DIGITIZATION REQUIREMENTS IN MAMMOGRAPHY - EFFECTS ON COMPUTER-AIDED DETECTION OF MICROCALCIFICATIONS

We have developed a computerized method for detection of microcalcifications on digitized mammograms. The program has achieved an accuracy that can detect subtle microcalcifications which may potentially be missed by radiologists. In this study, we evaluated the dependence of the detection accuracy on the pixel size and pixel depth of the digitized mammograms. The mammograms were digitized with a laser film scanner at a pixel size of 0.035 mm x0.035 mm and 12-bit gray levels. Digitization with larger pixel sizes or fewer number of bits was simulated by averaging adjacent pixels or by eliminating the least significant bits, respectively. The SNR enhancement filter and the signal-extraction criteria in the computer program were adjusted to maximize the accuracy of signal detection for each pixel size. The overall detection accuracy was compared using the free response receiver operating characteristic curves. The results indicate that the detection accuracy decreases significantly as the pixel size increases from 0.035 mm x 0.035 mm to 0.07 mm x 0.07 mm (P<0.007) and from 0.07 mm x 0.07 mm to 0.105 mm x 0.105 mm (P<0.002). The detection accuracy is essentially independent of pixel depth from 12 to 9 bits and decreases significantly (P<0.003) from 9 to 8 bits; a rapid decrease is observed as the pixel depth decreases further from 8 to 7 bits (P<0.03) or from 7 to 6 bits (P<0.02). Although the results are expected to depend on the noise and spatial resolution properties of the x-ray screen-film system and the film digitizer, as well as on the image processing methods, this study demonstrates that very high spatial resolution (a pixel size of 0.035 mm x 0.035 mm or smaller) will be required for digital mammographic systems in order to capture the very subtle microcalcifications detectable on screen-film mammograms.