High-power and time-reversed ultrasound based on FEM simulations and experiments

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Simulation of Forward Propagated Signals in Acoustic Time Reversal

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Phased laser diode array permits selective excitation of ultrasonic guided waves in coated bone-mimicking tubes

This paper validates simulation predictions, which state that specific modes could be enhanced in quantitative ultrasonic bone testing. Tunable selection of ultrasonic guided wave excitation is useful in non-destructive testing since it permits the mediation of energy into diagnostically useful modes while reducing the energy mediated into disturbing contributions. For instance, it is often challenging to distinguish and extract the useful modes from ultrasound signals measured in bone covered by a soft tissue. We show that a laser diode array can selectively excite ultrasound in bone mimicking phantoms. A fiber-coupled diode array (4 elements) illuminated two solid tubes (2-3mm wall thickness) embraced by an opaque soft-tissue mimicking elastomer coating (5mm thick). A predetermined time delay matching the selected mode and frequency was employed between the outputs of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results suggest that this array reduces the disturbances caused by the elastomer cover and so pave way to permit non-contacting in vivo guided wave ultrasound assessment of human bones. The implementation is small, inexpensive, and robust in comparison with the conventional pulsed lasers. Published by AIP Publishing.Peer reviewe

Coded excitation speeds up the detection of the fundamental flexural guided wave in coated tubes

The fundamental flexural guided wave (FFGW) permits ultrasonic assessment of the wall thickness of solid waveguides, such as tubes or, e.g., long cortical bones. Recently, an optical non-contact method was proposed for ultrasound excitation and detection with the aim of facilitating the FFGW reception by suppressing the interfering modes from the soft coating. This technique suffers from low SNR and requires iterative physical scanning across the source-receiver distance for 2D-FFT analysis. This means that SNR improvement achieved by temporal averaging becomes time-consuming (several minutes) which reduces the applicability of the technique, especially in time-critical applications such as clinical quantitative ultrasound. To achieve sufficient SNR faster, an ultrasonic excitation by a base-sequence-modulated Golay code (BSGC, 64-bit code pair) on coated tube samples (1-5 mm wall thickness and 5 mm soft coating layer) was used. This approach improved SNR by 21 dB and speeded up the measurement by a factor of 100 compared to using a classical pulse excitation with temporal averaging. The measurement now took seconds instead of minutes, while the ability to determine the wall thickness of the phantoms was maintained. The technique thus allows rapid noncontacting assessment of the wall thickness in coated solid tubes, such as the human bone. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).Peer reviewe

Deteriorated stabilization of walking in individuals with spastic cerebral palsy revealed by a simulated tripping perturbation

Abstract. The aim of this study was to make use of a pr eviously introduced method of a simulated tripping perturbation to examine stability of walking in individuals with and with out cerebral palsy. This tripping perturbation is a forward-dynamics analys is, and it works so that in a subjec t-specific muscle-driven simulation of walking, created from experimental gait data, a force is applied to the swin g-foot, and resulting changes in the kinematics are observed. Here, changes in the fore-aft position of the trunk were analyzed. Subjects were 10 children with spastic diplegic cerebral palsy and eight un impaired children walking at their self-sel ected speed. Several tripping perturbations throughout the swing phase were performed, and each perturbation was used to analyze stability at a respective instant of time. At a given instant of time, walking was defined as stable if after initially deviating from its unpertur bed position, because of the pertur bation, the trunk then approached and stayed close to that position. Walking was in turn de fined as unstable if the trunk moved away from its unperturbed position. All unimpaired subjects were stable at some point of their swi ng phases, wherea s six out of the ten CP subjects were never stable. The unimpaired subjects we re statistically signific antly stable for a larger percentage of the swing phase t han the CP subjects (p < 0. 05). On the average, unimpaired subjects were stable for 8.5% of the swing phase while CP subjects were st able for 1.5% of the sw ing phase. These findings demonstrate that a tripping perturbation is capable of detecting a balance impa irment. Thus, it shows promise as a tool for rehabilitation.peerReviewe

FEM-based time-reversal enhanced ultrasonic cleaning

Pipe fouling is a challenging problem in many industrial applications. Current cleaning techniques require halting the production during the cleaning phase and the existing methods are unable to do targeted cleaning, even though fouling is often localized to certain areas inside the pipeline. To address this issue, we use FEM-simulated, time-reversed signals to focus ultrasound power onto a pre-determined location: a fouled pipe residing inside a Plexiglas container. Ultrasound cleaning with similar acoustic power was compared to the time-reversal enhanced method in terms of cleaning efficiency. The cleaning efficiency was determined by measuring how much fouling, by mass, both protocols removed from the surface of a Plexiglas pipe, using similar input electric power and equal cleaning time. Our results indicate that the proposed time-reversal-based technique removes three times more fouling than the standard ultrasound cleaning without focusing. The study extends our previous paper on FEM-based time-reversal focusing [1].Peer reviewe

Association between low-frequency ultrasound and hip fractures - comparison with DXA-based BMD

Background: New methods for diagnosing osteoporosis and evaluating fracture risk are being developed. We aim to study the association between low-frequency (LF) axial transmission ultrasound and hip fracture risk in a population-based sample of older women. Methods: The study population consisted of 490 community-dwelling women (78 – 82 years). Ultrasound velocity (V LF ) at mid-tibia was measured in 2006 using a low-frequency scanning axial transmission device. Bone mineral density (BMD) at proximal femur measured using dual-energy x-ray absorptiometry (DXA) was used as the reference method. The fracture history of the participants was collected from December 1997 until the end of 2010. Lifestyle-related risk factors and mobility were assessed at 1997. Results: During the total follow-up period (1997 – 2010), 130 women had one or more fractures, and 20 of them had a hip fracture. Low V LF (the lowest quartile) was associated with increased hip fracture risk when compared with V LF in the normal range (Odds ratio, OR = 3.3, 95% confidence interval (CI) 1.3-8.4). However, V LF was not related to fracture risk when all bone sites were considered. Osteoporotic femoral neck BMD was associated with higher risk of a hip fracture (OR = 4.1, 95% CI 1.6-10.5) and higher risk of any fracture (OR = 2.4, 95% CI 1.6-3.8) compared to the non-osteoporotic femoral neck BMD. Decreased V LF remained a significant risk factor for hip fracture when combined with lifestyle-related risk factors (OR = 3.3, 95% CI 1.2-9.0). Conclusion: Low V LF was associated with hip fracture risk in older women even when combined with lifestyle-related risk factors. Further development of the method is needed to improve the measurement precision and to confirm the results.peerReviewe

FEM-based time-reversal technique for an ultrasonic cleaning application

Ultrasound provides a way to clean fouled pipes in industrial settings without interrupting the production. Ultrasonic clamp-on cleaners are used to clean pipes, but they typically cannot focus the cleaning power. This leads to insufficient cleaning results in cases where the fouling is localized to certain parts of the pipe. To solve this issue, we propose a finite-element method -based time-reversal (FEM-TR) technique for controlling the acoustic field produced by an ultrasonic clamp-on cleaner. We demonstrate by simulations and experimental validation that FEM-TR can be used to control the acoustic field in clamp-on cleaners featuring relatively few narrow-band and high-power transducers. The proposed method allows us to focus sound to arbitrary pre-selected locations inside the structure

Phase-delayed laser diode array allows ultrasonic guided wave mode selection and tuning

Selecting and tuning modes are useful in ultrasonic guided wave non-destructive testing (NDT) since certain modes at various center frequencies are sensitive to specific types of defects. Ideally one should be able to select both the mode and the center frequency of the launched waves. We demonstrated that an affordable laser diode array can selectively launch either the S0 or A0 ultrasonic wave mode at a chosen center frequency into a polymer plate. A fiber-coupled diode array (4 elements) illuminated a 2 mm thick acrylic plate. A predetermined time delay matching the selected mode and frequency was employed between the output of the elements. The generated ultrasound was detected by a 215 kHz piezo receiver. Our results imply that this array permits non-contacting guided wave ultrasonic NDT. The solution is small, affordable, and robust in comparison to conventional pulsed lasers. In addition, it does not require experienced operators.peerReviewe