Metasurfaces are planar optics designed by arranging artificially fabricated nanostructures, so-called meta-atoms that impose unique optical scattering characteristics. These meta-atoms perform a role as individual scatterers that spatially modulate the complex amplitude of incident light at will. [1][2][3][4][5][6][7][8] The geometric parameters and material composition of meta-atoms are determined arbitrarily to impose desired scattering characteristics. Thanks to its compact and lightweight characteristics with outstanding light modulation capabilities, it has a great deal of attention, especially in the wearable display market [9][10][11] and compact imaging systems. [12][13][14][15][16][17] These optical devices generally require optical elements satisfying two aspects of functionality: extensive spatial frequency modulation capabilities (e.g., high numerical aperture (NA) lens, large-angle deflector) for efficient usage of the light path, and the angle tolerable operation for reliable transport of image information with a broad range of spatial frequency. However, the conventional metasurface designed by the forward design process, i.e., the arrangement of predesigned meta-atom, is difficult to satisfy the abovementioned requirements. Because the forward design process does not reflect the influence of interaction between adjacent meta-atom, the so-called near-field coupling effect. [18,19] Due to this effect, a performance deterioration is severely induced by an undesired nearfield coupling effect in engineering. [20] Furthermore, when the angle of incidence deviates from normal incidence, the horizontal component of the incident light k-vector is generated and induces severe near-field coupling effects between adjacent meta-atom. [21] Therefore, to apply metasurface to complex and demanding applications, such as wearable displays and compact imaging systems, the abovementioned limitation should be handled through inverse design approaches of the metasurface. The inverse design method, which defines a target optical response as an objective function, can consider the aforementioned near-field coupling effect by calculating an entire field of metasurfaces at a time.
The paper presents an efficient and reliable feature descriptor for human detection in a top-view depth image that uses two statistical values of mean and standard deviation. Human detection performance of our descriptor outperform Rauter that use mean value and Simplified Local Ternary Pattern (SLTP). To evaluate the human detection performance using our descriptor, we capture 559 positive and 2382 negative top-view depth images. We get 98.1% precision and 94.2% recall rates. Run-time performance of our descriptor is reasonably fast. Human detection using the proposed descriptor can be applicable in detecting multiple human and tracking real time in top-view depth images.
Focused ultrasound has been applied in brain therapeutics. Although focusing ultrasonic beams on multiple arbitrary regions under the guidance of magnetic resonance imaging(MRI) is needed for precise treatments, current therapeutic transducers with large pitch sizes have been optimized to focus on deep brain regions. While annular arrays can adjust the beam foci from cortical to deep regions, their circular shape may generate eddy current-induced magnetic flux during MRI. In this study, a quadrisected annular array is proposed to address these limitations. Methods: Conventional and quadrisected annular arrays with three elements were implemented by loading the electrode patterns onto an 850kHz 1-3 composite PZT disc, with a diameter of 31mm, including three rings. MR compatibilities were demonstrated by imaging an MRI phantom with pulse sequences for B0 and B1 mapping and spin-echo imaging. Acoustic beam profiles, with and without a macaque monkey skull, were measured. A quadrisected transducer was also used to open the blood-brain barrier(BBB). Results: The flip angle distortion improved by 20% in spin-echo MR imaging. The acoustic beam distortions shifting the focal point from 36 to 41mm and elongating the focal zone from 10 to 15 mm could be recovered to nearly the original values. BBB openings in the hippocampus and basal region were also demonstrated. Conclusion: The MR compatibility was improved by the increased resistance of the electrodes in the quadrisected array maintaining dynamic focusing capabilities. Significance: The quadrisected annular design can be a fundamental structure for a larger MR-compatible segmented array transducer generating multiple acoustic foci.
Intravascular ultrasound (IVUS) imaging employs high imaging frequencies (from 20 to 60 MHz) to visualize microvascular structures by sacrificing the depth of penetration with an increased signal attenuation. To compensate for the reduced echo signal-to-noise ratio, chirp-coded excitations, which require complex system configurations in high-frequency ultrasound imaging, are considered as suitable methods for rotational IVUS. Golay-coded excitations, which only need simple electronics and have no range sidelobe artifacts if the incident locations of the Golay pair are the same, have not been considered for rotational IVUS. In this study, computer simulations and experiments with a custom-made IVUS imaging system running at 30 fps are performed to investigate on the optimal operational conditions of Golay-coded excitations in rotational IVUS imaging. While the code length was not a significant factor affecting the range sidelobe level (RSLL), a change in the imaging frequency from 25 MHz to 66 MHz increased the RSLL from −65 dB to −45 dB with 2048 beamlines. In addition, an increase in the number of beamlines from 64 to 2048 reduced the RSLL from −17 dB to −50 dB in the simulation and from −14 dB dB to −38 dB in the experiment. To further decrease the RSLL, a weighted averaging method that compressed the coded pulse was proposed. The RSLL could be suppressed from −50 dB to −80 dB and from −38 dB to −53 dB in the simulation and experiment, respectively. Golay-coded excitation imaging was also successfully demonstrated on freshly harvested pig coronary arteries. INDEX TERMS Intravascular ultrasound, Golay-coded excitation, range sidelobe level, weighted averaging, high-frequency ultrasound.
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