730 research outputs found
Ultrasound Signal Processing: From Models to Deep Learning
Medical ultrasound imaging relies heavily on high-quality signal processing
algorithms to provide reliable and interpretable image reconstructions.
Hand-crafted reconstruction methods, often based on approximations of the
underlying measurement model, are useful in practice, but notoriously fall
behind in terms of image quality. More sophisticated solutions, based on
statistical modelling, careful parameter tuning, or through increased model
complexity, can be sensitive to different environments. Recently, deep learning
based methods have gained popularity, which are optimized in a data-driven
fashion. These model-agnostic methods often rely on generic model structures,
and require vast training data to converge to a robust solution. A relatively
new paradigm combines the power of the two: leveraging data-driven deep
learning, as well as exploiting domain knowledge. These model-based solutions
yield high robustness, and require less trainable parameters and training data
than conventional neural networks. In this work we provide an overview of these
methods from the recent literature, and discuss a wide variety of ultrasound
applications. We aim to inspire the reader to further research in this area,
and to address the opportunities within the field of ultrasound signal
processing. We conclude with a future perspective on these model-based deep
learning techniques for medical ultrasound applications
Advanced Restoration Techniques for Images and Disparity Maps
With increasing popularity of digital cameras, the field of Computa-
tional Photography emerges as one of the most demanding areas of
research. In this thesis we study and develop novel priors and op-
timization techniques to solve inverse problems, including disparity
estimation and image restoration.
The disparity map estimation method proposed in this thesis incor-
porates multiple frames of a stereo video sequence to ensure temporal
coherency. To enforce smoothness, we use spatio-temporal connec-
tions between the pixels of the disparity map to constrain our solution.
Apart from smoothness, we enforce a consistency constraint for the
disparity assignments by using connections between the left and right
views. These constraints are then formulated in a graphical model,
which we solve using mean-field approximation. We use a filter-based
mean-field optimization that perform efficiently by updating the dis-
parity variables in parallel. The parallel updates scheme, however, is
not guaranteed to converge to a stationary point. To compare and
demonstrate the effectiveness of our approach, we developed a new
optimization technique that uses sequential updates, which runs ef-
ficiently and guarantees convergence. Our empirical results indicate
that with proper initialization, we can employ the parallel update
scheme and efficiently optimize our disparity maps without loss of
quality. Our method ranks amongst the state of the art in common
benchmarks, and significantly reduces the temporal flickering artifacts
in the disparity maps.
In the second part of this thesis, we address several image restora-
tion problems such as image deblurring, demosaicing and super-
resolution. We propose to use denoising autoencoders to learn an
approximation of the true natural image distribution. We parametrize
our denoisers using deep neural networks and show that they learn
the gradient of the smoothed density of natural images. Based on
this analysis, we propose a restoration technique that moves the so-
lution towards the local extrema of this distribution by minimizing
the difference between the input and output of our denoiser. Weii
demonstrate the effectiveness of our approach using a single trained
neural network in several restoration tasks such as deblurring and
super-resolution. In a more general framework, we define a new
Bayes formulation for the restoration problem, which leads to a more
efficient and robust estimator. The proposed framework achieves state
of the art performance in various restoration tasks such as deblurring
and demosaicing, and also for more challenging tasks such as noise-
and kernel-blind image deblurring.
Keywords. disparity map estimation, stereo matching, mean-field
optimization, graphical models, image processing, linear inverse prob-
lems, image restoration, image deblurring, image denoising, single
image super-resolution, image demosaicing, deep neural networks,
denoising autoencoder
An Adaptive and Robust Deep Learning Framework for THz Ultra-Massive MIMO Channel Estimation
Terahertz ultra-massive MIMO (THz UM-MIMO) is envisioned as one of the key
enablers of 6G wireless networks, for which channel estimation is highly
challenging. Traditional analytical estimation methods are no longer effective,
as the enlarged array aperture and the small wavelength result in a mixture of
far-field and near-field paths, constituting a hybrid-field channel. Deep
learning (DL)-based methods, despite the competitive performance, generally
lack theoretical guarantees and scale poorly with the size of the array. In
this paper, we propose a general DL framework for THz UM-MIMO channel
estimation, which leverages existing iterative channel estimators and is with
provable guarantees. Each iteration is implemented by a fixed point network
(FPN), consisting of a closed-form linear estimator and a DL-based non-linear
estimator. The proposed method perfectly matches the THz UM-MIMO channel
estimation due to several unique advantages. First, the complexity is low and
adaptive. It enjoys provable linear convergence with a low per-iteration cost
and monotonically increasing accuracy, which enables an adaptive
accuracy-complexity tradeoff. Second, it is robust to practical distribution
shifts and can directly generalize to a variety of heavily out-of-distribution
scenarios with almost no performance loss, which is suitable for the
complicated THz channel conditions. For practical usage, the proposed framework
is further extended to wideband THz UM-MIMO systems with beam squint effect.
Theoretical analysis and extensive simulation results are provided to
illustrate the advantages over the state-of-the-art methods in estimation
accuracy, convergence rate, complexity, and robustness.Comment: 15 pages, 11 figures, 5 tables, accepted by IEEE Journal of Selected
Topics in Signal Processing (JSTSP
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