China
Africa
Explore chapters and articles related to this topic
Bioengineering Aids to Reproductive Medicine
Published in Sujoy K. Guba, Bioengineering in Reproductive Medicine, 2020
Over a short axial distance from the face of the transducer, the ultrasonic field intensity varies in a complex manner with peaks and zeros. This zone is termed the “near field”. The ultrasonic wave undergoes multiple reflections in the region introducing “reverberation” signals. Further along the axis the “far field” pattern is established wherein the intensity gradually declines with increasing distance from the transducer. The non uniformity in the intensity in the near field produces the near field artifact, which leads to poor imaging of superficial structures. A synthetic gel spacer pad is now available16 for placement between the transducer and the body surface to minimize the reverberation artifacts and improve image quality.
Optical Fiber Ultrasonic Sensors
Published in Marvin C. Ziskin, Peter A. Lewin, Ultrasonic Exposimetry, 2020
Helen L. W. Chan, Kin S. Chiang
An experimental ultrasonic sensor is illustrated in Figure 6. Circularly polarized light derived from a He-Ne laser at λ = 633 nm followed by a quarter-wave plate is used to excite both polarized waves in a polarization-maintaining fiber. The fiber used is a bow-tie fiber from York VSOP with 4-μm core diameter and 80-μm cladding diameter. A short length of the fiber is supported by a rotatable fiber holder mounted on a three-axis translator, and submerged in distilled water. The ultrasonic wave to be measured is generated by a transducer in water whose beam is incident upon the fiber normally. The output light is passed through an analyzer, which is a linear polarizer with its principal axis set at 45° to the principal axes of the fiber, and is detected by a PIN photodiode. Since the phase difference between the two polarized waves is sensitive to the axial strain along the fiber,22 a section of the fiber is tensioned manually to control the bias phase Δφb in Equation 3). It should be noted that neither real-time control of the polarization state of the input light nor any alignment of the principal axes of the fiber to the input optics is required. Only stretching a short length of the fiber is required to compensate for bias shifting due to environmental effects. This sensor has been used in the characterization of low- and high-power ultrasonic transducers.23,24 Details are given in the next sections.
Safety of diagnostic ultrasound
Published in Peter R Hoskins, Kevin Martin, Abigail Thrush, Diagnostic Ultrasound, 2019
Temperature increase is the dominant hazard during diagnostic ultrasound procedures. However, a second hazard arises from the presence of gas within soft tissues. Gas may occur naturally, for instance air in the alveoli of lungs or gas in the intestines. Alternatively gas bubbles may be introduced deliberately in the form of gas contrast agents. Finally, gas nuclei may exist in crevices on the surface of solid bodies such as renal calculi. When such ‘gas bodies’ are exposed to ultrasound they can induce a variety of local mechanical effects that may cause damage to cells or tissue structures. The oscillation of the gas surface that causes the mechanical effects is termed ‘acoustic cavitation’ for a free bubble, or ‘gas body activation’ for the more general case. In these cases the hazard, a bubble caused to oscillate by the ultrasonic wave, gives a risk of tissue damage that will vary depending on the size of the oscillation, where the gas is, and what cellular changes result.
What happens to brain outside the thermal ablation zones? An assessment of needle-based therapeutic ultrasound in survival swine
Published in International Journal of Hyperthermia, 2022
Benjamin Szewczyk, Matthew Tarasek, Zahabiya Campwala, Rachel Trowbridge, Zhanyue Zhao, Phillip M. Johansen, Zachary Olmsted, Chitresh Bhushan, Eric Fiveland, Goutam Ghoshal, Tamas Heffter, Farid Tavakkolmoghaddam, Charles Bales, Yang Wang, Dhruv Kool Rajamani, Katie Gandomi, Christopher Nycz, Erin Jeannotte, Shweta Mane, Julia Nalwalk, E. Clif Burdette, Gregory Fischer, Desmond Yeo, Jiang Qian, Julie Pilitsis
The FEM model was implemented in COMSOL Multiphysics 5.6 (COMSOL AB, Stockholm, Sweden) to model the probe’s mechanical deformation, resultant applicator stationary acoustic pressure field, and the thermal dose threshold of 70 CEM43. A cross-sectional model of the probe’s geometry was created, surrounded by a 100 mm × 100 mm (L × W) 2 D acoustic medium. The probe material parameters were reported previously [19], and the brain tissue referenced acoustic medium material properties are described in Table 1. Far-field and thermally insulated boundary conditions were applied to the acoustic medium, and no reflection of ultrasonic waves from edges was assumed. A frequency domain study was conducted to obtain the acoustic pressure field produced by the applicator, applying a tetrahydral mesh with a maximum element size of λ/6, where λ represents the wavelength of the produced ultrasonic wave within the medium.
Incisionless MR-guided focused ultrasound: technical considerations and current therapeutic approaches in psychiatric disorders
Published in Expert Review of Neurotherapeutics, 2020
Thomas Kinfe, Andreas Stadlbauer, Klemens Winder, Rene Hurlemann, Michael Buchfelder
The different acoustic velocities of the cranial vault (outer layer = 3000 m/sec; diploie = 2700 m/sec; inner layer = 3000 m/sec) represent another considerable confounder leading to relevant heterogeneity in the ultrasonic wave speed, distribution and absorption rate [31,35,36]. These technical confounders of the past has been surmounted mainly by applying noninvasive aberration correction techniques, in particular a correcting phase (time-reversal processing; phase conjugation focusing) on the transmit signal of each component of the ultrasound array permitting the therapeutic use of FUS in a noninvasive manner. Thermal ablation (coagulation necrosis) induced by HIFU (MRgFUS at 650 kHz) leads to protein denaturation means of neuronal cell death. It is noteworthy that the ratio between the acoustic intensities of brain tissue (treatment target area) and the skull bone decreases with the intended targets distance to the skull bone/base. Hence, some proposed targets (e.g. for certain neurological indications (e.g. pedunculopontine nucleus (PPN) for Parkinson´s disease), which are located near bone structures (skull base heating), may not be suitable approaches for intracranial ultrasound therapy.
Ultrasound assisted-phytofabricated Fe3O4 NPs with antioxidant properties and antibacterial effects on growth, biofilm formation, and spreading ability of multidrug resistant bacteria
Published in Artificial Cells, Nanomedicine, and Biotechnology, 2019
Based on bottom-up and top-down approaches, there are many ways for preparation of Fe3O4 NPs including hydrothermal synthesis, thermal decomposition, ultrasound-assisted reduction, co-precipitation, electrochemical synthesis, and laser pyrolysis techniques as chemical and physical methods [15]. These methods have an advantage by the uniformity of MNPs distribution and disadvantages by consumption of toxic and expensive materials in MNPs preparation [16]. In recent years, green synthesis was introduced and applied by many scientists as a novel and effective process [17,18]. Several types of living organisms such as microorganisms (specifically magnetotactic bacteria), plants, and fungi were used for MNPs synthesis [19]. Among these organisms, plants have the advantages of more biocompatibility and availability than microbes and fungi [20]. These advantages are caused by this fact that plants have various secondary metabolites like flavonoids, flavonols, and terpenoids which they can contribute in the reduction and stabilizing of metal ions and MNPs structure, respectively [21]. Major disadvantages of green synthesis method are agglomeration and ununiform size and shape of NPs. For reducing these unsuitable results, we used ultrasonic wave as ultrasound-assisted reduction method. Advantages of this strategy were reported for the biosynthesis of Pd/Fe3O4 nanocatalyst by green tea leaves [22].