When monitoring a position of organ(s) at risk (OAR) near a target volume to be irradiated, a LINAC radiation beam source position is identified around a target volume, and a plane perpendicular to a radiation beam is identified. A first phase encoding direction within the plane is assigned, and a second phase encoding direction perpendicular to the first phase encoding direction is assigned. A read-out direction parallel to the radiation beam is assigned, and an MR image of the target volume and any OAR(s) is generated. The position of the target volume relative to the radiation beam is determined and a collimator between the LINAC source and the volume of interest and target volume is controlled to maintain the radiation beam on the target volume and away from any OAR(s) based on the determined position of the target volume.
