Institute of Fundamental Technological Research

Damage identification methods based on structural modal parameters are influenced by the structure form, number of measuring sensors and noise, resulting in insufficient modal data and low damage identification accuracy. The additional virtual mass method introduced in this study is based on the virtual deformation method for deriving the frequency-domain response equation of the virtual structure and identify its mode to expand the modal information of the original structure. Based on the initial condition assumption that the structural damage was sparse, the damage identification method based on sparsity with l1 and l2 norm of the damage-factor variation and the orthogonal matching pursuit (OMP) method based on the l0 norm were introduced. According to the characteristics of the additional virtual mass method, an improved OMP method (IOMP) was developed to improve the localization of optimal solution determined using the OMP method and the damage substructure selection process, analyze the damage in the entire structure globally, and improve damage identification accuracy. The accuracy and robustness of each damage identification method for multi-damage scenario were analyzed and verified through simulation and experiment.

structural health monitoring (SHM), damage identification, virtual mass, sparse constraint, IOMP method

Damage identification for liquid–solid coupling structures remains a challenging topic due to the influence of liquid and the limitation of experimental conditions. Therefore, the adding mass method for damage identification is employed in this study. Adding mass to structures is an effective method for damage identification, as it can increase not only the experimental data but also the sensitivity of experimental modes to local damage. First, the fundamental theory of the adding mass method for damage identification is introduced. After that, the method of equating the liquid to the attached mass is proposed by considering the liquid–solid coupling. Finally, the effectiveness and reliability of damage identification, based on adding mass for liquid–solid coupling structures, are verified through experiments of a submerged cantilever beam and liquid storage tank.

structural health monitoring, damage identification, liquid-solid coupling, adding mass, sensitivity

Introduction
High-intensity focused ultrasound is finding increasing therapeutic use. However, the frequencies at which it operates are typically limited to below 5 MHz, preventing research into therapy with ultrahigh spatial precision. A reason for this is that the design and fabrication of high-frequency biomedical ultrasound transducers to produce high intensities is an engineering challenge, especially for operating frequencies above 30 MHz, primarily because of the acoustic impedance mismatch and the high attenuation of water of 6dB/cm at 50 MHz leading to a low penetration depth. Commonly used materials such as PZT do not have the ability to produce a high enough intensity, due to de-poling or cracking. A potential application of high-intensity high-frequency ultrasound is non-invasive microsurgery.

Methods
To overcome these problems, we used Y-36o Lithium Niobate (LiNbO3). This crystal has a high Curie temperature and is much more difficult to de-pole at high-power inputs. In addition, Y-36o LiNbO3 has a resonant frequency of 3.3 MHz mm-1, thus allowing for much thicker elements at higher frequencies compared to PZT. A bowl transducer was manufactured using a total of 7 0.5-mm thick elements (4 hexagonal and 5 pentagonal) with a maximum width of 25 mm. The bowl had a curvature radius of 50 mm. The transducer was microballoon-backed in order to simplify the manufacturing process. The pentagonal elements were linked and driven by a 50-dB amplifier, whereas the hexagonal elements were linked and driven by a 55-dB amplifier. To test the available working frequency; single element transducers were manufactured with element thickness ranging from 500 μm to 200 μm, having working frequencies between 6.6 MHz and 20 MHz.

Results
The multi-element focused transducer generated a modulated sound field with an enveloped wavelength of 550 kHz at a frequency of 6.6 MHz with a maximum peak-to-peak pressure of 24.3 MPa; equivalent to mechanical index of 4.7. The modulation could be varied by changing the phase of either the pentagonal or hexagonal linked elements. The microballoon-backed transducers had a 5% reduced acoustic output compared to the air-backed transducer. Single- element transducers produced a maximum peak-to-peak pressure of 14 MPa at 6.3 MHz in the acoustic focus at 12 mm. These transducers were capable of producing over 6 MPa and 4 MPa at the 3rd and 5th harmonics, respectively, corresponding to frequencies of 21 MHz and 35 MHz.

Conclusion
We have established that manufacturing a high frequency, high intensity, multi-element, focused ultrasound transducer using LiNbO3 is feasible. We have also shown it is possible to use the resonant frequency and up to the 5th harmonic to achieve higher working frequencies.

High-frequency ultrasound, Ultrasound transducer, MR-guided Focussed Ultrasound Surgery

Focused ultrasound surgery (FUS) is usually based on frequencies below 5 MHz—typically around 1 MHz. Although this allows good penetration into tissue, it limits the minimum lesion dimensions that can be achieved. In this study, we investigate devices to allow FUS at much higher frequencies, in principle, reducing the minimum lesion dimensions. Furthermore, FUS can produce deep-sub-millimeter demarcation between viable and necrosed tissue; high-frequency devices may allow this to be exploited in super cial applications which may include dermatology, ophthalmology, treatment of the vascular system, and treatment of early dysplasia in epithelial tissue. In this paper, we explain the methodology we have used to build high-frequency high-intensity transducers using Y-36°-cut lithium niobate. This material was chosen because its low losses give it the potential to allow very-high- frequency operation at harmonics of the fundamental operating frequency. A range of single-element transducers with center frequencies between 6.6 and 20.0 MHz were built and the transducers’ e ciency and acoustic power output were measured. A focused 6.6-MHz transducer was built with multiple elements operating together and tested using an ultrasound phantom and MRI scans. It was shown to increase phantom temperature by 32°C in a localized area of 2.5 × 3.4 mm in the plane of the MRI scan. Ex vivo tests on poultry tissue were also performed and shown to create lesions of similar dimensions. This study, therefore, demonstrates that it is feasible to produce high-frequency transducers capable of high-resolution FUS using lithium niobate.

Lithium Niobite, Ultrasound Transducer, MRI-Guided ultrasound, Microsurgery

Focused ultrasound surgery (FUS) is usually based on frequencies below 5 MHz, typically around 1 MHz. Whilst this allows good penetration into tissue, it limits the minimum lesion dimensions that can be achieved. In the study reported here, we investigated devices to allow FUS at much higher frequencies, therefore in principle reducing the minimum lesion dimensions. We explain the methodology we have used to build high-frequency high-intensity transducers using Y-36o cut lithium niobate. This material was chosen as its low losses give it the potential to allow very high-frequency operation at harmonics of the fundamental operating frequency. A range of single element transducers with a centre frequency between 6.6 MHz and 20.0 MHz was built and the transducers’ efficiency and acoustic power output were measured. A focussed 6.6-MHz transducer was built with multiple elements operated together and tested using an ultrasound phantom and MRI scans. It was shown to increase phantom temperature by 32OC in a localised area of 2.5 mm × 3.4 mm in the plane of the MRI scan. This study therefore demonstrates that it is feasible to produce high-frequency transducers capable of high-resolution focused ultrasound surgery using lithium niobate.

FUS, high frequency, lithium niobate, high resolution, transducer manufacture, MRI compatibility