FIG. 2.

Pulse sequence diagrams of CODE for three different situations, i.e., BW > SW (or G_ss > G_ro), BW = SW (or G_ss = G_ro), and BW < SW (or G_ss < G_ro), which are referred to as case 1 (a), case 2 (b), and case 3, respectively. Particularly in case 3, the gradient area for the pre-dephasing is less than that for the readout, and as a result additional time for dephasing with G_ss is needed to keep the echo peak from occurring during the readout gradient ramp. Two sequence schemes are possible for this purpose. In the case of the basic scheme (c), which is referred to as case 3a, G_ss is simply extended having the same constant value as the original setup. However, in case 3b (d), a part of the extended gradient is replaced with gradient ramps (i.e., ramp-up and ramp-down) with an area (shaded triangle) equal to that of a part of the extended gradient to be replaced (shaded rectangle). The latter always provides a shorter TE than the former by the amount of time corresponding to the width of the shaded rectangle. As proven herein (see text), case 2 provides the shortest TE among the possible schemes, except in some specific circumstances (e.g., small FOV), case 3b provides the shortest TE.