6,793 research outputs found
Advanced signal processing methods in dynamic contrast enhanced magnetic resonance imaging
Tato dizertační práce představuje metodu zobrazování perfúze magnetickou rezonancí, jež je výkonným nástrojem v diagnostice, především v onkologii. Po ukončení sběru časové sekvence T1-váhovaných obrazů zaznamenávajících distribuci kontrastní látky v těle začíná fáze zpracování dat, která je předmětem této dizertace. Je zde představen teoretický základ fyziologických modelů a modelů akvizice pomocí magnetické rezonance a celý řetězec potřebný k vytvoření obrazů odhadu parametrů perfúze a mikrocirkulace v tkáni. Tato dizertační práce je souborem uveřejněných prací autora přispívajícím k rozvoji metodologie perfúzního zobrazování a zmíněného potřebného teoretického rozboru.This dissertation describes quantitative dynamic contrast enhanced magnetic resonance imaging (DCE-MRI), which is a powerful tool in diagnostics, mainly in oncology. After a time series of T1-weighted images recording contrast-agent distribution in the body has been acquired, data processing phase follows. It is presented step by step in this dissertation. The theoretical background in physiological and MRI-acquisition modeling is described together with the estimation process leading to parametric maps describing perfusion and microcirculation properties of the investigated tissue on a voxel-by-voxel basis. The dissertation is divided into this theoretical analysis and a set of publications representing particular contributions of the author to DCE-MRI.
Cross-talk and interference enhance information capacity of a signaling pathway
A recurring motif in gene regulatory networks is transcription factors (TFs)
that regulate each other, and then bind to overlapping sites on DNA, where they
interact and synergistically control transcription of a target gene. Here, we
suggest that this motif maximizes information flow in a noisy network. Gene
expression is an inherently noisy process due to thermal fluctuations and the
small number of molecules involved. A consequence of multiple TFs interacting
at overlapping binding-sites is that their binding noise becomes correlated.
Using concepts from information theory, we show that in general a signaling
pathway transmits more information if 1) noise of one input is correlated with
that of the other, 2) input signals are not chosen independently. In the case
of TFs, the latter criterion hints at up-stream cross-regulation. We
demonstrate these ideas for competing TFs and feed-forward gene regulatory
modules, and discuss generalizations to other signaling pathways. Our results
challenge the conventional approach of treating biological noise as
uncorrelated fluctuations, and present a systematic method for understanding TF
cross-regulation networks either from direct measurements of binding noise, or
bioinformatic analysis of overlapping binding-sites.Comment: 28 pages, 5 figure
From Galaxy-Galaxy Lensing to Cosmological Parameters
Galaxy-galaxy lensing measures the mean excess surface density DS(r) around a
sample of lensing galaxies. We develop a method for combining DS(r) with the
galaxy correlation function xi_gg(r) to constrain Omega_m and sigma_8, going
beyond the linear bias model to reach the level of accuracy demanded by current
and future measurements. We adopt the halo occupation distribution (HOD)
framework, and we test its applicability to this problem by examining the
effects of replacing satellite galaxies in the halos of an SPH simulation with
randomly selected dark matter particles from the same halos. The difference
between dark matter and satellite galaxy radial profiles has a ~10% effect on
DS(r) at r<1 Mpc/h. However, if radial profiles are matched, the remaining
impact of individual subhalos around satellite galaxies and environmental
dependence of the HOD at fixed halo mass is <5% in DS(r) for 0.1<r<15 Mpc/h. We
develop an analytic approximation for DS(r) that incorporates halo exclusion
and scale-dependent halo bias, and we demonstrate its accuracy with tests
against a suite of populated N-body simulations. We use the analytic model to
investigate the dependence of DS(r) and the galaxy-matter correlation function
xi_gm(r) on Omega_m and sigma_8, once HOD parameters for a given cosmological
model are pinned down by matching xi_gg(r). The linear bias prediction is
accurate for r>2 Mpc/h, but it fails at the 30-50% level on smaller scales. The
scaling of DS(r) ~ Omega_m^a(r) sigma_8^b(r) approaches the linear bias
expectation a=b=1 at r>10 Mpc/h, but a(r) and b(r) vary from 0.8 to 1.6 at
smaller r. We calculate a fiducial DS(r) and scaling indices a(r) and b(r) for
two SDSS galaxy samples; galaxy-galaxy lensing measurements for these samples
can be combined with our predictions to constrain Omega_m and sigma_8.Comment: 18 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Anomalous transport in the crowded world of biological cells
A ubiquitous observation in cell biology is that diffusion of macromolecules
and organelles is anomalous, and a description simply based on the conventional
diffusion equation with diffusion constants measured in dilute solution fails.
This is commonly attributed to macromolecular crowding in the interior of cells
and in cellular membranes, summarising their densely packed and heterogeneous
structures. The most familiar phenomenon is a power-law increase of the MSD,
but there are other manifestations like strongly reduced and time-dependent
diffusion coefficients, persistent correlations, non-gaussian distributions of
the displacements, heterogeneous diffusion, and immobile particles. After a
general introduction to the statistical description of slow, anomalous
transport, we summarise some widely used theoretical models: gaussian models
like FBM and Langevin equations for visco-elastic media, the CTRW model, and
the Lorentz model describing obstructed transport in a heterogeneous
environment. Emphasis is put on the spatio-temporal properties of the transport
in terms of 2-point correlation functions, dynamic scaling behaviour, and how
the models are distinguished by their propagators even for identical MSDs.
Then, we review the theory underlying common experimental techniques in the
presence of anomalous transport: single-particle tracking, FCS, and FRAP. We
report on the large body of recent experimental evidence for anomalous
transport in crowded biological media: in cyto- and nucleoplasm as well as in
cellular membranes, complemented by in vitro experiments where model systems
mimic physiological crowding conditions. Finally, computer simulations play an
important role in testing the theoretical models and corroborating the
experimental findings. The review is completed by a synthesis of the
theoretical and experimental progress identifying open questions for future
investigation.Comment: review article, to appear in Rep. Prog. Phy
Scanning SQUID Susceptometry of a paramagnetic superconductor
Scanning SQUID susceptometry images the local magnetization and
susceptibility of a sample. By accurately modeling the SQUID signal we can
determine the physical properties such as the penetration depth and
permeability of superconducting samples. We calculate the scanning SQUID
susceptometry signal for a superconducting slab of arbitrary thickness with
isotropic London penetration depth, on a non-superconducting substrate, where
both slab and substrate can have a paramagnetic response that is linear in the
applied field. We derive analytical approximations to our general expression in
a number of limits. Using our results, we fit experimental susceptibility data
as a function of the sample-sensor spacing for three samples: 1) delta-doped
SrTiO3, which has a predominantly diamagnetic response, 2) a thin film of
LaNiO3, which has a predominantly paramagnetic response, and 3) a
two-dimensional electron layer (2-DEL) at a SrTiO3/AlAlO3 interface, which
exhibits both types of response. These formulas will allow the determination of
the concentrations of paramagnetic spins and superconducting carriers from fits
to scanning SQUID susceptibility measurements.Comment: 11 pages, 13 figure
Cosmological Constraints from a Combination of Galaxy Clustering and Lensing -- I. Theoretical Framework
We present a new method that simultaneously solves for cosmology and galaxy
bias on non-linear scales. The method uses the halo model to analytically
describe the (non-linear) matter distribution, and the conditional luminosity
function (CLF) to specify the halo occupation statistics. For a given choice of
cosmological parameters, this model can be used to predict the galaxy
luminosity function, as well as the two-point correlation functions of
galaxies, and the galaxy-galaxy lensing signal, both as function of scale and
luminosity. In this paper, the first in a series, we present the detailed,
analytical model, which we test against mock galaxy redshift surveys
constructed from high-resolution numerical -body simulations. We demonstrate
that our model, which includes scale-dependence of the halo bias and a proper
treatment of halo exclusion, reproduces the 3-dimensional galaxy-galaxy
correlation and the galaxy-matter cross-correlation (which can be projected to
predict the observables) with an accuracy better than 10 (in most cases 5)
percent. Ignoring either of these effects, as is often done, results in
systematic errors that easily exceed 40 percent on scales of \sim 1
h^{-1}\Mpc, where the data is typically most accurate. Finally, since the
projected correlation functions of galaxies are never obtained by integrating
the redshift space correlation function along the line-of-sight out to
infinity, simply because the data only cover a finite volume, they are still
affected by residual redshift space distortions (RRSDs). Ignoring these, as
done in numerous studies in the past, results in systematic errors that easily
exceed 20 perent on large scales (r_\rmp \gta 10 h^{-1}\Mpc). We show that it
is fairly straightforward to correct for these RRSDs, to an accuracy better
than percent, using a mildly modified version of the linear Kaiser
formalism
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