They don't. As Stormer pointed out they just enhance the signal for the scan, giving you a better picture which the radiologist can read. And since tissue densities and water properties are different, adding contrast allows better visual differentiation.If the response of the healthy breast tissue and malignant tissue are both enhanced, how would the contrast agents be able to accentuate the region of malignancy?
Gadolinium and other paramagnetic contrast agents map blood flow. Malignant tumors are metabolically active and have "tumor vascularity" -- specific tumor growth factors recruit the growth of new blood vessels into the malignant tissue (neovascularity) to supply the increased metabolic needs of the malignant cells.Actually , the T1 and T2 relaxation times of protons in malignant vs. normal cells are different for some malignancies). The presence of , e.g.. gadolinium nuclei in some of the MRI 'contrast agents', injected into the patient some time before the contrast MRI image acquisition 'enhances' this difference and MR radiologists who 'read' these images, pre- and post- contrast can deduce whether that imaged tissue is malignant or not.