Gradient Coils Design For Open Superconducting MRI In Midfield

Magnetic resonance imaging(MRI)has been widely used in medical field.Superconducting open MRI has gradually become the research direction of open MRI because of its high field intensity.Compared with conventional MRI,superconducting MRI has more ferromagnetic material in its structure.If the gradient coil was designed by the target field method,the eddy current problem will be serious.In order to reduce the eddy currents caused by gradient coil in superconducting MRI,under this background,a new design method of gradient coil was proposed.In the proposed method,the imaging speed and the linearity of gradient magnetic field are both taken into consideration.First,the Fourier of the current density function was expanded as a trigonometric series.A uniformly distributed target points were set in the imaging region,and on these points the magnetic induction intensity Bz was determined according to the gradient magnetic field to be generated.The integral equation was derived from Biot-Savar law and the expansion coefficients of the current density function was obtained in reverse calculation,so as to determine the current density function.In order to reduce eddy currents,a pair of shielded coils were set outside the gradient coil and the magnetic induction intensity of points distributing outside the horizontal of the gradient coil and the shielded coil was restricted.Secondly,in order to verify the feasibility of the proposed method,the distributions of the gradient magnetic field on the X-axis,the shielding plane and the lateral horizontal magnetic induction intensity were calculated by biot-Savar law.The gradient magnetic field linearity and the gradient coil inductance were taken as the performance parameters,and the performance parameters of different expansion orders were compared.The gradient magnetic field linearity and the coil inductance of different current expansion orders are obtained,and the appropriate current density expansion order was determined according to the linearity and the inductance.After the current density function was determined,the winding was calculated by stream function method.The results showed that when the expansion orders of current density were appropriate,nonlinearity of gradient magnetic field was 4% and the inductance of the gradient coil was 0.49 m H.In the shield planes and the coils' lateral,the max magnetic induction intensity was less than 0.3m T.The coils system designed by the proposed method had a high-linearity gradient magnetic field and a good shielding effect.