Research On Gradient Amplifier Of Magnetic Resonance Imaging System

Magnetic resonance imaging (MRI) technology uses a unique imaging principle, so that it can use to image the internal structure of theorganism. Compared with other medical imaging technology, MRI has no harm to the human body. Therefore, magnetic resonance imaging device is more and more attention. Currently, the domestic medical industry is developing rapidly, there are a number of companies designed and produced MRI device with independent intellectual property rights, the gap between domestic and imported MRI is decreasing year by year.The gradient amplifier is one of the core parts of the MRI scanner, It provides drive current for the gradient coil, the load current follow and amplificate the pulse train reference signals generated by the computer spectrometer in X, Y, Z 3 axis, so that it can generate a gradient magnet field to achieve dynamic modification. Although the MRI scanner achieved domestically, but the gradient amplifier is still completely dependent on imports. Therefore, the independent research for gradient amplifier has a great significance for developing the domestic MRI system.This paper analyzed the hard-switched parallel phase shift gradient amplifier’s operation principle, and determined the overall system solution according to the demand of the MRI scanner. By the way of 4 full-bridge topology parallel with phase shift technology, the gradient amplifier’s equivalent switching frequency becomes four times the original switching frequency, reduce the output current ripple. At the same time, system uses the the inner and outerdual-loop control:the inner loop is the average current control to ensure the output current of each bridge arm is 1/4 of the load current; The outer loop is controlled by the traditional PWM technology. The feasibility of the proposed scheme is verified by simulation.Finally, a prototype of the gradient amplifier applied for the 1.5T magnetic resonance imaging instrument is completed. The prototype meets the requirements of the magnetic resonance imaging instrument.In order to meet the requirements of output current ripple frequency, the hard-switched gradient amplifier uses a muti-parallel full-bridge circuit, which makes the gradient amplifier circuit topology is very large and complex. In order to solve this problem, this paper introduces the auxiliary resonant commutated pole (ARCP) converter, proposes a soft-switched gradient amplifier circuit topology. It solves the increase switching frequency and reduce the switching losses of contradictions to a certain extent, the gradient amplifier reduces the number of parallel bridge module can meet the requirements of the frequency of the output current ripple. The commutation process of the ARCP module is analyzed in detail, a comparison of losses is made between a half-bridge circuit using conventional hard-switched phase legs and an ARCP phase legs, and the feasibility of the soft-switched scheme is verified by simulation.