Magnetic resonance imaging is a widely used medical imaging technique, and accelerated data acquisition is critical for clinical utility. In this thesis, new techniques that incorporate radial acquisition, compressed sensing and sparse regularization for improved rapid imaging are presented. Sufficiently accelerated imaging methods can lead to new applications. Here we demonstrate a solution to lung imaging during forced expiration using accelerated MRI. A technique for dynamic 3D imaging of the lungs from highly undersampled data is developed and tested on six subjects. This method takes advantage of image sparsity, both spatially and temporally, including the use of reference frames called bookends. Sparsity, with respect to total variation, and residual from the bookends, enables reconstruction from an extremely limited amount of data. Dynamic 3D images can be captured at an unprecedented sub-150 ms temporal resolution, using only three (or less) acquired radial lines per slice per time point. Lung volume calculations based on image segmentation are compared to those from simultaneously acquired spirometer measurements. Additionally, accelerated imaging methods can be used to improve upon widely used applications; we also present a technique for improved T₂-mapping. A novel model-based compressed sensing method is extended to include a sparse regularization that is learned from the principal component coefficients. The principal components are determined by a range of T₂ decay curves, and the coefficients of the principal components are reconstructed. These coefficient maps share coherent spatial structures, and a spatial patch--based dictionary is a learned for a sparse constraint. This transformation is learned from the coefficients themselves. The proposed reconstruction is suited for non-Cartesian, multi-channel data. The dictionary constraint leads to parameter maps with less noise and less aliasing for high amounts of acceleration.

Sector-Wise Golden Angle (SWIG) is a novel approach that was developed to address the limitations associated with Golden Angle radial imaging, commonly used for high temporal resolution flow measurements. Golden angle radial imaging is a time-efficient method that effectively reduces motion sensitivity. However, binned or retrospectively gated imaging where multiple heartbeats are utilized to acquire a single time series may lead to uneven coverage of k- space, ultimately resulting in poor image quality. In contrast, SWIG restricts the radial profiles to a sector of k-space per heartbeat, ensuring even distributions of spokes during retrospectively gated acquisitions. One drawback of SWIG is the loss of ability to reconstruct real-time images. The combination of sorted and unsorted acquisition simultaneously holds significant potential and could be applied in various domains. The goal of the thesis work was to design a trajectory that combines radial and spiral k-space sampling, enabling hybrid real-time and retrospectively gated imaging. The objective was to obtain an image series with comparable quality to a SWIG readout while retaining the ability to reconstruct a low-resolution real-time image series from the same data. To evaluate the hybrid trajectory, the numerical phantom XCAT was used to generate synthetic MRI images. Binned images were sampled using a hybrid-SWIG method, yielding similar image quality to a conventional SWIG image series, with the added benefit of being able to reconstruct a low-resolution real-time image series. Although the current method was only evaluated in a numerical phantom and may require additional adjustment to be suitable for a real MRI scanner, the results show that it is possible to combine radial and spiral imaging in a single readout.

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.

The aim of this thesis is to design and evaluate k-space weighting filters for simultaneously improving the spatial and temporal resolution of cardiovascular MRI, with the ultimate goal of improving the accuracy of quantitative flow measurements, which are important for diagnosis and follow-up of heart dysfunction. Two different k-space filters were implemented and evaluated retrospectively to already acquired data. In addition, evaluation was performed with respect to tapering of the filters in the radial k-space direction, as well as accelerated imaging using undersampling. To better utilize the properties of the golden-angle acquisition, a k-space filter was also implemented where the temporal footprint increased in discrete steps, referred to as rings. The temporal footprint of each ring was calculated according to the Fibonacci sequence, and the starting position for each ring was computed to satisfy the Nyquist criterion. The k-space filters were evaluated in comparison to non-filtered reconstructions of cine and phase-contrast images. Motion-mode images were created from the cine images and used to evaluate the edge sharpness of the septal wall indicating the spatial resolution of the image. Phase-contrast images were used to measure peak flow velocity over the mitral valve, and the myocardial velocity in the early and late filling phases. The resolution of the peak is highly dependent on the temporal resolution. Measuring the peak velocity gave an indication of the temporal resolution, which could be compared to non-filter reconstructions. This study showed that k-space filters adapted to the Nyquist criterion improve the temporal resolution of peak velocity measures. Further investigation is justified to conclude if the performance exceeded the best performing method without k-space filters. However, the k-space filter showed substantial agreement with the best performing temporal footprint without k-space filter. / Syftet med arbetet är att designa och utvärdera k-space viktade filter för att förbättra den spatiala och temporala upplösningen av kardiovaskulär MRI, med målet att förbättra noggrannheten i kvantitativa flödesmätningar, som är viktiga för diagnos och uppföljning av hjärtdysfunktion. Två typer av k-space filter skapades och utvärderades retrospektivt på redan inhämtade data. Dessutom utfördes utvärdering med avseende på avsmalning av filtren i den radiella k-rymdsriktningen, såväl som accelererad avbildning med undersampling. För att bättre utnyttja egenskaperna hos den gyllene vinkeln skapades det ena k-rumsfilter så att det temporala fotavtrycket ökade i diskreta steg, så kallade ringar. Det temporala fotavtrycket för varje ring beräknades enligt Fibonacci talen, och startpositionen för varje ring beräknades så att den uppfyllde Nyquistkriteriet. k-Spacefiltren utvärderades i jämförelse med icke-filtrerade rekonstruktioner av tidsupplösta, anatomiska bilder (cine) och tidsupplösta faskontrastbilder. Bilder i motion-mode skapades från cine-bilderna och användes för att utvärdera kantskärpan av hjärtats skiljevägg (septum), vilket användes som en indikator för bildens spatiala upplösning. Faskontrastbilder användes för att mäta den maximala flödeshastigheten över mitralisklaffen och myokardiets hastighet i den tidiga och sena fyllnadsfasen. Maximal flödeshastighet är starkt beroende av den temporala upplösningen och gav därav en indikation på den temporala upplösningen. Denna studie visade att k-rumsfilter anpassade till Nyquist-kriteriet förbättrar den temporala upplösningen av topphastigheten. Ytterligare undersökning behövs dock för att säkerställa att prestandan översteg den bäst presterande metoden utan k-rumsfilter. Bilder rekonstruerade med filtret visade dock god överensstämmelse med det minsta temporala fotavtrycket, utan filter.

One way of reducing the examination time in magnetic resonance imaging (MRI) is to reduce the amount of raw data acquired, by performing so-called undersampling. Conventionally, MRI data is acquired line-by-line on a Cartesian grid. In the field of Cardiovascular Magnetic Resonance (CMR), however, radial k-space sampling is seen as a promising emerging technique for rapid image acquisitions, mainly due to its robustness against motion disturbances occurring from the beating heart. Whereas Cartesian undersampling will result in image aliasing, radial undersampling will introduce streak artifacts. The objective of this work was to train the deep learning architecture, CycleGAN, to reduce streak artifacts in radially undersampled CMR images, and to evaluate the model performance. A benefit of using CycleGAN over other deep learning techniques for this application is that it can be trained on unpaired data. In this work, CycleGAN network was trained on 3060 radial and 2775 Cartesian unpaired CMR images acquired in human subjects to learn a mapping between the two image domains. The model was evaluated in comparison to images reconstructed using another emerging technique called GRASP. Whereas more investigation is warranted, the results are promising, suggesting that CycleGAN could be a viable method for effective streak-reduction in clinical applications.