The Design And Fabrication Of A MRI Insert-type Gradient Coil

In recent years, fast imaging velocity and high imaging resolution are the goals for the development of magnetic resonance imaging (MRI) technique. Faster imaging velocity is beneficial to decrease the discomfort of patients as well as the motion artifacts during the scanning process. Higher resolution images can provide more accurate information for clinical diagnosis. Gradient coil is a key component of MRI systems, stronger gradient strength and higher slew rate are needed to satisfy the above-mentioned requirements. To get more information from one image as well as providing image guidance during the radiotherapy, some hybrid system such as MRI-PET (Positron Emission Tomography) and MRI-LINAC (Linear Accelerator) arise. These systems usually have a split structure, which makes the split gradient coil develop gradually.In order to research further into gradient coil development, this paper carried out some simulations on the design of self-shielded gradient insert and the split gradient coil. The z-direction gradient coil was first designed using the simulated annealing algorithm, and the performances of the coil in different VOI (Volume of Interest) sizes as well as various loop numbers were studied. In order to get better gradient coil qualities, this paper used the finite difference method for the design of self-shielded gradient insert. This method was also used in the design of the split gradient coil due to its applicability for arbitrary shapes of gradient coil. According to the winding patterns of the gradient coil, this paper designed a whole process flow for the fabrication of gradient coil and did some initial measurements.Since the size of the gradient insert is reduced, the coil efficiency and gradient uniformity are improved significantly. The eddy current effects are greatly reduced since the distance between the gradient coil and the iron structure of the system increases, which makes it possible to only use main coils for fabrication. For the design of split gradient coil, the results show that both the coil efficiency and the gradient uniformity become worse compared with common gradient coil. To prevent this degradation, a flange-structure may be better for the design of split gradient coil. According to the initial measurements of the fabricated gradient coil, the inductance and the resistance are a little bit higher compared with the gradient coil of the 1.5T MRI system, because the diameter of the copper wire we use is small and the number of the coil loop is large. However, both the gradient strength and the slew rate are improved obviously because of the much higher coil efficiency.