Research On Magnetic Resonance Radio Frequency Generator Based On FPGA And Direct Digital Synthesis

Nowadays the research of magnetic resonance imaging(MRI)has developed rapidly in the world,especially in the field of medical diagnostic scanning.MRI has great potential in clinical diagnosis.MRI technology is different from traditional X-ray and CT scanning in that it has higher safety and higher imaging quality for scanning tissue.MRI images have higher contrast and clarity,especially on soft tissue of human body,can improve diagnostic efficiency,so MRI technology is more and more widely used in medicine.One of the core components of magnetic resonance imaging system is spectrometer,which is used to realize the generation,receiving and timing control of various signals such as radio frequency(RF)signal and gradient signal.The RF generation module is one of the key parts affecting the performance of magnetic resonance imaging spectrometer.The core device of RF generation module generally uses DDS(direct digital synthesis)chip as the main chip to generate RF pulse signal.Due to the limitation of special devices,the phase precision and amplitude precision of DDS signal generated by DDS chip with higher precision in existing devices are generally 12 bits or 14 bits.To studying high-performance RF generation module and realizing the further application of MRI,it is very important to design RF generation module with higher amplitude accuracy and phase accuracy.In this design,a RF signal generator based on FPGA and direct digital synthesis technology is studied to be used in magnetic resonance imaging spectrometer in the key technologies and realizing methods.This paper mainly uses FPGA combined with DSP(Digital Signal Processor)to complete the RF generation.DSP is used as the external sequence controller to set the waveform parameters of RF generation,and realize the direct digital synthesis function in FPGA,including phase accumulator,sine look-up table,soft pulse generation unit and corresponding control logic,The signal of FPGA is output to external high-precision DA to generate RF pulse,and the signal gain control,amplification and filtering are completed.In this design,the frequency resolution of DDS is 32 bits,the phase resolution is 16 bits,the amplitude resolution is 16 bits,and the time precision of soft pulse can reach 0.1 ?s.The frequency,amplitude and phase of the RF pulse signal can be flexibly controlled from the DSP control terminal according to the functional requirements.In some cases,such as the cost,or FPGA needs to integrate other functions,when the hardware resources are limited,the phase look-up table space is too small,resulting in high spurious in the spectrum.In this paper,the phase truncation error of the signal is analyzed in detail,and the injection phase jitter method is used to improve the spectrum purity,and the preset performance index is completed.The RF generation circuit has high integration and convenient debugging,which has important reference significance for the design of highperformance RF pulse generator.