Improved navigator-gated motion compensation in cardiac MR using additional constraint of magnitude of motion-corrupted data
Journal of Cardiovascular Magnetic Resonance volume 14, Article number: P245 (2012)
In conventional prospective respiratory navigator (NAV) acquisitions, 40-60% of the acquired data are discarded resulting in low efficiency and long scan times [1, 2]. Compressed-sensing Motion Compensation (CosMo) has a shorter fixed scan time by acquiring the full inner k-space and estimating the NAV-rejected outer k-space lines . Respiratory motion will mainly manifest itself as phase variation in the acquired k-space data. We sought to determine if the addition of the magnitude of the rejected k-space lines as a constraint in image reconstruction will improve the performance of CosMo.
To investigate the variability of the magnitude of k-space lines at different respiratory phases, free-breathing, ECG-triggered, targeted right coronary images with multiple averages were acquired from 10 healthy adult subjects. Magnitude variability was investigated quantitatively by calculating the cross-correlation between accepted and rejected k-space lines.
CosMo was implemented retrospectively on one acquisition from each subject. The inner k-space (31 ky by 7 kz lines) was filled with lines acquired within the 5 mm gating window from all acquisitions. The outer k-space was then filled only with lines from the first average acquired within the 5 mm gating window, resulting in an undersampled k-space with a fully sampled center. For reliable image reconstruction with CosMo, 10-20% of the inner k-space must be fully-sampled. The missing outer k-space lines were then estimated using LOST with an additional magnitude constraint within each estimation iteration or in the final iteration for each coil . The results were compared with prospective NAV-gating with a gating window of 5 mm and CosMo reconstruction without the magnitude constraint.
Figure 1 shows the cross-correlation between the accepted and worst rejected k-space lines for each position. The correlation is close to 1 at the center of k-space where the majority of image information is contained, indicating low variability in magnitude information at different respiratory phases. Figure 2 shows right coronary images acquired using a) fully-sampled, 5-mm gated data, b) the original CosMo, and CosMo with the additional magnitude constraint c) inside each iteration and d) in the final iteration. The relative signal-to-noise in the left ventricle blood pool is: 30.71±6.5; 40.32±14.2; 53.9±26.8; 56.8±25.9 for each reconstruction, respectively. Significant differences (p<0.05) are present for all measurements except between the original CosMo and the CosMo image with the magnitude constraint in each iteration (p=0.09).
The addition of the magnitude of rejected lines, readily available in all navigator-gated scans, as a constraint in CosMo results in improved image quality as measured by relative SNR.
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Akçakaya M: MRM. 2011
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Shaw, J.L., Moghari, M.H., Akcakaya, M. et al. Improved navigator-gated motion compensation in cardiac MR using additional constraint of magnitude of motion-corrupted data. J Cardiovasc Magn Reson 14 (Suppl 1), P245 (2012). https://doi.org/10.1186/1532-429X-14-S1-P245