Rationale and Objectives-In our earlier studies we reported an evidence-based Computer Assisted Decision (CAD) system for location-specific interrogation of mammograms. A contentbased image retrieval framework with information theoretic (IT) similarity measures serves as the foundation for this system. Specifically, the normalized mutual information (NMI) was shown to be the most effective similarity measure for reduction of false positive marks generated by other, prescreening mass detection schemes. The objective of this work was to investigate the importance of image filtering as a possible preprocessing step in our IT-CAD system. Materials and Methods-Different filters were applied, each one aiming to compensate for known limitations of the NMI similarity measure. The study was based on a region-of-interest database that included true masses and false positive regions from digitized mammograms. Results-ReceiverOperating Characteristics (ROC) analysis showed that IT-CAD is affected slightly by image filtering. Modest, yet statistically significant performance gain was observed with median filtering (overall ROC area index Az improved from 0.78 to 0.82). However, Gabor filtering improved performance for the high sensitivity portion of the ROC curve where a typical false positive reduction scheme should operate (partial ROC area index 0.90 A z improved from 0.33 to 0.37). Fusion of IT-CAD decisions from different filtering schemes markedly improved performance (A z =0.90 and 0.90 A z =0.55). At 95% sensitivity, the system's specificity improved by 36.6%.Conclusion-Additional improvement in false positive reduction can be achieved by incorporating image filtering as a preprocessing step in our information-theoretic CAD system.

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