The RF generation of MRI spectrometer using direct digital synthesis and phase dither based on FPGA

A radio frequency generation method in MRI based on monolithic field programmable gate array (FPGA) and direct digital synthesis (DDS) is designed. The RF generator can adjust soft pulse parameters flexibly and quickly, and the specified parameters of the RF pulse can be generated accurately. The DDS is generated inside the FPGA which is used as the core device of the RF pulse generation circuit. The external digital signal processor (DSP) realizes the control of the corresponding parameters in the RF pulse signal and high-speed switching of phase, frequency, and amplitude. The spurious influence of the phase look-up table under different conditions is analyzed through experiments. The suppression effect of the phase dither on the DDS spurious signal is evaluated after adding phase dither to the FPGA program. In view of the problem of large spurious signal amplitude when the look-up table is small, we propose a method of injecting phase dither to improve spectral purity. The experiment proves the effectiveness of the proposed method.