230 research outputs found
Reordering for Improved Constrained Reconstruction from Undersampled k-Space Data
Recently, there has been a significant interest in applying reconstruction techniques, like constrained reconstruction or compressed sampling methods, to undersampled k-space data in MRI. Here, we propose a novel reordering technique to improve these types of reconstruction methods. In this technique, the intensities of the signal estimate are reordered according to a preprocessing step when applying the constraints on the estimated solution within the iterative reconstruction. The ordering of the intensities is such that it makes the original artifact-free signal monotonic and thus minimizes the finite differences norm if the correct image is estimated; this ordering can be estimated based on the undersampled measured data. Theory and example applications of the method for accelerating myocardial perfusion imaging with respiratory motion and brain diffusion tensor imaging are presented
Development of whole-heart myocardial perfusion magnetic resonance imaging
Myocardial perfusion imaging is of huge importance for the detection of
coronary artery disease (CAD), one of the leading causes of morbidity
and mortality worldwide, as it can provide non-invasive detection at the
early stages of the disease. Magnetic resonance imaging (MRI) can assess
myocardial perfusion by capturing the rst-pass perfusion (FPP) of a
gadolinium-based contrast agent (GBCA), which is now a well-established
technique and compares well with other modalities. However, current MRI
methods are restricted by their limited coverage of the left ventricle. Interest
has therefore grown in 3D volumetric \whole-heart" FPP by MRI, although
many challenges currently limit this. For this thesis, myocardial perfusion
assessment in general, and 3D whole-heart FPP in particular, were reviewed
in depth, alongside MRI techniques important for achieving 3D FPP. From
this, a 3D `stack-of-stars' (SOS) FPP sequence was developed with the aim
of addressing some current limitations. These included the breath-hold
requirement during GBCA rst-pass, long 3D shot durations corrupted by
cardiac motion, and a propensity for artefacts in FPP. Parallel imaging and
compressed sensing were investigated for accelerating whole-heart FPP, with
modi cations presented to potentially improve robustness to free-breathing.
Novel sequences were developed that were capable of individually improving
some current sequence limits, including spatial resolution and signal-to-noise
ratio, although with some sacri ces. A nal 3D SOS FPP technique was
developed and tested at stress during free-breathing examinations of CAD
patients and healthy volunteers. This enabled the rst known detection of an
inducible perfusion defect with a free-breathing, compressed sensing, 3D FPP
sequence; however, further investigation into the diagnostic performance is
required. Simulations were performed to analyse potential artefacts in 3D
FPP, as well as to examine ways towards further optimisation of 3D SOS
FPP. The nal chapter discusses some limitations of the work and proposes
opportunities for further investigation.Open Acces
Scan Specific Artifact Reduction in K-space (SPARK) Neural Networks Synergize with Physics-based Reconstruction to Accelerate MRI
Purpose: To develop a scan-specific model that estimates and corrects k-space
errors made when reconstructing accelerated Magnetic Resonance Imaging (MRI)
data.
Methods: Scan-Specific Artifact Reduction in k-space (SPARK) trains a
convolutional-neural-network to estimate and correct k-space errors made by an
input reconstruction technique by back-propagating from the mean-squared-error
loss between an auto-calibration signal (ACS) and the input technique's
reconstructed ACS. First, SPARK is applied to GRAPPA and demonstrates improved
robustness over other scan-specific models, such as RAKI and residual-RAKI.
Subsequent experiments demonstrate that SPARK synergizes with residual-RAKI to
improve reconstruction performance. SPARK also improves reconstruction quality
when applied to advanced acquisition and reconstruction techniques like 2D
virtual coil (VC-) GRAPPA, 2D LORAKS, 3D GRAPPA without an integrated ACS
region, and 2D/3D wave-encoded images.
Results: SPARK yields 1.5x - 2x RMSE reduction when applied to GRAPPA and
improves robustness to ACS size for various acceleration rates in comparison to
other scan-specific techniques. When applied to advanced reconstruction
techniques such as residual-RAKI, 2D VC-GRAPPA and LORAKS, SPARK achieves up to
20% RMSE improvement. SPARK with 3D GRAPPA also improves performance by ~2x and
perceived image quality without a fully sampled ACS region. Finally, SPARK
synergizes with non-cartesian 2D and 3D wave-encoding imaging by reducing RMSE
between 20-25% and providing qualitative improvements.
Conclusion: SPARK synergizes with physics-based acquisition and
reconstruction techniques to improve accelerated MRI by training scan-specific
models to estimate and correct reconstruction errors in k-space
MRI reconstruction using Markov random field and total variation as composite prior
Reconstruction of magnetic resonance images (MRI) benefits from incorporating a priori knowledge about statistical dependencies among the representation coefficients. Recent results demonstrate that modeling intraband dependencies with Markov Random Field (MRF) models enable superior reconstructions compared to inter-scale models. In this paper, we develop a novel reconstruction method, which includes a composite prior based on an MRF model and Total Variation (TV). We use an anisotropic MRF model and propose an original data-driven method for the adaptive estimation of its parameters. From a Bayesian perspective, we define a new position-dependent type of regularization and derive a compact reconstruction algorithm with a novel soft-thresholding rule. Experimental results show the effectiveness of this method compared to the state of the art in the field
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