Computational models of traumatic brain injury (TBI) require accurate estimates of brain tissue mechanical properties, however properties of brain tissue ex vivo may differ from corresponding properties in vivo. To characterize possible differences, we performed MR elastography (MRE) on four Yucatan mini-pigs in vivo. Brain tissue samples from the same animals were later tested using ex vivo MRE. Shear moduli of both in vivo and ex vivo brain tissue were estimated using local direct inversion. Our results suggest that in vivo tissue is stiffer than ex vivo tissue at the same frequency, which has important implications for TBI model development.