Shift variance in digital radiographic imaging systems as a source of error in contrast detail measurements

The effects of the total inherent digital imaging system shift variance on the perception of image details were investigated. These effects are particularly important in clinical digital imaging systems when the sampling aperture is comparable to or greater than the resolution dimensions (width of the point spread function) of the input images. The total system shift variance of a commercial digital imaging system, System One, DigiRad, Inc. (Palo Alto, CA), was quantified using a multiple high contrast bar pattern image. The square wave response functions (SWRF) of several nominally randomly selected locations on the displayed images were obtained and linearized by the logit transformation. The Coltman equation was then used to calculate the corresponding modulation transfer functions. Contrast detail (CD) diagrams were obtained using images of Burger-Rose phantoms for different relative positions of the input images with respect to the sampling pixel matrix. The effects of the shift variance on the CD diagrams were found to depend not only on the relative location of the image with respect to the pixel matrix but also on the input image quality. Further studies need to be performed to assess the clinical importance of the inherent shift variance on clinical digital imaging systems.