Missing pulse steady-state free precession

Abstract

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: {-θ₁,-τ-θ₂-echo-τ-}. 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 θ₁ and θ₂, the interpulse interval τ, and the amount of interpulse dephase. The expression shows that θ₁ and θ₂ provide two degrees of freedom to optimize the signal-to-noise ratio and improve tissue contrast. Two different cases, θ₁=θ₂ and θ₁ = -θ₂, 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.