Automatically calculating the volumetric breast density (VBD) from an x-ray image produced during mammography is known. The volumetric breast density is defined as the ratio of the volume of the fibroglandular tissue to the overall volume of the breast. Below, the terms “fibroglandular tissue,” “glandular tissue,” and “mammographically dense tissue” or “dense tissue” are used synonymously. On the basis of this VBD value, the breast has until now been assigned a specific breast density category using fixed thresholds, e.g., a BI-RADS value from “1” to “4” according to the classification by the American College of Radiology (ACR). By way of example, this is described in U.S. Patent Publication No. 2011/0026791 A1 and in DE 10 2006 021 042 A1.
Women whose breast has a high VBD value have an increased risk of getting breast cancer. The increase in this risk is partly traced back to the fact that cancerous tissue is masked by mammographically dense tissue and therefore it is not identified during mammography.
It is known that the masking risk does not always correlate with the VBD value. FIGS. 1 and 2 depict a breast 3 compressed between two plates 1, 2 during mammography. After passing through the tissue of the breast 3, x-ray radiation 5 emanating from an x-ray source 4 is incident on an x-ray detector 6. As depicted in FIGS. 1 and 2, the same volume of fibroglandular tissue 7, 8 may cover small masses 9 in different ways. In the example depicted in FIG. 1, the fibroglandular tissue 7, which has a specific volume, is localized at a single position such that the small mass 9 is covered by the dense tissue 7. As a result of the volume of the fibroglandular tissue 7, the depicted region of the breast 3 is characterized by a specific VBD value. By contrast, the fibroglandular tissue 8, which has an identical volume, is distributed more uniformly in the volume of the breast 3 in the example depicted in FIG. 2, and so it is less likely for the small mass 9 to be covered by the dense tissue 8. Despite the VBD values being identical, the risk of masking of the small mass 9 is lower in FIG. 2. Therefore, the sole use of the VBD value is not sufficient for an accurate description of the masking effect by mammographically dense tissue 7, 8.
It is for this reason that the 5th edition of the ACR BI-RADS Atlas proposes new categories “a” to “d” with a verbal description of the breast density category, defined by the visually estimated portion of fibroglandular tissue in the breast. Taking into account the masking risk was proposed for the first time in this context. Thus, the category “c” may be assigned if small masses may be obscured as a result of a heterogeneous density distribution. It is proposed that the radiologist in such a case describes the position of the dense tissue in a further sentence.
Moreover, the nature of the breast may be described with the category “a” if the breasts are almost entirely fatty. The category “b” may be present if there are scattered areas of fibroglandular density. The category “d” may be assigned if the breasts are extremely dense, which lowers the sensitivity of mammography.
As depicted in FIG. 3, a glandularity map was previously calculated in act 101 using the x-ray image produced during mammography, with the glandularity denoting the proportion of the fibroglandular tissue in the overall tissue. The glandularity map defines the amount of glandular tissue in each image pixel of the x-ray image, either as a specification in millimeters or as a percentage specification, with the value lying in a range from, e.g., 1% to 50%. Then, the mean glandularity, the VBD value, is established in act 102. Subsequently, the categories “a” to “d” are determined in act 103 based purely on the VBD value. The acts are carried out either manually by a radiologist or already in an automated form with the aid of available systems. However, only experienced radiologists may undertake a reliable assessment of the masking risk and generate a complete breast density report, taking into account the masking risk, in accordance with the prescriptions of the ACR. Here, there is a risk of errors of judgment.