A novel non-invasive imaging method of unique k-space trajectory named “3D center-out EPI with cylindrical encoding” was developed and implemented for fast imaging of the human brain. The method based on a variant of 3D hybrid EPI combines advantages of the Cartesian and the radial encoding to achieve ultra-short echo time independent of spatial resolution and reasonably short echo train length yielding a quality image of high signal-to-noise ratio. Unlike rectilinear sampling, the method offers not only less motion and flow artifacts but enables also the undersampling capability. As a result, the method improves temporal resolution by shortening the measurement time. Nonetheless, artifacts induced from long-term drifts of the magnetic field as well as geometrical distortions caused by B0 inhomogeneity were removed with the average phase of the k-space center lines and an additional field map scan. Compared to other cylindrical k-space trajectories based on echo-planar imaging, which lead to progressively increasing echo time upon increasing the spatial resolution, the proposed method offers more benefits. As a significant application, imaging readout of the novel technique was applied to true 3D cine imaging which was later used in the combination of pseudo-continuous arterial spin labeling module in order to track a short arterial spin labeling (ASL) bolus of well-defined length along the fast passage through the large vessel compartment of the brain. Parametric maps of ASL signal change, estimated time-to-peak and ASL bolus width were extracted in order to characterize the macrovascular compartments of the brain-feeding arteries. Consequently, bolus dispersion within a single arterial branch was also assessed.

A novel non-invasive imaging method of unique k-space trajectory named “3D center-out EPI with cylindrical encoding” was developed and implemented for fast imaging of the human brain. The method based on a variant of 3D hybrid EPI combines advantages of the Cartesian and the radial encoding to achieve ultra-short echo time independent of spatial resolution and reasonably short echo train length yielding a quality image of high signal-to-noise ratio. Unlike rectilinear sampling, the method offers not only less motion and flow artifacts but enables also the undersampling capability. As a result, the method improves temporal resolution by shortening the measurement time. Nonetheless, artifacts induced from long-term drifts of the magnetic field as well as geometrical distortions caused by B0 inhomogeneity were removed with the average phase of the k-space center lines and an additional field map scan. Compared to other cylindrical k-space trajectories based on echo-planar imaging, which lead to progressively increasing echo time upon increasing the spatial resolution, the proposed method offers more benefits. As a significant application, imaging readout of the novel technique was applied to true 3D cine imaging which was later used in the combination of pseudo-continuous arterial spin labeling module in order to track a short arterial spin labeling (ASL) bolus of well-defined length along the fast passage through the large vessel compartment of the brain. Parametric maps of ASL signal change, estimated time-to-peak and ASL bolus width were extracted in order to characterize the macrovascular compartments of the brain-feeding arteries. Consequently, bolus dispersion within a single arterial branch was also assessed.

Die magnetische Blutbolusmarkierung (Spin-Labeling) ermöglicht die nichtinvasive quantitative Messung des Blutflusses im Gewebe. Beim Spin-Labeling wird arterielles Blut durch Radiofrequenzpulse magnetisch markiert und der Transport der Markierung MR-tomographisch gemessen. Am Modell einer unter physiologischen Bedingungen perfundierten extrakorporalen Schweineniere konnte die Quantifizierbarkeit der Messmethode nachgewiesen werden. In einer Studie an 36 Hirntumorpatienten wurde das Verfahren mit der kontrastmittelbasierten First-Pass-Bolus-Methode zur nicht-quantitativen Perfusionsmessung verglichen. Es zeigte sich eine sehr gute Übereinstimmung zwischen beiden Methoden, der lineare Korrelationskoeffizient des relativen Blutflusses in der Tumorregion lag bei R=0,83. Die mittels Spin-Labeling ermittelten Absolutwerte des Blutflusses spielen bei der Beurteilung des Tumorgrades eine untergeordnete Rolle, da die mittlere Perfusion individuell sehr verschieden ist. Ein zweiter Anwendungsbereich für das Spin-Labeling ist die Darstellung großer Arterien. Spin-Labeling ermöglicht die nichtinvasive dynamische Angiographie (Dynamische Spin-Labeling-Angiographie - DSLA). Analog zur digitalen Subtraktionsangiographie kann damit der Einstromvorgang des Blutes in den Gefäßbaum zeitaufgelöst gemessen werden, jedoch mit wesentlich höherer zeitlicher Auflösung und frei wählbarer Projektionsrichtung. In einer Studie an 18 Patienten mit einseitigen Carotisstenosen wurden die Zeitdifferenzen der Anflutung der zerebralen Gefäße zwischen der betroffenen und der nicht stenosierten Seite bestimmt. Die im Carotis-Siphon gemessenen Zeitdifferenzen korrelieren signifikant mit dem Stenosegrad, steigen aber erst ab einer Lumeneinengung oberhalb von 80 Prozent deutlich an. Im Vergleich zu den etablierten Methoden werden die Möglichkeiten und Grenzen der DSLA dargestellt. / Arterial spin labeling methods allow to determine quantitative tissue blood flow values noninvasively. Arterial blood is labelled by an inversion pulse and the distribution of this intrinsic tracer is measured using magnetic resonance imaging. Experiments using an extra corporal in-vitro porcine kidney in a MR compatible set-up were carried out to determine the accuracy of blood flow values calculated from arterial spin labeling measurements. In a study of 36 brain tumor patients, spin labeling was compared to non-quantitative contrast-enhanced dynamic susceptibility-weighted perfusion imaging. Relative blood flow values determined with both methods were in good agreement, the linear regression coefficient in the tumor region was R=0.83. Due to the variable individual perfusion state, quantitative blood flow values determined using spin labeling play a minor role in the assessment of tumor grade. Application of spin labeling to angiography of major arteries was investigated. Dynamic spin labeling angiography (DSLA) sequences were implemented and tested on a clinical scanner. This technique allows time-resolved depiction of blood flow in large vessels with very high temporal resolution. As opposed to digital subtraction angiography, the method allows arbitrary projection directions. In a study, 18 patients with one-sided carotid stenoses were examined. In these patients the time differences of blood bolus arrival at both hemispheres were determined. Time differences measured in the carotid siphon show a significant correlation with the degree of stenosis. However, a clear increase is not seen until 80% narrowing of a carotid. Possibilities and limitations of the DSLA method are discussed in comparison to established techniques.

Die zerebrale Perfusion ist eine wichtige physiologische Größe, die den Blutfluss in grauer bzw. weißer Hirnsubstanz beschreibt. Zur Perfusionsmessung in der klinischen Anwendung hat sich pseudo-kontinuierliches Arterielles Spin-Labeling (pCASL) als nichtinvasive Methode in der Magnetresonanzbildgebung etabliert. Das Anliegen der vorliegenden Arbeit ist es, pCASL zu charakterisieren und die Ursache für dessen Empfindlichkeit gegenüber intrinsischen Magnetfeldgradienten zu untersuchen. Anhand von Simulationen mit der Bloch-Gleichung konnten optimale Messparameter abgeleitet werden, um das Verfahren in dieser Hinsicht robuster zu machen. Die damit unabhängig von intrinsischen Magnetfeldgradienten bei 3\,T vorhergesagte hohe Labeling-Effizienz von 90\,\% wurde in vivo mittels eines eigens dafür entwickelten Messverfahrens experimentell validiert. / Cerebral perfusion is an important physiological parameter that describes the blood flow in brain tissue. To measure perfusion in a clinical setting, pseudo-continuous arterial spin labeling (pCASL) has been established as noninvasive method in magnetic resonance imaging. The purpose of this work is to characterize pCASL and to investigate its susceptibility to intrinsic magnetic field gradients. By simulations based on the Bloch equation, optimal parameter settings could be derived with particular focus on robustness against their impairing influence. As a result, a high labeling efficiency of 90\% was predicted independently of magnetic field gradients at 3\,T. This finding could finally be validated in vivo by a dedicated experimental approach.