Akustisches Synchronisationsverfahren für die Magnetresonanztomographie

In current clinical MR practice, cardiac motion is dealt with using electrocardiographic (ECG) gating to synchronize data acquisition with the cardiac cycle. ECG, being an inherently electrical measurement, is corrupted by interferences with electromagnetic fields and by magnetohydrodynamic effects, in particular at high magnetic field strengths. Consequently, artifacts in the ECG trace might be mis-interpreted as trigger pulse resulting in erroneous triggering together with motion corrupted image quality. For all of these reasons, a non-invasive, fully MR compatible cardiac monitoring and gating approach, which presents no risk of high voltage induction and patient burns, immunity to electromagnetic interferences, suitability for all magnetic field strengths, patient comfort and ease of use is conceptually appealing for the pursuit of robust and safe clinical cardiovascular MR (CVMR). To meet all these challenges, the first aim of this thesis is to develop a cardiac monitoring and gating device that employs acoustic signals. Next, the clinical efficacy and robustness of acoustic triggering in CVMR applications at 1.5 T, 3.0 T and 7.0 T including retrospective gating and prospective triggering regimes is demonstrated. The basic design of the MR-stethoscope is surveyed in Chapter 3. The acoustic gating device consists of three main components: an acoustic sensor, a signal processing unit and a coupler unit to the MR system. Signal conditioning and conversion are conducted outside the 0.5mT area using dedicated electronic circuits. The final waveform is delivered to the internal physiological signal controller circuitry of a clinical MR scanner. Chapter 4 presents the technical evaluation of the MR-stethoscope. In comparison, ECG waveforms were susceptible to T-wave elevation and other distortion, which were more pronounced at higher fieldstrengths. Chapter 5 describes the results derived from the clinical evaluation of the MR-stethoscope. For this purpose, cardiovascular MRI was performed on volunteers at 1.5 T, 3.0 T and 7.0 T whole body MR systems. Black blood imaging, 2D CINE imaging, 3D phase contrast MR angiography and myocardial T2* mapping were carried out. Obtaining a trigger signal from the target vessel territory would be also very beneficial to address motion artifact concerns raised by the traditional MRA approach, where missynchronization is frequently caused by the latency between the R-wave recognition used for gating and the motion of a target vessel distant to the heart. This applies in particular to peripheral ECG-gated MRA and to finger pulse oximetry gated acquisitions, where the mean travel time of blood between the heart and the target territory can be in the order of several hundreds of milliseconds. Hence, finger pulse oximetry gated prospective cardiac imaging of the systolic cardiac phase is elusive. For comparison, the latency between lectrophysiological and acoustic cardiac activity can be easily offset by reducing the trigger delay by 30ms in order not to miss the systolic cardiac phase. The acoustic approach also appears to be an excellent candidate for gating acquisitions in areas located distant from the heart. In clinical practice, areas of interest are usually positioned at the magnet’s isocenter. Consequently, the position of the heart in non-cardiac studies is off-center, closer to the ends off the gradient coils, which amplifies changes in dB/dt during gradient switching. This geometrical constraint increases the interference between the electrophysiological signals and electro-magnetic fields generated by the gradients, leading to pronounced distortion of the ECG signal. The capability of local acoustic gating can in principal serve to alleviate substantially the fundamental problem of erroneous ECG-gating for off-center positions of the heart. Fetal cardiovascular MRI is another emerging application [Sal08] which suffers from synchronization problems and hence is a driving force for further advancement of the MR-stethoscope towards capturing fetal phonocardiograms at (ultra)high magnetic fields. Further investigation is anticipated to ensure reliable performance of the acoustic synchronization approach in patients with heart or valvular diseases. Valvular defects need not be prohibitive for acoustic gating since the murmurs, which might be detected in the phonocardiogram, are of high frequency and hence can be suppressed by appropriate filtering. In conclusion, the efficacy of acoustic triggering for several common cardiovascular applications of MRI is demonstrated. The ACT’s intrinsic insensitivity to interference with electromagnetic fields renders it suitable for clinical imaging due to its excellent trigger reliability - even at ultra-high magnetic field strengths. For motion-synchronized MR imaging and spectroscopy, the acoustic gating approach offers versatility beyond that currently available for (ultra)-high field MR applications