Figure 2. Spin-echo 3D Imaging Sequence.

To confer insensitivity to view-to-view variations in M_z that result from inconsistent timing of the acquisition triggering, “trigger desensitizing” is performed. Trigger desensitizing is accomplished using an initial adiabatic half-passage (AHP) to prepare the magnetization in a consistent way (i.e., by nulling magnetization) immediately after the trigger. After a subsequent recovery period that remains constant, a second AHP is used to excite magnetization as the double spin-echo sequence begins. The FOV in the phase-encoded directions (y and z) is reduced using slice-selective 180° pulses of the hyperbolic secant (HS) type. The first echo, at 35 ms, is a non-phase encoded navigator echo. A BIR4 pulse is then used to generate a second echo at 52 ms which is phase encoded and used to reconstruct images. The lengths of the AHP, HS, and BIR4 pulses are 3, 2, and 2 ms, respectively. The bandwidth of the HS pulse is 10 kHz. The traces labeled AM and FM show the amplitude- and frequency-modulated functions of the adiabatic RF pulses, respectively. Gradient pulses used to spoil transverse magnetization are shown in gray. Gradient pulses used for 3D imaging are shown in dark gray.