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This article reports on the long-term variation of the spectral smile and the full-width at half-maximum (FWHM) of the sensor response function for the Hyperspectral Imager Suite (HISUI) onboard the International Space Station (ISS). HISUI is an optical hyperspectral imager comprising two subsystems: a visible-and-near-infrared (VNIR) system covering 400–980 nm and a short-wave infrared (SWIR) system covering 895–2481 nm. On the basis of atmospheric corrections for actual observation images, we found that the characteristics of the spectral smile and the FWHM can be classified into five major periods. The results show that the first two transitions of the periods are due to updates of the smile correction table used in HISUI and that the last one is related to the occurrence of vertical stripes in the HISUI images. However, two unexpected changes are observed. One is a sudden increase in the spectral smile in the SWIR data to 2.0–2.1 nm on September 30, 2021, which was just three days after the second update of the smile correction table. The other is a sudden improvement on January 5, 2022, showing almost zero spectral smile and a nearly constant FWHM. Because neither sudden change is associated with HISUI or ISS operations/events, the optical and electrical systems of HISUI might exhibit instability, leading to sporadic changes of the spectral smile and FWHM. The findings of this study therefore suggest that, although the postlaunch spectral smile correction has been completed, continued long-term evaluation of the spectral stability is important.

This article presents the design and implementation of the magnetic particle imaging (MPI) scanner for phantom imaging of the potential tracer agents for MPI applications. The field-free point (FFP)-based spatial encoding of 4.3 T/m was implemented so that it helps to localize the superparamagnetic iron oxide nanoparticles (SPIONs) response inside the field of view (FOV). Solenoid drive coil and gradiometric receive coil were constructed to achieve 15 mT (peak amplitude) at 9.3 kHz. Moreover, a magnetic particle spectrometer (MPS) was utilized at 9.9 kHz for the characterization of relaxation time, frequency spectrum, and full-width at half-maximum (FWHM) of the particle signals in response to the excitation field at 9.9 kHz. The X-space algorithm was designed for the post-processing of the particle signals in the MPI scanner. Dextran-coated Perimag and carboxydextran-coated Vivotrax plus were characterized with MPS followed by the phantom imaging with MPI scanner. Perimag was found as fast relaxation sample (low relaxation time), narrow pulsewidth (high spatial resolution), and higher signal intensity compared to the Vivotrax plus. [ABSTRACT FROM AUTHOR]

This article reports on the initial analysis of spectral smile calibration of the Hyperspectral Imager Suite (HISUI) onboard the International Space Station, which has been continuously acquiring data since September 4, 2020. HISUI is an optical hyperspectral imager consisting of two subsystems: VNIR covering 400–980 nm at intervals of 10 nm and SWIR covering 895–2481 nm at 12.5-nm intervals. Based on the atmospheric correction for actual observation images, we assessed cross-track dependences of the wavelength deviation (spectral smile) and the full-width at half-maximum (FWHM) of the HISUI response function. We found that significant spectral smile was observed, with maximum variations of 1.8 nm in VNIR and 4.3–4.5 nm in SWIR. In addition, the cross-track variation of FWHM was observed with maximum variations of 5.0 nm for VNIR and 2.5–3.5 nm for SWIR. We used the results to model the smile functions to update a smile correction table in the internal calibration system of HISUI. Then, we evaluated how the smile functions reduce the spectral smile in the data acquired after the update on September 27, 2021. We confirmed that VNIR showed a nearly flat profile within 0.25 nm with a nearly constant FWHM. For SWIR, although a slight amount of spectral smile and a variation of FWHM were still observed partly due to wavelength dependence in the spectral smile, the spectral smile was reduced to less than ~2.2 nm. This study demonstrated that wavelength calibration using actual observation images for ground surfaces is important for the characterization of hyperspectral sensors. [ABSTRACT FROM AUTHOR]

Ultrasound shear wave elastography (SWE) is an increasingly used noninvasive modality for quantitative evaluation of tissue mechanical properties. SWE typically uses an acoustic radiation force to produce laterally propagating shear waves that are tracked in the spatial and temporal domains, in order to obtain the wave velocity. One of the ways to study the viscoelasticity is through studying the shear wave phase velocity dispersion curves. Shear wave attenuation can be also characterized in viscoelastic tissues with methods that use multiple lateral data samples. In this article, we present an alternative method for measuring the shear wave attenuation without using a rheological model two-point frequency shift (2P-FS). The technique uses information related to the amplitude spectra FS of shear waves measured at only two lateral locations. The theoretical basis for the 2P-FS is derived and validated. We examined how the first signal position and the distance between the two locations affect the shear wave attenuation estimation in the 2P-FS method. We tested this new method on digital phantom data created using the local interaction simulation approach (LISA) in viscoelastic media. Moreover, we tested data acquired from custom-made tissue-mimicking viscoelastic phantom experiments and ex vivo porcine liver measurements. We compared results from the 2P-FS method with the other two techniques used for assessing a shear wave attenuation: the FS-based method and the attenuation-measuring ultrasound shear wave elastography (AMUSE) technique. In addition, we evaluated the 2P-FS algorithm with different levels of added white Gaussian noise to the shear wave particle velocity using numerical phantoms. Tests conducted showed that the 2P-FS method gives robust results based on only two measurements and can be used to measure attenuation of viscoelastic soft tissues. [ABSTRACT FROM AUTHOR]

Hyperspectral imaging has been widely applied in remote sensing scientific fields. For this study, hyperspectral imaging data covering the spectral region from 400 to 1000 nm were collected from an unmanned aerial vehicle visible/near-infrared imaging hyperspectrometer (UAV-VNIRIS). Theoretically, the spectral calibration parameters of the UAV-VNIRIS measured in the laboratory should be refined when applied to the hyperspectral data obtained from the UAV platform due to variations between the laboratory and actual flight environments. Therefore, accurate spectral calibration of the UAV-VNIRIS is essential to further applications of the hyperspectral data. Shifts in both the spectral center wavelength position and the full-width at half-maximum (FWHM) were retrieved using two different methods (Methods I and II) based on spectrum matching of atmospheric absorption features at oxygen bands near 760 nm and water vapor bands near 820 and 940 nm. Comparison of the spectral calibration results of these two methods over the calibration targets showed that the derived center wavelength and FWHM shifts are similar. For the UAV-VNIRIS observed data used here, the shifts in center wavelength derived from both Methods I and II over the three absorption bands are less than 0.13 nm, and less than 0.22 nm in terms of FWHM. The findings of this paper revealed: 1) the UAV-VNIRIS payload on the UAV platform performed well in terms of spectral calibration; and 2) the applied methods are effective for on-orbit spectral calibration of the hyper spectrometer.

Hyperspectral imaging has been widely applied in remote sensing scientific fields. For this study, hyperspectral imaging data covering the spectral region from 400 to 1000 nm were collected from an unmanned aerial vehicle visible/near-infrared imaging hyperspectrometer (UAV-VNIRIS). Theoretically, the spectral calibration parameters of the UAV-VNIRIS measured in the laboratory should be refined when applied to the hyperspectral data obtained from the UAV platform due to variations between the laboratory and actual flight environments. Therefore, accurate spectral calibration of the UAV-VNIRIS is essential to further applications of the hyperspectral data. Shifts in both the spectral center wavelength position and the full-width at half-maximum (FWHM) were retrieved using two different methods (Methods I and II) based on spectrum matching of atmospheric absorption features at oxygen bands near 760 nm and water vapor bands near 820 and 940 nm. Comparison of the spectral calibration results of these two methods over the calibration targets showed that the derived center wavelength and FWHM shifts are similar. For the UAV-VNIRIS observed data used here, the shifts in center wavelength derived from both Methods I and II over the three absorption bands are less than 0.13 nm, and less than 0.22 nm in terms of FWHM. The findings of this paper revealed: 1) the UAV-VNIRIS payload on the UAV platform performed well in terms of spectral calibration; and 2) the applied methods are effective for on-orbit spectral calibration of the hyper spectrometer. [ABSTRACT FROM PUBLISHER]

The magnetic particle imaging method allows for the quantitative determination of spatial distributions of superparamagnetic nanoparticles in vivo. Recently, it was shown that the 1-D magnetic particle imaging process can be formulated as a convolution. Analyzing the width of the convolution kernel allows for predicting the spatial resolution of the method. However, this measure does not take into account the noise of the measured data. Furthermore, it does not consider a reconstruction step, which can increase the resolution beyond the width of the convolution kernel. In this paper, the spatial resolution of magnetic particle imaging is investigated by analyzing the modulation transfer function of the imaging process. An expression for the spatial resolution is derived, which includes the noise level and which is validated in simulations and experiments. [ABSTRACT FROM PUBLISHER]

Accurate radiometric and spectral calibrations of hyperspectral remote sensing instruments are essential for optimum data processing and exploitation. Two improved methods for the refinement of the spectral calibration of air- and spaceborne imaging spectrometers are presented in this paper. Both spectral channel position and width can be retrieved by modeling the atmospheric absorption features around 760, 940, 1140, and 2060 nm without making use of external atmospheric or surface parameters. A sensitivity analysis based on synthetic data demonstrated that, for each of the two methods, the root-mean-square errors to be expected were less than 0.18 nm for the retrieval of channel wavelength center and less than 0.8 nm for channel full-width at half-maximum. The application of the proposed methods to a real Hyperion data set showed quite-similar cross-track variations in the spectral calibration for the two methods, although relatively large differences in magnitude were found near the 940- and 1140-nm \H2\O absorption features. The significant improvement of the reflectance spectra derived after the refinement of the instrument spectral calibration confirms the good performance of the proposed methods. [ABSTRACT FROM AUTHOR]