Dose dependence of mass and microcalcification detection in digital mammography: free response human observer studies

Abstract

The purpose of this study was to evaluate the effect of dose reduction in digital mammography on the detection of two lesion types-malignant masses and clusters of microcalcifications. Two free-response observer studies were performed-one for each lesion type. Ninety screening images were retrospectively selected; each image was originally acquired under automatic exposure conditions, corresponding to an average glandular dose of 1.3 mGy for a standard breast (50 mm compressed breast thickness with 50% glandularity). For each study, one to three simulated lesions were added to each of 40 images (abnormals) while 50 were kept without lesions (normals). Two levels of simulated system noise were added to the images yielding two new image sets, corresponding to simulated dose levels of 50% and 30% of the original images (100%). The manufacturer's standard display processing was subsequently applied to all images. Four radiologists experienced in mammography evaluated the images by searching for lesions and marking and assigning confidence levels to suspicious regions. The search data were analyzed using jackknife free-response (JA-FROC) methodology. For the detection of masses, the mean figure-of-merit (FOM) averaged over all readers was 0.74, 0.71, and 0.68 corresponding to dose levels of 100%, 50%, and 30%, respectively. These values were not statistically different from each other (F= 1.67, p=0.19) but showed a decreasing trend. In contrast, in the microcalcification study the mean FOM was 0.93, 0.67, and 0.38 for the same dose levels and these values were all significantly different from each other (F = 109.84, p < 0.0001). The results indicate that lowering the present dose level by a factor of two compromised the detection of microcalcifications but had a weaker effect on mass detection.

Figure 1

The five masses used in this study. The longest diameters (measured at FWHM) of the masses ranged from 8.7 mm to 9.4 mm with a mean of 9.1 mm. All size measurements are at the detector plane.

Figure 2

The five clusters of microcalcifications used in this study. There were, on the average, 36 microcalcifications per cluster. The average diameter of the individual microcalcifications was 264 μm. All size measurements are at the detector plane.

Figure 3

Appearance of lesions inserted into mammographic regions of interest (optimally windowed unprocessed images). The top row, left and right, consists of an image detail with and without a mass inserted, respectively (mass ‘e’ from Figure 1). The bottom row, left and right, consists of an image region with and without a microcalcification cluster inserted, respectively (microcalcification ‘a’ from Figure 2).

Figure 4

Display-processed regions of interest from two images (top and bottom rows) showing inserted lesions at the three dose levels used in the study. The top row (from left to right) consists of the same mass as in Figure 3 at dose levels 100%, 50% and 30%, respectively. The bottom row (from left to right) consists of the same microcalcification cluster as in Figure 3 at dose levels 100%, 50%, and 30%, respectively. Note the visible increase in noise as the dose is reduced from 100% to 30%.

Figure 5

Graphical user interface used in the human observer studies. In this example, there were two masses present in the image: the observer has marked an actual region (mass ‘b’ from Figure 1) with a ‘+’ and ranked it as a ‘3’, i.e. ‘likely to be a lesion’. The second mass in the image, mass ‘e’ from Figure 1, was not marked (actual location indicated with the arrow).

Figure 6

The reader-averaged free-response figure of merit (FOM) at each dose level for masses (gray) and microcalcifications (dotted). Note the steep decline for the microcalcifications as a function of dose and the relative constancy for the masses. The uncertainty bars represent 95% confidence intervals. The microcalcification FOM decline was statistically significant; that for masses was not