1. A magnetic resonance imaging (MRI) system comprising:an MRI scanner, a control unit, and a processing unit, wherein

the processing unit is configured to determine a flip angle schedule corresponding to an echo train having a plurality of phases by performing operations comprising:

for each phase of the plurality of phases of the echo train,

a) determining an initial condition relating to the phase, the initial condition including initial values of a starting flip angle of the phase, an ending flip angle of the phase, a spin echo number of the phase, and a characteristic parameter of the phase;

b) determining a function corresponding to the phase, the function relating to the starting flip angle of the phase, the ending flip angle of the phase, the spin echo number of the phase, and the characteristic parameter of the phase;

c) calculating, based on the function, one or more flip angles corresponding to one or more spin echoes of the phase;

d) assessing the one or more flip angles of the phase according to a criterion; and

e) if the criterion is not satisfied, revising the function by adjusting at least one of the starting flip angle, the ending flip angle, the spin echo number, or the characteristic parameter, and repeating (c) and (d);

wherein the function provides a portion of the flip angle schedule corresponding to the phase of the echo train for MRI, and

the control unit is configured to control the MRI scanner according to the flip angle schedule,

wherein the function includes:

where ?0 is the starting flip angle of the phase, ?N?1 is the ending flip angle of the phase, N is the spin echo number of the phase, P is the characteristic parameter of the phase, ?n is the one or more flip angles of the phase, n=0, 1, . . . , N?1;

where M=0 if ?N?1??0, or M=N?1 if?0>?N?1.