Abstract: A noise analyzer apparatus for a video signal produced by an X-ray imaging system corresponding to an image generates, in response to vertical and horizontal sync pulses, a two-level gating signal which is at a predetermined level solely during the times that intensity information contained in the video signal corresponds to pixels in an adjustably positionable window area of the image. A gated noise measuring circuit responsive to intensity information contained in the video signal and to the gating signal forms a signal which is a measure of a noise content solely within the window area of the image. A signal combiner combines the video signal and a signal derived from the gating signal to produce an augmented video signal containing in its intensity information a marker indicating the location of the window area.