- 标题
- 摘要
- 关键词
- 实验方案
- 产品
-
MR temperature imaging using PRF phase difference and a geometric model-based fat suppression method
摘要： BACKGROUND: Because protons in fat do not exhibit a temperature-dependent frequency shift, proton resonance frequency shift (PRFS)-based MR thermometry always suffers from disturbances due to the presence of fats or lipids. OBJECTIVE: A new fat suppression method for PRFS-based MR thermometry is proposed to obtain accurate variation of phase angle. METHODS: Similar to the approach of separating fat and water with the two-point Dixon technique, we first scan a complex MR image for reference and then scan another complex image varying with temperature at the same TE point. Based on the conventional PRFS method, we use geometric relationships to remove the effect of fat on the variation of the phase angle. RESULTS: Two phantoms with different water-to-fat ratios are involved in the temperature mapping test. Experimental results show that the temperature images of two phantoms are approximated under the same conditions. CONCLUSIONS: The proposed fat suppression method is simple and effective for PRFS-based MR thermometry.
关键词： geometric model,MR thermometry,fat suppression,PRFS
更新于2025-09-23 15:22:29
-
Analytical model of the fundamental mode of 3D square split ring resonators
摘要： An analytical model is developed for the charge and potential distributions of the fundamental mode of three-dimensional (3D) singly split ring resonators (SRRs) with square cross section, which nowadays can be conveniently fabricated by additive manufacturing techniques. This model allows the derivation of approximate formulas for the equivalent capacitance and the analysis of the resonant properties of this type of SRRs at any frequency. The total capacitance is expressed as the sum of the usual gap capacitance and a surface capacitance associated with the charges on the SRR walls, which are determined from the solution for the irrotational part of the electric field of a square split cylinder obtained by conformal mapping. The applicability of the proposed model for a broad range of SRR parameters is demonstrated by comparing the resonance frequency of sample SRR configurations found theoretically with the corresponding values obtained by numerical simulations and experiments. Furthermore, the functions describing the charge and current mode profiles provided in this work can be instrumental in estimating the near-field interaction and the coupling constants of an ensemble of resonant 3D square split rings and thus for tailoring the response of metamaterials and other devices formed by these elements.
关键词： capacitance,resonance frequency,analytical model,split ring resonators,metamaterials
更新于2025-09-23 15:22:29
-
A linear state space model for photoacoustic imaging in an acoustic attenuating media
摘要： In photoacoustic imaging, ultrasound waves generated by a temperature rise after illumination of light absorbing structures are measured on the sample surface. These measurements are then used to reconstruct the optical absorption. We develop a method for reconstructing the absorption inside the sample based on a discrete linear state space reformulation of a partial differential equation that describes the propagation of the ultrasound waves. Fundamental properties of the corresponding state space model such as stability, observability and controllability are also analyzed. By using Stokes’ equation, the frequency dependent attenuation of the ultrasound waves is incorporated into our model, therefore the proposed method is of general nature. This approach allows for inhomogeneous probes with arbitrary absorption profiles and it accounts for the decrease in laser intensity due to absorption. Furthermore, it provides a method for optimizing the laser modulation signal such that the accuracy of the estimated absorption profile is maximized. Utilizing the optimized laser modulation signal yields an increase in reconstruction accuracy compared to short laser pulses as well as chirp modulation in many scenarios.
关键词： optimal temporal laser excitation,photoacoustic image reconstruction,acoustic attenuation,linear state space model
更新于2025-09-23 15:22:29
-
Identification of Maize Kernel Vigor under Different Accelerated Aging Times Using Hyperspectral Imaging
摘要： Seed aging during storage is irreversible, and a rapid, accurate detection method for seed vigor detection during seed aging is of great importance for seed companies and farmers. In this study, an artificial accelerated aging treatment was used to simulate the maize kernel aging process, and hyperspectral imaging at the spectral range of 874–1734 nm was applied as a rapid and accurate technique to identify seed vigor under different accelerated aging time regimes. Hyperspectral images of two varieties of maize processed with eight different aging duration times (0, 12, 24, 36, 48, 72, 96 and 120 h) were acquired. Principal component analysis (PCA) was used to conduct a qualitative analysis on maize kernels under different accelerated aging time conditions. Second-order derivatization was applied to select characteristic wavelengths. Classification models (support vector machine?SVM) based on full spectra and optimal wavelengths were built. The results showed that misclassification in unprocessed maize kernels was rare, while some misclassification occurred in maize kernels after the short aging times of 12 and 24 h. On the whole, classification accuracies of maize kernels after relatively short aging times (0, 12 and 24 h) were higher, ranging from 61% to 100%. Maize kernels with longer aging time (36, 48, 72, 96, 120 h) had lower classification accuracies. According to the results of confusion matrixes of SVM models, the eight categories of each maize variety could be divided into three groups: Group 1 (0 h), Group 2 (12 and 24 h) and Group 3 (36, 48, 72, 96, 120 h). Maize kernels from different categories within one group were more likely to be misclassified with each other, and maize kernels within different groups had fewer misclassified samples. Germination test was conducted to verify the classification models, the results showed that the significant differences of maize kernel vigor revealed by standard germination tests generally matched with the classification accuracies of the SVM models. Hyperspectral imaging analysis for two varieties of maize kernels showed similar results, indicating the possibility of using hyperspectral imaging technique combined with chemometric methods to evaluate seed vigor and seed aging degree.
关键词： hyperspectral imaging technology,standard germination tests,support vector machine model,accelerated aging,principal component analysis,maize kernel
更新于2025-09-23 15:22:29
-
Multiple topological transitions in twisted bilayer graphene near the first magic angle
摘要： Recent experiments have observed strongly correlated physics in twisted bilayer graphene (TBG) at very small angles, along with nearly flat electron bands at certain fillings. A good starting point in understanding the physics is a continuum model (CM) proposed by Lopes dos Santos et al. [Phys. Rev. Lett. 99, 256802 (2007)] and Bistritzer et al. [Proc. Natl. Acad. Sci. USA 108, 12233 (2011)] for TBG at small twist angles, which successfully predicts the bandwidth reduction of the middle two bands of TBG near the first magic angle θ0 = 1.05°. In this paper, we analyze the symmetries of the CM and investigate the low energy flat band structure in the entire moiré Brillouin zone near θ0. Instead of observing flat bands at only one 'magic' angle, we notice that the bands remain almost flat within a small range around θ0, where multiple topological transitions occur. The topological transitions are caused by the creation and annihilation of Dirac points at either K, K', or Γ points or along the high symmetry lines in the moiré Brillouin zone. We trace the evolution of the Dirac points, which are very sensitive to the twist angle, and find that there are several processes transporting Dirac points from Γ to K and K'. At the Γ point, the lowest energy levels of the CM are doubly degenerate for some range of twisting angle around θ0, suggesting that the physics is not described by any two-band model. Based on this observation, we propose an effective six-band model (up to second order in quasimomentum) near the Γ point with the full symmetries of the CM, which we argue is the minimal model that explains the motion of the Dirac points around Γ as the twist angle is varied. By fitting the coefficients from the numerical results, we show that this six-band model captures the important physics over a wide range of angles near the first 'magic' angle.
关键词： flat bands,twisted bilayer graphene,continuum model,magic angle,topological transitions,Dirac points
更新于2025-09-23 15:22:29
-
A Comparative Assessment of Different Modeling Algorithms for Estimating Leaf Nitrogen Content in Winter Wheat Using Multispectral Images from an Unmanned Aerial Vehicle
摘要： Unmanned aerial vehicle (UAV)-based remote sensing (RS) possesses the significant advantage of being able to efficiently collect images for precision agricultural applications. Although numerous methods have been proposed to monitor crop nitrogen (N) status in recent decades, just how to utilize an appropriate modeling algorithm to estimate crop leaf N content (LNC) remains poorly understood, especially based on UAV multispectral imagery. A comparative assessment of different modeling algorithms (i.e., simple and non-parametric modeling algorithms alongside the physical model retrieval method) for winter wheat LNC estimation is presented in this study. Experiments were conducted over two consecutive years and involved different winter wheat varieties, N rates, and planting densities. A five-band multispectral camera (i.e., 490 nm, 550 nm, 671 nm, 700 nm, and 800 nm) was mounted on a UAV to acquire canopy images across five critical growth stages. The results of this study showed that the best-performing vegetation index (VI) was the modified renormalized difference VI (RDVI), which had a determination coefficient (R2) of 0.73 and a root mean square error (RMSE) of 0.38. This method was also characterized by a high processing speed (0.03 s) for model calibration and validation. Among the 13 non-parametric modeling algorithms evaluated here, the random forest (RF) approach performed best, characterized by R2 and RMSE values of 0.79 and 0.33, respectively. This method also had the advantage of full optical spectrum utilization and enabled flexible, non-linear fitting with a fast processing speed (2.3 s). Compared to the other two methods assessed here, the use of a look up table (LUT)-based radiative transfer model (RTM) remained challenging with regard to LNC estimation because of low prediction accuracy (i.e., an R2 value of 0.62 and an RMSE value of 0.46) and slow processing speed. The RF approach is a fast and accurate technique for N estimation based on UAV multispectral imagery.
关键词： UAV,multispectral imagery,radiative transfer model,LNC,vegetation index,non-parametric regression
更新于2025-09-23 15:22:29
-
[IEEE 2018 IEEE 2nd International Conference on Dielectrics (ICD) - Budapest (2018.7.1-2018.7.5)] 2018 IEEE 2nd International Conference on Dielectrics (ICD) - Modelling Charge Generation and Transport in Low Density Polyethylene Irradiated by an Electron-Beam
摘要： One way to bypass charge generation due to injection in an insulator sandwiched between parallel electrodes and submitted to an applied voltage is to implant charges in the material with the help of an electron beam. The electrons position and quantity is theoretically known as long as the beam energy and beam current are known. Low density polyethylene (LDPE) has been characterized with in-situ space charge measurements by pulsed electroacoustic method during irradiation, and with ex-situ measurements while a DC voltage is applied. A fluid charge transport model has been developed using a commercial software, to reproduce the space charge behaviour during and after irradiation. Simulated results during irradiation are first compared to in-situ space charge measurements, in order to validate the model parameters related to e-beam irradiation. Simulations are then performed on post-irradiated samples, polarized under different electric fields. Space charge measurements and current measurements are available for comparison. Simulated results are in relatively good agreement with experimental ones as long as the model parameters are adapted to irradiated low density polyethylene, compared to a best set of parameters adapted uniquely for non-irradiated polyethylene.
关键词： charge generation and transport,fluid model,LDPE,ageing,electron-beam irradiation
更新于2025-09-23 15:22:29
-
Decimeter-Level Geolocation Accuracy Updated by a Parametric Tropospheric Model with GF-3
摘要： GaoFen-3 (GF-3) is a multi-polarization C-band synthetic aperture radar (SAR) satellite in China with a resolution of up to 1 m. Up to now, the geolocation accuracy of GF-3 could be improved to 3 m. According to the current study, there still exist meter-level geolocation residuals caused by atmospheric path delay after compensating with a static tropospheric model. In this paper, we compensate the residuals with the sophisticated tropospheric model based on real meteorological data. The experimental results show that the tropospheric model has an accuracy on the millimeter level, which can increase GF-3’s geolocation accuracy to several decimeters compared with the static tropospheric model.
关键词： GF-3,tropospheric model,geolocation accuracy,SAR
更新于2025-09-23 15:22:29
-
Automatic Rectification of the Hybrid Stereo Vision System
摘要： By combining the advantages of 360-degree field of view cameras and the high resolution of conventional cameras, the hybrid stereo vision system could be widely used in surveillance. As the relative position of the two cameras is not constant over time, its automatic rectification is highly desirable when adopting a hybrid stereo vision system for practical use. In this work, we provide a method for rectifying the dynamic hybrid stereo vision system automatically. A perspective projection model is proposed to reduce the computation complexity of the hybrid stereoscopic 3D reconstruction. The rectification transformation is calculated by solving a nonlinear constrained optimization problem for a given set of corresponding point pairs. The experimental results demonstrate the accuracy and effectiveness of the proposed method.
关键词： projection model,hybrid stereo vision,automatic rectification
更新于2025-09-23 15:22:29
-
Side-Slither Data-Based Vignetting Correction of High-Resolution Spaceborne Camera with Optical Focal Plane Assembly
摘要： Optical focal plane assemblies are increasingly being used in high-resolution optical satellite systems to enhance the width of the image using linear push-broom imaging. With this system, vignetting occurs in the area of overlap, affecting image quality. In this paper, using the characteristics of the side-slither data, we propose side-slither data-based vignetting correction of a high-resolution spaceborne camera with an optical focal plane assembly. First, the raw side-slither data standardization is used to ensure that each row has the same features. Then, with the spatial correlation of a gray-level co-occurrence matrix, the gray-level co-occurrence matrix is proposed to identify the uniform regions, to extract the sample points. Finally, due to the characteristics of compatible linear response and non-linear response, the power-law model was used to fit, and the Levenberg–Marquardt algorithm was used to fit the model. In the experiment, polynomial fitting, laboratory coefficients and on-orbit coefficients were used for comparison with the proposed method. The side-slither data can be treated as a uniform scene due to their characteristics, and the side-slither image that was corrected using the proposed method showed less than 1% change in mean value, a root-mean-square deviation value better than 0.1%, and the average streaking metrics were superior to 0.02. The results showed that the proposed method performs significantly better in the vignetting area.
关键词： push-broom,gray level co-occurrence matrix,high-resolution optical satellite,side-slither data,vignetting correction,Levenberg–Marquardt,power-law model,optical focal plane assembly
更新于2025-09-23 15:22:29