Visual Requirements for High-Fidelity Display

The digital radiographic process involves (a) the attenuation of x rays along rays forming an orthographic projection, (b) the detection of the radiation beam by a two-dimensional recording device, (c) the processing of detector signals to produce a digital image for presentation, and (d) the display of the digital image. The performance of the process is typically considered in terms of the ability to present anatomic structures of importance to the person interpreting the image. Any steps in the process that limit contrast, blur detail, or add noise can limit the interpretative process. Image information from the patient is potentially degraded by large focal spots that produce blur or by inappropriate peak kilovoltage that produces poor contrast. The recording of images is potentially degraded by detectors with poor modulation transfer characteristics or by detectors that add instrument noise to the x-ray signal. Display of the digital image can similarly add instrument noise to the signal and further restrict modulation transfer. The image formation and radiographic recording processes are often capable of recording anatomic structures with very fine detail that produce very low contrast. If displayed with no further degradation, the detail and contrast in these signals can exceed the visual acuity limits of the human vision system. Interpretation in these circumstances requires some form of magnification, either geometric or produced by display processing. The visual acuity of the human vision system is considered in this chapter. The maximum spatial frequency that the eye can detect is used to suggest the maximum spatial frequency that is needed for a display (ie, the minimum picture element size [pixel] and maximum display array size). The minimum contrast to detect standard test targets is used to explain how display devices should be calibrated to produce an effective gray-scale response. The nonlinear characteristics of the globally adapted eye are used to derive conditions to achieve an equivalent appearance of a digital image when it is displayed on different devices.