a first plurality of mr signals from a patient &# 39 ; s tissue at respectively corresponding successive first time increments extending over a first time interval including a substantial majority of a subject &# 39 ; s cardiac cycle is acquired and analyzed to define a second time interval , shorter than the first time interval , during the cardiac cycle whereat there is a relatively steep rise in signal magnitudes as a function of time . a second plurality of mr signals is then acquired from tissue of the patient at respectively corresponding successive second time increments during the second time interval , the second time increments being substantially shorter than said first time increments . image data representing at least one contrast - free image of flowing fluid vessels is generated based on the second plurality of mr signals .