Limitations on contrast sensitivity and dynamic range imposed by human-eye response characteristics film-density and radiographic viewing conditions

摘要

Abstract: Models for characterizing the signal-to-noise ratio for radiographic imaging systems typically ignore the human interface. In this work, images of a calibrated mammographic step wedge and a very low contrast paper step wedge were made with mammography film-screen systems and with direct x-ray exposures. Large and small density differences between adjacent steps were thus obtained over a wide range of film optical densities. Experienced viewers determined large-area threshold contrast sensitivity as a function of film density and viewbox luminance at constant low ambient illuminance with masking to exclude transmission of bright viewbox light through any area other than that portion of the step-wedge image being examined. Then, at high and low levels of viewbox luminance, threshold contrast sensitivity was determined at four different levels of ambient illuminance ranging from lower to higher than levels ordinarily used in clinical practice. threshold contrast sensitivity with and without masking was determined for viewing on a typical radiographic viewbox at a constant luminance level. The resulting data give strong qualitative support to the conclusion that viewing conditions (viewbox luminance, masking, and ambient illuminance) typically used in clinical practice are insufficient for optimum detection of the diagnostic information that can be recorded and displayed with modern mammography film-screen systems.!3