Figure 1.

CS MRSI sequence, random walk and time delay scheme and undersampling pattern. (a) CS MRSI sequence for a single acquisition. Blip gradients with random amplitudes and time delays were applied on G_x and G_y during the readout period, which facilitated a random walk trajectory in the k_x-k_y plane. (b) The initial phase encoding locations followed a random spiral-out order. During the readout, some phase encodings were performing random walks, and some were not, due to the random time delays or the full sampling at the center. Random walks moved slower near the center, creating variable density sampling. Four phase encodings in the center were fully sampled along t dimension. (c) The probability distribution of undersampling at various time points. The central peak indicated full sampling. The sampling distribution due to the random walks in peripheral k-space became increasingly flat from t = 0 to 3.1 ms, and was relatively stationary with t > 3.1 ms. (d) The 3D k_x-k_y-t undersampling pattern (3.8-fold) for the first frame, with the cylindrical sampling boundary (dashed line).