FIG 1.

A, Diagram shows the TSI pulse sequence. The initial 90° excitation pulse is followed by two 180° refocusing pulses, and each of these pulses is slice-selective in orthogonal directions. The signal from this selected volume is then phase-encoded and acquired (echo1). The phase encoding is rewound and the signal is then refocused, phase-encoded, and acquired again (echo2).

B, The third echo is acquired in an analogous way. K-space is filled in such that the first echoes fill in the inner third of k-space while the second echoes fill out the middle third, and the third echo fills out the outer third. Phase-encoding must be performed in two orthogonal directions (Kx, Ky), and thus a full signal acquisition occurs at each point on this Kx-Ky grid. Kx-Ky points lying outside the largest circle are not acquired to reduce imaging time