Prospective Respiration Detection in Magnetic Resonance Imaging by a Non-Interfering Noise Navigator

Passive monitoring of the thermal noise variances of the channels of a receive array was shown to reveal respiratory motion of the underlying anatomy, a so called 'noise navigator'. There is, however, an inevitable trade off between the accuracy and temporal resolution of the noise navigator due to its passive nature. A temporal filter has to be added to the noise navigator to accurately reveal respiration and retain temporal resolution. For real-time applications of the noise navigator, e.g., prospective motion correction or motion tracking, the added filter must be prospective. Thus a prospective Kalman filter was designed to predict respiration from the noise navigator without a temporal delay. The performance of the noise navigator enhanced by this prospective Kalman filter was explored and the robustness of the proposed method was assessed on healthy volunteers. The respiratory signal could be measured by the noise navigator independent of magnetic resonance acquisition. The calculated respiratory signal was qualitatively compared with the respiratory bellows. In addition, a strong linear relationship was found between the prospective noise navigator and a quantitative 2-D image navigator for measurements, including free and tasked breathing

Evaluation and impact of gating methods for clinical and ultra-high-field magnetic resonance imaging

Hintergrund Die Synchronisation des kardialen Zyklus mit der Bildakquise (kardiales Gating) spielt in der kardiovaskulären Magnetresonanztomographie (CMR) eine große Rolle. Das dafür häufig verwendete Elektrokardiogram (EKG) unterliegt in der Umgebung des Magnetresonanztomographen häufig multiplen Störfaktoren. Insbesondere die Parameter Magnetfeldstärke, verwendete Gating-Methode und Positionierung der Elektroden scheinen die Güte des Gatings zu beeinflussen. Deren quantitativer Einfluss ist bis jetzt nicht hinreichend erforscht. Diesen abschätzen zu können, wäre jedoch für die Planung von Studien und die Evaluierung von neu entwickelten Gating-Methoden von großem Interesse. Daher war es Ziel dieser Studie, den Einfluss von 1.5T, 3T und insbesondere auch ultra-hohen Feldstärken von 7T, der EKG- und Vektorkardiogramm(VKG)-basierten Gating-Methode sowie von vier standardisierten Elektrodenpositionen auf die Güte des Gatings zu untersuchen. Methoden EKG-Signale von 16 gesunden Proband*innen wurden bei Feldstärken von 1.5T, 3T und 7T, jeweils mit zwei verschiedenen Gatingmethoden und vier standardisierten Elektrodenpositionen in einem standardisierten CMR-Protokoll aufgenommen. Die quantitative Güte des Gatings wurde mittels spezialisierter Software analysiert, wobei eine manuelle Annotation der QRS-Komplexe als Goldstandard zur Berechnung der Metriken für die Gating-Güte diente. Die statistische Analyse erfolgte in einem dreistufigen Testmodell. Ergebnisse In der globalen Analyse konnte ein signifikanter Einfluss der Magnetfeldstärke für alle untersuchten Gütemetriken gezeigt werden, wobei ultra-hohe Feldstärken zu einer deutlichen Verminderung der Gating-Güte führten (p<0.0001). Für die untersuchten Gating-Methoden zeigte sich in unseren Daten insgesamt eine Überlegenheit der EKG-Methode gegenüber der VKG-Methode für die Metriken: Sensitivität: p<0.0001, positiv prädiktiver Wert: p=0.0005, Propagation Delay: p>0.0001 und Jitter: p>0.0001 Die untersuchten alternativen Elektrodenpositionen zeigten in der Subgruppenanalyse eine ähnliche Gating-Güte wie die Standardpositionen. Zusammenfassung Zusammenfassend konnte der Einfluss von Magnetfeldstärken von 1.5T, 3T und 7T, der EKG- und VKG-Gating-Methode und von vier standardisierten Elektrodenposition auf die Gating-Güte quantifiziert werden. Die Ergebnisse erlauben eine verbesserte Abschätzung der Abbruchraten und Gesamtmesszeiten in CMR-Studien sowie eine Optimierung des Gatings für verschiedene Messkonfigurationen.Purpose Synchronizing the cardiac cycle with image acquisition (cardiac gating) is of paramount importance in cardiovascular magnetic resonance (CMR). Inside the scanner the electrocardiogram (ECG) generally utilized in gating is often compromised by several factors. Magnetic field-strength, gating method and electrode positioning seem to influence gating performance. Their quantitative impact, however, is not yet fully understood. An estimation of this impact, however, could facilitate study design and evaluation of newly developed gating methods. Therefore, the aim of this study was to systematically assess and quantify the impact of field-strengths of 1.5T, 3T and 7T, the ECG-based and the vectorcardiogram(VCG)-based gating-method, as well as four standardized electrode positions on gating performance. Methods ECG-Signals of 16 healthy volunteers were recorded at field-strengths of 1.5T, 3T and 7T, with the ECG-method and the VCG-method and with four standardized electrode positions during a standardized CMR-protocol. Gating-performance was assessed and quantified via specialized software with manual annotation of the QRS-complexes serving as the reference standard for the calculation of performance metrics. The statistical analysis was performed in a hierarchical test-strategy with three levels. Results In the global analysis a significant impact of magnetic field strengths on gating performance was found for all performance metrics, with ultra-high field-strengths leading to a significant decrease in gating performance (p0.0001 and jitter: p>0.0001. In the subgroup-analysis, the assessed alternative electrode positions provided similar gating-performance to the standard-positions. Conclusion We successfully quantified the influence of 1.5T, 3T and 7T, the ECG- and the VCG-method and four standardized electrode positions on gating-performance. Our results allow an improved estimation of drop-out rates and scan durations for CMR-studies, as well as an optimization of the gating procedure for specific gating-configurations

Surrogate-driven respiratory motion models for MRI-guided lung radiotherapy treatments

An MR-Linac integrates an MR scanner with a radiotherapy delivery system, providing non-ionizing real-time imaging of the internal anatomy before, during and after radiotherapy treatments. Due to spatio-temporal limitations of MR imaging, only high-resolution 2D cine-MR images can be acquired in real-time during MRI-guided radiotherapy (MRIgRT) to monitor the respiratory-induced motion of lung tumours and organs-at-risk. However, temporally-resolved 3D anatomical information is essential for accurate MR guidance of beam delivery and dose estimation of the actually delivered dose. Surrogate-driven respiratory motion models can estimate the 3D motion of the internal anatomy from surrogate signals, producing the required information. The overall aim of this thesis was to tailor a generalized respiratory motion modelling framework for lung MRIgRT. This framework can fit the model directly to unsorted 2D MR images sampling the 3D motion, and to surrogate signals extracted from the 2D cine-MR images acquired on an MR-Linac. It can model breath-to-breath variability and produce a motion compensated super-resolution reconstruction (MCSR) 3D image that can be deformed using the estimated motion. In this work novel MRI-derived surrogate signals were generated from 2D cine-MR images to model respiratory motion for lung cancer patients, by applying principal component analysis to the control point displacements obtained from the registration of the cine-MR images. An MR multi-slice interleaved acquisition potentially suitable for the MR-Linac was developed to generate MRI-derived surrogate signals and build accurate respiratory motion models with the generalized framework for lung cancer patients. The developed models and the MCSR images were thoroughly evaluated for lung cancer patients scanned on an MR-Linac. The results showed that respiratory motion models built with the generalized framework and minimal training data generally produced median errors within the MCSR voxel size of 2 mm, throughout the whole 3D thoracic field-of-view and over the expected lung MRIgRT treatment times