Figure 3.

(a): Overview of the proposed image reconstruction scheme; In Step 1, a low-resolution 2D image-based navigator is generated from the acquired projection data of the mid-ventricular slice capturing the cardiac motion in real time at a frame rate of 25 frames/s, hence the short-hand “real-time nav”; ROI detection in Step 2 and systolic-phase time stamps in Step 3 are automatically generated without the need for ECG signal; Following systolic-phase self gating, in Step 4 a set of sliding window (SW) “reference frames” are reconstructed using CG-SENSE for each of the 3 slices; Finally, in Step 5, a reference-constrained CS reconstruction scheme, dubbed TRACE, is employed to reconstruct one systolic frame per heartbeat per slice. (b): Description of the scheme used for forming the SW reference frames. For FPP time-frame n, systolic-phase projection data (21 projections around the time stamps generated in Step 3) from 7 heartbeats (frame n−3 to frame n+3) is used for reconstruction of the corresponding SW reference frame. (c): An example for the improvement in sparsity factor after application of the difference operator followed by the gradient operator (image-domain finite differences). Sparsity of the difference frame (FPP time frame subtracted from the reference frame) and its gradient are exploited in the TRACE reconstruction technique (Step 5).