Effects of heart valve prostheses on phase contrast flow measurements in Cardiovascular Magnetic Resonance - a phantom study

Background: Cardiovascular Magnetic Resonance is often used to evaluate patients after heart valve replacement. This study systematically analyses the influence of heart valve prostheses on phase contrast measurements in a phantom trial. Methods: Two biological and one mechanical aortic valve prostheses were integrated in a flow phantom. B-0 maps and phase contrast measurements were acquired at a 1.5 T MR scanner using conventional gradient-echo sequences in predefined distances to the prostheses. Results were compared to measurements with a synthetic metal-free aortic valve. Results: The flow results at the level of the prosthesis differed significantly from the reference flow acquired before the level of the prosthesis. The maximum flow miscalculation was 154 ml/s for one of the biological prostheses and 140 ml/s for the mechanical prosthesis. Measurements with the synthetic aortic valve did not show significant deviations. Flow values measured approximately 20 mm distal to the level of the prosthesis agreed with the reference flow for all tested all prostheses. Conclusions: The tested heart valve prostheses lead to a significant deviation of the measured flow rates compared to a reference. A distance of 20 mm was effective in our setting to avoid this influence

Flow measurement by cardiovascular magnetic resonance: a multi-centre multi-vendor study of background phase offset errors that can compromise the accuracy of derived regurgitant or shunt flow measurements

AIMS: Cardiovascular magnetic resonance (CMR) allows non-invasive phase contrast measurements of flow through planes transecting large vessels. However, some clinically valuable applications are highly sensitive to errors caused by small offsets of measured velocities if these are not adequately corrected, for example by the use of static tissue or static phantom correction of the offset error. We studied the severity of uncorrected velocity offset errors across sites and CMR systems. METHODS AND RESULTS: In a multi-centre, multi-vendor study, breath-hold through-plane retrospectively ECG-gated phase contrast acquisitions, as are used clinically for aortic and pulmonary flow measurement, were applied to static gelatin phantoms in twelve 1.5 T CMR systems, using a velocity encoding range of 150 cm/s. No post-processing corrections of offsets were implemented. The greatest uncorrected velocity offset, taken as an average over a 'great vessel' region (30 mm diameter) located up to 70 mm in-plane distance from the magnet isocenter, ranged from 0.4 cm/s to 4.9 cm/s. It averaged 2.7 cm/s over all the planes and systems. By theoretical calculation, a velocity offset error of 0.6 cm/s (representing just 0.4% of a 150 cm/s velocity encoding range) is barely acceptable, potentially causing about 5% miscalculation of cardiac output and up to 10% error in shunt measurement. CONCLUSION: In the absence of hardware or software upgrades able to reduce phase offset errors, all the systems tested appeared to require post-acquisition correction to achieve consistently reliable breath-hold measurements of flow. The effectiveness of offset correction software will still need testing with respect to clinical flow acquisitions

Acoustic cardiac triggering: a practical solution for synchronization and gating of cardiovascular magnetic resonance at 7 Tesla

<p>Abstract</p> <p>Background</p> <p>To demonstrate the applicability of acoustic cardiac triggering (ACT) for imaging of the heart at ultrahigh magnetic fields (7.0 T) by comparing phonocardiogram, conventional vector electrocardiogram (ECG) and traditional pulse oximetry (POX) triggered 2D CINE acquisitions together with (i) a qualitative image quality analysis, (ii) an assessment of the left ventricular function parameter and (iii) an examination of trigger reliability and trigger detection variance derived from the signal waveforms.</p> <p>Results</p> <p>ECG was susceptible to severe distortions at 7.0 T. POX and ACT provided waveforms free of interferences from electromagnetic fields or from magneto-hydrodynamic effects. Frequent R-wave mis-registration occurred in ECG-triggered acquisitions with a failure rate of up to 30% resulting in cardiac motion induced artifacts. ACT and POX triggering produced images free of cardiac motion artefacts. ECG showed a severe jitter in the R-wave detection. POX also showed a trigger jitter of approximately Δt = 72 ms which is equivalent to two cardiac phases. ACT showed a jitter of approximately Δt = 5 ms only. ECG waveforms revealed a standard deviation for the cardiac trigger offset larger than that observed for ACT or POX waveforms.</p> <p>Image quality assessment showed that ACT substantially improved image quality as compared to ECG (image quality score at end-diastole: ECG = 1.7 ± 0.5, ACT = 2.4 ± 0.5, p = 0.04) while the comparison between ECG vs. POX gated acquisitions showed no significant differences in image quality (image quality score: ECG = 1.7 ± 0.5, POX = 2.0 ± 0.5, p = 0.34).</p> <p>Conclusions</p> <p>The applicability of acoustic triggering for cardiac CINE imaging at 7.0 T was demonstrated. ACT's trigger reliability and fidelity are superior to that of ECG and POX. ACT promises to be beneficial for cardiovascular magnetic resonance at ultra-high field strengths including 7.0 T.</p

Rapid parametric mapping of the longitudinal relaxation time T1 using two- dimensional variable flip angle Magnetic Resonance Imaging at 1.5 Tesla, 3 Tesla, and 7 Tesla

Die visuelle und damit subjektive Auswertung T1 gewichteter (longitudinale Relaxationszeit) oder T2 gewichteter (transversale Relaxationszeit) Schnittbilder gehören zur täglichen klinischen Diagnostik in der kardialen Magnetresonanztomographie (MRT). Für diese nichtquantitative Bildgebungsmethoden hängt die Qualität der Diagnostik unter anderem von Aufnahmeparametern, Gerätekonfiguration, Homogenität des Grundmagnetfeldes (B0), vom Hochfrequenz-Sendefeld (B1+) sowie von der Erfahrung des Befunders ab. Die quantitative Erfassung der T1 und T2 Gewebeparameter birgt das Potential, sich von diesen äußeren Einflüssen unabhängig zu machen. Typischerweise sind T1 und T2 Akquisitions- und Quantifizierungsmethoden jedoch zeitaufwendig. Die vorgeschlagenen Techniken erzielen in der Literatur eine große Bandbreite an Normwerten, vorwiegend aufgrund technischer Hürden, unzureichenden Modellannahmen, oder physiologischen Einflussfaktoren wie Herzfrequenz, Hämodynamik als auch kardiale und respiratorische Bewegung. Aus diesen Gründen wurde in dieser Studie eine schnelle MR Technik entwickelt, die das Grundmagnetfeld, das HF-Sendefeld und die quantitative Kartierungen der T1 Zeit innerhalb weniger Sekunden ermitteln kann. Die entwickelte Methode wurde an einem statischen Phantom, sowie an gesunden Probanden im Gehirn bei magnetischen Feldstärken von 1.5 Tesla, 3 Tesla, und 7 Tesla getestet und gegen Referenzmessungen validiert. Exemplarisch wurde in Messungen am Patienten die klinische Anwendbarkeit demonstriert. Ein weiterer Schwerpunkt dieser Arbeit lag im Entwurf, der Konstruktion und der Evaluierung eines MR- kompatiblen bewegten Modells einer menschlichen linken Herzkammer. Diese diente der Ermöglichung standardisierter Messungen in der Präsenz von kardialer Bewegung und Blutfluss mit dem Ziel, die T1 Kartierung am Herzen zu ermöglichen und zu verbessern. Zum Zeitpunkt der Studie waren keine Normwerte für T1 und T2 Werte des Herzmuskels bei 3T vorhanden, sodass zunächst mittels eines alternativen Ansatzes bei gesunden Probanden Referenzwerte erhoben wurden. Während Messungen bei 1.5 Tesla und 3 Tesla zur klinischen Routine gehören, sind Untersuchungen bei 7 Tesla im humanen Bereich gegenwärtig als experimentell anzusehen. Für die kardiale Bildgebung im magnetischen Hochfeld (7T) waren zum Zeitpunkt der Entstehung dieser Arbeit notwendige Hochfrequenzspulen kommerziell nicht erhältlich. Verschiedene Spulenprototypen wurden deshalb entwickelt und auf die Machbarkeit diagnostischer Herzbildgebung geprüft und optimiert. Eine weitere Herausforderung im Hochfeld stellte die Synchronisation der Bildaufnahme mit dem Herzzyklus dar. Die konventionelle Synchronisationsmethode der Bildakquise mit dem Herzzyklus durch das Elektrokardiogramm ist in starken Magnetfeldern nicht ohne weiteres möglich. Die Ursachen wurden im Zuge dieser Arbeit untersucht, alternative Methoden entwickelt und für Humanuntersuchungen etabliert.Magnetic resonance imaging (MRI) is a non-invasive imaging technique free of ionizing radiation that allows medical imaging of the human body in any arbitrary orientation. Visual but subjective evaluations of longitudinal relaxation time (T1) or transversal relaxation time (T2) weighted images are commonly used in clinical diagnostics of cardiac MRI. For this non- quantitative measure, diagnostic quality depends on external influencing factors such as hardware configuration, sequence parameters, homogeneity of the static magnetic field (B0) and the radio frequency transmit field (B1+) and the observer experience. The quantitative determination of T1 and T2 has the potential to offset these external influences. However, quantification is typically time consuming and can exceed clinically acceptable scan times. Different techniques described in the literature show a large scatter of normal values mainly due to insufficient model assumptions or physiological factors such as heart rate, hemodynamics, or cardiac and respiratory motion. For these reasons, this study examines the feasibility of a rapid slice- selective T1 quantification using variable flip angles (VFA) at magnetic field strengths of 1.5 Tesla, 3 Tesla, and 7 Tesla. For this purpose, an MR sequence was developed, which enables measurement of B0, B1+ and T1 in scan times as short as few seconds. The developed method was validated in a static phantom as well as in healthy volunteers in the brain and benchmarked against reference measurements. Exemplarily, the clinical applicability was demonstrated in patient pilot studies. To allow measurements using this method in cardiac tissue, an MR compatible dynamic model of a human cardiac left ventricle was designed, constructed and tested. This setup resembled highly standardized physiological motion paradigms and dedicated MR signal properties to allow validation of the sequence in the presence of cardiac motion and blood flow. As no normal values for myocardium at 3T were available at the time of this study, reference values were collected in healthy volunteers using an alternative T1 mapping approach. Furthermore, radio-frequency (RF) coils necessary for cardiac imaging at high magnetic field strengths (7T) were not commercially available. Therefore, various multi-channel RF coil prototypes were developed, investigated, and optimized for diagnostic cardiac imaging. Another challenge in high magnetic fields was that the electrocardiogram, which is established at clinical field strengths for synchronizing the data acquisition with the cardiac cycle, could not be used due to signal distortions. The cause of these distortions was studied and alternative approaches for data synchronization were developed and investigated