Rationale and Objectives
American College of Radiology guidelines suggest that digital screening mammographic images should be viewed at the full resolution at which they were acquired. This slows interpretation speed. The aim of this study was to examine the effect of various levels of zooming on the detection and conspicuity of microcalcifications.
Materials and Methods
Six radiologists viewed 40 mammographic images five times in different random orders using five different levels of zooming: full resolution (100%) and 30%, 61%, 88%, and 126% of that size. Thirty-three images contained microcalcifications varying in subtlety, all associated with breast cancer. The clusters were circled. Seven images contained no malignant calcifications but also had randomly placed circles. The radiologists graded the presence or absence and visual conspicuity of any calcifications compared to calcifications in a reference image. They also counted the microcalcifications.
Results
The radiologists saw the microcalcifications in 94% of the images at 30% size and in either 99% or 100% of the other tested levels of zooming. Conspicuity ratings were worst for the 30% size and fairly similar for the others. Using the 30% size, two radiologists failed to see the microcalcifications on either the craniocaudal or mediolateral oblique view taken from one patient. Interobserver agreement regarding the number of calcifications was lowest for the 30% images and second lowest for the 100% images.
Conclusions
Images at 30% size should not be relied on alone for systematic scanning for microcalcifications. The other four levels of magnification all performed well enough to warrant further testing.
In 2007 the American College of Radiology promulgated recommendations that all digital screening mammograms should be viewed at the full resolution at which they were acquired. To do this, images must be zoomed so that they are physically larger than the area of the 5-megapixel monitors on which they are typically viewed. This zooming contributes to a near doubling of the time needed to interpret digital screening mammograms compared to film-screen screening mammograms . That increase in interpretation time may contribute to a relatively slow penetration of digital imaging into mammography, compared to its acceptance in other areas of radiology. We have studied the effect of varying degrees of image zooming on the visual conspicuity of microcalcifications present on digital screening mammograms to determine the contribution of zooming to the detection of microcalcifications and to guide our choice of levels of zooming to be tested in a subsequent trial. Eventually, we intend to study the effect of zooming on the detection of other signs of breast cancer, particularly masses. Because malignant microcalcifications are often small and subtle, any degradation of resolution caused by rebinning might have a greater impact on their detection than on the detection of masses. Therefore, we have first tested microcalcifications.
Materials and methods
This study was granted exemption by our institutional review board. The requirement for informed consent regarding patients whose images were used as part of the study was waived. Verbal consent to participate was deemed adequate for radiologists who agreed to take part as readers.
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Setting for the Experiment
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Scoring
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Statistical Analysis
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Results
Sensitivity
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Specificity
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Conspicuity
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Table 1
Conspicuity of Microcalcifications for Each of the 10 Pairs of Display Modes That Were Compared
Display Mode Four on One Fit Screen Partway 100% Resolution Slightly Larger Craniocaudal Four on one <.0001 <.0001 <.0001 <.0001 Fit screen <.0001 <.0001 <.0001 Partway .6442 .2616 100% resolution .0167 Slightly larger Mediolateral oblique Four on one <.0001 <.0001 <.0001 <.0001 Fit screen <.0001 <.0001 <.0001 Partway .0122 .0003 100% resolution .0075 Slightly larger
To allow for the several variables, such as differing readers, Bonferroni’s adjustment was used. This meant that the cutoff for significance was P < .005 rather than the more usual cutoff of P < .05.
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Number of Calcifications
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Table 2
Interreader Agreement Regarding the Number of Calcifications Comparing Readings for Each Display Mode, with Responses Cut Off Between More Than 10 and All Other Responses
Display mode Craniocaudal Mediolateral Oblique Four on one 0.24 0.21 Fit screen 0.68 0.67 Partway 0.64 0.65 100% resolution 0.55 0.56 Slightly larger 0.66 0.61
Table 3
Interreader Agreement Regarding the Number of Calcifications Comparing Readings for Each Display Mode, with Response Cut Off Between 5 and 6 to 10
Display mode Craniocaudal Mediolateral Oblique Four on one 0.28 0.29 Fit screen 0.62 0.60 Partway 0.67 0.46 100% resolution 0.40 0.58 Slightly larger 0.59 0.47
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Discussion
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