Chemical Shift Selective Missing Pulse Steady State

The missing pulse steady state free precession (MP-SSFP) sequence distinguishes itself from conventional steady state sequences by dropping every third RF pulse and acquiring when the missing pulse would have occurred. Both of the remaining two RF pulses were originally proposed for as slice selection pulses in the same imaging plane. However, the two RF pulses may be modified to select two distinct but intersecting spin groups. Only the spins at the intersection of these groups will refocus at the echo time, forming a selective MPSSFP sequence. By replacing one of the pulses with a chemical shift selective (CHESS) pulse, we have created a steady state sequence that is both slice and chemical shift selective.

Figure 2: Phantom experiment demonstrating the chemical shift selectivity of the chemical shift selective MPSSFP sequence. a) A localizer scan showing four phantoms (clockwise from left): a lipid phantom, two acetic acid phantoms (one of which was doped with contrast agent), and a water phantom. The chemical shift selective MPSSFP sequence was then applied with the center frequency of the CHESS pulse set on lipid (b), water (c), and acetic acid (d).

Figure 3: Chemical shift selective MPSSFP sequence applied to the knee of a healthy volunteer, with the CHESS pulse set on water (left) and fat (right).