1. A method for delivering radiation therapy to a mammal comprising: scanning an electron beam over an x-ray target to produce x-rays from a plurality of x-ray focal spots; directing said x-rays towards an imaging field of view and a specified object in said imaging field of view in said mammal; measuring amount of said x-rays striking a detector with an active imaging area smaller than an area of said imaging field of view; producing a first pixel representation of said object based on amount of said x-rays striking said detector from a first x-ray focal spot of said plurality of x-ray focal spots; producing a second pixel representation of said object based on amount of said x-rays striking said detector from a second x-ray focal spot of said plurality of x-ray focal spots; tracking movement of said object using said first pixel representation and said second pixel representation; analyzing image quality of said object in each plane of a plurality of planes; identifying a single plane of best focus of said object from said plurality of planes based on said analyzed image quality; identifying position of said object along a direction corresponding to plane displacement based on said single plane of best focus; and controlling radiation from a radiation therapy source towards said object.