1. A magnetic resonance imaging system comprising: a prep scan section configured to perform a prep scan acquiring plural sets of echo signals at a plurality of cardiac time phases corresponding to time delays in a cardiac cycle, wherein each of the plurality of cardiac time phases are mutually different from each other for each slice position of the prep scan; a prep image generating section configured to generate a plurality of prep images respectively corresponding to the plurality of cardiac time phases on the basis of the plural sets of echo signals; a reference information acquiring section configured to acquire reference information representing a graphable relationship between the time delays and pixel values based on the plurality of prep images for use in determining a first cardiac time phase and a second cardiac time phase; a reference information display section configured to display said reference information as a graph of the pixel values versus the time delays which represents the graphable relationship; a cardiac time phase setting section configured to set the first cardiac time phase and the second cardiac time phase in response to an operator's specification in conjunction with said displayed graphical reference information; a diagnostic imaging scan section configured to acquire imaging echo signals by performing a diagnostic imaging scan upon each of (a) the first cardiac time phase and (b) the second cardiac time phase which are set by the cardiac time phase setting section of the prep scan section; and a diagnostic imaging image generating section configured to generate a first image based on an echo signal of the first cardiac time phase obtained by the diagnostic imaging scan section, to generate a second image based on an echo signal of the second cardiac time phase obtained by the diagnostic imaging scan section, and to acquire a differential diagnostic image by calculating a difference between the first image and the second image.