A new shimming approach using the Equivalent Magnetizing Current (EMC) Method

Synopsis: This paper presents a new alternative shimming procedure to correct the magnetic field inhomogeneities generated by horizontal and C-shape biplanar MRI magnets. A magnetization map obtained through the application of the Equivalent Magnetizing Current (EMC) method is used to define the domain where the discrete iron shim set is placed to generate a given field harmonic [1]. Optionally, instead of iron a current pattern is used, then the magnetization is related to the stream function (SF) and the current pattern is placed at equally spaced contours of the SF [1]. If a set of discrete iron pieces with no reversible magnetization direction is employed to mimic the continuous magnetization function (MF) map, then iron shims of unit strength are placed only in the positive domain (valid domain) of the MF map. The field source matrix is calculated only for the discrete elements located in the valid domain, which leads to a better conditioned matrix and superior solutions. An LP algorithm is used to calculate the optimal thickness and location of the discrete ferroshims to produce the target harmonics. Examples of simulated shimming of horizontal/C-shape magnets are presented. In the case of permanent open magnets, the magnetic coupling among the iron pieces and its influence over the magnetic field harmonics is studied for linear and nonlinear iron cases. The influence of the selection and arrangement of individual shim sizes over the field source matrix conditioning is also analysed.