In cardiac MRI, the EKG signal is commonly used for synchronizing data acquisition to the cardiac cycle. However, the EKG suffers from multiple sources of interferences in MRI^[1], increases the complexity of scan setup, and the leads can be intimidating for children. Self-gating using the DC signal (center of k-space)^[1-3] provides a promising alternative. This DC signal can be repeatedly acquired with minimal scan time penalty for most sequences.

Especially in 3D imaging, the cardiac motion extracted from DC signal is mixed with motion of other structures (such as the liver) in the FOV, which may degrade the accuracy of self-gating triggers. Band-pass filtering^[4] can reduce the effect of respiratory motion, but may be unreliable during arrhythmias or irregular breathing. Here, we demonstrate the use of virtual coils (VC’s)^[5] to focus the DC signal on cardiac motion and to provide a robust, simple and generalized approach for self-gated 3D cardiac MRI.

Fitting for Virtual Coils The virtual coil is a linear combination of array coil elements, which has high sensitivity only inside the chosen ROI. The weights of the combination $$$w(c)$$$ are fitted by $$$\mathrm{minimizing}_{w}\|Sw-\mathrm{diag}(m)b\|_2$$$, where $$$S(\textbf{r},c)$$$ and $$$b(\textbf{r})$$$ are the multi-coil images and the root-sum-of-squares (RSOS) image, 3D spatial locations and coils are indexed with $$$\textbf{r}$$$ and $$$c$$$. $$$m(\textbf{r})$$$ is a mask selecting voxels inside the ROI, which has smooth transitions at the edge of the ROI. Since changes in the DC signal of the VC correspond to changes of the total signal in its sensitivity region, the ROI of the VC is placed around the edge of the left ventricle (Fig.1), so that the DC signal over time reflects cardiac motion.

Extraction of Self-Gating Signals For each TR, N samples of the DC signal are collected for each coil element. The multi-coil DC data are first linearly combined using the computed weights $$$w(c)$$$ in the coil dimension. The magnitude sum of the N virtual DC samples of each TR is taken as the DC signal. A bandpass filter with a 0.5-2.5Hz passband is applied to remove the baseline drift and high-frequency noise. A template matching algorithm^[3] is used to extract the cardiac triggers.

Experiments Free-breathing 4D-Flow scans were performed on 4 pediatric subjects in a 3T GE MR750 scanner using a 32-channel cardiac coil. Ferumoxytol contrast-enhancement and a 3D SPGR sequence with fat saturation were used with the parameters in Table 1. Both the DC signal and Butterfly navigators were acquired without modifying the gradient waveforms^[6,7] (Fig.2). The original array coil element with the highest DC energy in the frequency range of cardiac motion was used for comparison with the VC. The same filtering procedure was applied to the DC waveforms of the VC and the selected original coil element. The cardiac triggers extracted from the VC and this original coil element were compared with the EKG triggers by evaluating the trigger variability $$$\mathrm{TV}=\sqrt{\Sigma_{n=1}^{N}{\frac{(S_n-R_n)^2}{N-1}}}$$$, where N is the number of cardiac cycles, $$$S_n$$$ and $$$R_n$$$ represent the self-gating (mean trigger delay corrected) and EKG trigger points. A soft-gated ESPIRiT reconstruction^[8] with local translational motion correction^[6] was used to reconstruct the end-diastolic images, based on retrospective self-gating using the VC and EKG gating.

The virtual coil had superior spatial selectivity compared to the original coil (Fig.1) and suppressed most of the unwanted influence from liver in the DC waveform (Fig.3). The self-gating triggers from the VC had lower TVs of 10.3±2.2ms (mean ± standard deviation), compared with the results of the original coil (14.2±4.7ms). The self-gating TVs of VC were similar to the results in breath-hold 2D cine scans reported in a previous study^[3]. The EKG missed 1-3 trigger points in 3 studies, but there were no misses or false positives in self-gating triggers. Self-gated images had comparable quality to the EKG-gated reference (Fig.4).

The self-gating method is applicable to general cardiac imaging. It is especially useful for 4D-Flow, since the EKG gating is more likely to fail in the middle of a long scan, particularly in small children. The DC signal has a high temporal resolution of one TR for extracting accurate trigger times; the operator has more flexibility and control for tuning how the self-gating signal is extracted.