Method for optimizing flip angles in magnetic resonance imaging variable flip angle pulse sequence, cest imaging method, medium, and device

A method for optimizing flip angles in a magnetic resonance variable flip angle pulse sequence, comprising:

• S1, a flip angle optimization problem of its N−1 refocus pulses is modeled as the following objective function with respect to flip angles:
• min
• F
• ⁢
• A
• ⁢
• s
• [
• -
• a
• ⁢
• ∑
• i
• =
• 1
• N
• -
• 1
• (
• s
• i
• ⁢
• w
• i
• )
• 2
• +
• b
• ⁢
• ∑
• i
• =
• 1
• N
• -
• 1
• ⁢
• ∑
• j
• =
• 1
• N
• -
• 1
• ⁢
• (
• s
• i
• -
• s
• j
• )
• 2
• ∑
• i
• =
• 1
• N
• -
• 1
• ⁢
• s
• i
• 2
• ]
• ⁢
• s
• .
• t
• .
• 0
• ≤
• FA
• ≤
• 180
• ⁢
• °
• in the formula:
• si is a signal amplitude after an i-th refocus pulse is applied when there is no encoding gradient;
• wi is a signal attenuation coefficient related to the i-th refocus pulse;
• FAS=[α1, α2, . . . , αN−1] is a flip angle set to be optimized, FA represents a flip angle αi of the i-th refocus pulse, whose optimization range constraint is [0°, 180°];
• a and b are two weight coefficients in the objective function, and both a and b are greater than 0;
• S2, after rewriting the objective function using an extended phase graph (EPG) method, iterative solving is performed to obtain an optimal solution of an optimized flip angle set, which serves as a flip angle set value for the N−1 refocus pulses in the magnetic resonance variable flip angle pulse sequence, and the optimized magnetic resonance variable flip angle pulse sequence is sent to a magnetic resonance scanner, which is configured to perform magnetic resonance CEST imaging to obtain CEST images.