| Zusammenfassung |
Cardiovascular disease (CVD), including coronary artery disease, heart failure, cardiomyopathy, and myocarditis, accounts for 32% of deaths and remains the leading cause of death worldwide (17.9 million per year; WHO, 2019). In cardiology Magnetic resonance imaging (MRI) is an essential clinical imaging tool because it provides excellent soft-tissue contrast. MR examination can be used to examine non-invasively various parameters of the heart, providing information on, for example, functionality, blood flow, or tissue composition. Depending on the intended application, different cardiac MRI techniques are utilized. For example, cine MRI is suitable for visualizing cardiac motion and regional wall motion abnormalities, while the assessment of the progression of myocarditis can be achieved with the use of T1 mapping. Of disadvantage to many techniques are the long examination times mainly due to physiological motion such as breathing. To ensure that image quality is not compromised by motion artifacts, the breathhold strategy is commonly used in clinical practice. But for a complete examination, the patients must manage several instructed breath-holding phases, which can often be difficult for the sick, elderly, or children. Therefore, methods have been developed that allow cardiac MR examination under free breathing. Often these approaches are based on an additional measurement of an MR-navigator, which provides information about the respiratory motion state. However, the MR-navigator is not suitable for continuous measurements because its acquisition interrupts the steady-state during the measurement. A novel alternative for a motion surrogate is the pilot tone (PT). The PT is an additional RF signal introduced into the scanner, which can be extracted from the acquired MR data during the measurement. The intensity of the scale-free signal changes depending on the respiratory motion and can, therefore, be used as a motion surrogate. Nevertheless, quantitative motion information is required for motion correction. In this thesis, a new PT-based method for respiratory motion correction for cardiac MRI was developed. Using phantom- and in vivo data, it was demonstrated that motion correction using the PT leads to an improvement in image quality and accuracy of quantitative parameters compared with uncorrected images. The temporal stability of the PT was shown for at least 50 min. Subject-specific motion models were derived from a calibration scan, that allow to convert the qualitative PT into a quantitative signal providing information about respiratory motion. Furthermore, a comparison of the PT with other motion surrogates was performed.
For a 3D MR scan, retrospective motion correction using the PT improved the visibility of the coronary arteries similar to the MR-navigator. Thereafter, a novel prospective PT-based motion correction approach was developed, which enables slice tracking during the running sequence. The quantitative v PT was used to adapt the slice position during the measurement to ensure the current imaging slice follows the respiratory motion of the heart. Motion artifacts in functional cine images with Cartesian acquisition scheme could be strongly reduced with this prospective motion correction approach. The contrast-to-noise ratio with respect to motion artifacts and also the sharpness of the endocardium improved significantly compared with the uncorrected images. Furthermore, left ventricular blood pool areas were determined, and there was no significant difference between the reference breathhold method and the presented motion-corrected free-breathing approach. Similar improvements were achieved for quantitative T1-mapping of the myocardium. Here a radial acquisition scheme was used for data acquisition. Without motion correction respiratory motion led to an overestimation of T1 values compared to breathhold data, which was successfully corrected with the PT-based approach. The presented results demonstrate that PT-based respiratory motion correction is robust, accurate, and versatile, and thus may enable future developments such as high-resolution imaging strategies under free-breathing. |
