Implementation of MRI gradient generation system and controller on field programmable gate array(FPGA)

Magnetic Resonance Imaging(MRI) is an invasive technique used to form high-resolution images of internal tissues. Gradients are the small amplitude linearly varying field used to encode spatial information into the frequency domain. Most of the gradient system utilizes three gradients to perform Slice selection, Phase encodes and frequency encodes. Gradient waveforms can assume any shapes as application demands, most commonly used shapes are Trapezoidal, sinusoidal, Gaussian etc. Gradient envelope samples are generated by specifying some parameters viz. rise time, fall time, constant time and amplitude etc. Since Gradient Digital to Analog Converter's(DAC) operate at lower data rates so it is not necessary to pre-generate all the samples, stored it into registers and input to DAC. Gradient waveforms samples are generated in real time and inputted to DAC at the rate of 1 MSPS. The System consists of Gradient waveform synthesizer, 1MSPS DAC Serial Peripheral Interface(SPI) Interfaces and Timing and Signal generation unit implemented with VHDL code and converted into respective Intellectual Property (IP's). All IP's are interconnected with Block diagram functionality of Vivado software. Further, Necessary Timing and Amplitude levels of Gradient waveform to be generated at the specific instant are taken from Pulseq(Pulse Sequence File) file. Pulseq file contains Gradient waveform information compressed, run length encoded and converted into TCP/IP packets by third party software. Packets are received through FPGA MicroBlaze Soft core processor using Open source LWIP library. MicroBlaze is interfaced with Gradient Wave synthesizer by AXI interface. The result obtained shows minimum resolution of 12-bit with the minimum amplitude of 0.3 mV at 1.25 V DAC reference voltage.