Evaluation of low-energy contrast-enhanced spectral mammography images by comparing them to full-field digital mammography using EUREF image quality criteria

CESM is a promising new breast imaging modality that has been shown to be superior to conventional mammography in the detection of breast cancer [2, 3, 10]. No studies so far have compared the quality of the LE images to that of FFDM in a standardized and structured manner, using well established EUREF image quality criteria. We demonstrated that for the majority of image quality criteria there were neither clinically relevant nor statistically significant differences between these two modalities. Interestingly, we observed that the visualization of micro-calcifications is superior in LE images. Based on additional dose and contrast detail measurements, we could not find any physical explanation for these observations.

In 17 of 20 EUREF criteria no statistically significant differences were observed, indicating that there is hardly any difference between FFDM and LE in terms of image quality. The delineation of the pectoral muscle was scored significantly lower for LE when compared to FFDM. However, in our experience this will not cause any relevant diagnostic dilemmas, as the recombined image can also be used for assessment of lesions covering the pectoral muscle. In a worst-case scenario, an additional FFDM in mediolateral oblique view can still be considered if the lesion of interest is adjacent to the pectoral muscle. Figure 4 shows an example of the obscured pectoral muscle on LE versus FFDM. More interestingly, we observed a statistically significant improvement of micro-calcification visualization in LE images when compared to FFDM. This latter finding can indeed be of interest, as it suggests that ductal carcinoma in situ can be diagnosed more confidently or its extent might be assessed more accurately using LE CESM images. However, the limited number of DCIS cases (n = 7) in this study restrained us from further testing this hypothesis.

To further explain the differences in image quality scores we performed additional dose and contrast detail measurements. The average AGD of the FFDM exposure is 1.7 ± 0.4 mGy, while the AGD of the LE exposure is 2.3 ± 0.7 mGy. The difference reflects differences in AOP settings for FFDM and LE CESM, i.e. target/filter combination, tube voltage and tube load, where for LE CESM the prerequisite was that the LE acquisition should be suitable for reconstruction of the contrast-enhanced image. Furthermore, the doses used in these examinations were in agreement with results from a previous study [11] showing that the observed image quality differences could not be explained by using abnormal radiation doses in one of the techniques. We also performed CDMAM contrast detail measurements to investigate whether the observed differences in scoring could be explained. The different scores concerning pectoral muscle visibility could not be assessed by these measurements as this structure is not represented in the phantom. The detectability of smaller diameter gold disks in this phantom was found to relate to micro-calcification detection [12]. We did not observe a significant difference in the contrast detail curves, indicating that FFDM and LE CESM have similar contrast detail detection. FFDM and LE CESM both comply with the acceptable as well as the achievable limits for contrast threshold visibility. Since the CDMAM analysis was performed on the unprocessed images, it can be concluded that the observed differences were not due to hardware differences. A possible explanation for the improved calcification detection may be found in different post-processing techniques performed to obtain the final image from the unprocessed images. Unfortunately, we are not able to further evaluate the effect of these post-processing steps in our phantom or clinical measurements.

Only two previous papers focused on the potential of LE images as replacement for FFDM. Fallenberg et al. [13] focused on the ability of cancer detection and size estimation of bilateral CESM, digital mammography, and the combination of CESM and mammography in 107 patients. They did not use image quality criteria such as those suggested by EUREF. In their study, sensitivity of mammography was 77.9 %, which increased to approximately 95 % when using CESM alone or the combination of CESM and mammography, while CESM alone showed the best correlation with pathology regarding tumour size measurements. Although there are minimal and hardly noticeable differences in attenuation due to the iodine contrast agent already present in the breast during the LE acquisition, the improved sensitivity was suggested to be attributed to a slight enhancement of cancers even on the LE images. In addition, our observation suggests that micro-calcifications can be better visualized on LE images. Most importantly, neither Fallenberg et al.’s study nor our study showed any signs of inferiority of LE images compared to mammography.

Francescone et al. [14] studied 170 examinations in 88 women and compared the LE images with FFDM images that were obtained within 6 months. They mainly focused on technical parameters including posterior nipple line distance, compression thickness and compression force on the MLO projection. Similar to Fallenberg et al., they compared tumour size measurements on both modalities. An important limitation to this study was that the readers were not blinded and analysis was performed in a side-by-side comparison, thereby potentially introducing bias. No statistically significant differences were observed for all studies’ parameters between LE and FFDM, but this study also did not use standardized image quality criteria such as suggested by EUREF.

Although the studies of Fallenberg et al. and Francescone et al. suggested non-inferiority of LE CESM compared to FFDM, our study contains important additional information as it assessed differences by using well-defined image quality criteria in a structured and standardized manner in the largest study population so far. Nevertheless, when all current evidence is summarized, it is valid and safe to conclude that LE CESM images are indeed non-inferior to FFDM, and might even be superior in terms of breast cancer detection and micro-calcification evaluation. This means that FFDM can be omitted if a direct indication for CESM exists, for example in (postmenopausal) women with a suspicious palpable breast mass, or women with incidental suspicious breast findings observed in other imaging exams, such as chest CT examinations.

Our study has some limitations. First, FFDM images were made with a different mammography unit than the LE images. Both radiologists, however, were experienced in reading images from both mammography units. Consequently, they could distinguish the FFDM from the LE images based on their appearance alone. However, our current (random) reading strategy, including a 6-week interval, minimized the effect of any recall bias. Another limitation is that in our contrast detail phantom studies, we used the unprocessed images for data analysis. This is the standardized method, recommended by EUREF, as the post-processing may introduce image artefacts and may slightly differ from the mammography post-processing as an object-dependent histogram of texture analysis may be used. Therefore, it was not possible to determine whether post-processing improved the contrast-detail detection, and, thus, whether this would be an explanation for the significantly improved detection of micro-calcifications by LE images.