GRE sequences

Learning Objectives: Rapid imaging sequences are characterized by a very fast train of excitation and gradient pulses. Between excitation pulses or within TR, the magnetization is not able to return to its thermal equilibrium. As a consequence, excitation pulses will influence both the remaining transversal and the remaining longitudinal magnetization. The steady-state magnetization of a multi pulse experiment is thus a mixture or superposition of different transversal and longitudinal states, and the acquired image amplitude becomes a complex function of the investigated tissue's relaxation properties. Body: Steady state free precession (TR~T2. The steady state is characterized by a certain distribution of magnetization vectors within the spatial 3D space. If a steady state has been established this distribution will be exactly the same for each TR. This steady state distribution can also be described by a certain distribution of different configurations. The populations or amplitudes of these configurations are given by the Fourier components in k-space of the corresponding spatial distribution. It is obvious that a steady state can only be reached for sequences with a fixed TR (in order to have identical T1 and T2 relaxation within each TR period) constant flip angle (the phase of the excitation pulse may vary linearly or quadratically from pulse to pulse) constant dephasing within TR (the dephasing induced by switched gradients and susceptibility effects at a certain spatial position must be the same within each TR). From point 3 it can be deduced that the phase encoding gradient, which changes from excitation to excitation has to be refocused (compensated by a second gradient pulse with opposite gradient area). Sequences with nonrefocused phase encode gradients do not establish a steady state and additionally produce image artifacts. The analytical description of the steady state in the spatial domain can be achieved by solving the eigenvector equation that describes the three processes of excitation, dephasing and T1 and T2 relaxation. The steady state magnetization does not depend on the chosen gradient waveform that is switched between the excitation pulses. The magnetization generated just before and after the excitation pulse (sometimes called S+ and S-, respectively) is thus identical for all types of gradient echo sequences (except for some small details…)