Purpose: To compare T1- or T2-weighted magnetic resonanceimaging (MRI) and ultrasonography (US) as tools forin vivo mapping of different tissue components in spontaneoustumors of transgenic mice.Materials and Methods: Human-like mammary adenocarcinomasfrom FVB/neuT transgenic mice were analyzed byT2-weighted and T1-weighted MRI at 4.7 Tesla and US andthen, after excision, were paraffin-embedded for histologicanalysis. The histologic samples were prepared taking careto obtain sections that spatially matched the MRI and USimages as precisely as possible.Results: US can obtain basic information such as the sizeof developing tumors in experimental animals and canidentify necrotic areas. T2-weighted MRI, especially if comparedto T1-weighted MRI and/or US, allows advancedanalysis of morphologic aspects, with high resolution in thedifferentiation of details of necrotic areas such as coagulation,liquefaction, biphasic splitting of cysts, and fibroticand lipidic infiltration.Conclusion: Of the three methods, T2-weighted MRI providesthe most information about the anatomy of tumors.However, when distinctions between the different types ofnecrosis are not needed, US analysis is to be preferred forits practicality.