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Visual perception is such an intimate part of human experience that we assume that it is entirely
accurate. Yet, perception accounts for about half of the errors made by radiologists using adequate imaging
technology. The true incidence of errors that directly affect patient well being is not known but it is probably
at the lower end of the reported values of 3 to 25%. Errors in screening for lung and breast cancer are
somewhat better characterized than errors in routine diagnosis. About 25% of cancers actually recorded on
the images are missed and cancer is falsely reported in about 5% of normal people.
Radiologists must strive to decrease error not only because of the potential impact on patient care but
also because substantial variation among observers undermines confidence in the reliability of imaging
diagnosis. Observer variation also has a major impact on technology evaluation because the variation
between observers is frequently greater than the difference in the technologies being evaluated. This has
become particularly important in the evaluation of computer aided diagnosis (CAD).
Understanding the basic principles that govern the perception of medical images can provide a
rational basis for making recommendations for minimizing perceptual error. It is convenient to organize
thinking about perceptual error into five steps. 1) The initial acquisition of the image by the eye-brain
(contrast and detail perception). 2) The organization of the retinal image into logical components to produce
a literal perception (bottom-up, global, holistic). 3) Conversion of the literal perception into a preferred
perception by resolving ambiguities in the literal perception (top-down, simulation, synthesis). 4) Selective
visual scanning to acquire details that update the preferred perception. 5) Apply decision criteria to the
preferred perception.
The five steps are illustrated with examples from radiology with suggestions for minimizing error.
The role of perceptual learning in the development of expertise is also considered.
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