917 research outputs found
Parallel Magnetic Resonance Imaging as Approximation in a Reproducing Kernel Hilbert Space
In Magnetic Resonance Imaging (MRI) data samples are collected in the spatial
frequency domain (k-space), typically by time-consuming line-by-line scanning
on a Cartesian grid. Scans can be accelerated by simultaneous acquisition of
data using multiple receivers (parallel imaging), and by using more efficient
non-Cartesian sampling schemes. As shown here, reconstruction from samples at
arbitrary locations can be understood as approximation of vector-valued
functions from the acquired samples and formulated using a Reproducing Kernel
Hilbert Space (RKHS) with a matrix-valued kernel defined by the spatial
sensitivities of the receive coils. This establishes a formal connection
between approximation theory and parallel imaging. Theoretical tools from
approximation theory can then be used to understand reconstruction in k-space
and to extend the analysis of the effects of samples selection beyond the
traditional g-factor noise analysis to both noise amplification and
approximation errors. This is demonstrated with numerical examples.Comment: 28 pages, 7 figure
Characterization of the OCO-2 instrument line shape functions using on-orbit solar measurements
Accurately characterizing the instrument line shape (ILS) of the Orbiting Carbon Observatory-2 (OCO-2) is challenging and highly important due to its high spectral resolution and requirement for retrieval accuracy (0. 25 %) compared to previous spaceborne grating spectrometers. On-orbit ILS functions for all three bands of the OCO-2 instrument have been derived using its frequent solar measurements and high-resolution solar reference spectra. The solar reference spectrum generated from the 2016 version of the Total Carbon Column Observing Network (TCCON) solar line list shows significant improvements in the fitting residual compared to the solar reference spectrum currently used in the version 7 Level 2 algorithm in the O₂ A band. The analytical functions used to represent the ILS of previous grating spectrometers are found to be inadequate for the OCO-2 ILS. Particularly, the hybrid Gaussian and super-Gaussian functions may introduce spurious variations, up to 5 % of the ILS width, depending on the spectral sampling position, when there is a spectral undersampling. Fitting a homogeneous stretch of the preflight ILS together with the relative widening of the wings of the ILS is insensitive to the sampling grid position and accurately captures the variation of ILS in the O₂ A band between decontamination events. These temporal changes of ILS may explain the spurious signals observed in the solar-induced fluorescence retrieval in barren areas
Spectral cube extraction for the VLT/SPHERE IFS: Open-source pipeline with full forward modeling and improved sensitivity
We present a new open-source data-reduction pipeline to reconstruct spectral
data cubes from raw SPHERE integral-field spectrograph (IFS) data. The pipeline
is written in Python and based on the pipeline that was developed for the
CHARIS IFS. It introduces several improvements to SPHERE data analysis that
ultimately produce significant improvements in postprocessing sensitivity. We
first used new data to measure SPHERE lenslet point spread functions (PSFs) at
the four laser calibration wavelengths. These lenslet PSFs enabled us to
forward-model SPHERE data, to extract spectra using a least-squares fit, and to
remove spectral crosstalk using the measured lenslet PSFs. Our approach also
reduces the number of required interpolations, both spectral and spatial, and
can preserve the original hexagonal lenslet geometry in the SPHERE IFS. In the
case of least-squares extraction, no interpolation of the data is performed. We
demonstrate this new pipeline on the directly imaged exoplanet 51 Eri b and on
observations of the hot white dwarf companion to HD 2133. The extracted
spectrum of HD 2133B matches theoretical models, demonstrating
spectrophotometric calibration that is good to a few percent. Postprocessing on
two 51 Eri b data sets demonstrates a median improvement in sensitivity of 80%
and 30% for the 2015 and 2017 data, respectively, compared to the use of cubes
reconstructed by the SPHERE Data Center. The largest improvements are seen for
poorer observing conditions. The new SPHERE pipeline takes less than three
minutes to produce a data cube on a modern laptop, making it practical to
reprocess all SPHERE IFS data.Comment: 17 pages, 11 figures. Software available at:
https://github.com/PrincetonUniversity/charis-de
Gemini Frontier Fields: Wide-field Adaptive Optics -band Imaging of the Galaxy Clusters MACS J0416.1-2403 and Abell 2744
We have observed two of the six Frontier Fields galaxy clusters, MACS
J0416.1-2403 and Abell 2744, using the Gemini Multi-Conjugate Adaptive Optics
System (GeMS) and the Gemini South Adaptive Optics Imager (GSAOI). With
0.08"-0.10" FWHM our data are nearly diffraction-limited over a 100"x100" wide
area. GeMS/GSAOI complements the Hubble Space Telescope (HST) redwards of
1.6microns with twice the angular resolution. We reach a 5 sigma depth of Ks =
25.6 mag (AB) for compact sources. In this paper we describe the observations,
the data processing and the initial public data release. We provide fully
calibrated, co-added images matching the native GSAOI pixel scale as well as
the larger plate scales of the HST release, adding to the legacy value of the
Frontier Fields. Our work demonstrates that even for fields at high galactic
latitude, where natural guide stars are rare, current multi-conjugated adaptive
optics technology at 8m-telescopes has opened a new window on the distant
Universe. Observations of a third Frontier Field, Abell 370, are planned.Comment: Accepted for publication in ApJS; significantly revised compared to
the first submissio
Undersampled-Based Modulation Schemes for Optical Camera Communications
Widespread use of white light-emitting diodes and ubiquitous smart devices offer the opportunity to establish VLC, which has become a hot research topic based on the growing number of publications over the last decade. Camera-based VLC, namely OCC, provides many unique features when compared to a single-photodiode-based system, such as the ability to separate incident light in the spatial and color domains. OCC technology represents a promising approach to utilize the benefits of VLC in beyond-5G scenarios and is one of the key technologies of the Internet of Things. Establishing a long communication channel in OCC, as well as non-flickering illumination by using low-frame-rate camera detectors, requires special modulation schemes. This article provides an overview of the principles of three categories of modulation schemes for OCC systems using a low-frame-rate camera detector. In addition, a series of undersampled modulation schemes are proposed and discussed to achieve flicker-free OCC with higher spectral efficiency. In addition, framing structures are designed to solve problems occurring in OCC systems using particular modulation schemes. To evaluate the performance of these modulation schemes, measured bit error rate values are shown. Finally, challenges in the implementation of OCC systems are also outlined
Spatial Multiplexing of QPSK Signals with a Single Radio: Antenna Design and Over-the-Air Experiments
The paper describes the implementation and performance analysis of the first
fully-operational beam-space MIMO antenna for the spatial multiplexing of two
QPSK streams. The antenna is composed of a planar three-port radiator with two
varactor diodes terminating the passive ports. Pattern reconfiguration is used
to encode the MIMO information onto orthogonal virtual basis patterns in the
far-field. A measurement campaign was conducted to compare the performance of
the beam-space MIMO system with a conventional 2-by-?2 MIMO system under
realistic propagation conditions. Propagation measurements were conducted for
both systems and the mutual information and symbol error rates were estimated
from Monte-Carlo simulations over the measured channel matrices. The results
show the beam-space MIMO system and the conventional MIMO system exhibit
similar finite-constellation capacity and error performance in NLOS scenarios
when there is sufficient scattering in the channel. In comparison, in LOS
channels, the capacity performance is observed to depend on the relative
polarization of the receiving antennas.Comment: 31 pages, 23 figure
Accelerated MRI at 9.4 T with electronically modulated time-varying receive sensitivities
PURPOSE
To investigate how electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction at ultra-high field.
METHODS
Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B profiles, resulting in two distinct receive sensitivity configurations. A prototype 8-channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time-varying sensitivities was formulated, and g-factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time-varying sensitivities were compared with conventional static SENSE reconstructions.
RESULTS
The g-factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k-space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM = 0.81 for acceleration R = 4) and g-factors (max g = 2.4; R = 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM = 0.74 and max g = 3.2; R = 4). As the method relies on the short RF wavelength at ultra-high field, it does not yield significant benefits at 3T and below.
CONCLUSIONS
Time-varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra-high field
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