43 research outputs found
Anisotropic dynamics of a self-assembled colloidal chain in an active bath
Anisotropic macromolecules exposed to non-equilibrium (active) noise are very
common in biological systems, and an accurate understanding of their
anisotropic dynamics is therefore crucial. Here, we experimentally investigate
the dynamics of isolated chains assembled from magnetic microparticles at a
liquid-air interface and moving in an active bath consisting of motile E. coli
bacteria. We investigate both the internal chain dynamics and the anisotropic
center-of-mass dynamics through particle tracking. We find that both the
internal and center-of-mass dynamics are greatly enhanced compared to the
passive case, i.e., a system without bacteria, and that the center-of-mass
diffusion coefficient features a non-monotonic dependence as a function of
the chain length. Furthermore, our results show that the relationship between
the components of parallel and perpendicular with respect to the direction
of the applied magnetic field is preserved in the active bath compared to the
passive case, with a higher diffusion in the parallel direction, in contrast to
previous findings in the literature. We argue that this qualitative difference
is due to subtle differences in the experimental geometry and conditions and
the relative roles played by long-range hydrodynamic interactions and
short-range collisions
Simultaneous use of Individual and Joint Regularization Terms in Compressive Sensing: Joint Reconstruction of Multi-Channel Multi-Contrast MRI Acquisitions
Purpose: A time-efficient strategy to acquire high-quality multi-contrast
images is to reconstruct undersampled data with joint regularization terms that
leverage common information across contrasts. However, these terms can cause
leakage of uncommon features among contrasts, compromising diagnostic utility.
The goal of this study is to develop a compressive sensing method for
multi-channel multi-contrast magnetic resonance imaging (MRI) that optimally
utilizes shared information while preventing feature leakage.
Theory: Joint regularization terms group sparsity and colour total variation
are used to exploit common features across images while individual sparsity and
total variation are also used to prevent leakage of distinct features across
contrasts. The multi-channel multi-contrast reconstruction problem is solved
via a fast algorithm based on Alternating Direction Method of Multipliers.
Methods: The proposed method is compared against using only individual and
only joint regularization terms in reconstruction. Comparisons were performed
on single-channel simulated and multi-channel in-vivo datasets in terms of
reconstruction quality and neuroradiologist reader scores.
Results: The proposed method demonstrates rapid convergence and improved
image quality for both simulated and in-vivo datasets. Furthermore, while
reconstructions that solely use joint regularization terms are prone to
leakage-of-features, the proposed method reliably avoids leakage via
simultaneous use of joint and individual terms.
Conclusion: The proposed compressive sensing method performs fast
reconstruction of multi-channel multi-contrast MRI data with improved image
quality. It offers reliability against feature leakage in joint
reconstructions, thereby holding great promise for clinical use.Comment: 13 pages, 13 figures. Submitted for possible publicatio
DEQ-MPI: A Deep Equilibrium Reconstruction with Learned Consistency for Magnetic Particle Imaging
Magnetic particle imaging (MPI) offers unparalleled contrast and resolution
for tracing magnetic nanoparticles. A common imaging procedure calibrates a
system matrix (SM) that is used to reconstruct data from subsequent scans. The
ill-posed reconstruction problem can be solved by simultaneously enforcing data
consistency based on the SM and regularizing the solution based on an image
prior. Traditional hand-crafted priors cannot capture the complex attributes of
MPI images, whereas recent MPI methods based on learned priors can suffer from
extensive inference times or limited generalization performance. Here, we
introduce a novel physics-driven method for MPI reconstruction based on a deep
equilibrium model with learned data consistency (DEQ-MPI). DEQ-MPI reconstructs
images by augmenting neural networks into an iterative optimization, as
inspired by unrolling methods in deep learning. Yet, conventional unrolling
methods are computationally restricted to few iterations resulting in
non-convergent solutions, and they use hand-crafted consistency measures that
can yield suboptimal capture of the data distribution. DEQ-MPI instead trains
an implicit mapping to maximize the quality of a convergent solution, and it
incorporates a learned consistency measure to better account for the data
distribution. Demonstrations on simulated and experimental data indicate that
DEQ-MPI achieves superior image quality and competitive inference time to
state-of-the-art MPI reconstruction methods
TranSMS: Transformers for Super-Resolution Calibration in Magnetic Particle Imaging
Magnetic particle imaging (MPI) offers exceptional contrast for magnetic
nanoparticles (MNP) at high spatio-temporal resolution. A common procedure in
MPI starts with a calibration scan to measure the system matrix (SM), which is
then used to set up an inverse problem to reconstruct images of the MNP
distribution during subsequent scans. This calibration enables the
reconstruction to sensitively account for various system imperfections. Yet
time-consuming SM measurements have to be repeated under notable changes in
system properties. Here, we introduce a novel deep learning approach for
accelerated MPI calibration based on Transformers for SM super-resolution
(TranSMS). Low-resolution SM measurements are performed using large MNP samples
for improved signal-to-noise ratio efficiency, and the high-resolution SM is
super-resolved via model-based deep learning. TranSMS leverages a vision
transformer module to capture contextual relationships in low-resolution input
images, a dense convolutional module for localizing high-resolution image
features, and a data-consistency module to ensure measurement fidelity.
Demonstrations on simulated and experimental data indicate that TranSMS
significantly improves SM recovery and MPI reconstruction for up to 64-fold
acceleration in two-dimensional imaging
Joint reconstruction of multi-contrast images: compressive sensing reconstruction using both joint and individual regularization functions
In many clinical settings, multi-contrast images of a patient are acquired to maximize complementary information. With the underlying anatomy being the same, the mutual information in multi-contrast data can be exploited to improve image reconstruction, especially in accelerated acquisition schemes such as Compressive Sensing (CS). This study proposes a CS-reconstruction algorithm that uses four regularization functions; joint L1-sparsity and TV-regularization terms to exploit the mutual information, and individual L1-sparsity and TV-regularization terms to recover unique features in each image. The proposed method is shown to be robust against leakage-of-features across contrasts, and is demonstrated using simulations and in-vivo experiments