Stefan Kroboth
Published: 2021
Total Pages:
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Abstract: Magnetic Resonance Imaging (MRI) is a tomographic imaging modality commonly used for diagnosis in medicine. In an attempt to push the limits of MRI, quadratic magnetic fields were recently added to the image encoding process. While this concept was shown to aid the image acquisition, it is yet unclear, which properties such nonlinear fields should have in order to fully exploit their potential. Therefore in the recent past so-called matrix gradient coils, which consist of a large number of small coils, were introduced. The current in each such coil element can be adjusted individually, and the final field shape is given by the superposition of the fields of all coil elements. Such an approach on one hand allows for a wide range of different field shapes. On the other hand it necessitates as many amplifiers as coil elements, which can be expensive and technically challenging. The first part of this thesis introduces a method for overcoming the above-mentioned problem by driving the matrix gradient coil with fewer amplifiers than coil elements. This is achieved by first finding a so-called configuration, which defines a network of coils capable of approximating a desired field shape. Since most image encoding strategies in MRI require more than a single field, one configuration per target field is obtained. Then a switching circuit is optimized, which is able to switch between the set of configurations with a low number of switches. While nonlinear fields have shown to add additional degrees of freedom to the image acquisition process, it remained unclear how to utilize them for image encoding in MRI most efficiently. Therefore the second part this thesis introduces an algorithm, which obtains ways to drive the acquisition of the MR signal by efficiently utilizing the available hardware (gradient coils with arbitrary field geometries, radio-frequency receiver coils) such that the overall acquired information content is maximized. This approach can also be used as a means to investigate the interplay of spatial encoding steps and local radio-frequency receiver coils, which may help to find ways of driving the available hardware, such that imperfections of one component are compensated for by another component while reducing the number of required encoding steps. In the past, hardware components where typically designed independent of each other, but with the insights gained from this method, it may in the future be possible to design components in parallel while considering their interactions with each other. This may in the future lead to faster and higher quality image acquisition, which is beneficial for both the operation of the MRI as well as the patients