Valdez MA, et al. - In order to contrast the distributions of MRI contrast and dextran molecules of various molecular weights (3, 70 and 500 kDa) following focused ultrasound (FUS)-mediated blood-brain barrier opening through a range of ultrasound pressures (0.18–0.46 MPa) in laboratory mice, experts combined in vivo MRI and ex vivo multispectral fluorescence imaging. Via the MRI and fluorescence images, the volume of brain exposed was measured, which was markedly reliant on both molecular weight and ultrasound pressure. From the fluorescence images, diffusion coefficients of the various-molecular-weight dextran molecules in the brain parenchyma were also measured which were negatively associated with the molecular weight of the dextran molecules. Therefore, the outcomes of this work build on a foundation of knowledge is critically significant for the FUS method to be practiced in the clinical delivery of therapeutics to the brain.