Design of Halbach shim coils for human finger blood glucose detection based on harmonic analysis

Shim coils are essential components to improve the accuracy of finger blood glucose detection in magnetic resonance image (MRI). Halbach magnet is popular in the field of portable MRI due to the magnetic concentration effect, but shim coils are needed to improve its poor magnetic field uniformity in the region of interest (ROI). Based on the harmonic analysis method, the high linearity of shim coils are considered as the objective function and the power consumption of coils are taken as the penalty function. The appropriate current density coefficient is determined by adjusting the penalty factor and minimizing the objective function. The stream function is applied to the current density, and the specific winding pattern on the cylindrical-surface is obtained by using MATLAB. In order to verify the accuracy of the method, the finite element software COMSOL was used to calculate the magnetic field distribution in the ROI The results illustrate that eight shim coils (x, y, z, (z
²
-(x
²
+y
²
)/2), xz, yz, xy, (x
²
-y
²
)) well meet the linearity requirements and the performance requirements of the human finger blood glucose MRI detection apparatus. Finally, the y and z directions of the first-order shim coils are fabricated with flexible PCB.