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144 Prospective self-gating for simultaneous compensation of cardiac and respiratory motion
Journal of Cardiovascular Magnetic Resonance volume 10, Article number: A45 (2008)
Data acquisition in cardiac imaging has to be synchronized with heart and respiratory motion. For this purpose an electrocardiogram (ECG) in combination with a breath-hold or a respiratory navigator is usually applied. However, these motion monitoring techniques, necessitate patient cooperation and increase the complexity of the examination. Furthermore, the ECG signal may be distorted by radio-frequency and gradient action and the navigator signal might not precisely represent the respiratory motion of the heart. In this work a prospective, self-gated approach for free breathing cardiac imaging is presented requiring neither of the monitoring techniques described above. The motion data used for cardiac triggering and respiratory gating are extracted and processed in real-time from repeatedly acquired data at the k-space center. Image quality with the proposed method was found to be comparable to ECG triggered breathheld acquisitions while the scan efficiency was significantly increased.
To repeatedly measure the k-space center a modified SSFP sequence  was implemented, providing a signal which contains both cardiac and respiratory synchronous variations (Figure 1). Cardiac variations arise from changes of the blood volume in the heart during the cardiac cycle. Respiratory-related variations stem from moving structures such as the liver or abdomen which shift in and out of the sensitive imaging volume during the respiratory cycle. Since cardiac and respiratory signals are superimposed, real-time separation is necessary in order to use the motion information for prospective cardiac triggering and respiratory gating. To this end, two efficient filters were designed complying with real-time requirements. A Butterworth band-pass filter extracts the cardiac related signal component followed by a peak-finding algorithm providing the cardiac trigger. For extracting the respiratory motion variation an adaptive averaging filter was implemented which uses the detected trigger-points to average over one cardiac cycle. Also an adaptive threshold was implemented for definition of the respiratory gating window. In image reconstruction data were re-filtered based on the stored motion signal in order to correct for the latency of real-time filtering.
To test the feasibility of the approach a cardiac four-chamber view of a healthy volunteer was acquired (TR = 4.5 ms, TE = 2.6 ms, flip angle = 60°, scan matrix = 192 × 186, FOV = 320 × 320 mm2, slice thickness = 8 mm, 11 lines/segment, 30 cardiac phases) using a five channel cardiac coil array. To asses the quality of the extracted self-gating motion curves, signals from an ECG and a respiratory belt were simultaneously acquired and stored. As reference a retrospective self-gating acquisition described in  was performed. In the retrospective approach the data are evaluated after the acquisition, therefore requiring temporal oversampling of the data to ensure that every profile is acquired at least once in an acceptable motion state. Additionally, a standard ECG triggered breathheld acquisition was acquired as second reference.
Cardiac and respiratory signals could be extracted in real-time and were comparable to the wired ECG and respiratory belt signals (Figure 2). The standard deviation of differences between the wired ECG signal and the self-gating trigger was less than 11 msec. Reconstructed images were of comparable quality relative to the retrospective self-gated approach. At the same time, scan efficiency of the prospective method was significantly increased relative to the retrospective approach (prospective: 64 sec + 10 sec preparation period, retrospective:160 sec). Both, the prospective and the retrospective methods compared well with the standard ECG, breathheld acquisition (Figure 3).
It has been shown that cardiac and respiratory variations can be accurately detected prospectively using signals originating from the k-space center and thereby eliminating the need for external cardiac and respiratory signal detection. Compared to the retrospective self-gating approach total scan time could be reduced while image quality was well maintained.
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Curcic, J., Buehrer, M., Boesiger, P. et al. 144 Prospective self-gating for simultaneous compensation of cardiac and respiratory motion. J Cardiovasc Magn Reson 10 (Suppl 1), A45 (2008). https://doi.org/10.1186/1532-429X-10-S1-A45
- Respiratory Motion
- Respiratory Signal
- Respiratory Gating
- Cardiac Trigger
- Gating Window