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A major difficulty with currently available display systems is contrast
rendition. The number of gray steps that can be displayed by a digital system
driving an analog display is determined by the luminance range of the display
and the signal-to-noise ratio (Schreiber, 1986). For a good display this might
be 128 steps (7 bits). The minimal detectable contrast is determined by the
average luminance at the eye and the image noise. Pizer and Chan (1979) using
ROC techniques found the "perceived dynamic range" of a "high quality W
monitor" to be about 87 steps. In their experiment, the eye was adapted to each
luminance level. In a display like the chest with large bright and dark areas
(heart and mediastinum vs. lungs) the eye will be adapted to some intermediate
level and not optimally coupled to either the bright or the dark area. For this
reason gray scale image processing is used for matching correctly the display
to the observer' s perceptual system (Kundel, 1986). There have been some
attempts to develop "perceptually relevant" gray scales (Johnston et al.,1985).
Clinicians do not seem willing to spend the time needed to adjust optimally the
controls at a display console. Even when the controls are fairly simple they
still prefer images that technicians have recorded on film using some
prescription for window and level. Perhaps they do not know how to optimize the
contrast in the image and prefer to leave it to someone else or perhaps the
controls are not as simple as they look.
We have been working on a rule-based system that will select the proper gray
scale transfer characteristic (GSTC) for the initial look at the image. It is
based on the notion that there is an optimal GSTC for each diagnostic finding
on the image and that for most images the diagnostic findings can be predicted
from prompts derived directly from the clinical history. If this is the case,
radiologists who receive diagnostic prompts should be able to predict the
appropriate GSTC.
This experiment was done to determine if radiologists can agree about the GSTC
that will optimize the video display for particular diagnoses.
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