COSESS and INSESS: coherent and incoherent spin echoes in the steady state.

A new fast sequence type, called COSESS/INSESS, is described. The steady state for COSESS is coherent but incoherent for INSESS. COSESS/INSESS combine the contrast behavior of fast imaging with steady-state free precession (FISP) and fast low angle shot (FLASH), respectively, to the robustness of spin-echo (SE) sequences with regard to susceptibility and field inhomogeneities. In contrast to other fast sequences such as rapid spin-echo excitation (RASEE), fast low angle spin echo with short echo time (FATE) or the missing pulse sequence, which also uses a spin echo, COSESS/ INSESS can additionally handle time-dependent disturbances arising from eddy currents, hysteresis or B(zero) instability. The acquisition time is 2 to 3 times longer than with gradient-based steady-state sequences. The principle of echo generation is evaluated and shows a superposition of several subechoes in the spin echo. Different solutions were elaborated to attain a coherent superposition of sub-echo signals for COSESS. Signal and contrast behavior were investigated both by simulations and experiments and demonstrate the performance of COSESS/INSESS.