title: Compensation of Magnetic Field Inhomogeneities with Dynamic Fat Saturation for Magnetic Resonance Imaging at 1.5 Tesla
Creators: name: Eisen, Christian Karl
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2024

description: Fat suppression is fundamental to magnetic resonance imaging (MRI) in clinical and research applications. Insufficiently suppressed fat signal can degrade image quality and the overall usability of a measurement. A versatile and fast method to manipulate fat independently of water is the application of pre-saturation pulses tuned to the resonance frequency of fat. However, the resonance frequency is proportional to the main magnetic field (B0). Therefore, conventional pre-saturation pulses are sensitive to B0 inhomogeneities. Furthermore, since the flip angle of the applied pulse is dependent on the transmit magnetic field (B1), deviations from the nominal B1 influence the degree of suppression. The aim of this work was to develop a dynamic pulse design incorporating information about field inhomogeneities to determine B0- and B1-insensitive pulses for the purpose of fat suppression at 1.5 Tesla. Therefore, a pulse design process was implemented directly on the scanner to allow a simple workflow without the need for operator interaction. A turbo spin echo (TSE) sequence was modified to replace the conventional pre-saturation pulse with the dynamic fat saturation pulse (dFx). After starting a measurement with the modified TSE sequence, maps with information about B0 and B1 inhomogeneities are acquired automatically. The saturation pulse is calculated based on these maps, a specified gradient trajectory and defined optimization settings. The gradients are applied simultaneously to the dFx pulses and manipulate the magnetic field distribution while the radio-frequency pulse induces the saturation. An offline optimization approach was developed to find suitable gradient trajectory parameters for specific targets (e.g., body regions). In addition, various options for accelerating the optimization procedure were investigated. The first experiments focused on 2D dFx pulses, which were evaluated in measurements with multiple slices of a phantom and in vivo (i.e., the cervical spine and the foot in sagittal orientation). For this purpose, a novel spiral trajectory design was found and optimized. The efficiency of the dFx pulses, which were determined using subject-specific field information, was compared with a conventional Gaussian pulse and the Dixon method. A reduction of the remaining signal in fat-dominated regions of at least 70%±2% (phantom), 76% ± 10% (cervical spine) and 44% ± 19% (foot) was observed relative to the Gaussian pulse. In addition, no severe suppression of water signal by the dFx pulses was detected. Further investigations revealed that the quality of fat suppression with dFx is more affected by the design scheme for multiple pulses in multi-slice acquisitions than by the actual B1 information. Additional measurements of the axial cervical spine and the coronal foot demonstrated a general applicability of dFx to other target applications. A numerical optimization approach for the gradient trajectory was tested as a basis for 3D dFx pulses. Although the method could not provide reproducible results, investigations were performed with the best trajectory according to simulations (flip angle deviations of 2.7° ± 2.0° for fat and 3.8° ± 1.4° for water) compared to two conventional suppression pulses in two configurations. Individually calculated dFx pulses outperformed all other pulses, including universal dFx pulses optimized with field information from multiple data sets. Simulations showed that a novel parameterized 3D double SPINS trajectory provides a similar fat suppression performance compared to the numerically found solution. Additional experiments on encountered artifacts, a further acceleration option and effects of possible discrepancies between simulation and real measurements give a deeper insight into the proposed method.