Systems, methods and apparatus for protecting sensitive data in a file that has been replaced with pointer for each sensitive data. A pulse string which monotonically decreases the flip angle is inserted between the prior pulse in one round and the imaging pulse in the previous round, and a pulse string which monotonically increases the flip angle is inserted between the prior pulse and the imaging pulse in one round. The displacement caused by the waveform over time includes displacements associated with response to different frequencies.