Cardiac imaging at 7.0 T: comparison of pulse oximetry, electrocardiogram and phonocardiogram triggered 2D-CINE for LV-function assessment
Journal of Cardiovascular Magnetic Resonance volume 12, Article number: O15 (2010)
One important development which is looming on the (pre)clinical research horizon is the move towards CMR at 7.0 T . At (ultra)highfields, the sensitivity of ECG recordings to interference from electromagnetic fields and to magneto-hydrodynamic effects increases and with it the ECG failure rate together with the motivation for a robust, practical gating/triggering alternative. Realizing the constraints of conventional ECG, an MR-stethoscope has been proposed to meet the demands of cardiac triggered MRI .
Motivated by the challenges and limitations of conventional ECG together with the advantages of acoustic cardiac triggering (ACT), this study compares phonocardiogram, electrocardiogram and pulse oximetry triggered MRI for LV-function assessment at 7.0 T. For this purpose, breath-held 2D-CINE imaging in conjunction with a retrospective triggering regime was conducted.
The acoustic gating device comprises three main components: an acoustic sensor, an acoustic wave guide and a signal processing unit . Short axis views of the heart were acquired with a 2D-CINE-FLASH technique (TE = 2 ms, TR = 4 ms, matrix = 256 × 192, FOV = 36 cm2, 25 cardiac phases, slice thickness) on a 7.0 T MR-system (Siemens, Erlangen, Germany) using a dedicated 4-element TX/RX cardiac coil array. Vector-ECG, pulse oximetry and ACT traces were recorded simultaneously.
ECG waveforms were susceptible to severe T-wave elevation which was pronounced at the isocenter of the 7.0 T magnet (Fig. 1). The MR-stethoscope provided phonocardiograms at 7.0 T free of interferences from electromagnetic fields or magneto-hydraulic effects (Fig. 1) even in the isocenter. This renders ACT suitable for reliable synchronization at ultrahigh fields as demonstrated in Fig. 2. Conversely, R-wave mis-registration occurred in ECG-triggered acquisitions with a failure rate of appr. 50% which manifest itself in a severe jitter of the R-wave recognition tickmarks (Fig. 2b). Failure to detect the onset of the cardiac cycle was reduced for pulse oximetry, though a temporal inaccuracy of the triggering of app. 20% of a R-R-interval was observed (Fig. 2). Acoustically triggered CINE imaging at 7.0 T produced images free of motion artifacts (Fig. 3a). In contrast, ECG triggered CINE imaging was prone to severe cardiac motion artifacts if R-wave misregistration occurred (Fig. 1a). Pulse oximetry triggered 2D-CINE imaging was found to be less sensitive to cardiac motion effects although the jitter in the pulse-oximetry recognition constituted a synchronization problem.
This work examined the feasibility of LV-function assessment at 7.0 T using acoustically triggered 2D-CINE imaging. ACT's superior robustness has been demonstrated by eliminating the frequently-encountered difficulty of mis-triggering due to ECG-waveform distortions or temporal jittering in the pulse-oximetry synchronization.
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Frauenrath, T., Renz, W., Patel, N. et al. Cardiac imaging at 7.0 T: comparison of pulse oximetry, electrocardiogram and phonocardiogram triggered 2D-CINE for LV-function assessment. J Cardiovasc Magn Reson 12 (Suppl 1), O15 (2010). https://doi.org/10.1186/1532-429X-12-S1-O15