Shaoying Huang
Email Address
surhshao@nus.edu.sg
Organizational Units
SURGERY
dept
YONG LOO LIN SCH OF MEDICINE
faculty
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Publication A review on reconfigurable liquid dielectric antennas(MDPI AG, 2020) Motovilova, E.; Huang, S.Y.; SURGERYThe advancements in wireless communication impose a growing range of demands on the antennas performance, requiring multiple functionalities to be present in a single device. To satisfy these different application needs within a limited space, reconfigurable antennas are often used which are able to switch between a number of states, providing multiple functions using a single antenna. Electronic switching components, such as PIN diodes, radio-frequency micromechanical systems (RF-MEMS), and varactors, are typically used to achieve antenna reconfiguration. However, some of these approaches have certain limitations, such as narrow bandwidth, complex biasing circuitry, and high activation voltages. In recent years, an alternative approach using liquid dielectric materials for antenna reconfiguration has drawn significant attention. The intrinsic conformability of liquid dielectric materials allows us to realize antennas with desired reconfigurations with different physical constraints while maintaining high radiation efficiency. The purpose of this review is to summarize different approaches proposed in the literature for the liquid dielectric reconfigurable antennas. It facilitates the understanding of the advantages and limitations of this technology, and it helps to draw general design principals for the development of reconfigurable antennas in this category. © 2020 by the authors.Publication Water-Tunable Highly Sub-Wavelength Spiral Resonator for Magnetic Field Enhancement of MRI Coils at 1.5 T(Institute of Electrical and Electronics Engineers Inc., 2019) Motovilova, E.; Sandeep, S.; Hashimoto, M.; Huang, S.Y.; SURGERYIn magnetic resonance imaging (MRI), several studies have demonstrated that the metamaterial-based structures can effectively improve the sensitivity, and thus the signal-to-noise ratio (SNR), of receiving radio-frequency (RF) coils. However, the use of metamaterials for this type of the MRI application is often limited due to the bulkiness of the metamaterial structure at RF wavelengths and a lack of frequency tunability of the final design. In this work, we propose a planar compact sub-wavelength (?/50) spiral resonator to increase the sensitivity of a receive coil with frequency tunability for the 1.5 T MRI. Its double-layered spiral design with a cavity embedded in the substrate between the two spirals allows water deposition for frequency tuning. At the resonance frequency of 64 MHz, the spiral resonator shows a 24% improvement in terms of the B1- field at the depth of 30 mm into a load experimentally. Even at a penetration depth as much as 60 mm (deep brain in the case of head imaging), an enhancement of 9% was observed. Moreover, the magnetic field enhancement comes with a decrease (10%) in specific absorption rate (SAR). In terms of tuning, by controlling the water level in the cavity, the proposed spiral resonator shows a wide tuning range of 35 MHz, centered around 64 MHz, with high tunability sensitivity (2.4-0.75 MHz/ml or 15-4.8 MHz/mm), which is due to the fact that the tuning cavity is located between the two spirals, where the fields are highly confined. © 2013 IEEE.Publication Image quality improvement and memory-saving in a permanent-magnet-array-based MRI system(MDPI AG, 2020) Gong, J.; Yu, W.; Huang, S.Y.; SURGERYPoint-of-care magnetic resonance imaging (MRI) requires clear images within a short scanning time, a small footprint of the scanner, and relatively low memory required for image reconstruction. A permanent magnet array (PMA)-based MRI system is a good candidate to supply a magnetic field due to its compactness and low power consumption. However, it has relatively inhomogeneous magnetic field and thus non-linear gradients, which results in location-dependent k-spaces (so called local k-spaces) and uneven signal point populations in the local k-spaces, compromising the image quality. Moreover, owing to the non-linearity, imaging reconstruction using Fourier transform does not work, which leads to an increase in the required computation memory. In this study, in order to improve the image quality, the approaches of compensating the uneven signal point population by increasing the numbers of sampling points or rotation angles are investigated in terms of their impacts on image quality improvement, acquisition time, image reconstruction time, and memory consumption. Both methods give a significant improvement on image image quality although they result in a large and dense encoding matrix and thus a large memory consumption. To lower the memory consumption, it is further proposed to transform such a matrix to frequency domain where the matrix could be sparse. Moreover, a row-wise truncation to the transformed encoding matrix is applied to further reduce the memory consumption. Through the results of numerical experiments, it is shown that the required memory for calculation can effectively be reduced by 71.6% while the image becomes clearer by increasing the number of sampling point and/or the number of rotation angles. With the successful demonstration where improved image quality and a lowered memory required can be obtained simultaneously, the proposed study is one step forward for a PMA-based MRI system towards its targeted point-of-care application scenario. © 2020 by authors.Publication Effects of Encoding Fields of Permanent Magnet Arrays on Image Quality in Low-Field Portable MRI Systems(Institute of Electrical and Electronics Engineers Inc., 2019) Gong, J.; Huang, S.Y.; Ren, Z.H.; Yu, W.; SURGERYLow-cost and portable magnetic resonance imaging (MRI) may make this imaging modality more accessible. Permanent-magnet-array is an option to supply a static magnetic field (B-field) with portability, low cost, and no power consumption. However, it has low field strength. Moreover, it does not have linear gradients, thus the signals and the images are not linked by the Fourier transformation as they are in a conventional system. The B-field generated by an array and called spatial-encoding-magnetic-field (SEM), is spatially non-linear and always on. Such an SEM, in terms of the field strength, direction, homogeneity, pattern and its field pattern variation, is related to the image quality. This relation is crucial because it can be used to guide the magnet and system design for high image quality and portability. However, it has not been systematically studied. In this paper, the characteristics of the SEMs from different magnet array designs are identified. Due to the non-linearity of the SEMs, local structural similarity (SSIM) index is proposed to evaluate the region-dependent image quality, and local k-space is applied to analyze the region-dependent effects of these SEMs on image reconstruction. Moreover, point spread function is applied to analyze the overall effect of the SEMs on the quality of reconstructed images. Besides the intrinsic effects of the SEMs, those of the external factors, e.g. the receive coil sensitivity, are analyzed. This study identifies the unique characteristics of the SEMs in a permanent-magnet-array-based MRI system, and offers methods to analyze the unique relation between the image quality and the field. It can not only guide the magnet designs but also trigger more design ideas, e.g., the design of the mechanical movement of the magnet array, and that of the static magnetic field shimming coils, paving the way towards a low-field MRI system with practical portability. © 2013 IEEE.Publication An Irregular-Shaped Inward-Outward Ring-Pair Magnet Array with a Monotonic Field Gradient for 2D Head Imaging in Low-Field Portable MRI(Institute of Electrical and Electronics Engineers Inc., 2019) Ren, Z.H.; Gong, J.; Huang, S.Y.; SURGERYWe present a design and the optimization of an irregular-shaped inward-outward (IO) ring-pair magnet array that generates a 1D monotonic field pattern for 2D head imaging in a low-field portable magnetic resonance imaging (MRI) system. The magnet rings are discretized into fan-shaped ring segments with varying outer radii for the design and optimization. Besides, the inner radii of ring-pairs are tapered from outside in to provide the controlled field inhomogeneity. Genetic algorithm (GA) was used, and a current model for a fan-shaped ring segment was derived to have a fast forward calculation in the optimization. A monotonic field pattern is successfully obtained along the x-direction in a cylindrical field of view (FoV), with a relatively strong magnetic field (132.98 mT) and the homogeneity of 151840 ppm. The proposed array was further evaluated by applying its field as a spatial encoding magnetic field (SEM) for imaging by using simulation. Due to the field monotonicity, the reconstructed image by applying the fields of the proposed array shows clearer features (a higher structural similarity index) with a reduced error rate compared to that using a sparse dipolar Halbach array. The proposed magnet array is a promising alternative to supply SEM for imaging in a permanent-magnet-based low-field portable MRI system. © 2013 IEEE.