Some researchers proposed that CAD can reduce the interpretation time if radiologists focus on a small number of regions indicated by a CAD scheme. A reduction in time is most likely to occur when CAD is used as a first reader. Such a paradigm, often called "CAD as a first reader" (Evancho 2002; Mani et al. 2004), can possibly be used for separating out negative CTC cases even before radiologists read the cases. However, such a separation is likely to come with an increased number of missed abnormalities, because there is always a trade-off between sensitivity and specificity in CAD due to the fact that the diagnostic performance of a CAD scheme is represented by a receiver-operating characteristic curve (Metz 2000). In other words, when a CAD scheme is set to yield a high true-negative rate (i.e., a high specificity), the price we need to pay is a high false-negative rate (i.e., a low sensitivity).
Thus far, no study has shown convincingly that CAD can shorten the interpretation time of CTC examinations, although some commercial CAD systems are advertised to be used in a similar manner to a first reader, and thus they imply that their CAD systems would reduce radiologists' interpretation time. In fact, conventional use of CAD as a second reader may increase the interpretation time, because additional time is needed for examining the possible
Fig. 11.11. Example of a true positive in CAD for fecal-tagging CTC. (Courtesy of Michael Zalis, M.D., Massachusetts General Hospital, Boston, MA)
lesions found by CAD but not by a human reader. However, such an increase in time is expected to be small, as demonstrated in CAD for mammography (Jiang et al. 1999), or the total reading time may be unchanged, as demonstrated by a recent observer study (Mani et al. 2004). Further studies need to be conducted for evaluation of the effect of CAD for rapid interpretation of CTC examinations as well as the efficacy of the use of CAD as a first reader.
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