MRI receiver frequency response as a contributor to Nyquist ghosting in echo planar imaging

PURPOSE: To study the frequency response characteristic of the MRI signal receiver system as a contributing factor to the formation of Nyquist ghosting in echo-planar imaging (EPI). MATERIALS AND METHODS: Experimental work was undertaken on a 1.5 T system. A cylindrical test object filled with water was imaged axially with EPI in the center of the quadrature, transmit-receive head coil. In the first set of experiments, the water conductivity was increased progressively with the addition of salt between EPI acquisitions. In the second set of experiments, the conductivity of the water in the test object was kept constant and EPI images were acquired at several different bandwidths. A computer simulation was also implemented to demonstrate the impact of changes in the frequency response characteristic of the signal receiver system on EPI Nyquist ghosting. RESULTS: Experimental and simulation results showed that Nyquist ghosting increased with the variation of the frequency response characteristic within the effective frequency range determined by the image bandwidth. One can increase the variation in the frequency response characteristic by increasing its steepness over the image's bandwidth window when coil loading is decreased, or by increasing the effective frequency range when image bandwidth is increased. CONCLUSIONS: The results of this research may help reduce Nyquist ghosting in EPI studies when the imaging coil is not sufficiently loaded, such as in pediatric and phantom studies.