1,412 research outputs found
Deep unfolding as iterative regularization for imaging inverse problems
Recently, deep unfolding methods that guide the design of deep neural
networks (DNNs) through iterative algorithms have received increasing attention
in the field of inverse problems. Unlike general end-to-end DNNs, unfolding
methods have better interpretability and performance. However, to our
knowledge, their accuracy and stability in solving inverse problems cannot be
fully guaranteed. To bridge this gap, we modified the training procedure and
proved that the unfolding method is an iterative regularization method. More
precisely, we jointly learn a convex penalty function adversarially by an
input-convex neural network (ICNN) to characterize the distance to a real data
manifold and train a DNN unfolded from the proximal gradient descent algorithm
with this learned penalty. Suppose the real data manifold intersects the
inverse problem solutions with only the unique real solution. We prove that the
unfolded DNN will converge to it stably. Furthermore, we demonstrate with an
example of MRI reconstruction that the proposed method outperforms conventional
unfolding methods and traditional regularization methods in terms of
reconstruction quality, stability and convergence speed
Enhancing the number of modes in metasurfaced reverberation chambers for field uniformity improvement
The use of metasurfaces to increase the number of modes, lower the operating frequency, and improve the field uniformity in reverberation chambers (RCs) is investigated in this paper. The method used to improve the field uniformity and decrease the resonance frequencies is based on increasing the number of modes by using the concept of subwavelength cavities. The resonance frequencies of a RC with metasurface wall are derived and expressed analytically in terms of macroscopic characteristics. Simulation of the reflection phase of the unit cell is then given as a guideline to choose the required microscopic parameters of the designed metasurface. The mode density in such subwavelength RCs is then obtained using a numerical eigenmode solver. Compared to traditional RCs, a much higher modal density is obtained at low frequencies. The standard deviation of the field uniformity in the test volume of the RC corresponding to different types of metasurface walls is finally compared. It is shown that by increasing the number of modes in the RC at the lower band, the operating frequency decreases and the field uniformity of the RC is improved
SPIRiT-Diffusion: Self-Consistency Driven Diffusion Model for Accelerated MRI
Diffusion models are a leading method for image generation and have been
successfully applied in magnetic resonance imaging (MRI) reconstruction.
Current diffusion-based reconstruction methods rely on coil sensitivity maps
(CSM) to reconstruct multi-coil data. However, it is difficult to accurately
estimate CSMs in practice use, resulting in degradation of the reconstruction
quality. To address this issue, we propose a self-consistency-driven diffusion
model inspired by the iterative self-consistent parallel imaging (SPIRiT),
namely SPIRiT-Diffusion. Specifically, the iterative solver of the
self-consistent term in SPIRiT is utilized to design a novel stochastic
differential equation (SDE) for diffusion process. Then -space data
can be interpolated directly during the reverse diffusion process, instead of
using CSM to separate and combine individual coil images. This method indicates
that the optimization model can be used to design SDE in diffusion models,
driving the diffusion process strongly conforming with the physics involved in
the optimization model, dubbed model-driven diffusion. The proposed
SPIRiT-Diffusion method was evaluated on a 3D joint Intracranial and Carotid
Vessel Wall imaging dataset. The results demonstrate that it outperforms the
CSM-based reconstruction methods, and achieves high reconstruction quality at a
high acceleration rate of 10
Convex Latent-Optimized Adversarial Regularizers for Imaging Inverse Problems
Recently, data-driven techniques have demonstrated remarkable effectiveness
in addressing challenges related to MR imaging inverse problems. However, these
methods still exhibit certain limitations in terms of interpretability and
robustness. In response, we introduce Convex Latent-Optimized Adversarial
Regularizers (CLEAR), a novel and interpretable data-driven paradigm. CLEAR
represents a fusion of deep learning (DL) and variational regularization.
Specifically, we employ a latent optimization technique to adversarially train
an input convex neural network, and its set of minima can fully represent the
real data manifold. We utilize it as a convex regularizer to formulate a
CLEAR-informed variational regularization model that guides the solution of the
imaging inverse problem on the real data manifold. Leveraging its inherent
convexity, we have established the convergence of the projected subgradient
descent algorithm for the CLEAR-informed regularization model. This convergence
guarantees the attainment of a unique solution to the imaging inverse problem,
subject to certain assumptions. Furthermore, we have demonstrated the
robustness of our CLEAR-informed model, explicitly showcasing its capacity to
achieve stable reconstruction even in the presence of measurement interference.
Finally, we illustrate the superiority of our approach using MRI reconstruction
as an example. Our method consistently outperforms conventional data-driven
techniques and traditional regularization approaches, excelling in both
reconstruction quality and robustness
Matrix Completion-Informed Deep Unfolded Equilibrium Models for Self-Supervised k-Space Interpolation in MRI
Recently, regularization model-driven deep learning (DL) has gained
significant attention due to its ability to leverage the potent
representational capabilities of DL while retaining the theoretical guarantees
of regularization models. However, most of these methods are tailored for
supervised learning scenarios that necessitate fully sampled labels, which can
pose challenges in practical MRI applications. To tackle this challenge, we
propose a self-supervised DL approach for accelerated MRI that is theoretically
guaranteed and does not rely on fully sampled labels. Specifically, we achieve
neural network structure regularization by exploiting the inherent structural
low-rankness of the -space data. Simultaneously, we constrain the network
structure to resemble a nonexpansive mapping, ensuring the network's
convergence to a fixed point. Thanks to this well-defined network structure,
this fixed point can completely reconstruct the missing -space data based on
matrix completion theory, even in situations where full-sampled labels are
unavailable. Experiments validate the effectiveness of our proposed method and
demonstrate its superiority over existing self-supervised approaches and
traditional regularization methods, achieving performance comparable to that of
supervised learning methods in certain scenarios
Sex‐specific activation of SK current by isoproterenol facilitates action potential triangulation and arrhythmogenesis in rabbit ventricles
Sex has a large influence on cardiac electrophysiological properties. Whether sex differences exist in apamin‐sensitive small conductance Ca2+‐activated K+ (SK) current (IKAS) remains unknown. We performed optical mapping, transmembrane potential, patch clamp, western blot and immunostaining in 62 normal rabbit ventricles, including 32 females and 30 males. IKAS blockade by apamin only minimally prolonged action potential (AP) duration (APD) in the basal condition for both sexes, but significantly prolonged APD in the presence of isoproterenol in females. Apamin prolonged APD at the level of 25% repolarization (APD25) more prominently than APD at the level of 80% repolarization (APD80), consequently reversing isoproterenol‐induced AP triangulation in females. In comparison, apamin prolonged APD to a significantly lesser extent in males and failed to restore the AP plateau during isoproterenol infusion. IKAS in males did not respond to the L‐type calcium current agonist BayK8644, but was amplified by the casein kinase 2 (CK2) inhibitor 4,5,6,7‐tetrabromobenzotriazole. In addition, whole‐cell outward IKAS densities in ventricular cardiomyocytes were significantly larger in females than in males. SK channel subtype 2 (SK2) protein expression was higher and the CK2/SK2 ratio was lower in females than in males. IKAS activation in females induced negative intracellular Ca2+–voltage coupling, promoted electromechanically discordant phase 2 repolarization alternans and facilitated ventricular fibrillation (VF). Apamin eliminated the negative Ca2+–voltage coupling, attenuated alternans and reduced VF inducibility, phase singularities and dominant frequencies in females, but not in males. We conclude that β‐adrenergic stimulation activates ventricular IKAS in females to a much greater extent than in males. IKAS activation plays an important role in ventricular arrhythmogenesis in females during sympathetic stimulation
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