Soft Tissue Contrast
X-rays can only discriminate between different tissues by the number of electrons capable of receiving the x-ray photon's energy†. Unfortunately this number does not vary much in soft tissues, and contrast between different soft tissues can only be enhanced by the injection of contrast media. In MRI, however, other tissue-specific parameters affect the MR signal (T1, T2*, proton density). There is enough variation between tissues in these parameters to permit acquisition of images with greater contrast between soft tissues, and between normal and abnormal morphology / pathology. Furthermore, the effect on the contrast in an MR image of the tissue-specific parameters can be suppressed or enhanced by another set of operator-selectable parameters (such as TR, TE and flip angle). Drastic manipulation of MR images of the same anatomical site is possible, order to provide different information (morphological, functional, pathological).
†this varies with material density and atomic number, but for different reasons. Greater material density means more electrons and hence more interactions. Higher atomic number implies more electrons as well, but it also means more electrons with a suitable photoelectric cross section.