Missing pulse steady-state free precession.

A fast imaging technique, missing pulse steady-state free precession (MP-SSFP), is described. MP-SSFP is one of a class of steady-state free precession techniques in which every nth RF pulse is missing. MP-SSFP has been implemented for the case where every third RF pulse is omitted: [-theta 1-r-theta 2-r-echo-r-]. A RF-refocused echo forms at the time of the missing pulse. This echo is less sensitive to field inhomogeneities than the gradient-recalled echoes used in most fast imaging methods. An analytical expression is obtained for the signal strength as a function of the flip angles theta 1 and theta 2, the interpulse interval tau, and the amount of interpulse dephase. The expression shows that theta 1 and theta 2 provide two degrees of freedom to optimize the signal-to-noise ratio and improve tissue contrast. Two different cases, theta 1 = theta 2 and theta 1 = -theta 2, are described to demonstrate the difference in contrast. The first case gives a strong signal from spins with short T1 and short T2 while the second case has contrast very similar to a conventional SSFP technique. The theoretical expression predicts that the signal consists of multiple components which may be observed experimentally by adjusting the gradient pulses.