1. A method of determining a tissue perfusion response, comprising: acquiring data comprising voxels using an imaging device in a predetermined modality as a function of time; determining a time-related parameter value comprising a time-to-peak parameter value for each of a predetermined set of the voxels consisting of adjacent voxels and at least one voxel of interest in the acquired data; excluding based upon the time-related parameter value any one of the adjacent voxels along a predetermined blood flow direction to the voxel of interest, the remaining acquired data defining selected adjacent voxels, wherein any one of the adjacent voxels is excluded if the time-to-peak parameter value of the adjacent voxel exceeds the time-to-peak parameter value of the voxel of interest; determining a local input function based the acquired data of the selected adjacent voxels; and determining the tissue perfusion response based upon the local input function and an output function according to the acquired data.