1. A method using magnetic resonance imaging to characterize ventilation and perfusion in a lung, comprising: acquiring a magnetic resonance (MR) image of the lung that includes MR data in a voxel of the MR image, wherein the acquiring includes obtaining a breathing frequency parameter; determining a water density value, a specific ventilation value, and a perfusion value in at least one voxel of the MR image based on the MR data, wherein the water density value is used to determine an air content value; and determining a ventilation-perfusion ratio value that is the product of the specific ventilation value, the air content value, the inverse of the perfusion value, and the breathing frequency.