ECG gating is affected when the triggering software erroneously triggers on MHD-peaks instead of the R-wave. V MHD is dominant during the systolic S-wave-to-T-wave (S-T) segment ( Fig. This voltage is superimposed on the real ECG (ECG real). The V MHD amplitude is frequently comparable to, or larger than, the R-wave peak in the ECG signal. Since blood is electrically conductive, an MHD voltage (V MHD) arises according to the Lorentz’s law. 1(a), blood is ejected from the LV and flows into the aortic arch, rapidly moving in a direction perpendicular to the B 0. When the Left Ventricle (LV) contracts, as shown in Fig. This approach frequently fails since the linear relationship between gradient strength and the noise generated is not always valid. Previous efforts ( 5, 9- 11) to reduce gradient noise have used adaptive filters, which receive as input the gradient patterns generated by the pulse sequence, and assume that the noise generated is linearly correlated to these patterns. The magnitude and spatial distribution of the electric fields induced by time-varying MRI gradients in the human body have been derived theoretically and demonstrated experimentally ( 8). Gradient-ramp-induced and body-coil-decoupling-induced voltages have components within the ECG frequency band, so they are difficult to remove without affecting the true ECG, especially in high slew-rate sequences such as Steady State Free Precession (SSFP), which are frequently used in CMR imaging. Furthermore, low-pass (<30-50 Hz) filtering of the ECG signals, performed in the 4-lead ECG receivers to reduce this noise, leads to low-fidelity waveforms, so that current intra-MRI 4-lead ECGs are primarily applicable to acquisition synchronization, and not for physiological monitoring ( 1). ![]() However, relative to conventional 12-lead ECGs, this circuitry reduces the peak-to-peak ECG voltages by >50% ( 1), and increases the noise floor. By placing the leads in close proximity, and using high-impedance leads, RF and gradient induction into the body and the ECG leads is reduced. These utilize MRI-conditional ECG electrodes, such as Quadtrode® from Invivo Inc., which are placed at small inter-electrode distances of 5-10cm, and use high-impedance (50-100 kOhm) lead wires. In order to allow for cardiac imaging, MRI scanner manufacturers and several third-party vendors currently provide 3-4 lead MRI-conditional ECG systems. These issues are larger when the MRI sequences employ fast rising and larger peak gradients, as the magnetic field increases, and when the RF amplifiers deliver more power. If a conventional 12-lead ECG system is used inside an MRI, these three effects result in ECG traces that are frequently unreadable, may possess spectral peaks that are higher than the R-wave, and can potentially cause surface burns. ![]() The three main obstacles to acquiring undistorted ECG signals within MR scanners are: superimposed Magneto-Hydro-Dynamic (MHD) voltages in the MR static magnetic field (B 0) ( 2), gradient- and body-coil-decoupling- switching induced voltages during imaging ( 5), and potential heating during radio-frequency (RF) transmission at the ECG electrodes, which can cause skin burns ( 6, 7). In Cardiac Magnetic Resonance (CMR) imaging, where imaging is synchronized to the heartbeat, obtaining accurate single- or multiple-cardiac-phase MRI images depends largely on the reliability of the ECG gating ( 3, 4). The 12-lead electrocardiogram (ECG) is a standard of care for cardiac physiological monitoring ( 1), and may be required during intervention on cardiac patients( 2, 3).
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