Current FDA approved CAD systems are registered for use as a second reader. In the second reader paradigm, the radiologist first interprets the image, whereupon suggested suspicious areas by CAD can be accepted or dismissed. In this thesis we have shown that CAD as a second reader has a significant added value for the detection of lung nodules, even when bone suppressed images together with original CXRs are being used as a standard. However, there exists a contradiction in the literature about the improvement in performance with the use of CAD for the detection of lung nodules in CXRs. Most studies were able to find an increase in sensitivity for
the detection of lung nodules using CAD12,30-35. However, several studies also reported
a decrease in specificity, resulting in negligible effect of CAD on the overall detection
performance36-39. The improvement with CAD seems to be highly dependent on
image quality, the case set, and the expertise of the observers. Radiologists find it hard to discriminate between TP and FP CAD marks. Therefore we explored new ways of using CAD information that are less dependent on the human-CAD interaction.
The low specificity of CAD systems results in many marks to be inspected by the radiologist, which leads to a lower specificity when radiologists accept FP CAD marks. Therefore we explored the use of an interactive CAD system to reduce the risk of lowering specificity. In this interactive system CAD marks remain hidden unless their location is queried by the radiologist. In our experiment we found that such a system indeed decreased the exposure to FP CAD marks. On average only 84 FP CAD marks, instead of the 299 FP CAD marks in the traditional reading with CAD, were reviewed. However, this did not result in an improved performance, since also fewer true lesions were found in the reading with CAD. Also the radiologists did not query many lesion locations. This result suggests that the proportion of oversight errors in the detection of lung nodules in chest radiographs might be larger than previously assumed. The assumptions made on the proportion of oversight and interpretation errors are based on some eye-tracking studies. In these studies they
considered a dwell time of 1 second enough for conscious perception of the lesion. If the radiologists did not mark lesions that exceeded this dwell time threshold was counted as an interpretation error. It may be that this method underestimates the true number of oversight errors. Longer dwell times might therefore be needed for conscious perception of lesions in chest radiographs.
Another alternative to reduce the detrimental effects of a low specificity of CAD systems is to exclude the human interpretation of CAD marks from the reading process. We tested such an approach by combining the interpretation of the radiologist with the results of the CAD system in the background. The CAD system now functions as a second independent reader. This reading paradigm would require a change in reporting by the radiologist. In order to combine the results of the radiologist and CAD system, the radiologist would need to assign ratings to suspicious areas in the image, similar to the BI-RADS scoring system in breast imaging. When the interpretations of radiologist and CAD are being combined we found a significant increase in detection performance that exceeded the performance with CAD as a second reader. Although the stand-alone performance of the CAD system we had available is still worse than the weakest radiologist in our study, performance increased for 11 out of 12 participating radiologists. In contrast, many radiologists decreased in performance when combined with a weaker human observer. The combination of CAD and radiologist represents therefore a strong and more objective combination. With an improved CAD system that reaches the performance of a weak radiologist, as we demonstrated in mammography, the combination of CAD and observer will further improve, and be on average similar to a combination of two radiologists. Practical usage of such system has yet to be investigated. As a first step it would require a change in reporting by the radiologists. Also potential legal issues have to be resolved, since in this scenario the final result will be a computed result. Moreover, the CAD systems have to be tested in a different way before being approved as clinically acceptable devices. All in all, using CAD as an independent second reader offers the potential to improve reader performance, without drastically interfering with the reading process. Even the best observers increase in performance, and this increase in performance comes with little costs.
Since application of a CAD system that influences the input of the radiologist may be problematic in the near future, we looked at other options for implementing CAD in current clinical practice. It was found that a CAD system at very high specificity settings was still able to detect lesions that were missed in clinical practice. Such a system may help the radiologist to detect the more suspicious lesions, those that probably would be classified as obvious lesions since CAD detects them so well. Missing obvious lesions is to be avoided at all times and may have costly legal implications. Although such use of the CAD system will not help to detect all lesions, only minor effort by the radiologist is needed with a high chance to pick up few additional important findings.
CAD systems can be easily integrated into clinical practice, and used in the PACS environment. However, the FDA approved system we used in this thesis is currently only meant to be used as a second reader. Therefore, the other proposed CAD applications should be further tested in a clinical environment. With the proposed use of CAD as a second reader the review time will slightly increase. The workflow with a CAD system demands that the radiologist reviews the CAD marks after the normal interpretation process. Reading time therefore inevitably lengthens. Alternative use of CAD, such as the interactive CAD system, may shorten the reading time. Besides, more experience with the CAD system will also shorten the reading time. However, clinical studies are needed to quantify the burden on the workflow. But more importantly future research will have to show the effect on detection performance in clinical practice, and the effect of CAD usage on the number of secondary referrals to CT for further diagnostic work-up.