A z-gradient coil array system for magnetic resonance imaging

Abstract

In this thesis, a z-gradient array system for use in MRI is proposed. In MRI scanners, in order to encode the MR signal gradient coils are used. Conventionally, encoding in the z-direction (along the bore of the magnet) carried out using a single gradient coil. Proposed z-gradient coil array is a exible and novel design to achieve high gradient strength while observing the peripheral nerve stimulation (PNS) limits. The design has a potential to decrease both the complexity and the cost of the gradient power ampli ers (GPA). In order to prove the concept, a three-channel system was designed and built. By using di erent combination of the generated elds of these channels, e ective length of the coil can be modi ed according to the application. For the same PNS limitations, gradient strength can be increased in a smaller region, hence decreasing the dead-time in most sequences and acquiring images with smaller pixel sizes. In order to proof of concept, this system is designed and implemented in the MRI scanner. In our preliminary experiment, each channel of the system is composed of two coils with 16 turns each on a cylindrical former with the diameter of 7.5 cm, placed symmetrically about the center with equal current in opposite direction at each side. The overall length of the structure is 245 mm and the current ratio and the distance between the coils optimized such that three sizes of volume of interest (VOI) with more than 95% linearity is achievable. In order to drive each channel independently, three H-bridge GPAs with 30 A current capability at 50 V providing 0:5 A= s slew-rate for the load with 46 H inductance, 30 H maximum coupling and 390 m resistance is built which are controlled digitally by an FPGA (Xilinx XUPV 5) board. Although in this prototype the distance and the current ratio between the coils are the only parameters that were optimized, other parameters of the system can also be optimized according to the di erent criteria as well. In addition to the coil simulations, phantom experiments conducted to show the feasibility of this system. 2D coronal slices were acquired for three sizes of VOI; large, mid-size and small volume and di erent advantages of each mode veri ed experimentally. The gradient array is a promising design for the future of the gradient systems in magnetic resonance imaging. In addition to linear elds, gradient array system is also exible in terms of producing non-linear elds by proper combination of the elds generated by each channel. This is noticeable when higher number of independent channels are used. Gradient array system provides exible and e ective eld generation along with less complex ampli er design.