Sequence Controller And RF Generator Circuit Design Of MRI Spectrometer

Spectrometer is the core equipment of the Magnetic Resonance Imaging (MRI) system. It controls the whole system of signal timing, transmitting and receiving of radio frequency (RF) pulse, gradient signal generation, and data acquisition and processing. With the promotion of MRI application, the development of the spectrometer has been the research focus in MRI areas both at home and abroad. One of the key development directions is to raise the level of digitalization and the control flexibility of spectrometer. Sequence controller and RF pulse generator is an important part of spectrometer. Their performance determines the property (e.g. precision of frequency and phase) of RF pulse which has significant influence on imaging quality. The development of flexible control and high performance sequence controller and RF generator is of great significance for spectrometer.Since the running speed of the existing spectrometer sequence controller is more than12ns, the instruction cycle of sequence controller is designed to10ns. RF generator uses the DDS chip to realize pulse signal,The accuracy of the signal is restricted by DDS chip. In order to improve the pulse sequence signal phase, amplitude accuracy, use FPGA to generate RF pulse. The design of sequence controller and RF pulse generator circuit based on DSP (TMS320VC5509A) and FPGA (EP3C25Q240C8) is studied in this paper. It can be used in the self-developed MRI spectrometer system applicable to0-1.5T. DSP is used as the key equipment of sequence controller. DSP configures parameters and data of FPGA through local bus to control its operations. The instruction cycle of sequence controller is set to10ns, so that high-speed operation sequence and convenient and flexible sequential control to each module of the spectrometer can be realized. Based on the abundant hardware resources of FPGA, digital dynamics simulator (DDS), waveform memory, multiplier and related control logic can build within FPGA. Frequency, phase and amplitude resolution of DDS are32bit,16bit and14bit, respectively (compared with the special DDS devices, such as the AD9852, the phase and amplitude accuracy has improved). And for soft pulse waveform, time precision can reach0.5us. As the system clock is set to be150MHz, the out frequencies can range from DC to67.5MHz. Integrating sequence controller and RF generator on one circuit board has contribution to the compactness of spectrometer.