188 research outputs found
Recovering missing slices of the discrete fourier transform using ghosts
The discrete Fourier transform (DFT) underpins the solution to many inverse problems commonly possessing missing or unmeasured frequency information. This incomplete coverage of the Fourier space always produces systematic artifacts called Ghosts. In this paper, a fast and exact method for deconvolving cyclic artifacts caused by missing slices of the DFT using redundant image regions is presented. The slices discussed here originate from the exact partitioning of the Discrete Fourier Transform (DFT) space, under the projective Discrete Radon Transform, called the discrete Fourier slice theorem. The method has a computational complexity of O(n\log-{2}n) (for an n=N\times N image) and is constructed from a new cyclic theory of Ghosts. This theory is also shown to unify several aspects of work done on Ghosts over the past three decades. This paper concludes with an application to fast, exact, non-iterative image reconstruction from a highly asymmetric set of rational angle projections that give rise to sets of sparse slices within the DFT
Universal mask for hard X rays
The penetrating power of X rays underpins important applications such as
medical radiography. However, this same attribute makes it challenging to
achieve flexible on-demand patterning of X-ray beams. One possible path to this
goal is ``ghost projection'', a method which may be viewed as a reversed form
of classical ghost imaging. This technique employs multiple exposures, of a
single illuminated non-configurable mask that is transversely displaced to a
number of specified positions, to create any desired pattern. An experimental
proof-of-concept is given for this idea, using hard X rays. The written pattern
is arbitrary, up to a tunable constant offset, and its spatial resolution is
limited by both (i) the finest features present in the illuminated mask and
(ii) inaccuracies in mask positioning and mask exposure time. In principle, the
method could be used to make a universal lithographic mask in the hard-X-ray
regime. Ghost projection might also be used as a dynamically-configurable
beam-shaping element, namely the hard-X-ray equivalent of a spatial light
modulator. The underpinning principle can be applied to gamma rays, neutrons,
electrons, muons, and atomic beams. Our flexible approach to beam shaping gives
a potentially useful means to manipulate such fields.Comment: Revised for resubmission to Optica; numerous clarifications
throughout the paper; Sec. 4 (numbered item 2) and Supplement 1 Sec. 2
significantly extended; all figures and ancillary movies unchange
Optimizing illumination patterns for classical ghost imaging
Classical ghost imaging is a new paradigm in imaging where the image of an
object is not measured directly with a pixelated detector. Rather, the object
is subject to a set of illumination patterns and the total interaction of the
object, e.g., reflected or transmitted photons or particles, is measured for
each pattern with a single-pixel or bucket detector. An image of the object is
then computed through the correlation of each pattern and the corresponding
bucket value. Assuming no prior knowledge of the object, the set of patterns
used to compute the ghost image dictates the image quality. In the
visible-light regime, programmable spatial light modulators can generate the
illumination patterns. In many other regimes -- such as x rays, electrons, and
neutrons -- no such dynamically configurable modulators exist, and patterns are
commonly produced by employing a transversely-translated mask. In this paper we
explore some of the properties of masks or speckle that should be considered to
maximize ghost-image quality, given a certain experimental classical
ghost-imaging setup employing a transversely-displaced but otherwise
non-configurable mask.Comment: 28 pages, 17 figure
Ghost Tomography
Ghost tomography using single-pixel detection extends the emerging field of
ghost imaging to three dimensions, with the use of penetrating radiation. In
this work, a series of spatially random x-ray intensity patterns is used to
illuminate a specimen in various tomographic angular orientations with only the
total transmitted intensity being recorded by a single-pixel camera (or bucket
detector). The set of zero-dimensional intensity readings, combined with
knowledge of the corresponding two-dimensional illuminating patterns and
specimen orientations, is sufficient for three-dimensional reconstruction of
the specimen. The experimental demonstration of ghost tomography is presented
here using synchrotron hard x-rays. This result expands the scope of ghost
imaging to encompass volumetric imaging (i.e., tomography), of optically opaque
objects using penetrating radiation. For hard x-rays, ghost tomography has the
potential to decouple image quality from dose rate as well as image resolution
from detector performance
Inherent dose-reduction potential of classical ghost imaging
Classical ghost imaging is a computational imaging technique that employs patterned illumination. It is very similar in concept to the single-pixel camera in that an image may be reconstructed from a set of measurements even though all imaging photons or particles that pass through that sample are never recorded with a position resolving detector. The method was first conceived and applied for visible-wavelength photons and was subsequently translated to other probes such as x rays, atomic beams, electrons, and neutrons. In the context of classical ghost imaging using penetrating probes that enable transmission measurement, we here consider several questions relating to the achievable signal-to-noise ratio (SNR). This is compared with the SNR for conventional imaging under scenarios of constant radiation dose and constant experiment time, considering both photon shot noise and per-measurement electronic readout noise. We show that inherent improved SNR capabilities of classical ghost imaging are limited to a subset of these scenarios and are actually due to increased dose (Fellgett advantage). An explanation is also presented for recent results published in the literature that are not consistent with these findings
Neutron ghost imaging
Ghost imaging is demonstrated using a polyenergetic reactor source of thermal neutrons. This enables position resolution to be incorporated into a variety of neutron instruments that are not position resolving. Such a proof of concept enables several further applications. For example, in an imaging context, neutron ghost imaging can be beneficial for dose reduction and resolution enhancement. We explore the principle of resolution enhancement
by employing a variant of the method in which each pixel of a position-sensitive detector is regarded as an independent bucket detector; a neutron ghost image is then computed for each pixel. We demonstrate the principle that this parallel form of neutron ghost imaging can significantly increase the spatial resolution of
a pixelated detector such as a CCD or CMOS camera. Further applications and extensions of our neutron ghost-imaging protocol are discussed. These include neutron ghost tomography, neutron ghost microscopy, dark-field neutron ghost imaging, and isotope-resolved color neutron ghost imaging via prompt-gamma-ray bucket detection.A.M.K. and G.R.M. acknowledge the financial support
of the Australian Research Council and FEI-Thermo Fisher
Scientific through Linkage Project No. LP150101040, and
the use of supercomputer time provided by Australia’s National Computational Infrastructure (NCI)
DAF-16/FOXO employs the chromatin remodeller SWI/SNF to promote stress resistance and longevity
Organisms are constantly challenged by stresses and privations and require adaptive responses for their survival. The transcription factor DAF-16/FOXO is central nexus in these responses, but despite its importance little is known about how it regulates its target genes. Proteomic identification of DAF-16/FOXO binding partners in Caenorhabditis elegans and their subsequent functional evaluation by RNA interference (RNAi) revealed several candidate DAF-16/FOXO cofactors, most notably the chromatin remodeller SWI/SNF. DAF-16/FOXO and SWI/SNF form a complex and globally colocalize at DAF-16/FOXO target promoters. We show that specifically for gene-activation, DAF-16/FOXO depends on SWI/SNF, facilitating SWI/SNF recruitment to target promoters, in order to activate transcription by presumed remodelling of local chromatin. For the animal, this translates into an essential role of SWI/SNF for DAF-16/FOXO-mediated processes, i.e. dauer formation, stress resistance, and the promotion of longevity. Thus we give insight into the mechanisms of DAF-16/FOXO-mediated transcriptional regulation and establish a critical link between ATP-dependent chromatin remodelling and lifespan regulation
- …