| Magnetic resonance imaging(MRI)is an imaging technique with high soft tissue resolution and no ionizing radiation damage.Due to its own advantages,it has wide applications in medical diagnosis,neurological and metabolic research,and other fields.The magnetic resonance imaging spectrometer is the core equipment of the MRI system,and the RF generation module is an important component of the spectrometer.Its performance directly affects the quality of imaging.Currently,the application of ultra-high field MRI(with field strength of7 T and above)is becoming increasingly common,and it has a significant advantage of high image signal-to-noise ratio.In addition to the most frequently imaged hydrogen nuclei,other nuclei with spin,such as 13 C,23Na,39 P,etc.,have gradually achieved ultra-high field imaging.The resonance frequencies of different nuclei vary greatly,and fast imaging switching between different nuclei is required,which puts high demands on the RF generation module of the spectrometer.However,there is currently a lack of literature on radio frequency generation technologies that support multi nuclear imaging.In response to the above situation,this project has studied and designed a spectrometer RF generation module that supports multi core imaging.The main work includes:(1)The overall structure of the RF generation module has been designed,and a multi-core RF generation scheme based on a single intermediate frequency source and multiple gating circuits,combined with the upper mixing circuit of each imaging core,has been proposed.(2)We have developed an intermediate frequency generation circuit based on a single chip field programmable gate array(FPGA)and direct digital synthesis(DDS).Using FPGA as the core of the entire RF generation circuit,DDS module,waveform data memory,and other functional units are implemented within FPGA.Using a digital signal processor(DSP)as an external sequence controller,the waveform parameters of the FPGA are configured to control frequency synthesis.Combined with an external high-speed and high-precision analog-to-digital converter,intermediate frequency pulses with specified parameters(amplitude,phase,frequency,pulse width,etc.)are generated.The word lengths of frequency,phase,and amplitude can reach 32 bit,16bit,and 16 bit,respectively.(3)A high-performance multiplexer based multi-channel gating circuit has been developed,which can achieve the gating of intermediate frequency pulses to the upper mixing circuit of the selected imaging core under the control of an external DSP.This circuit has the advantages of fast switching speed and high channel isolation.(4)We have developed a 7T hydrogen core up mixing circuit that can output 298 MHz RF pulses and can be applied to 7T system imaging.(5)We conducted experiments on intermediate frequency signal generation,multi-channel switching,and radio frequency generation,and analyzed the spectrum of DDS output intermediate frequency signals under different signal output bits,phase lookup table size,and system clock conditions.Through spectrum analysis,it is found that when the signal output bits vary between 12 bit,14bit,and 16 bit,there is no significant impact on the signal spectrum.The size of the phase lookup table can affect the quality of the spectrum.When the phase lookup table is 16384,the stray energy in the spectrum is already very low.Increasing the clock frequency can also improve spectral quality to a certain extent.The research in this topic has certain reference significance for the design of multi nuclear imaging spectrometers,and is helpful for the development and application of ultra-high field MRI systems in China. |
