9 research outputs found
Development of Advanced Acquisition and Reconstruction Techniques for Real-Time Perfusion MRI
Diese Doktorarbeit befasst sich mit der methodischen Entwicklung von Akquisition- und Rekonstruktionstechniken zur Anwendung von Echtzeit-Bildgebungstechniken auf das Gebiet der dynamischen kontrastmittelgestützten Magentresonanztomographie. Zur Unterdrückung unerwünschter Bildartefakte wird eine neue Spoiling-Technik vorgeschlagen, die auf randomisierten Phasen der Hochfrequenzanregung basiert. Diese Technik erlaubt eine schnelle, artefaktfreie Aufnahme von T1-gewichteten Rohdaten bei radialer Abtastung. Die Rekonstruktion quantitativer Parameterkarten aus solchen Rohdaten kann als nichtlineares, inverses Problem aufgefasst werden. In dieser Arbeit wird eine modellbasierte Rekonstruktionstechnik zur quantitativen T1-Kartierung entwickelt, die dieses inverse Problem mittels der iterativ regularisierten Gauß-Newton-Methode mit parameterspezifischer Regularisierung löst. In Simulationen sowie in-vitro- und in-vivo-Studien wird Genauigkeit und Präzision dieser neuen Methode geprüft, die ihre direkte Anwendung in in-vitro-Experimenten zur "first-pass"-Perfusion findet. In diesen Experimenten wird ein kommerziell verfügbares Phantom verwendet, dass in-vivo-Perfusion simuliert und gleichzeitig vollständige Kontrolle über die vorherrschenden Austauschraten erlaubt
Joint T1 and T2 Mapping with Tiny Dictionaries and Subspace-Constrained Reconstruction
Purpose: To develop a method that adaptively generates tiny dictionaries for
joint T1-T2 mapping.
Theory: This work breaks the bond between dictionary size and representation
accuracy (i) by approximating the Bloch-response manifold by piece-wise linear
functions and (ii) by adaptively refining the sampling grid depending on the
locally-linear approximation error.
Methods: Data acquisition was accomplished with use of an 2D radially sampled
Inversion-Recovery Hybrid-State Free Precession sequence. Adaptive dictionaries
are generated with different error tolerances and compared to a heuristically
designed dictionary. Based on simulation results, tiny dictionaries were used
for T1-T2 mapping in phantom and in vivo studies. Reconstruction and parameter
mapping were performed entirely in subspace.
Results: All experiments demonstrated excellent agreement between the
proposed mapping technique and template matching using heuristic dictionaries.
Conclusion: Adaptive dictionaries in combination with manifold projection
allow to reduce the necessary dictionary sizes by one to two orders of
magnitude
Physics-based Reconstruction Methods for Magnetic Resonance Imaging
Conventional Magnetic Resonance Imaging (MRI) is hampered by long scan times
and only qualitative image contrasts that prohibit a direct comparison between
different systems. To address these limitations, model-based reconstructions
explicitly model the physical laws that govern the MRI signal generation. By
formulating image reconstruction as an inverse problem, quantitative maps of
the underlying physical parameters can then be extracted directly from
efficiently acquired k-space signals without intermediate image reconstruction
-- addressing both shortcomings of conventional MRI at the same time. This
review will discuss basic concepts of model-based reconstructions and report
about our experience in developing several model-based methods over the last
decade using selected examples that are provided complete with data and code.Comment: 8 figures, review accepted to Philos. Trans. R. Soc.
Quantitative Magnetic Resonance Imaging by Nonlinear Inversion of the Bloch Equations
Purpose: Development of a generic model-based reconstruction framework for
multi-parametric quantitative MRI that can be used with data from different
pulse sequences.
Methods: Generic nonlinear model-based reconstruction for quantitative MRI
estimates parametric maps directly from the acquired k-space by numerical
optimization. This requires numerically accurate and efficient methods to solve
the Bloch equations and their partial derivatives. In this work, we combine
direct sensitivity analysis and pre-computed state-transition matrices into a
generic framework for calibrationless model-based reconstruction that can be
applied to different pulse sequences. As a proof-of-concept, the method is
implemented and validated for quantitative and mapping with
single-shot inversion-recovery (IR) FLASH and IR bSSFP sequences in
simulations, phantoms, and the human brain.
Results: The direct sensitivity analysis enables a highly accurate and
numerically stable calculation of the derivatives. The state-transition
matrices efficiently exploit repeating patterns in pulse sequences, speeding up
the calculation by a factor of 10 for the examples considered in this work,
while preserving the accuracy of native ODE solvers. The generic model-based
method reproduces quantitative results of previous model-based reconstructions
based on the known analytical solutions for radial IR FLASH. For IR bSFFP it
produces accurate and maps for the NIST phantom in numerical
simulations and experiments. Feasibility is also shown for human brain,
although results are affected by magnetization transfer effects.
Conclusion: By developing efficient tools for numerical optimizations using
the Bloch equations as forward model, this work enables generic model-based
reconstruction for quantitative MRI.Comment: 30 pages, 7 Figures, 1 Table, Research Pape
Free-Breathing Myocardial T1 Mapping using Inversion-Recovery Radial FLASH and Motion-Resolved Model-Based Reconstruction
Purpose: To develop a free-breathing myocardial T1 mapping technique using
inversion-recovery (IR) radial fast low-angle shot (FLASH) and calibrationless
motion-resolved model-based reconstruction. Methods: Free-running
(free-breathing, retrospective cardiac gating) IR radial FLASH is used for data
acquisition at 3T. First, to reduce the waiting time between inversions, an
analytical formula is derived that takes the incomplete T1 recovery into
account for an accurate T1 calculation. Second, the respiratory motion signal
is estimated from the k-space center of the contrast varying acquisition using
an adapted singular spectrum analysis (SSA-FARY) technique. Third, a
motion-resolved model-based reconstruction is used to estimate both parameter
and coil sensitivity maps directly from the sorted k-space data. Thus,
spatio-temporal total variation, in addition to the spatial sparsity
constraints, can be directly applied to the parameter maps. Validations are
performed on an experimental phantom, eleven human subjects, and a young
landrace pig with myocardial infarction. Results: In comparison to an IR
spin-echo reference, phantom results confirm good T1 accuracy, when reducing
the waiting time from five seconds to one second using the new correction. The
motion-resolved model-based reconstruction further improves T1 precision
compared to the spatial regularization-only reconstruction. Aside from showing
that a reliable respiratory motion signal can be estimated using modified
SSA-FARY, in vivo studies demonstrate that dynamic myocardial T1 maps can be
obtained within two minutes with good precision and repeatability. Conclusion:
Motion-resolved myocardial T1 mapping during free-breathing with good accuracy,
precision and repeatability can be achieved by combining inversion-recovery
radial FLASH, self-gating and a calibrationless motion-resolved model-based
reconstruction.Comment: Part of this work has been presented at the ISMRM Annual Conference
2021 (Virtual), submitted to Magnetic Resonance in Medicin