1. A magnetic resonance imaging (MRI) system comprising: an MRI gantry having static and gradient magnet assemblies and at least one radio frequency (RF) coil defining an image volume; MRI control circuits connected to control components within said MRI gantry and configured to effect MRI data acquisition sequences of RF and gradient magnetic pulses which elicit MRI signals, acquire and process said elicited MRI signals into MR image data of cardiac tissue; said MRI control circuits being configured to (a) acquire multi-dimensional MR k-space data using multiple data acquisition subsequences for each of multiple time to inversion (TI) intervals without using an injected contrast agent, each TI interval being an elapsed time interval occurring after a fixed time delay from an R-wave ECG signal and before a slice selective excitation pulse of a data acquisition subsequence; (b) reconstruct said acquired k-space data into a series of spatial domain MR image data each having TI intervals of different lengths of time; (c) perform an analysis of a peaked curve shape in a plot of signal intensity as a function of the TI intervals based on the series of the spatial domain MR image data, to determine a cardiac tissue type based on an analysis of the peaked curve shape; and (d) display a result of the analysis, wherein the MRI control circuits are configured to perform the analysis of curve shape by an analysis that includes calculation of a curve shape width value based on a predetermined measurement analysis of curve shape.