424 research outputs found
Fundamentals of turbulent flow spectrum imaging
Purpose
To introduce a mathematical framework and in-silico validation of turbulent flow spectrum imaging (TFSI) of stenotic flow using phase-contrast MRI, evaluate systematic errors in quantitative turbulence parameter estimation, and propose a novel method for probing the Lagrangian velocity spectra of turbulent flows.
Theory and Methods
The spectral response of velocity-encoding gradients is derived theoretically and linked to turbulence parameter estimation including the velocity autocorrelation function spectrum. Using a phase-contrast MRI simulation framework, the encoding properties of bipolar gradient waveforms with identical first gradient moments but different duration are investigated on turbulent flow data of defined characteristics as derived from computational fluid dynamics. Based on theoretical insights, an approach using velocity-compensated gradient waveforms is proposed to specifically probe desired ranges of the velocity autocorrelation function spectrum with increased accuracy.
Results
Practical velocity-encoding gradients exhibit limited encoding power of typical turbulent flow spectra, resulting in up to 50% systematic underestimation of intravoxel SD values. Depending on the turbulence level in fluids, the error due to a single encoding gradient spectral response can vary by 20%. When using tailored velocity-compensated gradients, improved quantification of the Lagrangian velocity spectrum on a voxel-by-voxel basis is achieved and used for quantitative correction of intravoxel SD values estimated with velocity-encoding gradients.
Conclusion
To address systematic underestimation of turbulence parameters using bipolar velocity-encoding gradients in phase-contrast MRI of stenotic flows with short correlation times, tailored velocity-compensated gradients are proposed to improve quantitative mapping of turbulent blood flow characteristics
Relaxation enhancement by microwave irradiation may limit dynamic nuclear polarization
Dynamic nuclear polarization enables the hyperpolarization of nuclear spins
beyond the thermal-equilibrium Boltzmann distribution. However, it is often
unclear why the experimentally measured hyperpolarization is below the
theoretical achievable maximum polarization. We report a (near-) resonant
relaxation enhancement by microwave (MW) irradiation, leading to a significant
increase in the nuclear polarization decay compared to measurements without MW
irradiation. For example, the increased nuclear relaxation limits the
achievable polarization levels to around 35% instead of hypothetical 60%,
measured in the DNP material TEMPO in 1H glassy matrices at 3.3 K and 7 T.
Applying rate-equation models to published build-up and decay data indicates
that such relaxation enhancement is a common issue in many samples when using
different radicals at low sample temperatures and high Boltzmann polarizations
of the electrons. Accordingly, quantification and a better understanding of the
relaxation processes under MW irradiation might help to design samples and
processes towards achieving higher nuclear hyperpolarization levels.better
understanding of the relaxation processes under MW irradiation might help to
design samples and processes towards achieving higher nuclear hyperpolarization
levels
Assessment of 3D velocity vector fields and turbulent kinetic energy in a realistic aortic phantom using multi-point variable-density velocity encoding
ISSN:1097-6647ISSN:1532-429
Validation of Finite Element Image Registration-based Cardiac Strain Estimation from Magnetic Resonance Images
International audienceAccurate assessment of regional and global function of the heart is an important readout for the diagnosis and routine evaluation of cardiac patients. Indeed, recent clinical and experimental studies suggest that compared to global metrics, regional measures of function could allow for more accurate diagnosis and early intervention for many cardiac diseases. Although global strain measures derived from tagged magnetic resonance (MR) imaging have been shown to be reproducible for the majority of image registration techniques, the measurement of regional heterogeneity of strain is less robust. Moreover, radial strain is underestimated with the current techniques even globally. Finite element (FE)-based techniques offer a mechanistic approach for the regularization of the ill-posed registration problem. This paper presents the validation of a recently proposed FE-based image registration method with mechanical regularization named equilibrated warping. For this purpose, synthetic 3D-tagged MR images are generated from a reference biomechanical model of the left ventricle (LV). The performance of the registration algorithm is consequently tested on the images with different signal-to-noise ratios (SNRs), revealing the robustness of the method
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