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Department of Radiology, Brigham and Women's Hospital
Harvard Medical School |
Contrast is a measure of differences in brightness, counts, or optical density in adjacent regions of the image:
C = (D1 - D2) / {0.5*(D1+D2)} or C = (D1 - D2) / D2
The contrast inherent in the image information, independent of the means of display.
Varies with the means used to capture and display the latent image information. For filmed images, the displayed contrast will depend on the H&D curve, which relates optical density to film exposure. For computer displays, the displayed contrast will depend on the type of grayscale or colorscale used to transform the counts in each pixel to luminance on the display monitor. This transformation can also be changed with the "window" and "level" (or contrast and brightness) computer controls.
The total range of counts that can be recorded between the minimum and maximum displayable brightnesses. For film, the dynamic range is called latitude , the range of exposures which are visible on the film without saturation (overexposure). Displays (films) with wide dynamic range (latitude) usually present image features with worse contrast than narrow dynamic-range systems, but they are more forgiving of sloppy technique.
a. For small lesions, the peak lesion contrast may be reduced by the system's spatial resolution (FWHM):
b. For all lesion sizes, contrast is reduced by scatter in the patient (and other materials). The scatter tails on the PSF give rise to a broad, uniform (almost flat) scatter background which is added to the image of primary (unscattered) photons. This is perhaps the most significant source of contrast reduction in nuclear medicine.
Lesson Author: Stephen C Moore, PhD, scmoore@bwh.harvard.edu
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Updated June 1, 1998