145 research outputs found
Real-time motion and main magnetic field correction in MR spectroscopy using an EPI volumetric navigator
In population groups where subjects do not lie still during Magnetic Resonance Spectroscopy (MRS) scans, real-time volume of interest (VOI), frequency, and main magnetic field (B0) shim correction may be necessary. This work demonstrates firstly that head movement causes significant B0 disruption in both single voxel spectroscopy and spectroscopic imaging
An Approximate Message Passing Algorithm for Rapid Parameter-Free Compressed Sensing MRI
For certain sensing matrices, the Approximate Message Passing (AMP) algorithm
efficiently reconstructs undersampled signals. However, in Magnetic Resonance
Imaging (MRI), where Fourier coefficients of a natural image are sampled with
variable density, AMP encounters convergence problems. In response we present
an algorithm based on Orthogonal AMP constructed specifically for variable
density partial Fourier sensing matrices. For the first time in this setting a
state evolution has been observed. A practical advantage of state evolution is
that Stein's Unbiased Risk Estimate (SURE) can be effectively implemented,
yielding an algorithm with no free parameters. We empirically evaluate the
effectiveness of the parameter-free algorithm on simulated data and find that
it converges over 5x faster and to a lower mean-squared error solution than
Fast Iterative Shrinkage-Thresholding (FISTA).Comment: 5 pages, 5 figures, IEEE International Conference on Image Processing
(ICIP) 202
Approximate Message Passing with a Colored Aliasing Model for Variable Density Fourier Sampled Images
The Approximate Message Passing (AMP) algorithm efficiently reconstructs
signals which have been sampled with large i.i.d. sub-Gaussian sensing
matrices. Central to AMP is its "state evolution", which guarantees that the
difference between the current estimate and ground truth (the "aliasing") at
every iteration obeys a Gaussian distribution that can be fully characterized
by a scalar. However, when Fourier coefficients of a signal with non-uniform
spectral density are sampled, such as in Magnetic Resonance Imaging (MRI), the
aliasing is intrinsically colored, AMP's scalar state evolution is no longer
accurate and the algorithm encounters convergence problems. In response, we
propose the Variable Density Approximate Message Passing (VDAMP) algorithm,
which uses the wavelet domain to model the colored aliasing. We present
empirical evidence that VDAMP obeys a "colored state evolution", where the
aliasing obeys a Gaussian distribution that can be fully characterized with one
scalar per wavelet subband. A benefit of state evolution is that Stein's
Unbiased Risk Estimate (SURE) can be effectively implemented, yielding an
algorithm with subband-dependent thresholding that has no free parameters. We
empirically evaluate the effectiveness of VDAMP on three variations of Fast
Iterative Shrinkage-Thresholding (FISTA) and find that it converges in around
10 times fewer iterations on average than the next-fastest method, and to a
comparable mean-squared-error.Comment: 13 pages, 7 figures, 3 tables. arXiv admin note: text overlap with
arXiv:1911.0123
Optimization of Undersampling Parameters for 3D Intracranial Compressed Sensing MR Angiography at 7 Tesla
Purpose: 3D Time-of-flight (TOF) MR Angiography (MRA) can accurately
visualize the intracranial vasculature, but is limited by long acquisition
times. Compressed sensing (CS) reconstruction can be used to substantially
accelerate acquisitions. The quality of those reconstructions depends on the
undersampling patterns used in the acquisitions. In this work, optimized sets
of undersampling parameters using various acceleration factors for Cartesian 3D
TOF-MRA are established.
Methods: Fully-sampled datasets acquired at 7T were retrospectively
undersampled using variable-density Poisson-disk sampling with various
autocalibration region sizes, polynomial orders, and acceleration factors. The
accuracy of reconstructions from the different undersampled datasets was
assessed using the vessel-masked structural similarity index. Results were
compared for four imaging volumes, acquired from two different subjects.
Optimized undersampling parameters were validated using additional
prospectively undersampled datasets.
Results: For all acceleration factors, using a fully-sampled calibration area
of 12x12 k-space lines and a polynomial order of around 2-2.4 resulted in the
highest image quality. The importance of sampling parameter optimization was
found to increase for higher acceleration factors. The results were consistent
across resolutions and regions of interest with vessels of varying sizes and
tortuosity. In prospectively undersampled acquisitions, using optimized
undersampling parameters resulted in a 7.2% increase in the number of visible
small vessels at R = 7.2.
Conclusion: The image quality of CS TOF-MRA can be improved by appropriate
choice of undersampling parameters. The optimized sets of parameters are
independent of the acceleration factor.Comment: Manuscript to be submitted to Magnetic Resonance in Medicin
A temperature-controlled cooling system for accurate quantitative post-mortem MRI
Purpose:Â To develop a temperature-controlled cooling system to facilitate accurate quantitative post-mortem MRI and enable scanning of unfixed tissue.
Methods:Â A water cooling system was built and integrated with a 7T scanner to minimize temperature drift during MRI scans. The system was optimized for operational convenience and rapid deployment to ensure efficient workflow, which is critical for scanning unfixed post-mortem samples. The performance of the system was evaluated using a 7-h diffusion MRI protocol at 7T with a porcine tissue sample. Quantitative T1, T2, and ADC maps were interspersed with the diffusion scans at seven different time points to investigate the temperature dependence of MRI tissue parameters. The impact of temperature changes on biophysical model fitting of diffusion MRI data was investigated using simulation.
Results:Â Tissue T1, T2, and ADC values remained stable throughout the diffusion MRI scan using the developed cooling system, but varied substantially using a conventional scan setup without temperature control. The cooling system enabled accurate estimation of biophysical model parameters by stabilizing the tissue temperature throughout the diffusion scan, while the conventional setup showed evidence of significantly biased estimation.
Conclusion:Â A temperature-controlled cooling system was developed to tackle the challenge of heating in post-mortem imaging, which shows potential to improve the accuracy and reliability of quantitative post-mortem imaging and enables long scans of unfixed tissue
Haemodynamic flow abnormalities in bicuspid aortic valve disease improve with aortic valve replacement
Aortic 4D flow: quantifying the effects of contrast and field strength at 1.5 T, 3T and 7T
Left Ventricular Flow Analysis
Background Cardiac remodeling, after a myocardial insult, often causes progression to heart failure. The relationship between alterations in left ventricular blood flow, including kinetic energy (KE), and remodeling is uncertain. We hypothesized that increasing derangements in left ventricular blood flow would relate to (1) conventional cardiac remodeling markers, (2) increased levels of biochemical remodeling markers, (3) altered cardiac energetics, and (4) worsening patient symptoms and functional capacity.
Methods Thirty-four dilated cardiomyopathy patients, 30 ischemic cardiomyopathy patients, and 36 controls underwent magnetic resonance including 4-dimensional flow, BNP (brain-type natriuretic peptide) measurement, functional capacity assessment (6-minute walk test), and symptom quantification. A subgroup of dilated cardiomyopathy and control subjects underwent cardiac energetic assessment. Left ventricular flow was separated into 4 components: direct flow, retained inflow, delayed ejection flow, and residual volume. Average KE throughout the cardiac cycle was calculated.
Results Patients had reduced direct flow proportion and direct-flow average KE compared with controls (Pandlt;0.0001). The residual volume proportion and residual volume average KE were increased in patients (Pandlt;0.0001). Importantly, in a multiple linear regression model to predict the patientandrsquo;s 6-minute walk test, the independent predictors were age (andbeta;=andminus;0.3015;andnbsp;P=0.019) and direct-flow average KE (andbeta;=0.280,andnbsp;P=0.035; R2andnbsp;model, 0.466,andnbsp;P=0.002). In contrast, neither ejection fraction nor left ventricular volumes were independently predictive.
Conclusions This study demonstrates an independent predictive relationship between the direct-flow average KE and a prognostic measure of functional capacity. Intracardiac 4-dimensional flow parameters are novel biomarkers in heart failure and may provide additive value in monitoring new therapies and predicting prognosis.</p
In the presence of a patent foramen ovale paroxysmal embolism risk increases with non-vortical right atrial blood flow
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