1. A method of performing physiological gating in a medical procedure, wherein the method is at least partially performed using a hardware processor, comprising: acquiring a sequence of images, wherein each of the images in the sequence is generated using a same imaging device, the sequence of images comprises at least a first image and a second image of a target region; determining a first composite image based at least in part on the first and second images, wherein the determining comprises performing an image subtraction using the first and second images from the sequence; gating a medical procedure based at least in part on the first composite image, wherein the act of gating the medical procedure is performed in real time using the processor; and determining a first value associated with a contrast of the first composite image; wherein the gating the medical procedure is performed based at least on the first value; and wherein the gating comprises de-activating a radiation beam when the first value is above a prescribed threshold value.