This study obtained institutional review board approval, and the need for informed patient consent was waived for this retrospective study. A prospectively enrolled patients in our institution that consisted of 577 consecutive patients who underwent CDT-VIBE DCE-MRI and RS-EPI DWI of the breast between January 2016 and August 2018 was queried for patients who fulfilled the following inclusion criteria: postoperative histopathology confirmed BC with receptor status, no recurrence, no previous treatment, not pregnant, and not breastfeeding. A total of 262 consecutive patients matched our search criteria. The exclusion criteria were as follows: (1) nonmass enhancement or multiple masses; (2) lesions< 1.0 cm; and (3) poor image quality or no lesion visible on MRI. The patient selection is outlined in Figure 1 .

IHC techniques were introduced to classify the different receptor statuses and molecular subtypes of BC. ER- and PR-positive tumors were defined as ≥1% positively stained tumor cells (15). HER2-positive tumors were defined as tumors with IHC staining of 3+ or gene amplification using fluorescence in situ hybridization demonstrated gene amplification >2.0 of tumors when IHC staining of 2+. Tumors with Ki-67≥20% were considered high Ki-67 expression tumors (16). According to the expression status of ER, PR, HER-2 and Ki-67, BC can be divided into four subtypes: luminal A, luminal B, HER-2 enriched, and TNBC. Luminal A was positive for ER and/or PR, Ki-67< 20% and HER-2-negative. Luminal B was ER- and/or PR-positive, HER-2-positive, or HER-2-negative when Ki-67 was ≥20%. HER-2 enrichment was ER- and PR-negative with HER-2 positivity. TNBC was all negative for ER, PR, and HER-2 (16).

All examinations were performed on a 3.0 T MRI scanner (Magnetom Skyra, Siemens, Germany) using a 16-channel bilateral breast coil with the patient positioned in the center of the magnet in the prone position. The MRI protocols included CDT-VIBE DCE-MRI, RS-EPI DWI, and conventional T₁-and T₂-weighted imaging. The RS-EPI sequence was performed prior to DCE-MRI. The parameters of the CDT-VIBE sequence (80-85 slices) were as follows: TR/TE = 6.4/3.3 ms, FA = 9°, FOV = 288 mm × 384 mm, matrix =448×314, bandwidth = 870 Hz,4 factor CAIPIRINHA acceleration, 2 mm slice thickness without slice gap. A total of 34 phases with a temporal resolution of 8.7 s were obtained within 5 min 5 s, starting 17.7 s after intravenous injection of 0.2 ml/kg of gadopentetate dimeglumine (Bayer Pharma AG) at an injection flow rate of 3.0 ml/s. An RS-EPI sequence (50-55 slices) with 2 b-values (50, 800 s/mm²) (TR/TE = 4800/56 ms, FA = 180°, FOV = 170 mm × 340 mm, matrix=96×192, bandwidth = 868 Hz, 4 mm slice thickness with 0.8 slice gap, and a total acquisition time of 4 min 58 s) was performed. T₁-weighted acquisition (TR = 5.5 ms, TE = 2.5 ms, FOV = 341 mm×341 mm, slice thickness = 1.5 mm, slice gap = 0.3 mm, bandwidth = 430 Hz, total acquisition time = 1 min 40 s) and T₂-weighted acquisition (TR = 3570 ms, TE = 74 ms, FOV = 341 mm×341 mm, slice thickness = 1.5 mm, slice gap = 0.3 mm, bandwidth = 248 Hz, total acquisition time =2 min 23 s) was also performed.

MRI images were independently evaluated by two senior radiologists with more than ten years of breast MRI experience. They were both blinded to the patients’ clinical history and histopathological results. DCE-MRI, DWI and ADC maps from each patient were transferred to the Siemens Syngo workstation. DCE-derived parametric maps of quantitative and semiquantitative parameters were automatically generated after motion correction, and registration was performed by using Tissue 4D software tool (Syngo via VB10, Siemens Healthcare).The pharmacokinetic parameters (17), including the volume transfer constant (K^trans, min^-1), extravascular-extracellular space volume ratio (V_e, min^-1), rate constant (K_ep=K^trans/V^e), rate of contrast enhancement for contrast agent inflow (W_-in, min^-1), rate of contrast decay for contrast agent outflow (W_-out, min^-1) and time-to-peak enhancement after contrast agent injection (TTP, min),were assessed with parametric maps by using the conventional Tofts model (9) and were then generated for each voxel defined by the region of interest (ROI). The RS-EPI DWI image (b= 800 s/mm²) and ADC maps were opened as read-only files, and the ADC value (×10 ^-3mm ²/s) for the RS-EPI DWI was measured in the same region of the lesion as the DCE-derived parametric maps. The breast tumors were identified on DCE-MRI images with the prominent area of enhancement corresponding to the hyperintense regions on DWI (b=800 s/mm²) and the hypointense regions on ADC maps. One 2D-ROI with a minimum area of 10 mm² was placed on the greatest representative slice of the tumor, avoiding obvious bleeding, visible necrosis, vessels, calcifications, and cystic appearing areas. The mean values were recorded after 3 repeated measurements.

To validate the stability of the measured parameters, Reader 1 performed tumor measurements in all 165 patients, and Reader 2 performed tumor measurements in 111 patients who were randomly selected from the whole cohort to assess interreader agreement. Only those parameters with an interclass correlation coefficient (ICC) value greater than 0.75 were selected for further experiments (18).

The statistical analysis was performed with R (version 3.5.1). The Mann–Whitney U test and t-test were used to compare the quantitative MRI parameters between the different receptor expression statuses. The Kruskal–Wallis H test and one-way ANOVA test were performed for different molecular subtypes.Multivariate analysis was performed to explore confounder adjusted associations among quantitative parameters, receptor status and molecular subtypes. We included all factors that were significant (P<0.05) upon univariate analysis. In addition, receiver operating characteristic (ROC) curve analysis was also carried out, and the optimal cutoff values of the quantitative parameters to predict receptor status and molecular subtype were determined according to the largest Youden index. The area under the curve (AUC), accuracy, sensitivity and specificity were also calculated.A P value <0.05 was considered to be statistically significant.

A total of 165 patients were included in this study, and the mean patient age was 50 years (range:25~81 years). Most had invasive ductal carcinoma (150 lesions), and there were five ductal carcinomas in situ, two medullary carcinomas, one adenosquamous carcinoma and seven invasive lobular carcinomas. The table of patient’s characteristics are presented in Table 1 . PR-positive patients were younger than PR-negative patients (P<0.01).

The ICC values of K^trans, K_ep, V_e, TTP, W_-in, W_-out, and ADC were 0.962 (95% CI: 0.946-0.974), 0.874 (95% CI: 0.822-0.912), 0.919 (95% CI: 0.884-0.944), 0.924 (95% CI: 0.891-0.947),0.878 (95% CI: 0.826-0.914), 0.901 (95% CI: 0.855-0.933), and 0.797 (95% CI: 0.718-0.856), respectively. All the ICC values were greater than the threshold value of 0.75.

The quantitative parameters of DCE-MRI and ADC for tumors stratified by receptor status are summarized in Table 1 . The K_ep values were lower when ER and PR were positive (all P<0.05). In contrast, the TTP value was higher when ER and PR were positive (all P<0.01). The K_ep and ADC values were higher when HER-2 was positive (all P<0.05).The W_-out value was higher when PR was positive (P<0.05). Box plot graphs revealing statistically significant differences in values for different receptor statuses are shown in Figure 2 . The parameters K^trans, W_-in, and TTP were not significantly different in the expression statuses of ER, PR, HER-2 or Ki-67 (all P>0.05).

According to the univariate analysis ( Table 2 ), a lower K_ep value (<0.704, OR=0.19, P=0.019) and a higher TTP value (> 0.585 min, OR=47.73, P=0.002) were significantly associated with ER-positive status. Younger age (<49.5 years, OR=0.96, P=0.008), a lower K_ep value (<0.704,OR=0.08, P=0.001), a higher W_-out value (<-0.005 min^-1,OR=13.5, P=0.031) and a higher TTP value (> 0.629 min, OR=41.9, P<0.001) were significantly associated with PR-positive status. A higher ADC value (> 0.757×10⁻³mm²/s, OR=215.9, P<0.001) was significantly associated with HER-2-positive status. In the multivariate analysis, a higher TTP value (adjusted OR=28.19, P=0.01) remained independently associated with ER-positive status, while younger age (adjusted OR=0.95, P=0.004), a lower K_ep value (adjusted OR=0.14, P =0.044), and a higher TTP value (adjusted OR=24.62, P =0.011) remained independently associated with PR-positive status.

Table 3 shows the values of the quantitative parameters for different molecular subtypes. We found that K_ep was the highest in HER-2-enriched cells and the lowest in luminal A cells (P<0.05). TTP was the highest in luminal A and the lowest in TNBC (P<0.01). ADC was the highest in HER-2-enriched cells and the lowest in TNBC (P<0.001).

Table 4 shows the pairwise comparison analysis of the parameters of different subtypes. Further paired comparisons revealed that the K_ep values were significantly different between luminal A and HER-2 enrichment and between luminal A and TNBC (all P<0.05), but only the difference between luminal A and TNBC was confirmed in the univariate analysis (P<0.05). The TTP value was significantly different between luminal A and TNBC, between luminal B and TNBC, and between HER-2-enriched BC and TNBC (all P< 0.05), and those differences were confirmed in the univariate analysis (all P<0.05).The ADC value was significantly different between luminal A and TNBC, between luminal B and HER-2-enriched BC, and between HER-2-enriched BC and TNBC (all P<0.05), and those differences were confirmed in the univariate analysis (all P<0.05). In the multivariate analysis lower TTP (<0.582 min, adjusted OR<0.001,P =0.004) and lower ADC values (<0.719×10^-3 mm²/s, adjusted OR<0.001,P =0.048) were significantly associated with TNBC compared to luminal A; a lower ADC value (<0.759 ×10^-3 mm²/s, adjusted OR<0.001, P =0.002) was significantly associated with TNBC compared to HER-2 enrichment. Representative K_ep, TTP and ADC images in luminal A and TNBC are shown in Figure 3 .

The cutoff value, AUC, accuracy, sensitivity and specificity of these MR parameters with significant differences were further calculated using ROC curves ( Table 5 and Figure 4 ). In identifying TNBC and luminal A, the combination model of K_ep, TTP and ADC achieved the highest performance in the differential diagnosis, with an AUC of 0.811 (95% CI: 0.708–0.915). In identifying HER-2-enriched BC and TNBC, the combination model of ADC and TTP achieved the highest performance in the differential diagnosis, with an AUC of 0.793 (95% CI: 0.681–0.904).