Passive Shimming for a Permanent MRI Magnet

A new method for allowing a permanent magnet for MR imaging to meet its stringent field homogeneity requirements has been developed. The method is called passive shimming and uses the permanent magnetic pieces instead of the ferromagnetic material in the magnet bore at locations of the hardware. Software and hardware have facilitated this shimming procedure with high efficiency and stabilization. The method can be considered as a function approximation. In this paper, the function of a magnetic dipole is presented as a basis function. From the functional space that consists of simple functions of a series of magnetic dipoles, the specific function that approximates the negative quantity of the error function in the homogeneous magnetic field with the desired accuracy can be found. The series of magnetic dipoles are realized by the permanent magnetic pieces located in the hardware. The result shows that this method is fixable and normative over the traditional method.