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The use of high-intensity focused ultrasound (HIFU) to heat polymer films is getting more in focus of interest in the industry due to some advantages e.g. low damages of thin polymer films during film processing or cost reduction of the processing tool. The capability to heat thin polymer films using a solid waveguide transducer was already reported in the literature. However, some significant hurdles persist: the physical principles responsible for the heating of thin polymer films are not fully understood. This study aims to investigate the sound field occurring in thin polymer films between a solid waveguide transducer (having a resonance frequency of 1.1 MHz) and a solid counterpart and compared it to the sound field in mm-thick polymer sample. An analogy between the sound field measured in water and the sound field inside polymer films during the heating process is done. The sound field between counterpart and solid waveguide is analyzed by means of membrane hydrophone sound field measurements and finite element simulations. The impact of the transducer-counterpart gap size is studied. This investigation reveals that the behavior and characteristics of the sound field in water in a transducer-counterpart gap differ considerably from each other depending on the gap size. When the gap is smaller than a wavelength, the sound cannot be focused within the waveguide-counterpart gap and complex interferences occur inside the small gap. Furthermore, the investigation showed that similarities occur between the sound field measured in water and the sound field calculated in polymer. These findings are relevant for the understanding of HIFU-based processes involving thin polymer films.

This article presents the study of the thermal and acoustic effects occurring in polymer samples of different thicknesses receiving high-intensity focused ultrasound (HIFU). Whereas the heating mechanisms in polymer plates immersed in water are well known, the physical mechanisms enabling the heating of polymer films using a solid waveguide transducer remain not fully understood. A coupled acoustothermal finite-element simulation is conducted to model the sound field and the heat generation inside polymer samples of different thicknesses. To validate the acoustic model, the acoustic particle velocities at the transducer waveguide tip are measured by vibrometry and compared with the simulation results. The heating effects in the samples are monitored using an infrared thermography system and compared with the measured particle velocities and with the acoustic and thermal simulation results. Correlations among particle velocities, sound intensity, and polymer heating are investigated. A qualitative and quantitative correlation between the simulation and the measurement results is found. Experiments show that the heating effects depend on the sample thickness. In samples thinner than 1 mm, the maximum temperature is lower than the one observed in samples thicker than 1 mm but rises faster. The simulation shows that the sound intensity in polymer samples thinner than 1 mm decreases sharply with the decrease in the thickness of the sample. This study contributes to the understanding of the challenges in heating thin polymer films by HIFU in a dry environment, using only a force to couple the transducer to the polymer films.

Previous studies showed that using a 1 MHz solid waveguide transducer based on High-Intensity Focused Ultrasound (HIFU)enables 320 μm thick low density polyethylene (LDPE)films heating above the melting temperature within 200 ms. However, the physical mechanisms causing the heating of such thin polymer films are not fully understood. It is not clear in detail, how the waves are transmitted and concentrated. Therefore, in this contribution, the physical principles responsible for polymer films heating are investigated using sound field, vibrometry (sLDV)and infrared measurements. The sound field distribution measured does not look like the typical focused sound field for an unbounded wave propagation. When the gap between waveguide and counterpart is small, the waves cannot propagate to focus and generate an energy concentration in the gap. Depending on the gap size, an irregular superposed wave field occurs. A correlation between the acoustic particle velocity at the waveguide tip and the heating inside the polymer is found. From these findings, it can be concluded that the heating in LDPE-films is caused by waveguide tip hammering on the films leading to intermolecular frictions rather than by the absorption of the focused ultrasonic wave inside the thin films.

Quantitative ultrasound (QUS) is a promising technique for bone tissue evaluation. Highly focused transducers used for QUS also have the capability to be applied for tissue-regenerative purposes and can provide spatially limited deposition of acoustic energy. We describe a focused low-intensity pulsed ultrasound (FLIPUS) system, which has been developed for the stimulation of cell monolayers in the defocused far field of the transducer through the bottom of the well plate. Tissue culture well plates, carrying the cells, were incubated in a special chamber, immersed in a temperature-controlled water tank. A stimulation frequency of 3.6 MHz provided an optimal sound transmission through the polystyrene well plate. The ultrasound was pulsed for 20 min daily at 100-Hz repetition frequency with 27.8% duty cycle. The calibrated output intensity corresponded to ${{{I}}_{{\text{SATA}}}} = 44.5 \pm 7.1\;{\text{mW}}/\text{cm}^{2}$ , which is comparable to the most frequently reported nominal output levels in LIPUS studies. No temperature change by the ultrasound exposure was observed in the well plate. The system was used to stimulate rat mesenchymal stem cells (rMSCs). The applied intensity had no apoptotic effect and enhanced the expression of osteogenic markers, i.e., osteopontin (OPN), collagen 1 (Col-1), the osteoblast-specific transcription factor—Runx-2 and E11 protein, an early osteocyte marker, in stimulated cells on day 5. The proposed FLIPUS setup opens new perspectives for the evaluation of the mechanistic effects of LIPUS.

High-intensity focused ultrasound (HIFU) is increasingly getting in focus of interest for polymer layer joining [1]. It has been found that HIFU is able to heat polymer layers with a temperature rise of over 680 K · s−1 [1]. However the physical mechanisms causing the heating of thin polymer layers using ultrasound with a solid wave guide ultrasonic transducer are still unknown. In this paper, the correlation between sample thickness, particle velocity at the top of the solid wave guide and heating behaviors are investigated by means of scanning Doppler laser vibrometry and infrared thermography. The study showed that the heat generation induced by high-intensity focused ultrasound with a solid wave guide depends on the distribution of the particle velocities at the top of the wave guide for samples thinner than a half wavelength in PMMA. However for the samples thicker than a half wavelength, the heating of the sample depends on the particle velocities distribution at the top of the wave guide, as well as on the sound waves focused by the transducer. The findings are relevant for processes in which solid polymers need to be heated quickly and locally such as in welding or forming.

The pressure fields of two different high intensity focused ultrasound (HIFU) transducers operated in burst mode were measured at acoustical power levels of 25 and 50 W (continuous wave equivalent) with three different hydrophones: A fiber-optic displacement sensor, a commercial HIFU needle hydrophone, and a prototype of a membrane hydrophone with a protective coating against cavitation effects. Additionally, the fields were modeled using a freely available simulations software package. The measured waveforms, the peak pressure profiles, as well as the spatial-peak temporal-average intensities from the different devices and from the modeling are compared and possible reasons for differences are discussed. The results clearly show that reliable pressure measurements in HIFU fields remain a difficult task concerning both the reliability of the measured values and the robustness of the sensors used: Only the fiber-optic hydrophone survived all four exposure regimes and the measured spatial-peak temporal-average intensities varied by a factor of up to 1.5 between the measurements and the modeling and between the measurements among themselves.

A fiber-optic sensor is presented that is capable of measuring the particle displacement in high-intensity focused ultrasound (HIFU) fields. For this probe, a secondary calibration was performed, and the resulting complex frequency response is discussed. As a first practical application, the setup was used to measure the pressure in the field of a weakly focusing ultrasound transducer. The result is compared with that of a membrane hydrophone measurement. The feasibility of measurements in HIFU fields is demonstrated by means of measurements of the spatial distribution of the peak particle velocity within the focus of a HIFU transducer and of the dependence of the peak values on the acoustical power level.

Ultrasound cleaning baths fitting the full range from micromechanical components up to large machine parts, are regularly used in industry and in the lab. Despite the large number of applications, generally approved principles and objective criteria for parameter settings which allow an efficient operation are non-existent. The empirical selections of the running parameters often impede an optimization in terms of produce and reproducibility. One proposal for an objective description of the processes is the characterization of the sound field in the cleaning bath, which causes cavities, and subsequently, the cleaning process. Sound field measurements in the appropriate frequency range from 20 kHz up to more then 1 MHz incorporate a number of problems, such as large sensors disturbing the sound field, a lack of accuracy and the risk of being destroyed by cavitation bubbles. Measurement systems based on optical fiber tips and piezo-electrical hydrophones will be presented, which fulfil the accuracy requirements and withstand ultrasound fields with high power and cavitation. The spatial distribution of sound field parameters such as positive and negative peak pressure, amplitudes of fundamentals, harmonics and sub-harmonics as well as the energy density and spectral density in several frequency ranges are determined in experiments. Finally, the determined field parameters are related to the cavitation effects by means of photometric analysis of perforated aluminium foil. Perforations as well as intentions are analyzed and quantified from scanner images of the exposed foil samples using special image processing software. The experiments indicate clear differences in the structure of the sound fields and the spectral properties between the several types of cleaning baths, transducer arrangements and excitations.

The spatial and temporal distribution of the temperature elevation caused by high-intensity therapeutic ultrasound (HITU) in a tissue-mimicking material (TMM) has been determined with magnetic resonance (MR) thermometry, infrared (IR) thermometry and a thermal test object with an integrated thin-film thermocouple at three different National Metrological Institutes (PTB/Germany, NPL/UK, INRIM/Italy). Results obtained from the different types of measurement are compared and some general aspects of the methods are discussed, particularly with regard to their suitability for the in vitro characterization of transducers for treatment planning.

The thermo acoustic sensor measures ultrasound field intensities by evaluating the warming of an absorber material. In the range of medical used ultrasound intensity there is a linear dependency between intensity and absorber warming. By using different absorber materials the sensitivity of the sensors can be influenced. Several materials were tested to make the absorber most sensitive. In addition to sensitivity measurements, measurements of linearity, response time and spatial resolution were performed for all sensors. Transfer functions between temperature differences and intensities were evaluated. Furthermore investigations on sensors with nonmetal housings show an increase in sensitivity. Combining materials and housings enables sensors for several intensities. A new sensor concept containing a half-bridge measurement circuit improves the evaluation and leads to handy and user- friendly intensity measurement applications in future.

In this work, a calibration phantom based on an electrical resistor heating rod is presented, which creates a temperature distribution similar to those in typical HIFU fields (diameter ∼2 mm, heating rate ∼10 K/s). The temperature distribution is measured first with a calibrated reference device and then with MR thermometry. Equal conditions for both measurements can be ensured by monitoring the voltage and current across the resistor during the heating. From the comparison of both measurements, the accuracy of the MR measurement can be assessed. The calibration phantom is MR compatible, and it has high reproducibility and low production costs. A wide variety of possible heating sequences can be employed and the phantom has a compact and easy setup. Thus, it can be used to quickly verify the results from newly developed MR thermometry sequences as well as to evaluate the uncertainties of existing sequences.

The characterization and quantification of High Intensity Therapeutic Ultrasound (HITU) fields is a difficult challenge, as appropriate sensors need to withstand very high local intensities in small focal regions and to measure them with a high reliability. Due to their high robustness and their small dimensions, fiber‐optic probes have proven to be a promising approach for this purpose. In this paper, a fiber‐optic displacement sensor and an interferometric optical set‐up are presented and the potential as well as the limitations of this set‐up are discussed.

To ensure the success of high-intensity focused ultrasound (HIFU) treatment, lesion formation and focus dislocation has to be monitored online. For an automatic and immediate stop of HIFU exposure in case of irregularities of the lesion formation a monitoring technique allowing real time feedback is needed. For this purpose, we propose the analysis of shear waves which are radiated from treated tissue when the exposure with HIFU is intermitted. By A-scans close to the focal region the changes in time-of-flight and magnitude of shear waves are investigated. It is shown that dislocation of the focus can be detected by a decrease of shear wave magnitude in regions below the original focus. Deformation of lesion comes along with considerable changes in magnitude and time-of-flight of the shear wave.

Thermal intensity sensors based on the transformation of the incident ultrasonic energy into heat inside a small cylindrical absorber have been developed at PTB in the past, in particular to determine the acoustic output of medical diagnostic ultrasound equipment. Currently, this sensor technique is being expanded to match the measurement challenges of high intensity therapeutic ultrasound (HITU) fields. At the high acoustic power levels as utilized in the clinical application of HITU transducers, beam characterization using hydrophones is critical due to the possible damage of the sensitive and expensive measurement devices. Therefore, the low-cost and robust thermal sensors developed offer a promising alternative for the determination of high intensity output beam profiles. A sensor prototype with a spatial resolution of 0.5 mm was applied to the beam characterization of an HITU transducer operated at several driving amplitude levels. Axial beam plots and lateral profiles at focus were acquired. The absolute continuous wave output power was, in addition, determined using a radiation force balance.