Abstract: It is clearly clarified the conceptions of quadrature modulation, quadrature exciting, quadrature receiving, and quadrature demodulation in the MRI system. The purpose of quadrature modulation is to generate the SSB RF exciting pulse that avoids exciting it's mirror slice. Quadrature demodulation can distinguish positive and negative frequency so it can overcome position uncertainties on imaging data space. Quadrature exciting and quadrature receiving refer to make use of circular polarization RF coils. These quadrature operations are necessary for the optimization of MRI system and they bring some significant benefit.