According to the remote detection scheme, conventional MRI could be spatially separated into prepolarization, encoding and detection. Magnetometers with high sensitivity, such as optically pumped magnetometer, could then be applied in detection to improve the capability of MRI. By analyzing how the output of the servo circuit depend on modulation, optimum modulation parameters could be obtained for maximum signal amplitude and best sensitivity. In our point of view, it could provide a way to make the optically pumped magnetometer more applicable to MRI.