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Clinical practice in CMR with respect to cardiovascular disease is currently focused on tissue characterization, and cardiac function, in particular. In recent years MRI based diffusion tensor imaging (DTI) has been shown to enable the assessment of microstructure based on the analysis of Brownian motion of water molecules in anisotropic tissue, such as the myocardium. With respect to both functional and structural imaging, 7T MRI may increase SNR, providing access to information beyond the reach of clinically applied field strengths. To date, cardiac 7T MRI is still a research modality that is only starting to develop towards clinical application.
In this thesis we primarily aimed to advance methods of ultrahigh field CMR using the latest 7T technology and its application towards the functional and structural characterization of the myocardium.
Regarding the assessment of myocardial microstructure at 7T, feasibility of ex vivo DTI of large animal hearts was demonstrated. In such hearts a custom sequence implemented for in vivo DTI was evaluated and fixation induced alterations of derived diffusion metrics and tissue properties were assessed. Results enable comparison of prior and future ex vivo DTI studies and provide information on measurement parameters at 7T.
Translating developed methodology to preclinical studies of mouse hearts, ex vivo DTI provided highly sensitive surrogates for microstructural remodeling in response to subendocardial damage. In such cases echocardiography measurements revealed mild diastolic dysfunction and impaired longitudinal deformation, linking disease induced structural and functional alterations. Complementary DTI and echocardiography data also improved our understanding of structure-function interactions in cases of loss of contractile myofiber tracts, replacement fibrosis, and LV systolic failure.
Regarding the functional characterization of the myocardium at 7T, sequence protocols were expanded towards a dedicated 7T routine protocol, encompassing accurate cardiac planning and the assessment of cardiac function via cine imaging in humans.
This assessment requires segmentation of myocardial contours. For that, artificial intelligence (AI) was developed and trained, enabling rapid automatic generation of cardiac segmentation in clinical data. Using transfer learning, AI models were adapted to cine data acquired using the latest generation 7T system. Methodology for AI based segmentation was translated to cardiac pathology, where automatic segmentation of scar tissue, edema and healthy myocardium was achieved.
Developed radiofrequency hardware facilitates translational studies at 7T, providing controlled conditions for future method development towards cardiac 7T MRI in humans.
In this thesis the latest 7T technology, cardiac DTI, and AI were used to advance methods of ultrahigh field CMR. In the long run, obtained results contribute to diagnostic methods that may facilitate early detection and risk stratification in cardiovascular disease.
Cardiovascular disease is one of the leading causes of death worldwide and, so far, echocardiography, nuclear cardiology, and catheterization are the gold standard techniques used for its detection. Cardiac magnetic resonance (CMR) can replace the invasive imaging modalities and provide a "one-stop shop" characterization of the cardiovascular system by measuring myocardial tissue structure, function and perfusion of the heart, as well as anatomy of and flow in the coronary arteries. In contrast to standard clinical magnetic resonance imaging (MRI) scanners, which are often operated at a field strength of 1.5 or 3 Tesla (T), a higher resolution and subsequent cardiac parameter quantification could potentially be achieved at ultra-high field, i.e., 7 T and above.
Unique insights into the pathophysiology of the heart are expected from ultra-high field MRI, which offers enhanced image quality in combination with novel contrast mechanisms, but suffers from spatio-temporal B0 magnetic field variations. Due to the resulting spatial misregistration and intra-voxel dephasing, these B0-field inhomogeneities generate a variety of undesired image artifacts, e.g., artificial image deformation. The resulting macroscopic field gradients lead to signal loss, because the effective transverse relaxation time T2* is shortened. This affects the accuracy of T2* measurements, which are essential for myocardial tissue characterization. When steady state free precession-based pulse sequences are employed for image acquisition, certain off-resonance frequencies cause signal voids. These banding artifacts complicate the proper marking of the myocardium and, subsequently, systematic errors in cardiac function measurements are inevitable. Clinical MR scanners are equipped with basic shim systems to correct for occurring B0-field inhomogeneities and resulting image artifacts, however, these are not sufficient for the advanced measurement techniques employed for ultra-high field MRI of the heart.
Therefore, this work focused on the development of advanced B0 shimming strategies for CMR imaging applications to correct the spatio-temporal B0 field variations present in the human heart at 7 T. A novel cardiac phase-specific shimming (CPSS) technique was set up, which featured a triggered B0 map acquisition, anatomy-matched selection of the shim-region-of-interest (SROI), and calibration-based B0 field modeling. The influence of technical limitations on the overall spherical harmonics (SH) shim was analyzed. Moreover, benefits as well as pitfalls of dynamic shimming were debated in this study. An advanced B0 shimming strategy was set up and applied in vivo, which was the first implementation of a heart-specific shimming approach in human UHF MRI at the time.
The spatial B0-field patterns which were measured in the heart throughout this study contained localized spots of strong inhomogeneities. They fluctuated over the cardiac cycle in both size and strength, and were ideally addressed using anatomy-matched SROIs. Creating a correcting magnetic field with one shim coil, however, generated eddy currents in the surrounding conducting structures and a resulting additional, unintended magnetic field. Taking these shim-to-shim interactions into account via calibration, it was demonstrated for the first time that the non-standard 3rd-order SH terms enhanced B0-field homogeneity in the human heart. However, they were attended by challenges for the shim system hardware employed in the presented work, which was indicated by the currents required to generate the optimal 3rd-order SH terms exceeding the dynamic range of the corresponding shim coils. To facilitate dynamic shimming updated over the cardiac cycle for cine imaging, the benefit of adjusting the oscillating CPSS currents was found to be vital. The first in vivo application of the novel advanced B0 shimming strategy mostly matched the simulations.
The presented technical developments are a basic requirement to quantitative and functional CMR imaging of the human heart at 7 T. They pave the way for numerous clinical studies about cardiac diseases, and continuative research on dedicated cardiac B0 shimming, e.g., adapted passive shimming and multi-coil technologies.