled, even where measurement of an input plane is not possible.

Models of ultrasound propagation in biologically relevant media have applications in planning and verification of ultrasound therapies and computational dosimetry. To be effective, the models must be able to accurately predict both the spatial distribution and amplitude of the acoustic pressure. This requires that the models are validated in absolute terms, which for arbitrarily heterogeneous media should be performed by comparison with measurements of the acoustic field. In this study, simulations performed using the open-source k-Wave acoustics toolbox, with a measurement-based source definition, were quantitatively validated against measurements of acoustic pressure in water and layered absorbing fluid media. In water, the measured and simulated spatial peak pressures agreed to within 3% under linear conditions and 7% under non-linear conditions. After propagation through a planar or wedge shaped glycerolfilled phantom, the difference in spatial peak pressure was 8.5% and 10.7%, respectively. These differences are within or close to the expected uncertainty of the acoustic pressure measurement. The -6 dB width and length of the focus agreed to within 4% in all cases, and the focal positions were within 0.7 mm for the planar phantom and 1.2 mm for the wedge shaped phantom. These results demonstrate that when the acoustic medium properties and geometry are well known, accurate quantitative predictions of the acoustic field can be made using k-Wave.

Measurement of high acoustic pressures is necessary in order to fully characterize clinical high-intensity focused ultrasound (HIFU) fields, and for accurate validation of computational models of ultrasound propagation. However, many existing measurement devices are unable to withstand the extreme pressures generated in these fields, and those that can often exhibit low sensitivity. Here, a planar Fabry-Pérot interferometer with hard dielectric mirrors and spacer was designed, fabricated, and characterized, and its suitability for measurement of nonlinear focused ultrasound fields was investigated. The noise equivalent pressure (NEP) of the scanning system scaled with the adjustable pressure detection range between 49 kPa for pressures up to 8 MPa and 152 kPa for measurements up to 25 MPa, over a 125 MHz measurement bandwidth. Measurements of the frequency response of the sensor showed that it varied by less than 3 dB in the range 1-62 MHz. The effective element size of the sensor was 65 and waveforms were acquired at a rate of 200 Hz. The device was used to measure the acoustic pressure in the field of a 1.1 MHz single-element spherically focused bowl transducer. Measurements of the acoustic field at low pressures compared well with measurements made using a Polyvinylidene difluoride needle hydrophone. At high pressures, the measured peak focal pressures agreed well with the focal pressure modeled using the Khokhlov-Zabolotskaya-Kuznetsov equation. Maximum peak positive pressures of 25 MPa and peak negative pressures of 12 MPa were measured, and planar field scans were acquired in scan times on the order of 1 min. The properties of the sensor and scanning system are well suited to measurement of nonlinear focused ultrasound fields, in both the focal region and the low-pressure peripheral regions. The fast acquisition speed of the system and its low NEP are advantageous, and with further development of the sensor, it has potential in application to HIFU metrology.

Accurate measurements of acoustic pressure are required for characterisation of ultrasonic transducers and for experimental validation of models of ultrasound propagation. Errors in measured pressure can arise from a variety of sources, including variations in the properties of the source and measurement equipment, calibration uncertainty, and processing of measured data. In this study, the repeatability of measurements made with four probe and membrane hydrophones was examined. The pressures measured by these hydrophones in three different ultrasound fields, with both linear and nonlinear, pulsed and steady state driving conditions, were compared to assess the reproducibility of measurements. The coefficient of variation of the focal peak positive pressure was less than 2% for all hydrophones across five repeated measurements. When comparing hydrophones, pressures measured in a spherically focused 1.1 MHz field were within 7% for all except 1 case, and within 10% for a broadband 5 MHz pulse from a diagnostic linear array. Larger differences of up to 55% were observed between measurements of a tightly focused 3.3 MHz field, which were reduced for some hydrophones by the application of spatial averaging corrections. Overall, the major source of these differences was spatial averaging and uncertainty in the complex frequency response of the hydrophones.

During transvaginal ultrasound scanning, the fetus and other sensitive tissues are placed close to the transducer. Heating of these tissues occurs by direct conduction from the transducer and by absorption of ultrasound in the tissue. The extent of any heating will depend on the equipment and settings used, the duration of the scan, imaging modes and other aspects of scanning practice. To ensure that scans are performed with minimum risk, staff should have an appropriate knowledge of safety and follow guidelines issued by professional bodies. An online survey aiming to document current practice in transvaginal ultrasound in the UK was created and distributed to individuals performing this type of scanning. The survey posed questions about the respondents, the departments where scans were performed, the equipment used, knowledge of ultrasound safety, scanning practice and the frequency, duration and mode of transvaginal ultrasound scans for gynaecology, obstetrics and fertility applications. In all, 294 responses were obtained, mostly from sonographers (94%). From the analysis of the responses, it was clear that there was a good understanding of the general meaning of thermal and mechanical index and high awareness of guidelines issued by professional bodies. However, 40% of respondents stated that they rarely or never monitor Thermal or Mechanical indices during scanning. Scanning practice was consistent in terms of the duration of scans, scan protocols followed and use of imaging modes. The results highlight the importance of continued ultrasound safety training and promotion of safety guidelines to users.

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