A computerized method for the measurement of conspicuity. Comparison of film and digital nephrograms

We have designed a computer program appropriate for comparing the conspicuity of an object of interest portrayed on film-based and photoelectronic images. To test this program, we evaluated nephrograms generated by exposing matched film tomograms and digital images of rabbit kidneys following contrast administration. We then evaluated their conspicuity--a measure of how well an object can be distinguished from its surround--by a computer program which automatically traced a concentric and equal area of surrounding background around operator-defined nephrograms. Computerized evaluation of these areas provides the formula elements (mean object and background density and variance values and the number of pixels) from which conspicuity is automatically calculated. Nephrogram conspicuity was greatest for digital subtraction images. This approach is useful for comparing film and digital techniques.