2,064 research outputs found
Quantum Monte Carlo calculation of entanglement Renyi entropies for generic quantum systems
We present a general scheme for the calculation of the Renyi entropy of a
subsystem in quantum many-body models that can be efficiently simulated via
quantum Monte Carlo. When the simulation is performed at very low temperature,
the above approach delivers the entanglement Renyi entropy of the subsystem,
and it allows to explore the crossover to the thermal Renyi entropy as the
temperature is increased. We implement this scheme explicitly within the
Stochastic Series expansion as well as within path-integral Monte Carlo, and
apply it to quantum spin and quantum rotor models. In the case of quantum
spins, we show that relevant models in two dimensions with reduced symmetry (XX
model or hardcore bosons, transverse-field Ising model at the quantum critical
point) exhibit an area law for the scaling of the entanglement entropy.Comment: 5+1 pages, 4+1 figure
Wireless, Customizable Coaxially-shielded Coils for Magnetic Resonance Imaging
Anatomy-specific RF receive coil arrays routinely adopted in magnetic
resonance imaging (MRI) for signal acquisition, are commonly burdened by their
bulky, fixed, and rigid configurations, which may impose patient discomfort,
bothersome positioning, and suboptimal sensitivity in certain situations.
Herein, leveraging coaxial cables' inherent flexibility and electric field
confining property, for the first time, we present wireless, ultra-lightweight,
coaxially-shielded MRI coils achieving a signal-to-noise ratio (SNR) comparable
to or surpassing that of commercially available cutting-edge receive coil
arrays with the potential for improved patient comfort, ease of implementation,
and significantly reduced costs. The proposed coils demonstrate versatility by
functioning both independently in form-fitting configurations, closely adapting
to relatively small anatomical sites, and collectively by inductively coupling
together as metamaterials, allowing for extension of the field-of-view of their
coverage to encompass larger anatomical regions without compromising coil
sensitivity. The wireless, coaxially-shielded MRI coils reported herein pave
the way toward next generation MRI coils
Wearable Coaxially-shielded Metamaterial for Magnetic Resonance Imaging
Recent advancements in metamaterials have yielded the possibility of a
wireless solution to improve signal-to-noise ratio (SNR) in magnetic resonance
imaging (MRI). Unlike traditional closely packed local coil arrays with rigid
designs and numerous components, these lightweight, cost-effective
metamaterials eliminate the need for radio frequency (RF) cabling, baluns,
adapters, and interfaces. However, their clinical adoption has been limited by
their low sensitivity, bulky physical footprint, and limited, specific use
cases. Herein, we introduce a wearable metamaterial developed using
commercially available coaxial cable, designed for a 3.0 T MRI system. This
metamaterial inherits the coaxially-shielded structure of its constituent
coaxial cable, effectively containing the electric field within the cable,
thereby mitigating the electric coupling to its loading while ensuring safer
clinical adoption, lower signal loss, and resistance to frequency shifts.
Weighing only 50g, the metamaterial maximizes its sensitivity by conforming to
the anatomical region of interest. MRI images acquired using this metamaterial
with various pulse sequences demonstrate an up to 2-fold SNR enhancement when
compared to a state-of-the-art 16-channel knee coil. This work introduces a
novel paradigm for constructing metamaterials in the MRI environment, paving
the way for the development of next-generation wireless MRI technology
Bayesian Reconstruction of Magnetic Resonance Images using Gaussian Processes
A central goal of modern magnetic resonance imaging (MRI) is to reduce the
time required to produce high-quality images. Efforts have included hardware
and software innovations such as parallel imaging, compressed sensing, and deep
learning-based reconstruction. Here, we propose and demonstrate a Bayesian
method to build statistical libraries of magnetic resonance (MR) images in
k-space and use these libraries to identify optimal subsampling paths and
reconstruction processes. Specifically, we compute a multivariate normal
distribution based upon Gaussian processes using a publicly available library
of T1-weighted images of healthy brains. We combine this library with
physics-informed envelope functions to only retain meaningful correlations in
k-space. This covariance function is then used to select a series of
ring-shaped subsampling paths using Bayesian optimization such that they
optimally explore space while remaining practically realizable in commercial
MRI systems. Combining optimized subsampling paths found for a range of images,
we compute a generalized sampling path that, when used for novel images,
produces superlative structural similarity and error in comparison to
previously reported reconstruction processes (i.e. 96.3% structural similarity
and <0.003 normalized mean squared error from sampling only 12.5% of the
k-space data). Finally, we use this reconstruction process on pathological data
without retraining to show that reconstructed images are clinically useful for
stroke identification
K-space Cold Diffusion: Learning to Reconstruct Accelerated MRI without Noise
Deep learning-based MRI reconstruction models have achieved superior
performance these days. Most recently, diffusion models have shown remarkable
performance in image generation, in-painting, super-resolution, image editing
and more. As a generalized diffusion model, cold diffusion further broadens the
scope and considers models built around arbitrary image transformations such as
blurring, down-sampling, etc. In this paper, we propose a k-space cold
diffusion model that performs image degradation and restoration in k-space
without the need for Gaussian noise. We provide comparisons with multiple deep
learning-based MRI reconstruction models and perform tests on a well-known
large open-source MRI dataset. Our results show that this novel way of
performing degradation can generate high-quality reconstruction images for
accelerated MRI.Comment: 22 pages, 5 figures, 3 table
Internal Friction and Vulnerability of Mixed Alkali Glasses
Based on a hopping model we show how the mixed alkali effect in glasses can
be understood if only a small fraction c_V ofthe available sites for the mobile
ions is vacant. In particular, we reproduce the peculiar behavior of the
internal friction and the steep fall (''vulnerability'') of the mobility of the
majority ion upon small replacements by the minority ion. The single and mixed
alkali internal friction peaks are caused by ion-vacancy and ion-ion exchange
processes. If c_V is small, they can become comparable in height even at small
mixing ratios. The large vulnerability is explained by a trapping of vacancies
induced by the minority ions. Reasonable choices of model parameters yield
typical behaviors found in experiments.Comment: 4 pages, 4 figure
MRI Field-transfer Reconstruction with Limited Data: Regularization by Neural Style Transfer
Recent works have demonstrated success in MRI reconstruction using deep
learning-based models. However, most reported approaches require training on a
task-specific, large-scale dataset. Regularization by denoising (RED) is a
general pipeline which embeds a denoiser as a prior for image reconstruction.
The potential of RED has been demonstrated for multiple image-related tasks
such as denoising, deblurring and super-resolution. In this work, we propose a
regularization by neural style transfer (RNST) method to further leverage the
priors from the neural transfer and denoising engine. This enables RNST to
reconstruct a high-quality image from a noisy low-quality image with different
image styles and limited data. We validate RNST with clinical MRI scans from
1.5T and 3T and show that RNST can significantly boost image quality. Our
results highlight the capability of the RNST framework for MRI reconstruction
and the potential for reconstruction tasks with limited data.Comment: 30 pages, 8 figures, 2 tables, 1 algorithm char
Hubbard model versus t-J model: The one-particle spectrum
The origin of the apparent discrepancies between the one-particle spectra of
the Hubbard and t-J models is revealed: Wavefunction corrections, in addition
to the three-site terms, should supplement the bare t-J. In this way a
quantitative agreement between the two models is obtained, even for the
intermediate- values appropriate for the high-Tc cuprate superconductors.
Numerical results for clusters of up to 20 sites are presented. The momentum
dependence of the observed intensities in the photoemission spectra of
Sr2CuO2Cl2 are well described by this complete strong-coupling approach.Comment: 4 two-column RevTeX pages, including 4 Postscript figures. Uses epsf.
Accepted for publication in Physical Review B, Rapid Communicatio
Dynamic Exponent of t-J and t-J-W Model
Drude weight of optical conductivity is calculated at zero temperature by
exact diagonalization for the two-dimensional t-J model with the two-particle
term, . For the ordinary t-J model with =0, the scaling of the Drude
weight for small doping concentration is
obtained, which indicates anomalous dynamic exponent =4 of the Mott
transition. When is switched on, the dynamic exponent recovers its
conventional value =2. This corresponds to an incoherent-to-coherent
transition associated with the switching of the two-particle transfer.Comment: LaTeX, JPSJ-style, 4 pages, 5 eps files, to appear in J. Phys. Soc.
Jpn. vol.67, No.6 (1998
Statin use and incident cardiovascular events in renal transplant recipients
BACKGROUND: Statins achieve potent LDL lowering in the general population leading to a significant cardiovascular (CV) risk reduction. In renal transplant recipients (RTR) statins are included in treatment guidelines, however, conclusive evidence of improved cardiovascular outcomes has not been uniformly provided and concerns have been raised about simultaneous use of statins and the immunosuppressant cyclosporine. This study aimed to elucidate the effect of statins on a compound CV endpoint, comprised of ischaemic CV events and CV mortality in RTR, with subgroup analysis focussing on cyclosporine users. METHOD: 622 included RTR (follow‐up 5.4 years) were matched based on propensity scores and dichotomized by statin use. Survival analysis was conducted. RESULTS: Cox regression showed that statin use was not significantly associated with the compound CV endpoint in a fully adjusted model (HR = 0.81, 95% CI = 0.53‐1.24, P = .33). Subgroup analyses in RTR using cyclosporine revealed a strong positive association of statin use with the CV compound outcome in a fully adjusted model (HR = 6.60, 95% CI 1.75‐24.9, P = .005). Furthermore, statin use was positively correlated with cyclosporine trough levels (correlation coefficient 0.11, P = .04). CONCLUSION: In conclusion, statin use does not significantly decrease incident CV events in an overall RTR cohort, but is independently associated with CV‐specific mortality and events in cyclosporine using RTR, possibly due to a bilateral pharmacological interaction
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