Susceptibility compensated fMRI study using a tailored RF echo planar imaging sequence.

PURPOSE To implement a method using a tailored radiofrequency (TRF) pulse with a quadratic phase profile to recover susceptibility-induced signal losses in gradient-recalled echo-planar images (EPI). MATERIALS AND METHODS A functional magnetic resonance imaging (fMRI) experiment for compensation of susceptibility artifacts, known as the TRF pulse EPI sequence (TRF-EPI), was used. TRF pulse compensates the susceptibility effect with a reduced signal-to-noise ratio (SNR) to one-half when the maximum phase distribution is 2pi. We demonstrate theoretically that the maximum phase distribution can also be reduced to pi rather than 2pi, improving the SNR accordingly. An analysis was conducted comparing this newly proposed strategy using a standard RF excitation with a linear phase distribution and a quadratic TRF excitation with a pi phase distribution. RESULTS Thorough experimental comparisons were also made between the TRF quadratic excitation with a pi phase profile and conventional EPI with a standard excitation in human subjects during ventral brain activation. CONCLUSION With reduced maximum phase distribution in the TRF pulse, signals in the susceptibility-affected areas, such as the orbitofrontal and inferior temporal cortex, were increased, suggesting that the technique could be a useful adjunct to fMRI.