A method for correcting geometric distortion in a multi-camera flat panel X-Ray detector, the method comprising: placing internal markers at known locations adjacent to a scintillator of the detector, inside a casing of the detector; placing external markers at known locations outside the casing of the detector adjacent to a cover of the detector; acquiring an X-Ray image depicting the external markers and the internal markers; finding the location of the external markers and the internal markers on the X-Ray image; calculating a parallax free transformation for correcting geometric distortion by: calculating an initial transformation for correcting geometric distortion based on the relation between physical location of the external markers and location of external markers on the X-Ray image; applying the transformation to location of the internal markers on the X-Ray image; calculating a projective transformation relating the external markers to their respective projections on the scintillator based on comparison between the true location of the internal markers and location after applying the initial transformation; estimating location of projection of the external markers on the scintillator using the projective transformation; and calculating the parallax free transformation based on the relation between the location of projections of the external markers on the scintillator and location of the external markers on the X-Ray image. placing internal markers at known locations adjacent to a scintillator of the detector, inside a casing of the detector; placing external markers at known locations outside the casing of the detector adjacent to a cover of the detector; acquiring an X-Ray image depicting the external markers and the internal markers; finding the location of the external markers and the internal markers on the X-Ray image; calculating a parallax free transformation for correcting geometric distortion by: calculating an initial transformation for correcting geometric distortion based on the relation between physical location of the external markers and location of external markers on the X-Ray image; applying the transformation to location of the internal markers on the X-Ray image; calculating a projective transformation relating the external markers to their respective projections on the scintillator based on comparison between the true location of the internal markers and location after applying the initial transformation; estimating location of projection of the external markers on the scintillator using the projective transformation; and calculating the parallax free transformation based on the relation between the location of projections of the external markers on the scintillator and location of the external markers on the X-Ray image. calculating an initial transformation for correcting geometric distortion based on the relation between physical location of the external markers and location of external markers on the X-Ray image; applying the transformation to location of the internal markers on the X-Ray image; calculating a projective transformation relating the external markers to their respective projections on the scintillator based on comparison between the true location of the internal markers and location after applying the initial transformation; estimating location of projection of the external markers on the scintillator using the projective transformation; and calculating the parallax free transformation based on the relation between the location of projections of the external markers on the scintillator and location of the external markers on the X-Ray image.